Dropwizard Core

The dropwizard-core module provides you with everything you’ll need for most of your applications.

It includes:

Jetty, a high-performance HTTP server.
Jersey, a full-featured RESTful web framework.
Jackson, the best JSON library for the JVM.
Metrics, an excellent library for application metrics.
Logback, the successor to Log4j, Java’s most widely-used logging framework.
Hibernate Validator, the reference implementation of the Java Bean Validation standard.

Dropwizard consists mostly of glue code to automatically connect and configure these components.

Organizing Your Project

If you plan on developing a client library for other developers to access your service, we recommend you separate your projects into three Maven modules: project-api, project-client, and project-application.

project-api should contain your Representations; project-client should use those classes and an HTTP client to implement a full-fledged client for your application, and project-application should provide the actual application implementation, including Resources.

To give a concrete example of this project structure, let’s say we wanted to create a Stripe-like API where clients can issue charges and the server would echo the charge back to the client. stripe-api project would hold our Charge object as both the server and client want to work with the charge and to promote code reuse, Charge objects are stored in a shared module. stripe-app is the Dropwizard application. stripe-client abstracts away the raw HTTP interactions and deserialization logic. Instead of using a HTTP client, users of stripe-client would just pass in a Charge object to a function and behind the scenes, stripe-client will call the HTTP endpoint. The client library may also take care of connection pooling, and may provide a more friendly way of interpreting error messages. Basically, distributing a client library for your app will help other developers integrate more quickly with the service.

If you are not planning on distributing a client library for developers, one can combine project-api and project-application into a single project, which tends to look like this:

com.example.myapplication:
- api: Representations. Request and response bodies.
- cli: Commands
- client: Client code that accesses external HTTP services.
- core: Domain implementation; where objects not used in the API such as POJOs, validations, crypto, etc, reside.
- db: Database access classes
- health: Health Checks
- resources: Resources
- MyApplication: The application class
- MyApplicationConfiguration: configuration class

Application

The main entry point into a Dropwizard application is, unsurprisingly, the Application class. Each Application has a name, which is mostly used to render the command-line interface. In the constructor of your Application you can add Bundles and Commands to your application.

Configuration

Dropwizard provides a number of built-in configuration parameters. They are well documented in the example project’s configuration and configuration reference.

Each Application subclass has a single type parameter: that of its matching Configuration subclass. These are usually at the root of your application’s main package. For example, your User application would have two classes: UserApplicationConfiguration, extending Configuration, and UserApplication, extending Application<UserApplicationConfiguration>.

When your application runs Configured Commands like the server command, Dropwizard parses the provided YAML configuration file and builds an instance of your application’s configuration class by mapping YAML field names to object field names.

Note

If your configuration file doesn’t end in .yml or .yaml, Dropwizard tries to parse it as a JSON file.

To keep your configuration file and class manageable, we recommend grouping related configuration parameters into independent configuration classes. If your application requires a set of configuration parameters in order to connect to a message queue, for example, we recommend that you create a new MessageQueueFactory class:

Source file

public final class MessageQueueFactory {
    @NotEmpty
    private String host;

    @Min(1)
    @Max(65535)
    private int port = 5672;

    @JsonProperty
    public String getHost() {
        return host;
    }

    @JsonProperty
    public void setHost(String host) {
        this.host = host;
    }

    @JsonProperty
    public int getPort() {
        return port;
    }

    @JsonProperty
    public void setPort(int port) {
        this.port = port;
    }

    public MessageQueueClient build(Environment environment) {
        MessageQueueClient client = new MessageQueueClient(getHost(), getPort());
        environment.lifecycle().manage(new Managed() {
            @Override
            public void start() {
            }

            @Override
            public void stop() {
                client.close();
            }
        });
        return client;
    }
}

In this example our factory will automatically tie our MessageQueueClient connection to the lifecycle of our application’s Environment.

Your main Configuration subclass can then include this as a member field:

Source file

public class ExampleConfiguration extends Configuration {
    @Valid
    @NotNull
    private MessageQueueFactory messageQueue = new MessageQueueFactory();

    @JsonProperty("messageQueue")
    public MessageQueueFactory getMessageQueueFactory() {
        return messageQueue;
    }

    @JsonProperty("messageQueue")
    public void setMessageQueueFactory(MessageQueueFactory factory) {
        this.messageQueue = factory;
    }
}

And your Application subclass can then use your factory to directly construct a client for the message queue:

Source file

public void run(ExampleConfiguration configuration,
                Environment environment) {
    MessageQueueClient messageQueue = configuration.getMessageQueueFactory().build(environment);
}

Then, in your application’s YAML file, you can use a nested messageQueue field:

Source file

messageQueue:
  host: mq.example.com
  port: 5673

The @NotNull, @NotEmpty, @Min, @Max, and @Valid annotations are part of Dropwizard Validation functionality. If your YAML configuration file’s messageQueue.host field was missing (or was a blank string), Dropwizard would refuse to start and would output an error message describing the issues.

Once your application has parsed the YAML file and constructed its Configuration instance, Dropwizard then calls your Application subclass to initialize your application’s Environment.

Note

You can override configuration settings by passing special Java system properties when starting your application. Overrides must start with prefix dw., followed by the path to the configuration value being overridden.

For example, to override the Logging level, you could start your application like this:

java -Ddw.logging.level=DEBUG server my-config.json

This will work even if the configuration setting in question does not exist in your config file, in which case it will get added.

You can override configuration settings in arrays of objects like this:

java -Ddw.server.applicationConnectors[0].port=9090 server my-config.json

You can override configuration settings in maps like this:

java -Ddw.database.properties.hibernate.hbm2ddl.auto=none server my-config.json

If you need to use the ‘.’ character in one of the values, you can escape it by using ‘\.’ instead.

You can also override a configuration setting that is an array of strings by using the ‘,’ character as an array element separator. For example, to override a configuration setting myapp.myserver.hosts that is an array of strings in the configuration, you could start your service like this: java -Ddw.myapp.myserver.hosts=server1,server2,server3 server my-config.json

If you need to use the ‘,’ character in one of the values, you can escape it by using ‘\,’ instead.

The array override facility only handles configuration elements that are arrays of simple strings. Also, the setting in question must already exist in your configuration file as an array; this mechanism will not work if the configuration key being overridden does not exist in your configuration file. If it does not exist or is not an array setting, it will get added as a simple string setting, including the ‘,’ characters as part of the string.

Environment variables

The dropwizard-configuration module also provides the capabilities to substitute configuration settings with the value of environment variables using a SubstitutingSourceProvider and EnvironmentVariableSubstitutor.

Source file

public void initialize(Bootstrap<ExampleConfiguration> bootstrap) {
    // Enable variable substitution with environment variables
    EnvironmentVariableSubstitutor substitutor = new EnvironmentVariableSubstitutor(false);
    SubstitutingSourceProvider provider =
            new SubstitutingSourceProvider(bootstrap.getConfigurationSourceProvider(), substitutor);
    bootstrap.setConfigurationSourceProvider(provider);
}

The configuration settings which should be substituted need to be explicitly written in the configuration file and follow the substitution rules of StringSubstitutor from the Apache Commons Text library.

Source file

mySetting: ${DW_MY_SETTING}
defaultSetting: ${DW_DEFAULT_SETTING:-default value}

In general SubstitutingSourceProvider isn’t restricted to substitute environment variables but can be used to replace variables in the configuration source with arbitrary values by passing a custom StringSubstitutor implementation.

SSL

SSL support is built into Dropwizard. You will need to provide your own java keystore, which is outside the scope of this document (keytool is the command you need, and Jetty’s documentation can get you started). There is a test keystore you can use in the Dropwizard example project.

Source file

---
server:
  applicationConnectors:
    - type: https
      port: 8443
      keyStorePath: example.keystore
      keyStorePassword: example
      validateCerts: false

By default, only secure TLSv1.2 cipher suites are allowed. Older versions of cURL, Java 6 and 7, and other clients may be unable to communicate with the allowed cipher suites, but this was a conscious decision that sacrifices interoperability for security.

Dropwizard allows a workaround by specifying a customized list of cipher suites. If no lists of supported protocols or cipher suites are specified, then the JVM defaults are used. If no lists of excluded protocols or cipher suites are specified, then the defaults are inherited from Jetty.

The following list of excluded cipher suites will allow for TLSv1 and TLSv1.1 clients to negotiate a connection similar to pre-Dropwizard 1.0.

Source file

---
server:
  applicationConnectors:
    - type: https
      port: 8443
      excludedCipherSuites:
        - SSL_RSA_WITH_DES_CBC_SHA
        - SSL_DHE_RSA_WITH_DES_CBC_SHA
        - SSL_DHE_DSS_WITH_DES_CBC_SHA
        - SSL_RSA_EXPORT_WITH_RC4_40_MD5
        - SSL_RSA_EXPORT_WITH_DES40_CBC_SHA
        - SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
        - SSL_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA

Since the version 9.4.8 (Dropwizard 1.2.3) Jetty supports native SSL via Google’s Conscrypt which uses BoringSSL (Google’s fork of OpenSSL) for handling cryptography. You can enable it in Dropwizard by registering the provider in your app:

Source file

<dependency>
    <groupId>org.conscrypt</groupId>
    <artifactId>conscrypt-openjdk-uber</artifactId>
    <version>${conscrypt.version}</version>
</dependency>

Source file

static {
    Security.insertProviderAt(new OpenSSLProvider(), 1);
}

and setting the JCE provider in the configuration:

Source file

---
server:
  type: simple
  connector:
    type: https
    jceProvider: Conscrypt

For HTTP/2 servers you need to add an ALPN Conscrypt provider as a dependency.

Source file

<dependency>
    <groupId>org.eclipse.jetty</groupId>
    <artifactId>jetty-alpn-conscrypt-server</artifactId>
</dependency>

Note

If you are using Conscrypt with Java 8, you must exclude TLSv1.3 protocol as it is now enabled per default with Conscrypt 2.0.0 but not supported by Java 8.

Bootstrapping

Before a Dropwizard application can provide the command-line interface, parse a configuration file, or run as a server, it must first go through a bootstrapping phase. This phase corresponds to your Application subclass’s initialize method. You can add Bundles, Commands, or register Jackson modules to allow you to include custom types as part of your configuration class.

Environments

A Dropwizard Environment consists of all the Resources, servlets, filters, Health Checks, Health, Jersey providers, Managed Objects, Tasks, and Jersey properties which your application provides.

Each Application subclass implements a run method. This is where you should be creating new resource instances, etc., and adding them to the given Environment class:

@Override
public void run(ExampleConfiguration config,
                Environment environment) {
    // encapsulate complicated setup logic in factories
    final Thingy thingy = config.getThingyFactory().build();

    environment.jersey().register(new ThingyResource(thingy));
    environment.healthChecks().register("thingy", new ThingyHealthCheck(thingy));
}

It’s important to keep the run method clean, so if creating an instance of something is complicated, like the Thingy class above, extract that logic into a factory.

Health Checks

A health check is a runtime test which you can use to verify your application’s behavior in its production environment. For example, you may want to ensure that your database client is connected to the database:

Source file

public class DatabaseHealthCheck extends HealthCheck {
    private final Database database;

    public DatabaseHealthCheck(Database database) {
        this.database = database;
    }

    @Override
    protected Result check() throws Exception {
        if (database.isConnected()) {
            return Result.healthy();
        } else {
            return Result.unhealthy("Cannot connect to " + database.getUrl());
        }
    }
}

You can then add this health check to your application’s environment:

Source file

environment.healthChecks().register("database", new DatabaseHealthCheck(database));

By sending a GET request to /healthcheck on the admin port you can run these tests and view the results:

$ curl http://dw.example.com:8081/healthcheck
{"deadlocks":{"healthy":true},"database":{"healthy":true}}

If all health checks report success, a 200 OK is returned. If any fail, a 500 Internal Server Error is returned with the error messages and exception stack traces (if an exception was thrown).

Note

This behavior overlaps in many ways with the new Health functionality. If you wish to disable the admin health servlet, a new flag was introduced into the health check configuration Health checks to allow disabling it.

All Dropwizard applications ship with the deadlocks health check installed by default, which uses Java 1.6’s built-in thread deadlock detection to determine if any threads are deadlocked.

Health

The health checks described in Health Checks can be configured to create a holistic view of your service health, which can then be used to drive decision making by things like Kubernetes readiness & liveness checks, or to dictate whether or not a load balancer should forward traffic to your service.

This can be done by running these dependency health checks periodically in the background on some schedule, and then aggregating the results of all of those checks into a single indicator of overall health. Certain dependencies may be critical to your application functioning, like a database that your service can’t function without, but other dependencies may be more non-critical to your service being able to function (let’s say a cache, that could be considered more of a nice to have than a necessity).

Define the following health check configurations in your config.yml file:

health:
  delayedShutdownHandlerEnabled: true
  shutdownWaitPeriod: 10s
  healthChecks:
    - name: user-database
      critical: true
    - name: user-notifications-queue
      critical: false
      schedule:
        checkInterval: 2500ms
        downtimeInterval: 10s
        failureAttempts: 2
        successAttempts: 1
    - name: user-cache
      critical: false

Note

This behavior was integrated from the Dropwizard Health module. If you are migrating from that module to the new Dropwizard core framework health code, you will want to refer to Migrating from dropwizard-health the migration guide.

Application Status

There are two types of status that are supported currently: Alive and Ready.

An alive status indicates the application is operating normally and does not need to be restarted to recover from a stuck state. Long-running applications can eventually reach a broken state and cannot recover except by being restarted (e.g. deadlocked threads).
A ready status indicates the application is ready to serve traffic. Applications can temporarily be unable to serve traffic due to a variety of reasons, for example, an application might need to build/compute large caches during startup or can critically depend on an external service.

An example of how you might query the health check, assuming you’re using the default responder/responseProvider settings in configuration:

https://<hostname>:<port>/health-check?type=<type>&name=<name>

replace <type> with ready or alive; defaults to ready if the type parameter is not provided
replace <name> with the name of the health check to query. Multiple names can be provided, or no no names. If all checks are desired, name=all can be specified to retrieve all checks

HTTP & TCP Checks

Should your service have any dependencies that it needs to perform health checks against that expose either an HTTP or TCP health check interface, you can use the HttpHealthCheck or TcpHealthCheck classes to do so easily.

