How to write a Java filter plugin

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

Start by copying the example filter 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 filter plugin allows one to configure a field in each event that will be reversed. For example, if the filter were configured to reverse the day_of_week field, an event with day_of_week: "Monday" would be transformed to day_of_week: "yadnoM". Let’s look at the main class in that example filter:

@LogstashPlugin(name = "java_filter_example")
public class JavaFilterExample implements Filter {

    public static final PluginConfigSpec<String> SOURCE_CONFIG =
            PluginConfigSpec.stringSetting("source", "message");

    private String id;
    private String sourceField;

    public JavaFilterExample(String id, Configuration config, Context context) {
        this.id = id;
        this.sourceField = config.get(SOURCE_CONFIG);
    }

    @Override
    public Collection<Event> filter(Collection<Event> events, FilterMatchListener matchListener) {
        for (Event e : events) {
            Object f = e.getField(sourceField);
            if (f instanceof String) {
                e.setField(sourceField, StringUtils.reverse((String)f));
                matchListener.filterMatched(e);
            }
        }
        return events;
    }

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

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

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

@LogstashPlugin(name = "java_filter_example")
public class JavaFilterExample implements Filter {

Notes about the class declaration:

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

public static final PluginConfigSpec<String> SOURCE_CONFIG =
        PluginConfigSpec.stringSetting("source", "message");

@Override
public Collection<PluginConfigSpec<?>> configSchema() {
    return Collections.singletonList(SOURCE_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 source setting defines the name of the field in each event that will be reversed. It is not a required setting and if it is not explicitly set, its default value will be message.

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 String id;
private String sourceField;

public JavaFilterExample(String id, Configuration config, Context context) {
    this.id = id;
    this.sourceField = config.get(SOURCE_CONFIG);
}

All Java filter 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 name of the field to be reversed in each event is retrieved from its setting and stored in a local variable so that it can be used later in the filter method.

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

@Override
public Collection<Event> filter(Collection<Event> events, FilterMatchListener matchListener) {
    for (Event e : events) {
        Object f = e.getField(sourceField);
        if (f instanceof String) {
            e.setField(sourceField, StringUtils.reverse((String)f));
            matchListener.filterMatched(e);
        }
    }
    return events;

Finally, we come to the filter method that is invoked by the Logstash execution engine on batches of events as they flow through the event processing pipeline. The events to be filtered are supplied in the events argument and the method should return a collection of filtered events. Filters may perform a variety of actions on events as they flow through the pipeline including:

In the example above, the value of the source field is retrieved from each event and reversed if it is a string value. Because each event is mutated in place, the incoming events collection can be returned.

The matchListener is the mechanism by which filters indicate which events "match". The common actions for filters such as add_field and add_tag are applied only to events that are designated as "matching". Some filters such as the grok filter have a clear definition for what constitutes a matching event and will notify the listener only for matching events. Other filters such as the UUID filter have no specific match criteria and should notify the listener for every event filtered. In this example, the filter notifies the match listener for any event that had a String value in its source field and was therefore able to be reversed.

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

For filter 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 filter 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 filter plugin is correctly installed and functioning.

input {
  generator { message => "Hello world!" count => 1 }
}
filter {
  java_filter_example {}
}
output {
  stdout { codec => rubydebug }
}

Copy the above Logstash configuration to a file such as java_filter.conf. Start Logstash with:

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

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

{
      "sequence" => 0,
      "@version" => "1",
       "message" => "!dlrow olleH",
    "@timestamp" => yyyy-MM-ddThh:mm:ss.SSSZ,
          "host" => "<yourHostName>"
}

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