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Developing with the Java Cloud Config API

Assumption: a def file, which is the schema for one of your configs, is created and put in src/main/resources/configdefinitions/.

To generate source code for the def-file, invoke the config-class-plugin from pom.xml, in the <build>, <plugins> section:


The generated classes will be saved to target/generated-sources/vespa-configgen-plugin, when the generate-sources phase of the build is executed. The def-file motd.def is used in this tutorial, and a class called MotdConfig was generated (in the package myproject). It is a subtype of ConfigInstance.

When using only the config system (and not other parts of Vespa or the JDisc container), pull in that by using this in pom.xml:


Subscribing and getting config

To retrieve the config in the application, create a ConfigSubscriber. A ConfigSubscriber is capable of subscribing to one or more configs. The example shown here uses simplified error handling:

ConfigSubscriber subscriber = new ConfigSubscriber();
ConfigHandle<MotdConfig> handle = subscriber.subscribe(MotdConfig.class, "motdserver2/0");
if (!subscriber.nextConfig()) throw new RuntimeException("Config timed out.");
if (handle.isChanged()) {
  String message = handle.getConfig().message();
  int port = handle.getConfig().port();

Note that isChanged() always will be true after the first call to nextConfig(), it is included here to illustrate the API.

In many cases one will do this from a thread which loops the nextConfig() call, and reconfigures your application if isChanged() is true.

The second parameter to subscribe(), "motdserver2/0", is the config id.

If one ConfigSubscriber subscribes to multiple configs, nextConfig() will only return true if the configs are of the same generation, i.e. they are "in sync".

See the com.yahoo.config javadoc for details. Example:

ConfigSubscriber subscriber = new ConfigSubscriber();
ConfigHandle<MotdConfig> motdHandle = subscriber.subscribe(MotdConfig.class, "motdserver2/0");
ConfigHandle<AnotherConfig> anotherHandle = subscriber.subscribe(AnotherConfig.class, "motdserver2/0");
if (!subscriber.nextConfig()) throw new RuntimeException("Config timed out.");
// We now have a synchronized new generation for these two configs.
if (motdHandle.isChanged()) {
  String message = motdHandle.getConfig().message();
  int port = motdHandle.getConfig().port();
if (anotherHandle.isChanged()) {
  String myfield = anotherHandle.getConfig().getMyField();

Simplified subscription

In cases like the first example above, where you only subscribe to one config, you may also subscribe using the ConfigSubscriber.SingleSubscriber interface. In this case, you define a configure() method from the interface, and call a special subscribe(). The method will start a dedicated config fetcher thread for you. The method will throw an exception in the user thread if initial configuration fails, and print a warning in the config thread if it fails afterwards. Example:

public class MyConfigSubscriber implements ConfigSubscriber.SingleSubscriber<MotdConfig> {

  public MyConfigSubscriber(String configId) {
    new ConfigSubscriber().subscribe(this, MotdConfig.class, configId);

  public void configure(MotdConfig config) {
    // configuration logic here

The disadvantage to using this is that one cannot implement custom error handling or otherwise track config changes. If needed, use the generic method above.

Unit testing config

When instantiating a ConfigSubscriber, one can give it a ConfigSource. One such source is a ConfigSet. It consists of a set of Builders. This is an example of instantiating a subscriber using this - it uses 2 types of config, that were generated from files app.def and string.def:

ConfigSet myConfigs = new ConfigSet();
AppConfig.Builder a0builder = new AppConfig.Builder().message("A message, 0").times(88);
AppConfig.Builder a1builder = new AppConfig.Builder().message("A message, 1").times(89);
myConfigs.add("app/0", a0builder);
myConfigs.add("app/1", a1builder);
myConfigs.add("bar", new StringConfig.Builder().stringVal("StringVal"));
ConfigSubscriber subscriber = new ConfigSubscriber(myConfigs);

To help with unit testing, each config type has a corresponding builder type. The Builder is mutable whereas the ConfigInstance is not. Use this to set up config fixtures for unit tests. The ConfigSubscriber has a reload() method which is used in tests to force the subscriptions into a new generation. It emulates a vespa activate operation after you have updated the ConfigSet.

A full example can be found in ConfigSetSubscriptionTest.java.