You will need to register your health check(s) in your Application class run() method.

HTTP

@Override
public void run(final AppConfiguration configuration, final Environment environment) {
    ...
    environment.healthChecks().register("some-http-dependency", new HttpHealthCheck("http://some-http-dependency.com:8080/health-check"));
}

TCP

@Override
public void run(final AppConfiguration configuration, final Environment environment) {
    ...
    environment.healthChecks().register("some-tcp-dependency", new TcpHealthCheck("some-tcp-dependency.com", 443));
}

Health Data Access

In the Application.run() method, you can access views of the health state data in two different ways:

Accessing data directly

@Override
public void run(final AppConfiguration configuration, final Environment environment) {
    ...
    Collection<HealthStateView> views = environment.health().healthStateAggregator().healthStateViews();
}

Listening to data changes

@Override
public void run(final AppConfiguration configuration, final Environment environment) {
    ...
    HealthStateListener myListener = new HealthStateListener() {
        @Override
        public void onStateChanged(String healthCheckName, boolean healthy) {
            System.out.println(healthCheckName + "changed state to " + healthy);
        }

        @Override
        public void onHealthyCheck(String healthCheckName) {
            System.out.println(healthCheckName + "is healthy! :)");
        }

        @Override
        public void onUnhealthyCheck(String healthCheckName) {
            System.out.println(healthCheckName + "is unhealthy! :(");
        }
    };
    environment.health().addHealthStateListener(myListener);
}

Managed Objects

Most applications involve objects which need to be started and stopped: thread pools, database connections, etc. Dropwizard provides the Managed interface for this. You can either have the class in question implement the #start() and/or #stop() methods, or write a wrapper class which does so. Adding a Managed instance to your application’s Environment ties that object’s lifecycle to that of the application’s HTTP server. Before the server starts, the #start() method is called. After the server has stopped (and after its graceful shutdown period) the #stop() method is called.

For example, given a theoretical Riak client which needs to be started and stopped:

Source file

public class RiakClientManager implements Managed {
    private final RiakClient client;

    public RiakClientManager(RiakClient client) {
        this.client = client;
    }

    @Override
    public void start() throws Exception {
        client.start();
    }

    @Override
    public void stop() throws Exception {
        client.stop();
    }
}

Source file

public class ManagedApp extends Application<Configuration> {
    @Override
    public void run(Configuration configuration, Environment environment) {
        RiakClient client = new RiakClient();
        RiakClientManager riakClientManager = new RiakClientManager(client);
        environment.lifecycle().manage(riakClientManager);
    }
}

If RiakClientManager#start() throws an exception–e.g., an error connecting to the server–your application will not start and a full exception will be logged. If RiakClientManager#stop() throws an exception, the exception will be logged but your application will still be able to shut down.

It should be noted that Environment has built-in factory methods for ExecutorService and ScheduledExecutorService instances which are managed. These managed instances use InstrumentedThreadFactory that monitors the number of threads created, running and terminated

Source file

public void run(Configuration configuration, Environment environment) {
    ExecutorService executorService = environment.lifecycle()
            .executorService(nameFormat)
            .maxThreads(maxThreads)
            .build();

    ScheduledExecutorService scheduledExecutorService = environment.lifecycle()
            .scheduledExecutorService(nameFormat)
            .build();
}

Bundles

A Dropwizard bundle is a reusable group of functionality, used to define blocks of an application’s behavior by implementing the ConfiguredBundle interface.

For example, AssetBundle from the dropwizard-assets module provides a simple way to serve static assets from your application’s src/main/resources/assets directory as files available from /assets/* (or any other path) in your application.

Given the bundle MyConfiguredBundle and the interface MyConfiguredBundleConfig below, your application’s Configuration subclass would need to implement MyConfiguredBundleConfig.

Source file

public class MyConfiguredBundle implements ConfiguredBundle<MyConfiguredBundleConfig> {
    @Override
    public void run(MyConfiguredBundleConfig applicationConfig, Environment environment) {
        applicationConfig.getBundleSpecificConfig();
    }

    @Override
    public void initialize(Bootstrap<?> bootstrap) {
    }
}

Source file

public interface MyConfiguredBundleConfig {
    String getBundleSpecificConfig();
}

Serving Assets

Either your application or your static assets can be served from the root path, but not both. The latter is useful when using Dropwizard to back a Javascript application. To enable it, move your application to a sub-URL.

server:
  rootPath: /api/

Note

If you use the Simple server configuration, then rootPath is calculated relatively from applicationContextPath. So, your API will be accessible from the path /application/api/

Then use an extended AssetsBundle constructor to serve resources in the assets folder from the root path. index.htm is served as the default page.

Source file

bootstrap.addBundle(new AssetsBundle("/assets/", "/"));

When an AssetBundle is added to the application, it is registered as a servlet using a default name of assets. If the application needs to have multiple AssetBundle instances, the extended constructor should be used to specify a unique name for the AssetBundle.

Source file

bootstrap.addBundle(new AssetsBundle("/assets/css", "/css", null, "css"));
bootstrap.addBundle(new AssetsBundle("/assets/js", "/js", null, "js"));
bootstrap.addBundle(new AssetsBundle("/assets/fonts", "/fonts", null, "fonts"));

SSL Reload

By registering the SslReloadBundle your application can have new certificate information reloaded at runtime, so a restart is not necessary.

Source file

public void initialize(Bootstrap<Configuration> bootstrap) {
    bootstrap.addBundle(new SslReloadBundle());
}

To trigger a reload send a POST request to ssl-reload

curl -k -X POST 'https://localhost:<admin-port>/tasks/ssl-reload'

Dropwizard will use the same exact https configuration (keystore location, password, etc) when performing the reload.

Note

If anything is wrong with the new certificate (eg. wrong password in keystore), no new certificates are loaded. So if the application and admin ports use different certificates and one of them is invalid, then none of them are reloaded.

A http 500 error is returned on reload failure, so make sure to trap for this error with whatever tool is used to trigger a certificate reload, and alert the appropriate admin. If the situation is not remedied, next time the app is stopped, it will be unable to start!

Commands

Commands are basic actions which Dropwizard runs based on the arguments provided on the command line. The built-in server command, for example, spins up an HTTP server and runs your application. Each Command subclass has a name and a set of command line options which Dropwizard will use to parse the given command line arguments.

Below is an example on how to add a command and have Dropwizard recognize it.

Source file

public class MyCommand extends Command {
    public MyCommand() {
        // The name of our command is "hello" and the description printed is
        // "Prints a greeting"
        super("hello", "Prints a greeting");
    }

    @Override
    public void configure(Subparser subparser) {
        // Add a command line option
        subparser.addArgument("-u", "--user")
                .dest("user")
                .type(String.class)
                .required(true)
                .help("The user of the program");
    }

    @Override
    public void run(Bootstrap<?> bootstrap, Namespace namespace) throws Exception {
        System.out.println("Hello " + namespace.getString("user"));
    }
}

Dropwizard recognizes our command once we add it in the initialize stage of our application.

