:register_method:	true
:multi_receive_method:	true
:plugintype:    	output
:pluginclass:   	Outputs
:pluginname:    	example
:pluginnamecap: 	Example
:plugintypecap:   Output
:sversion:        '0.0.1'

:pluginrepo:      https://github.com/logstash-plugins/logstash-output-java_output_example[example output plugin]

:blockfilter:		true


[[java-output-plugin]]
=== How to write a Java output plugin

experimental[]

// Pulls in shared section: Setting Up Environment
include::include/javapluginsetup.asciidoc[]

[float]
=== Code the plugin

The example output plugin prints events in JSON format to the console. Let's
look at the main class in that example output:
 
[source,java]
-----
@LogstashPlugin(name="java_output_example")
public class JavaOutputExample implements Output {

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

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

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

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

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

    @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);
    }
}
-----

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

[float]
==== Class declaration

[source,java]
-----
@LogstashPlugin(name="java_output_example")
public class JavaOutputExample implements Output {
-----

Notes about the class declaration:

* All Java plugins must be annotated with the `@LogstashPlugin` annotation. Additionally:
** The `name` property of the annotation must be supplied and defines the name of the plugin as it will be used
   in the Logstash pipeline definition. For example, this output would be referenced in the output section of the
   Logstash pipeline definition as `output { java_output_example => { .... } }`
** The value of the `name` property must match the name of the class excluding casing and underscores.
* The class must implement the `co.elastic.logstash.api.v0.Output` interface.

[float]
==== Plugin settings

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

[source,java]
-----
public static final PluginConfigSpec<String> PREFIX_CONFIG =
        Configuration.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.

[float]
==== Constructor and initialization

[source,java]
-----
private String prefix;
private PrintStream printer;

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

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

All Java output plugins must have a constructor taking both 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.

[float]
==== Output method

[source,java]
-----
@Override
public void output(final Collection<Event> events) {
    try {
        Iterator<Event> z = events.iterator();
        while (z.hasNext() && !stopped) {
            String s = prefix + z.next().toJson();
            printer.println(s);
        }
    } catch (JsonProcessingException e) {
        throw new IllegalStateException(e);
    }
}
-----

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.

[float]
==== Stop and awaitStop methods

[source,java]
-----
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.

[float]
==== Unit tests

Lastly, but certainly not least importantly, unit tests are strongly encouraged.
The example output plugin includes an 
https://github.com/logstash-plugins/logstash-output-java_output_example/blob/master/src/test/java/org/logstash/javaapi/JavaOutputExampleTest.java[example unit
test] that you can use as a template for your own.


// Pulls in shared section about Packaging and Deploying
include::include/javapluginpkg.asciidoc[]

[float]
=== Running Logstash with the Java output plugin

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

[source,java]
-----
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:

[source,txt]
-----
bin/logstash --java-execution -f /path/to/java_output.conf
-----

Note that the `--java-execution` flag to enable the Java execution engine is
required as Java plugins are not supported in the Ruby execution engine.

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

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

[float]
=== Feedback

If you have any feedback on Java plugin support in Logstash, please comment on our 
https://github.com/elastic/logstash/issues/9215[main Github issue] or post in the 
https://discuss.elastic.co/c/logstash[Logstash forum].

:pluginrepo!:
:sversion!:  
:plugintypecap!:
:pluginnamecap!:
:pluginname!:
:pluginclass!:
:plugintype!:
