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<h2 class="title"><a id="java-input-plugin"></a>How to write a Java input plugin<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h2>
</div></div></div>
<p>To develop a new Java input for Logstash, you write a new Java class that
conforms to the Logstash Java Inputs API, package it, and install it with the
logstash-plugin utility. We’ll go through each of those steps.</p>
<h3><a id="_set_up_your_environment"></a>Set up your environment<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h3>
<h4><a id="_copy_the_example_repo"></a>Copy the example repo<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<p>Start by copying the <a href="https://github.com/logstash-plugins/logstash-input-java_input_example" class="ulink" target="_top">example input plugin</a>. 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 <code class="literal">git</code> command:</p>
<div class="pre_wrapper lang-shell">
<pre class="programlisting prettyprint lang-shell">git clone --branch <branch_name> --single-branch https://github.com/elastic/logstash.git <target_folder></pre>
</div>
<p>The <code class="literal">branch_name</code> should correspond to the version of Logstash containing the
preferred revision of the Java plugin API.</p>
<div class="note admon">
<div class="icon"></div>
<div class="admon_content">
<p>The GA version of the Java plugin API is available in the <code class="literal">7.2</code>
and later branches of the Logstash codebase.</p>
</div>
</div>
<p>Specify the <code class="literal">target_folder</code> for your local copy of the Logstash codebase. If you
do not specify <code class="literal">target_folder</code>, it defaults to a new folder called <code class="literal">logstash</code>
under your current folder.</p>
<h4><a id="_generate_the_jar_file"></a>Generate the .jar file<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<p>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 ($LS_HOME), you
can compile it with <code class="literal">./gradlew assemble</code> (or <code class="literal">gradlew.bat assemble</code> if you’re
running on Windows). This should produce the
<code class="literal">$LS_HOME/logstash-core/build/libs/logstash-core-x.y.z.jar</code> where <code class="literal">x</code>, <code class="literal">y</code>, and
<code class="literal">z</code> refer to the version of Logstash.</p>
<p>After you have successfully compiled Logstash, you need to tell your Java plugin
where to find the <code class="literal">logstash-core-x.y.z.jar</code> file. Create a new file named
<code class="literal">gradle.properties</code> in the root folder of your plugin project. That file should
have a single line:</p>
<div class="pre_wrapper lang-txt">
<pre class="programlisting prettyprint lang-txt">LOGSTASH_CORE_PATH=<target_folder>/logstash-core</pre>
</div>
<p>where <code class="literal">target_folder</code> is the root folder of your local copy of the Logstash codebase.</p>
<h3><a id="_code_the_plugin"></a>Code the plugin<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h3>
<p>The example input plugin generates a configurable number of simple events before
terminating. Let’s look at the main class in the example input.</p>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">@LogstashPlugin(name="java_input_example")
public class JavaInputExample implements Input {
public static final PluginConfigSpec<Long> EVENT_COUNT_CONFIG =
PluginConfigSpec.numSetting("count", 3);
public static final PluginConfigSpec<String> PREFIX_CONFIG =
PluginConfigSpec.stringSetting("prefix", "message");
private String id;
private long count;
private String prefix;
private final CountDownLatch done = new CountDownLatch(1);
private volatile boolean stopped;
public JavaInputExample(String id, Configuration config, Context context) {
this.id = id;
count = config.get(EVENT_COUNT_CONFIG);
prefix = config.get(PREFIX_CONFIG);
}
@Override
public void start(Consumer<Map<String, Object>> consumer) {
int eventCount = 0;
try {
while (!stopped && eventCount < count) {
eventCount++;
consumer.accept.push(Collections.singletonMap("message",
prefix + " " + StringUtils.center(eventCount + " of " + count, 20)));
}
} finally {
stopped = true;
done.countDown();
}
}
@Override
public void stop() {
stopped = true; // set flag to request cooperative stop of input
}
@Override
public void awaitStop() throws InterruptedException {
done.await(); // blocks until input has stopped
}
@Override
public Collection<PluginConfigSpec<?>> configSchema() {
return Arrays.asList(EVENT_COUNT_CONFIG, PREFIX_CONFIG);
}
@Override
public String getId() {
return this.id;
}
}</pre>
</div>
<p>Let’s step through and examine each part of that class.</p>
<h4><a id="_class_declaration_5"></a>Class declaration<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">@LogstashPlugin(name="java_input_example")
public class JavaInputExample implements Input {</pre>
</div>
<p>Notes about the class declaration:</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
<p>All Java plugins must be annotated with the <code class="literal">@LogstashPlugin</code> annotation. Additionally:</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
The <code class="literal">name</code> 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 input would be referenced in the input section of the
Logstash pipeline defintion as <code class="literal">input { java_input_example => { .... } }</code>
</li>
<li class="listitem">
The value of the <code class="literal">name</code> property must match the name of the class excluding casing and underscores.