Source file

public class CustomCommandApp extends Application<Configuration> {
    @Override
    public void initialize(Bootstrap<Configuration> bootstrap) {
        bootstrap.addCommand(new MyCommand());
    }

    @Override
    public void run(Configuration configuration, Environment environment) {
    }
}

To invoke the new functionality, run the following:

java -jar <jarfile> hello dropwizard

Configured Commands

Some commands require access to configuration parameters and should extend the ConfiguredCommand class, using your application’s Configuration class as its type parameter. By default, Dropwizard will treat the last argument on the command line as the path to a YAML configuration file, parse and validate it, and provide your command with an instance of the configuration class.

A ConfiguredCommand can have additional command line options specified, while keeping the last argument the path to the YAML configuration.

Source file

public class MyConfiguredCommand extends ConfiguredCommand<Configuration> {
    public MyConfiguredCommand() {
        // The name of our command is "hello" and the description printed is
        // "Prints a greeting"
        super("hello", "Prints a greeting");
    }

    @Override
    public void configure(Subparser subparser) {
        // Add a command line option
        subparser.addArgument("-u", "--user")
                .dest("user")
                .type(String.class)
                .required(true)
                .help("The user of the program");
    }

    @Override
    protected void run(Bootstrap<Configuration> bootstrap,
                       Namespace namespace,
                       Configuration configuration) throws Exception {
        System.out.println("Hello " + namespace.getString("user"));
    }
}

For more advanced customization of the command line (for example, having the configuration file location specified by -c), adapt the ConfiguredCommand class as needed.

Note

If you override the configure method, you must call super.override(subparser) (or call addFileArgument) in order to preserve the configuration file parameter in the subparser.

Tasks

A Task is a run-time action your application provides access to on the administrative port via HTTP. All Dropwizard applications start with: the gc task, which explicitly triggers the JVM’s garbage collection (This is useful, for example, for running full garbage collections during off-peak times or while the given application is out of rotation.); and the log-level task, which configures the level of any number of loggers at runtime (akin to Logback’s JmxConfigurator). The execute method of a Task can be annotated with @Timed, @Metered, and @ExceptionMetered. Dropwizard will automatically record runtime information about your tasks. Here’s a basic task class:

Source file

public class TruncateDatabaseTask extends Task {
    private final Database database;

    public TruncateDatabaseTask(Database database) {
        super("truncate");
        this.database = database;
    }

    @Override
    public void execute(Map<String, List<String>> parameters, PrintWriter output) throws Exception {
        this.database.truncate();
    }
}

You can then add this task to your application’s environment:

Source file

public void run(Configuration configuration, Environment environment) {
    Database database = new Database();
    environment.admin().addTask(new TruncateDatabaseTask(database));
}

Running a task can be done by sending a POST request to /tasks/{task-name} on the admin port. The task will receive any query parameters as arguments. For example:

$ curl -X POST http://dw.example.com:8081/tasks/gc
Running GC...
Done!

You can also extend PostBodyTask to create a task which uses the body of the post request. Here’s an example:

Source file

public class EchoTask extends PostBodyTask {
    public EchoTask() {
        super("echo");
    }

    @Override
    public void execute(Map<String, List<String>> parameters, String body, PrintWriter output) throws Exception {
        output.write(body);
        output.flush();
    }
}

Logging

Dropwizard uses Logback for its logging backend. It provides an slf4j implementation, and even routes all java.util.logging, Log4j, and Apache Commons Logging usage through Logback.

slf4j provides the following logging levels:

ERROR: Error events that might still allow the application to continue running.
WARN: Potentially harmful situations.
INFO: Informational messages that highlight the progress of the application at coarse-grained level.
DEBUG: Fine-grained informational events that are most useful to debug an application.
TRACE: Finer-grained informational events than the DEBUG level.

Note

If you don’t want to use Logback, you can exclude it from Dropwizard and use an alternative logging configuration:

Exclude Logback from the dropwizard-core artifact

<dependency>
    <groupId>io.dropwizard</groupId>
    <artifactId>dropwizard-core</artifactId>
    <version>{$dropwizard.version}</version>
    <exclusions>
        <exclusion>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
        </exclusion>
        <exclusion>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-access</artifactId>
        </exclusion>
        <exclusion>
            <groupId>org.slf4j</groupId>
            <artifactId>log4j-over-slf4j</artifactId>
        </exclusion>
    </exclusions>
</dependency>

Mark the logging configuration as external in your Dropwizard config

server:
  type: simple
  applicationContextPath: /application
  adminContextPath: /admin
  requestLog:
    type: external
logging:
  type: external

Disable bootstrapping Logback in your application

public class ExampleApplication extends Application<ExampleConfiguration> {

    @Override
    protected void bootstrapLogging() {
    }
}

Log Format

Dropwizard’s log format has a few specific goals:

Be human readable.
Be machine parsable.
Be easy for sleepy ops folks to figure out why things are pear-shaped at 3:30AM using standard UNIXy tools like tail and grep.

The logging output looks like this:

TRACE [2010-04-06 06:42:35,271] com.example.dw.Thing: Contemplating doing a thing.
DEBUG [2010-04-06 06:42:35,274] com.example.dw.Thing: About to do a thing.
INFO  [2010-04-06 06:42:35,274] com.example.dw.Thing: Doing a thing
WARN  [2010-04-06 06:42:35,275] com.example.dw.Thing: Doing a thing
ERROR [2010-04-06 06:42:35,275] com.example.dw.Thing: This may get ugly.
! java.lang.RuntimeException: oh noes!
! at com.example.dw.Thing.run(Thing.java:16)
!

A few items of note:

All timestamps are in UTC and ISO 8601 format.
You can grep for messages of a specific level really easily:
```
tail -f dw.log | grep '^WARN'
```
You can grep for messages from a specific class or package really easily:
```
tail -f dw.log | grep 'com.example.dw.Thing'
```
You can even pull out full exception stack traces, plus the accompanying log message:
```
tail -f dw.log | grep -B 1 '^\!'
```
The ! prefix does not apply to syslog appenders, as stack traces are sent separately from the main message. Instead, t is used (this is the default value of the SyslogAppender that comes with Logback). This can be configured with the stackTracePrefix option when defining your appender.

To apply the prefixing with the ! symbol Dropwizard introduces several new Logback conversion words. These are dwEx, dwException, dwThrowable, dwXEx, dwXException, dwXThrowable, dwREx and dwRootException. These conversion words work like the ones from Logback, except that the first tab of a stack trace is replaced by a !.

The default Dropwizard logging layout uses the Dropwizard specific conversion words.

Configuration

You can specify a default logger level, override the levels of other loggers in your YAML configuration file, and even specify appenders for them. The latter form of configuration is preferable, but the former is also acceptable.

# Logging settings.
logging:

  # The default level of all loggers. Can be OFF, ERROR, WARN, INFO, DEBUG, TRACE, or ALL.
  level: INFO

  # Logger-specific levels.
  loggers:

    # Overrides the level of com.example.dw.Thing and sets it to DEBUG.
    "com.example.dw.Thing": DEBUG

    # Enables the SQL query log and redirect it to a separate file
    "org.hibernate.SQL":
      level: DEBUG
      # This line stops org.hibernate.SQL (or anything under it) from using the root logger
      additive: false
      appenders:
        - type: file
          currentLogFilename: ./logs/example-sql.log
          archivedLogFilenamePattern: ./logs/example-sql-%d.log.gz
          archivedFileCount: 5

Asynchronous Logging

By default, all logging in Dropwizard is asynchronous, even to typically synchronous sinks such as files and the console. When a slow logger (like file logger on an overloaded disk) is coupled with a high load, Dropwizard will seamlessly drop events of lower importance (TRACE, DEBUG, INFO) in an attempt to maintain reasonable latency.

Tip

Instead of logging business critical statements under INFO, insert them into a database, durable message queue, or another mechanism that gives confidence that the request has satisfied business requirements before returning the response to the client.

This logging behavior can be configured:

Set discardingThreshold to 0 so that no events are dropped
At the opposite end, set neverBlock to true so that even WARN and ERROR levels will be discarded from logging under heavy load

Request access logging has the same logging behavior, and since all request logging is done under INFO, each log statement has an equal chance of being dropped if the log queue is nearing full.

Console Logging

By default, Dropwizard applications log INFO and higher to STDOUT. You can configure this by editing the logging section of your YAML configuration file:

logging:
  appenders:
    - type: console
      threshold: WARN
      target: stderr

In the above, we’re instead logging only WARN and ERROR messages to the STDERR device.

File Logging

Dropwizard can also log to an automatically rotated set of log files. This is the recommended configuration for your production environment:

logging:

  appenders:
    - type: file
      # The file to which current statements will be logged.
      currentLogFilename: ./logs/example.log

      # When the log file rotates, the archived log will be renamed to this and gzipped. The
      # %d is replaced with the previous day (yyyy-MM-dd). Custom rolling windows can be created
      # by passing a SimpleDateFormat-compatible format as an argument: "%d{yyyy-MM-dd-hh}".
      archivedLogFilenamePattern: ./logs/example-%d.log.gz

      # The number of archived files to keep.
      archivedFileCount: 5

      # The timezone used to format dates. HINT: USE THE DEFAULT, UTC.
      timeZone: UTC

Syslog Logging

Finally, Dropwizard can also log statements to syslog.

Note

Because Java doesn’t use the native syslog bindings, your syslog server must have an open network socket.

logging:

  appenders:
    - type: syslog
      # The hostname of the syslog server to which statements will be sent.
      # N.B.: If this is the local host, the local syslog instance will need to be configured to
      # listen on an inet socket, not just a Unix socket.
      host: localhost

      # The syslog facility to which statements will be sent.
      facility: local0

You can combine any number of different appenders, including multiple instances of the same appender with different configurations:

logging:

  # Permit DEBUG, INFO, WARN and ERROR messages to be logged by appenders.
  level: DEBUG

  appenders:
    # Log warnings and errors to stderr
    - type: console
      threshold: WARN
      target: stderr

    # Log info, warnings and errors to our apps' main log.
    # Rolled over daily and retained for 5 days.
    - type: file
      threshold: INFO
      currentLogFilename: ./logs/example.log
      archivedLogFilenamePattern: ./logs/example-%d.log.gz
      archivedFileCount: 5

    # Log debug messages, info, warnings and errors to our apps' debug log.
    # Rolled over hourly and retained for 6 hours
    - type: file
      threshold: DEBUG
      currentLogFilename: ./logs/debug.log
      archivedLogFilenamePattern: ./logs/debug-%d{yyyy-MM-dd-hh}.log.gz
      archivedFileCount: 6

JSON Log Format

You may prefer to produce logs in a structured format such as JSON, so it can be processed by analytics or BI software. For that, add a module to the project for supporting JSON layouts:

<dependency>
    <groupId>io.dropwizard</groupId>
    <artifactId>dropwizard-json-logging</artifactId>
    <version>${dropwizard.version}</version>
</dependency>

Setup the JSON layout in the configuration file.

For general logging:

logging:
  appenders:
    - type: console
      layout:
        type: json

The json layout will produces the following log message:

{"timestamp":1515002688000, "level":"INFO","logger":"org.eclipse.jetty.server.Server","thread":"main","message":"Started @6505ms"}

For request logging:

server:
  requestLog:
    appenders:
      - type: console
        layout:
          type: access-json

The access-json layout will produces the following log message:

{"timestamp":1515002688000, "method":"GET","uri":"/hello-world", "status":200, "protocol":"HTTP/1.1","contentLength":37,"remoteAddress":"127.0.0.1","requestTime":5, "userAgent":"Mozilla/5.0"}

Logging Configuration via HTTP

Active log levels can be changed during the runtime of a Dropwizard application via HTTP using the LogConfigurationTask. For instance, to configure the log level for a single Logger. The logger parameter may be repeated. The optional duration parameter must be an ISO 8601 duration format. When the duration elapses the level will revert to the effective level of the parent logger.:

# Configure com.example.helloworld to INFO
curl -X POST -d "logger=com.example.helloworld&level=INFO" http://localhost:8081/tasks/log-level
# Configure com.example.helloworld and com.example.helloearth to INFO
curl -X POST -d "logger=com.example.helloworld&logger=com.example.helloearth&level=INFO" http://localhost:8081/tasks/log-level
# Configure com.example.helloworld to INFO, then revert to default level after 10 minutes
curl -X POST -d "logger=com.example.helloworld&level=INFO&duration=PT10M" http://localhost:8081/tasks/log-level
# Revert com.example.helloworld to the default level
curl -X POST -d "logger=com.example.helloworld" http://localhost:8081/tasks/log-level

Note

Chaining log level changes on the same package may have unexpected consequences due to the naive implementation of a simple FIFO timer.

Logging Filters

Just because a statement has a level of INFO, doesn’t mean it should be logged with other INFO statements. One can create logging filters that will intercept log statements before they are written and decide if they’re allowed. Log filters can work on both regular statements and request log statements. The following example will be for request logging as there are many reasons why certain requests may be excluded from the log:

Only log requests that have large bodies
Only log requests that are slow
Only log requests that resulted in a non-2xx status code
Exclude requests that contain sensitive information in the URL
Exclude healthcheck requests

The example will demonstrate excluding /secret requests from the log.

@JsonTypeName("secret-filter-factory")
public class SecretFilterFactory implements FilterFactory<IAccessEvent> {
    @Override
    public Filter<IAccessEvent> build() {
        return new Filter<IAccessEvent>() {
            @Override
            public FilterReply decide(IAccessEvent event) {
                if (event.getRequestURI().equals("/secret")) {
                    return FilterReply.DENY;
                } else {
                    return FilterReply.NEUTRAL;
                }
            }
        };
    }
}

Reference SecretFilterFactory type in our configuration.

server:
  requestLog:
    appenders:
      - type: console
        filterFactories:
          - type: secret-filter-factory

The last step is to add our class (in this case com.example.SecretFilterFactory) to META-INF/services/io.dropwizard.logging.common.filter.FilterFactory in our resources folder.

