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.

public class MyApplication extends Application<MyConfiguration> {
    // [...]
    @Override
    public void initialize(Bootstrap<MyConfiguration> bootstrap) {
        // Enable variable substitution with environment variables
        bootstrap.setConfigurationSourceProvider(
                new SubstitutingSourceProvider(bootstrap.getConfigurationSourceProvider(),
                                                   new EnvironmentVariableSubstitutor(false)
                )
        );

    }

    // [...]
}

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

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 StrSubstitutor 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). There is a test keystore you can use in the Dropwizard example project.

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

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/

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.

@Override
public void initialize(Bootstrap<HelloWorldConfiguration> bootstrap) {
    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.

@Override
public void initialize(Bootstrap<HelloWorldConfiguration> bootstrap) {
    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"));
}

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.

@Override
public void configure(Subparser subparser) {
    super.configure(subparser);

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

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

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.

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

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.

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

Paths

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 this case, our resource only supports JSON, and so the NotificationList is serialized to JSON using Jackson.

Metrics

Every resource method can be annotated with @Timed, @Metered, and @ExceptionMetered. Dropwizard augments Jersey to automatically record runtime information about your resource methods.

• @Timed measures the duration of requests to a resource
• @Metered measures the rate at which the resource is accessed
• @ExceptionMetered measures how often exceptions occur processing the resource

Parameters

The annotated methods on a resource class can accept parameters which are mapped to from aspects of the incoming request. The *Param annotations determine which part of the request the data is mapped, and the parameter type determines how the data is mapped.

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 IntParam parameter takes the first count value from the request’s query string and passes it as a String to IntParam‘s constructor. IntParam (and all other io.dropwizard.jersey.params.* classes) 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") LongParam userId,
                    @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.

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

If your resource class unintentionally throws an exception, Dropwizard will log that exception (including stack traces) and return a terse, safe text/plain 500 Internal Server Error response.

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.

If you throw a subclass of WebApplicationException jersey will map that to a defined response.

If you want more control, you can also declare JerseyProviders in your Environment to map Exceptions to certain responses by calling JerseyEnvironment#register(Object) with an implementation of javax.ws.rs.ext.ExceptionMapper. e.g. Your resource throws an InvalidArgumentException, but the response would be 400, bad request.

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.

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>.

@JsonSnakeCase
public class Person {
    private final String firstName;

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

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

{
    "first_name": "Coda"
}

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.

Jersey filters

There might be cases when you want to filter out requests or modify them before they reach your Resources. 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 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 javax.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 should active. Here’s an example:

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