Updating Dropwizard configuration at runtime

A guide to dynamically updating your configuration at runtime via a Dropwizard task endpoint

20 Nov 2020•3 min read

An example Dropwizard application demonstrating runtime configuration

Although this feature isn’t supported out of the box by dropwizard, you’re able to update your configuration at runtime fairly easy with the tools they give you.

Before I get started, note that this doesn’t persist the updated config values to the config.yml. However, this would be easy enough to implement yourself simply by writing to the config file from the application. If anyone would like to write this implementation feel free to open a PR on the example project I’ve linked below.

An example application can be found at scottg489/dropwizard-runtime-config-example

Code

Start off with a minimal config:

config.yml

myConfigValue: "hello"

And it’s corresponding configuration file:

ExampleConfiguration.java

public class ExampleConfiguration extends Configuration {
    private String myConfigValue;

    public String getMyConfigValue() {
        return myConfigValue;
    }

    public void setMyConfigValue(String value) {
        myConfigValue = value;
    }
}

Then create a task which updates the config:

UpdateConfigTask.java

public class UpdateConfigTask extends Task {
    ExampleConfiguration config;

    public UpdateConfigTask(ExampleConfiguration config) {
        super("updateconfig");
        this.config = config;
    }

    @Override
    public void execute(Map<String, List<String>> parameters, PrintWriter output) {
        config.setMyConfigValue("goodbye");
    }
}

Also, for demonstration purposes, create a resource which allows you to get the config value:

ConfigResource.java

@Path("/config")
public class ConfigResource {
    private final ExampleConfiguration config;

    public ConfigResource(ExampleConfiguration config) {
        this.config = config;
    }

    @GET
    public Response handleGet() {
        return Response.ok().entity(config.getMyConfigValue()).build();
    }
}

Finally, wire everything up in your application:

ExampleApplication.java

public class ExampleApplication extends Application<ExampleConfiguration> {
    public static void main(String[] args) throws Exception {
        new ExampleApplication().run(args);
    }

    @Override
    public String getName() {
        return "dropwizard-runtime-config-example";
    }

    @Override
    public void run(ExampleConfiguration configuration, Environment environment) {
        environment.jersey().register(new ConfigResource(configuration));
        environment.admin().addTask(new UpdateConfigTask(configuration));
    }
}

Usage

Start up the application

./gradlew clean build install && \
    ./build/install/dropwizard-runtime-config-example/bin/dropwizard-runtime-config-example server config.yml

Then verify everything is working as expected:

$ curl 'http://localhost:8080/config'
hello
$ curl -X POST 'http://localhost:8081/tasks/updateconfig'
$ curl 'http://localhost:8080/config'
goodbye

How it works

This works simply by passing the same reference to the constructor of ConfigResource.java and UpdateConfigTask.java. If you aren’t familiar with the concept see here: https://stackoverflow.com/questions/40480/is-java-pass-by-reference-or-pass-by-value

The linked classes above are to a project I’ve created which demonstrates this as a complete solution:

scottg489/dropwizard-runtime-config-example

Footnote: I haven’t verified this works with the built in configuration. However, the dropwizard Configuration class (which you need to extend for your own configuration) does have various “setters” for the internal configuration.