Filtering Request Logs for a Specific URI

Reference UriFilterFactory type in your configuration.

server:
  requestLog:
    appenders:
      - type: console
        filterFactories:
          - type: uri
            uris:
              - "/health-check"

Testing Applications

All of Dropwizard’s APIs are designed with testability in mind, so even your applications can have unit tests:

Source file

@ExtendWith(MockitoExtension.class)
class MyApplicationTest {
    @Mock
    private Environment environment;
    @Mock
    private JerseyEnvironment jersey;
    private MyApplication application;
    private MyConfiguration config;

    @BeforeEach
    void setup() throws Exception {
        config = new MyConfiguration();
        config.setMyParam("yay");
        application = new MyApplication();
        when(environment.jersey()).thenReturn(jersey);
    }

    @Test
    void buildsMyResource() throws Exception {
        application.run(config, environment);

        verify(jersey).register(eq(MyResource.class));
    }
}

We highly recommend Mockito for all your mocking needs.

Banners

We think applications should print out a big ASCII art banner on startup. Yours should, too. It’s fun. Just add a banner.txt class to src/main/resources and it’ll print it out when your application starts:

INFO  [2011-12-09 21:56:37,209] io.dropwizard.core.cli.ServerCommand: Starting hello-world
                                                 dP
                                                 88
  .d8888b. dP.  .dP .d8888b. 88d8b.d8b. 88d888b. 88 .d8888b.
  88ooood8  `8bd8'  88'  `88 88'`88'`88 88'  `88 88 88ooood8
  88.  ...  .d88b.  88.  .88 88  88  88 88.  .88 88 88.  ...
  `88888P' dP'  `dP `88888P8 dP  dP  dP 88Y888P' dP `88888P'
                                        88
                                        dP

INFO  [2011-12-09 21:56:37,214] org.eclipse.jetty.server.Server: jetty-7.6.0
...

We could probably make up an argument about why this is a serious devops best practice with high ROI and an Agile Tool, but honestly we just enjoy this.

We recommend you use TAAG for all your ASCII art banner needs.

Resources

Unsurprisingly, most of your day-to-day work with a Dropwizard application will be in the resource classes, which model the resources exposed in your RESTful API. Dropwizard uses Jersey for this, so most of this section is just re-hashing or collecting various bits of Jersey documentation.

Jersey is a framework for mapping various aspects of incoming HTTP requests to POJOs and then mapping various aspects of POJOs to outgoing HTTP responses. Here’s a basic resource class:

@Path("/{user}/notifications")
@Produces(MediaType.APPLICATION_JSON)
@Consumes(MediaType.APPLICATION_JSON)
public class NotificationsResource {
    private final NotificationStore store;

    public NotificationsResource(NotificationStore store) {
        this.store = store;
    }

    @GET
    public NotificationList fetch(@PathParam("user") OptionalLong userId,
                                  @QueryParam("count") @DefaultValue("20") OptionalInt count) {
        final List<Notification> notifications = store.fetch(userId.get(), count.get());
        if (notifications != null) {
            return new NotificationList(userId, notifications);
        }
        throw new WebApplicationException(Status.NOT_FOUND);
    }

    @POST
    public Response add(@PathParam("user") OptionalLong userId,
                        @NotNull @Valid Notification notification) {
        final long id = store.add(userId.get(), notification);
        return Response.created(UriBuilder.fromResource(NotificationResource.class)
                                          .build(userId.get(), id))
                       .build();
    }
}

This class provides a resource (a user’s list of notifications) which responds to GET and POST requests to /{user}/notifications, providing and consuming application/json representations. There’s quite a lot of functionality on display here, and this section will explain in detail what’s in play and how to use these features in your application.

Paths

Important

Every resource class must have a @Path annotation.

The @Path annotation isn’t just a static string, it’s a URI Template. The {user} part denotes a named variable, and when the template matches a URI the value of that variable will be accessible via @PathParam-annotated method parameters.

For example, an incoming request for /1001/notifications would match the URI template, and the value "1001" would be available as the path parameter named user.

If your application doesn’t have a resource class whose @Path URI template matches the URI of an incoming request, Jersey will automatically return a 404 Not Found to the client.

Methods

Methods on a resource class which accept incoming requests are annotated with the HTTP methods they handle: @GET, @POST, @PUT, @DELETE, @HEAD, @OPTIONS, @PATCH.

Support for arbitrary new methods can be added via the @HttpMethod annotation. They also must be added to the list of allowed methods. This means, by default, methods such as CONNECT and TRACE are blocked, and will return a 405 Method Not Allowed response.

If a request comes in which matches a resource class’s path but has a method which the class doesn’t support, Jersey will automatically return a 405 Method Not Allowed to the client.

The return value of the method (in this case, a NotificationList instance) is then mapped to the negotiated media type. In this case, our resource only supports JSON, and so the NotificationList is serialized to JSON using Jackson.

Metrics

Every resource method or the class itself can be annotated with @Timed, @Metered, @ResponseMetered and @ExceptionMetered. If the annotation is placed on the class, it will apply to all its resource methods. Dropwizard augments Jersey to automatically record runtime information about your resource methods.

public class ExampleApplication extends ResourceConfig {
    .
    .
    .
    register(new InstrumentedResourceMethodApplicationListener (new MetricRegistry()));
    config = config.register(ExampleResource.class);
    .
    .
    .
}

@Path("/example")
@Produces(MediaType.TEXT_PLAIN)
public class ExampleResource {
    @GET
    @Timed
    public String show() {
        return "yay";
    }

    @GET
    @Metered(name = "fancyName") // If name isn't specified, the meter will given the name of the method it decorates.
    @Path("/metered")
    public String metered() {
        return "woo";
    }

    @GET
    @ExceptionMetered(cause = IOException.class) // Default cause is Exception.class
    @Path("/exception-metered")
    public String exceptionMetered(@QueryParam("splode") @DefaultValue("false") boolean splode) throws IOException {
        if (splode) {
            throw new IOException("AUGH");
        }
        return "fuh";
    }

    @GET
    @ResponseMetered
    @Path("/response-metered")
    public Response responseMetered(@QueryParam("invalid") @DefaultValue("false") boolean invalid) {
        if (invalid) {
            return Response.status(Response.Status.INTERNAL_SERVER_ERROR).build();
        }
        return Response.ok().build();
    }
}

@Timed measures the duration of requests to a resource
@Metered measures the rate at which the resource is accessed
@ResponseMetered measures rate for each class of response codes (1xx/2xx/3xx/4xx/5xx)
@ExceptionMetered measures how often exceptions occur processing the resource

Important

@Timed and @Metered can only be used on the same resource method at the same time, if their name is unique, also see the annotation parameter name. Otherwise, the generated metrics names will be identical which will cause an IllegalArgumentException.

Parameters

The annotated methods on a resource class can accept parameters which are mapped to from aspects of the incoming request.

For example:

A @PathParam("user")-annotated String takes the raw value from the user variable in the matched URI template and passes it into the method as a String.
A @QueryParam("count")-annotated OptionalInt parameter takes the first count value from the request’s query string and passes it as a String to OptionalInt’s constructor. OptionalInt parses the string as an Integer, returning a 400 Bad Request if the value is malformed.
A @FormParam("name")-annotated Set<String> parameter takes all the name values from a posted form and passes them to the method as a set of strings.
A *Param–annotated NonEmptyStringParam will interpret empty strings as absent strings, which is useful in cases where the endpoint treats empty strings and absent strings as interchangeable.

What’s noteworthy here is that you can actually encapsulate the vast majority of your validation logic using specialized parameter objects. See AbstractParam for details.

Request Entities

If you’re handling request entities (e.g., an application/json object on a PUT request), you can model this as a parameter without a *Param annotation. In the example code, the add method provides a good example of this:

@POST
public Response add(@PathParam("user") OptionalLong userId,
                    @NotNull @Valid Notification notification) {
    final long id = store.add(userId.get(), notification);
    return Response.created(UriBuilder.fromResource(NotificationResource.class)
                                      .build(userId.get(), id))
                   .build();
}

Jersey maps the request entity to any single, unbound parameter. In this case, because the resource is annotated with @Consumes(MediaType.APPLICATION_JSON), it uses the Dropwizard-provided Jackson support which, in addition to parsing the JSON and mapping it to an instance of Notification, also runs that instance through Dropwizard’s Constraining Entities.

If the deserialized Notification isn’t valid, Dropwizard returns a 422 Unprocessable Entity response to the client.

Note

If a request entity parameter is just annotated with @Valid, it is still allowed to be null, so to ensure that the object is present and validated @NotNull @Valid is a powerful combination.

Media Types

Jersey also provides full content negotiation, so if your resource class consumes application/json but the client sends a text/plain entity, Jersey will automatically reply with a 406 Not Acceptable. Jersey’s even smart enough to use client-provided q-values in their Accept headers to pick the best response content type based on what both the client and server will support.

Responses

If your clients are expecting custom headers or additional information (or, if you simply desire an additional degree of control over your responses), you can return explicitly-built Response objects:

return Response.noContent().language(Locale.GERMAN).build();

In general, though, we recommend you return actual domain objects if at all possible. It makes testing resources much easier.

Error Handling

Almost as important as an application’s happy path (receiving expected input and returning expected output) is an application’s behavior when something goes wrong.

If your resource class unintentionally throws an exception, Dropwizard will log that exception under the ERROR level (including stack traces) and return a terse, safe application/json 500 Internal Server Error response. The response will contain an ID that can be grepped out the server logs for additional information.

If your resource class needs to return an error to the client (e.g., the requested record doesn’t exist), you have two options: throw a subclass of Exception or restructure your method to return a Response. If at all possible, prefer throwing Exception instances to returning Response objects, as that will make resource endpoints more self describing and easier to test.

The least intrusive way to map error conditions to a response is to throw a WebApplicationException:

@GET
@Path("/{collection}")
public Saying reduceCols(@PathParam("collection") String collection) {
    if (!collectionMap.containsKey(collection)) {
        final String msg = String.format("Collection %s does not exist", collection);
        throw new WebApplicationException(msg, Status.NOT_FOUND)
    }

    // ...
}

In this example a GET request to /foobar will return

{"code":404,"message":"Collection foobar does not exist"}

One can also take exceptions that your resource may throw and map them to appropriate responses. For instance, an endpoint may throw IllegalArgumentException and it may be worthy enough of a response to warrant a custom metric to track how often the event occurs. Here’s an example of such an ExceptionMapper

public class IllegalArgumentExceptionMapper implements ExceptionMapper<IllegalArgumentException> {
    private final Meter exceptions;
    public IllegalArgumentExceptionMapper(MetricRegistry metrics) {
        exceptions = metrics.meter(name(getClass(), "exceptions"));
    }

    @Override
    public Response toResponse(IllegalArgumentException e) {
        exceptions.mark();
        return Response.status(Status.BAD_REQUEST)
                .header("X-YOU-SILLY", "true")
                .type(MediaType.APPLICATION_JSON_TYPE)
                .entity(new ErrorMessage(Status.BAD_REQUEST.getStatusCode(),
                        "You passed an illegal argument!"))
                .build();
    }
}

and then registering the exception mapper:

@Override
public void run(final MyConfiguration conf, final Environment env) {
    env.jersey().register(new IllegalArgumentExceptionMapper(env.metrics()));
    env.jersey().register(new Resource());
}

Overriding Default Exception Mappers

To override a specific exception mapper, register your own class that implements the same ExceptionMapper<T> as one of the default. For instance, we can customize responses caused by Jackson exceptions:

public class JsonProcessingExceptionMapper implements ExceptionMapper<JsonProcessingException> {
    @Override
    public Response toResponse(JsonProcessingException exception) {
        // create the response
    }
}

With this method, one doesn’t need to know what the default exception mappers are, as they are overridden if the user supplies a conflicting mapper. While not preferential, one can also disable all default exception mappers, by setting server.registerDefaultExceptionMappers to false. See the class ExceptionMapperBinder for a list of the default exception mappers.

URIs

While Jersey doesn’t quite have first-class support for hyperlink-driven applications, the provided UriBuilder functionality does quite well.

Rather than duplicate resource URIs, it’s possible (and recommended!) to initialize a UriBuilder with the path from the resource class itself:

UriBuilder.fromResource(UserResource.class).build(user.getId());

Testing

As with just about everything in Dropwizard, we recommend you design your resources to be testable. Dependencies which aren’t request-injected should be passed in via the constructor and assigned to final fields.

Testing, then, consists of creating an instance of your resource class and passing it a mock. (Again: Mockito.)

public class NotificationsResourceTest {
    private final NotificationStore store = mock(NotificationStore.class);
    private final NotificationsResource resource = new NotificationsResource(store);

    @Test
    public void getsReturnNotifications() {
        final List<Notification> notifications = mock(List.class);
        when(store.fetch(1, 20)).thenReturn(notifications);

        final NotificationList list = resource.fetch(new LongParam("1"), new IntParam("20"));

        assertThat(list.getUserId(),
                  is(1L));

        assertThat(list.getNotifications(),
                   is(notifications));
    }
}

Caching

Adding a Cache-Control statement to your resource class is simple with Dropwizard:

@GET
@CacheControl(maxAge = 6, maxAgeUnit = TimeUnit.HOURS)
public String getCachableValue() {
    return "yay";
}

The @CacheControl annotation will take all of the parameters of the Cache-Control header.

Sessions

Although Dropwizard’s main purpose is to build stateless RESTful APIs, a stateful web service can be built using HTTP sessions. As most users won’t profit from having session support enabled by default, session support is implemented as opt-in.

The underlying Jetty server will handle sessions only if a SessionHandler is provided at application startup. Therefore the following code has to be added to the run method of the Application class:

@Override
public void run(final TestConfiguration configuration, final Environment environment) {
    environment.servlets().setSessionHandler(new org.eclipse.jetty.server.session.SessionHandler());
}

This will provide Jetty’s default SessionHandler to the servlet environment and session support is enabled. To get an HttpSession object injected into a Jersey resource method, Dropwizard provides a @Session annotation:

public Response doSomethingWithSessions(@Session HttpSession httpSession) {
    return Response.ok().build();
}

Representations

Representation classes are classes which, when handled to various Jersey MessageBodyReader and MessageBodyWriter providers, become the entities in your application’s API. Dropwizard heavily favors JSON, but it’s possible to map from any POJO to custom formats and back.

