How to write a Java output plugin

To develop a new Java output for Logstash, you write a new Java class that conforms to the Logstash Java Outputs API, package it, and install it with the logstash-plugin utility. We’ll go through each of those steps.

Start by copying the example output plugin. The plugin API is currently part of the Logstash codebase so you must have a local copy of that available. You can obtain a copy of the Logstash codebase with the following git command:

git clone --branch <branch_name> --single-branch https://github.com/elastic/logstash.git <target_folder>

The branch_name should correspond to the version of Logstash containing the preferred revision of the Java plugin API.

Specify the target_folder for your local copy of the Logstash codebase. If you do not specify target_folder, it defaults to a new folder called logstash under your current folder.

After you have obtained a copy of the appropriate revision of the Logstash codebase, you need to compile it to generate the .jar file containing the Java plugin API. From the root directory of your Logstash codebase ($LS_HOME), you can compile it with ./gradlew assemble (or gradlew.bat assemble if you’re running on Windows). This should produce the $LS_HOME/logstash-core/build/libs/logstash-core-x.y.z.jar where x, y, and z refer to the version of Logstash.

After you have successfully compiled Logstash, you need to tell your Java plugin where to find the logstash-core-x.y.z.jar file. Create a new file named gradle.properties in the root folder of your plugin project. That file should have a single line:

LOGSTASH_CORE_PATH=<target_folder>/logstash-core

where target_folder is the root folder of your local copy of the Logstash codebase.

The example output plugin prints events to the console using the event’s toString method. Let’s look at the main class in the example output:

@LogstashPlugin(name = "java_output_example")
public class JavaOutputExample implements Output {

    public static final PluginConfigSpec<String> PREFIX_CONFIG =
            PluginConfigSpec.stringSetting("prefix", "");

    private final String id;
    private String prefix;
    private PrintStream printer;
    private final CountDownLatch done = new CountDownLatch(1);
    private volatile boolean stopped = false;

    public JavaOutputExample(final String id, final Configuration configuration, final Context context) {
        this(id, configuration, context, System.out);
    }

    JavaOutputExample(final String id, final Configuration config, final Context context, OutputStream targetStream) {
        this.id = id;
        prefix = config.get(PREFIX_CONFIG);
        printer = new PrintStream(targetStream);
    }

    @Override
    public void output(final Collection<Event> events) {
      Iterator<Event> z = events.iterator();
      while (z.hasNext() && !stopped) {
          String s = prefix + z.next();
          printer.println(s);
        }
    }

    @Override
    public void stop() {
        stopped = true;
        done.countDown();
    }

    @Override
    public void awaitStop() throws InterruptedException {
        done.await();
    }

    @Override
    public Collection<PluginConfigSpec<?>> configSchema() {
        return Collections.singletonList(PREFIX_CONFIG);
    }

    @Override
    public String getId() {
        return id;
    }
}

Let’s step through and examine each part of that class.

@LogstashPlugin(name="java_output_example")
public class JavaOutputExample implements Output {

Notes about the class declaration:

The snippet below contains both the setting definition and the method referencing it:

public static final PluginConfigSpec<String> PREFIX_CONFIG =
        PluginConfigSpec.stringSetting("prefix", "");

@Override
public Collection<PluginConfigSpec<?>> configSchema() {
    return Collections.singletonList(PREFIX_CONFIG);
}

The PluginConfigSpec class allows developers to specify the settings that a plugin supports complete with setting name, data type, deprecation status, required status, and default value. In this example, the prefix setting defines an optional prefix to include in the output of the event. The setting is not required and if it is not explicitly set, it defaults to the empty string.

The configSchema method must return a list of all settings that the plugin supports. In a future phase of the Java plugin project, the Logstash execution engine will validate that all required settings are present and that no unsupported settings are present.

private final String id;
private String prefix;
private PrintStream printer;

public JavaOutputExample(final String id, final Configuration configuration, final Context context) {
    this(configuration, context, System.out);
}

JavaOutputExample(final String id, final Configuration config, final Context context, OutputStream targetStream) {
    this.id = id;
    prefix = config.get(PREFIX_CONFIG);
    printer = new PrintStream(targetStream);
}

All Java output plugins must have a constructor taking a String id and a Configuration and Context argument. This is the constructor that will be used to instantiate them at runtime. The retrieval and validation of all plugin settings should occur in this constructor. In this example, the values of the prefix setting is retrieved and stored in a local variable for later use in the output method. In this example, a second, pacakge private constructor is defined that is useful for unit testing with a Stream other than System.out.

Any additional initialization may occur in the constructor as well. If there are any unrecoverable errors encountered in the configuration or initialization of the output plugin, a descriptive exception should be thrown. The exception will be logged and will prevent Logstash from starting.

@Override
public void output(final Collection<Event> events) {
    Iterator<Event> z = events.iterator();
    while (z.hasNext() && !stopped) {
        String s = prefix + z.next();
        printer.println(s);
    }
}

Outputs may send events to local sinks such as the console or a file or to remote systems such as Elasticsearch or other external systems. In this example, the events are printed to the local console.

private final CountDownLatch done = new CountDownLatch(1);
private volatile boolean stopped;

@Override
public void stop() {
    stopped = true;
    done.countDown();
}

@Override
public void awaitStop() throws InterruptedException {
    done.await();
}

The stop method notifies the output to stop sending events. The stop mechanism may be implemented in any way that honors the API contract though a volatile boolean flag works well for many use cases. Because this output example is so simple, its output method does not check for the stop flag.

Outputs stop both asynchronously and cooperatively. Use the awaitStop method to block until the output has completed the stop process. Note that this method should not signal the output to stop as the stop method does. The awaitStop mechanism may be implemented in any way that honors the API contract though a CountDownLatch works well for many use cases.

@Override
public String getId() {
    return id;
}

For output plugins, the getId method should always return the id that was provided to the plugin through its constructor at instantiation time.

Lastly, but certainly not least importantly, unit tests are strongly encouraged. The example output plugin includes an example unit test that you can use as a template for your own.

Java plugins are packaged as Ruby gems for dependency management and interoperability with Ruby plugins. Once they are packaged as gems, they may be installed with the logstash-plugin utility just as Ruby plugins are. Because no knowledge of Ruby or its toolchain should be required for Java plugin development, the procedure for packaging Java plugins as Ruby gems has been automated through a custom task in the Gradle build file provided with the example Java plugins. The following sections describe how to configure and execute that packaging task as well as how to install the packaged Java plugin in Logstash.

The following section appears near the top of the build.gradle file supplied with the example Java plugins:

// ===========================================================================
// plugin info
// ===========================================================================
group                      'org.logstashplugins' // must match the package of the main plugin class
version                    "${file("VERSION").text.trim()}" // read from required VERSION file
description                = "Example Java filter implementation"
pluginInfo.licenses        = ['Apache-2.0'] // list of SPDX license IDs
pluginInfo.longDescription = "This gem is a Logstash plugin required to be installed on top of the Logstash core pipeline using \$LS_HOME/bin/logstash-plugin install gemname. This gem is not a stand-alone program"
pluginInfo.authors         = ['Elasticsearch']
pluginInfo.email           = ['info@elastic.co']
pluginInfo.homepage        = "http://www.elastic.co/guide/en/logstash/current/index.html"
pluginInfo.pluginType      = "filter"
pluginInfo.pluginClass     = "JavaFilterExample"
pluginInfo.pluginName      = "java_filter_example"
// ===========================================================================

You should configure the values above for your plugin.

Several Ruby source files along with a gemspec file and a Gemfile are required to package the plugin as a Ruby gem. These Ruby files are used only for defining the Ruby gem structure or at Logstash startup time to register the Java plugin. They are not used during runtime event processing. The Gradle packaging task automatically generates all of these files based on the values configured in the section above.

You run the Gradle packaging task with the following command:

./gradlew gem

For Windows platforms: Substitute gradlew.bat for ./gradlew as appropriate in the command.

That task will produce a gem file in the root directory of your plugin’s codebase with the name logstash-{plugintype}-<pluginName>-<version>.gem

After you have packaged your Java plugin as a Ruby gem, you can install it in Logstash with this command:

bin/logstash-plugin install --no-verify --local /path/to/javaPlugin.gem

For Windows platforms: Substitute backslashes for forward slashes as appropriate in the command.

The following is a minimal Logstash configuration that can be used to test that the Java output plugin is correctly installed and functioning.

input {
  generator { message => "Hello world!" count => 1 }
}
output {
  java_output_example {}
}

Copy the above Logstash configuration to a file such as java_output.conf. Logstash should then be started with:

bin/logstash -f /path/to/java_output.conf

The expected Logstash output (excluding initialization) with the configuration above is:

{"@timestamp":"yyyy-MM-ddTHH:mm:ss.SSSZ","message":"Hello world!","@version":"1","host":"<yourHostname>","sequence":0}

If you have any feedback on Java plugin support in Logstash, please comment on our main Github issue or post in the Logstash forum.