</li>
</ul>
</div>
</li>
<li class="listitem">
The class must implement the <code class="literal">co.elastic.logstash.api.Input</code> interface.
</li>
<li class="listitem">
Java plugins may not be created in the <code class="literal">org.logstash</code> or <code class="literal">co.elastic.logstash</code> packages to prevent potential
clashes with classes in Logstash itself.
</li>
</ul>
</div>
<h4><a id="_plugin_settings"></a>Plugin settings<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<p>The snippet below contains both the setting definition and the method referencing it.</p>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">public static final PluginConfigSpec<Long> EVENT_COUNT_CONFIG =
PluginConfigSpec.numSetting("count", 3);
public static final PluginConfigSpec<String> PREFIX_CONFIG =
PluginConfigSpec.stringSetting("prefix", "message");
@Override
public Collection<PluginConfigSpec<?>> configSchema() {
return Arrays.asList(EVENT_COUNT_CONFIG, PREFIX_CONFIG);
}</pre>
</div>
<p>The <code class="literal">PluginConfigSpec</code> 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 <code class="literal">count</code> setting defines
the number of events that will be generated and the <code class="literal">prefix</code> setting defines an
optional prefix to include in the event field. Neither setting is required and
if it is not explicitly set, the settings default to <code class="literal">3</code> and <code class="literal">message</code>,
respectively.</p>
<p>The <code class="literal">configSchema</code> 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.</p>
<h4><a id="_constructor_and_initialization"></a>Constructor and initialization<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">private String id;
private long count;
private String prefix;
public JavaInputExample(String id, Configuration config, Context context) {
this.id = id;
count = config.get(EVENT_COUNT_CONFIG);
prefix = config.get(PREFIX_CONFIG);
}</pre>
</div>
<p>All Java input plugins must have a constructor taking a <code class="literal">String</code> id and
<code class="literal">Configuration</code> and <code class="literal">Context</code> 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
two plugin settings are retrieved and stored in local variables for later use in
the <code class="literal">start</code> method.</p>
<p>Any additional initialization may occur in the constructor as well. If there are
any unrecoverable errors encountered in the configuration or initialization of
the input plugin, a descriptive exception should be thrown. The exception will
be logged and will prevent Logstash from starting.</p>
<h4><a id="_start_method"></a>Start method<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">@Override
public void start(Consumer<Map<String, Object>> consumer) {
int eventCount = 0;
try {
while (!stopped && eventCount < count) {
eventCount++;
consumer.accept.push(Collections.singletonMap("message",
prefix + " " + StringUtils.center(eventCount + " of " + count, 20)));
}
} finally {
stopped = true;
done.countDown();
}
}</pre>
</div>
<p>The <code class="literal">start</code> method begins the event-producing loop in an input. Inputs are flexible and may produce events through
many different mechanisms including:</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
a pull mechanism such as periodic queries of external database
</li>
<li class="listitem">
a push mechanism such as events sent from clients to a local network port
</li>
<li class="listitem">
a timed computation such as a heartbeat
</li>
<li class="listitem">
any other mechanism that produces a useful stream of events. Event streams may be either finite or infinite.
If the input produces an infinite stream of events, this method should loop until a stop request is made through
the <code class="literal">stop</code> method. If the input produces a finite stream of events, this method should terminate when the last
event in the stream is produced or a stop request is made, whichever comes first.