Basic JSON

Jackson is awesome at converting regular POJOs to JSON and back. This file:

public class Notification {
    private String text;

    public Notification(String text) {
        this.text = text;
    }

    @JsonProperty
    public String getText() {
        return text;
    }

    @JsonProperty
    public void setText(String text) {
        this.text = text;
    }
}

gets converted into this JSON:

{
    "text": "hey it's the value of the text field"
}

If, at some point, you need to change the JSON field name or the Java field without affecting the other, you can add an explicit field name to the @JsonProperty annotation.

If you prefer immutable objects rather than JavaBeans, that’s also doable:

public class Notification {
    private final String text;

    @JsonCreator
    public Notification(@JsonProperty("text") String text) {
        this.text = text;
    }

    @JsonProperty("text")
    public String getText() {
        return text;
    }
}

Advanced JSON

Not all JSON representations map nicely to the objects your application deals with, so it’s sometimes necessary to use custom serializers and deserializers. Just annotate your object like this:

@JsonSerialize(using=FunkySerializer.class)
@JsonDeserialize(using=FunkyDeserializer.class)
public class Funky {
    // ...
}

Then make a FunkySerializer class which implements JsonSerializer<Funky> and a FunkyDeserializer class which implements JsonDeserializer<Funky>.

Snake Case

A common issue with JSON is the disagreement between camelCase and snake_case field names. Java and Javascript folks tend to like camelCase; Ruby, Python, and Perl folks insist on snake_case. To make Dropwizard automatically convert field names to snake_case (and back), just annotate the class with @JsonSnakeCase:

@JsonSnakeCase
public class Person {
    private final String firstName;

    @JsonCreator
    public Person(@JsonProperty String firstName) {
        this.firstName = firstName;
    }

    @JsonProperty
    public String getFirstName() {
        return firstName;
    }
}

This gets converted into this JSON:

{
    "first_name": "Coda"
}

Unknown properties

If the name of a JSON property cannot be mapped to a Java property (or otherwise handled), that JSON property will simply be ignored.

You can change this behavior by configuring Dropwizard’s object mapper:

public void initialize(Bootstrap<ExampleConfiguration> bootstrap) {
    bootstrap.getObjectMapper().enable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
}

Note

The YAML configuration parser will fail on unknown properties regardless of the object mapper configuration.

Streaming Output

If your application happens to return lots of information, you may get a big performance and efficiency bump by using streaming output. By returning an object which implements Jersey’s StreamingOutput interface, your method can stream the response entity in a chunk-encoded output stream. Otherwise, you’ll need to fully construct your return value and then hand it off to be sent to the client.

HTML Representations

For generating HTML pages, check out Dropwizard’s views support.

Custom Representations

Sometimes, though, you’ve got some wacky output format you need to produce or consume and no amount of arguing will make JSON acceptable. That’s unfortunate but OK. You can add support for arbitrary input and output formats by creating classes which implement Jersey’s MessageBodyReader<T> and MessageBodyWriter<T> interfaces. (Make sure they’re annotated with @Provider and @Produces("text/gibberish") or @Consumes("text/gibberish").) Once you’re done, just add instances of them (or their classes if they depend on Jersey’s @Context injection) to your application’s Environment on initialization.

Filters

There might be cases when you want to filter out requests or modify them before they reach your Resources.

Jersey filters

Jersey has a rich api for filters and interceptors that can be used directly in Dropwizard. You can stop the request from reaching your resources by throwing a WebApplicationException. Alternatively, you can use filters to modify inbound requests or outbound responses.

@Provider
public class DateNotSpecifiedFilter implements ContainerRequestFilter {
    @Override
    public void filter(ContainerRequestContext requestContext) throws IOException {
        String dateHeader = requestContext.getHeaderString(HttpHeaders.DATE);

        if (dateHeader == null) {
            Exception cause = new IllegalArgumentException("Date Header was not specified");
            throw new WebApplicationException(cause, Response.Status.BAD_REQUEST);
        }
    }
}

This example filter checks the request for the “Date” header, and denies the request if was missing. Otherwise, the request is passed through.

Filters can be dynamically bound to resource methods using DynamicFeature:

@Provider
public class DateRequiredFeature implements DynamicFeature {
    @Override
    public void configure(ResourceInfo resourceInfo, FeatureContext context) {
        if (resourceInfo.getResourceMethod().getAnnotation(DateRequired.class) != null) {
            context.register(DateNotSpecifiedFilter.class);
        }
    }
}

The DynamicFeature is invoked by the Jersey runtime when the application is started. In this example, the feature checks for methods that are annotated with @DateRequired and registers the DateNotSpecified filter on those methods only.

You typically register the feature in your Application class, like so:

environment.jersey().register(DateRequiredFeature.class);

Servlet filters

Another way to create filters is by creating servlet filters. They offer a way to register filters that apply both to servlet requests as well as resource requests. Jetty comes with a few bundled filters which may already suit your needs. If you want to create your own filter, this example demonstrates a servlet filter analogous to the previous example:

public class DateNotSpecifiedServletFilter implements jakarta.servlet.Filter {
    // Other methods in interface omitted for brevity

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        if (request instanceof HttpServletRequest) {
            String dateHeader = ((HttpServletRequest) request).getHeader(HttpHeaders.DATE);

            if (dateHeader != null) {
                chain.doFilter(request, response); // This signals that the request should pass this filter
            } else {
                HttpServletResponse httpResponse = (HttpServletResponse) response;
                httpResponse.setStatus(HttpStatus.BAD_REQUEST_400);
                httpResponse.getWriter().print("Date Header was not specified");
            }
        }
    }
}

This servlet filter can then be registered in your Application class by wrapping it in FilterHolder and adding it to the application context together with a specification for which paths this filter will be active. Here’s an example:

environment.servlets().addFilter("DateNotSpecifiedServletFilter", new DateNotSpecifiedServletFilter())
                      .addMappingForUrlPatterns(EnumSet.of(DispatcherType.REQUEST), true, "/*");

How it’s glued together

When your application starts up, it will spin up a Jetty HTTP server, see DefaultServerFactory. This server will have two handlers, one for your application port and the other for your admin port. The admin handler creates and registers the AdminServlet. This has a handle to all of the application healthchecks and metrics via the ServletContext.

The application port has an HttpServlet as well, this is composed of DropwizardResourceConfig, which is an extension of Jersey’s resource configuration that performs scanning to find root resource and provider classes. Ultimately when you call env.jersey().register(new SomeResource()), you are adding to the DropwizardResourceConfig. This config is a jersey Application, so all of your application resources are served from one Servlet

DropwizardResourceConfig is where the various ResourceMethodDispatchAdapter are registered to enable the following functionality:

Resource method requests with @Timed, @Metered, @ExceptionMetered are delegated to special dispatchers which decorate the metric telemetry

Resources that return Optional are unboxed. Present returns underlying type, and non-present 404s

Resource methods that are annotated with @CacheControl are delegated to a special dispatcher that decorates on the cache control headers

Enables using Jackson to parse request entities into objects and generate response entities from objects, all while performing validation