</li>
</ul>
</div>
<p>Events should be constructed as instances of <code class="literal">Map<String, Object></code> and pushed into the event pipeline via the
<code class="literal">Consumer<Map<String, Object>>.accept()</code> method. To reduce allocations and GC pressure, inputs may reuse the same
map instance by modifying its fields between calls to <code class="literal">Consumer<Map<String, Object>>.accept()</code> because the event
pipeline will create events based on a copy of the map’s data.</p>
<h4><a id="_stop_and_awaitstop_methods"></a>Stop and awaitStop methods<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">private final CountDownLatch done = new CountDownLatch(1);
private volatile boolean stopped;
@Override
public void stop() {
stopped = true; // set flag to request cooperative stop of input
}
@Override
public void awaitStop() throws InterruptedException {
done.await(); // blocks until input has stopped
}</pre>
</div>
<p>The <code class="literal">stop</code> method notifies the input to stop producing events. The stop
mechanism may be implemented in any way that honors the API contract though a
<code class="literal">volatile boolean</code> flag works well for many use cases.</p>
<p>Inputs stop both asynchronously and cooperatively. Use the <code class="literal">awaitStop</code> method to
block until the input has completed the stop process. Note that this method
should <span class="strong strong"><strong>not</strong></span> signal the input to stop as the <code class="literal">stop</code> method does. The
awaitStop mechanism may be implemented in any way that honors the API contract
though a <code class="literal">CountDownLatch</code> works well for many use cases.</p>
<h4><a id="_getid_method"></a>getId method<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">@Override
public String getId() {
return id;
}</pre>
</div>
<p>For input plugins, the <code class="literal">getId</code> method should always return the id that was provided to the plugin through its
constructor at instantiation time.</p>
<h4><a id="_unit_tests"></a>Unit tests<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<p>Lastly, but certainly not least importantly, unit tests are strongly encouraged.
The example input plugin includes an
<a href="https://github.com/logstash-plugins/logstash-input-java_input_example/blob/master/src/test/java/org/logstash/javaapi/JavaInputExampleTest.java" class="ulink" target="_top">example unit
test</a> that you can use as a template for your own.</p>
<h3><a id="_package_and_deploy"></a>Package and deploy<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h3>
<p>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 <code class="literal">logstash-plugin</code> 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.</p>
<h4><a id="_configuring_the_gradle_packaging_task"></a>Configuring the Gradle packaging task<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<p>The following section appears near the top of the <code class="literal">build.gradle</code> file supplied
with the example Java plugins:</p>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">// ===========================================================================
// 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 = ['[email protected]']
pluginInfo.homepage = "http://www.elastic.co/guide/en/logstash/current/index.html"
pluginInfo.pluginType = "filter"
pluginInfo.pluginClass = "JavaFilterExample"
pluginInfo.pluginName = "java_filter_example"
// ===========================================================================</pre>
</div>
<p>You should configure the values above for your plugin.</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
The <code class="literal">version</code> value will be automatically read from the <code class="literal">VERSION</code> file in the
root of your plugin’s codebase.
</li>
<li class="listitem">
<code class="literal">pluginInfo.pluginType</code> should be set to one of <code class="literal">input</code>, <code class="literal">filter</code>, <code class="literal">codec</code>,
or <code class="literal">output</code>.
</li>
<li class="listitem">
<code class="literal">pluginInfo.pluginName</code> must match the name specified on the <code class="literal">@LogstashPlugin</code>
annotation on the main plugin class. The Gradle packaging task will validate
that and return an error if they do not match.
</li>
</ul>
</div>
<h4><a id="_running_the_gradle_packaging_task"></a>Running the Gradle packaging task<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<p>Several Ruby source files along with a <code class="literal">gemspec</code> file and a <code class="literal">Gemfile</code> 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.</p>
<p>You run the Gradle packaging task with the following command:</p>
<div class="pre_wrapper lang-shell">
<pre class="programlisting prettyprint lang-shell">./gradlew gem</pre>
</div>
<p>For Windows platforms: Substitute <code class="literal">gradlew.bat</code> for <code class="literal">./gradlew</code> as appropriate in the command.</p>
<p>That task will produce a gem file in the root directory of your
plugin’s codebase with the name <code class="literal">logstash-{plugintype}-<pluginName>-<version>.gem</code></p>
<h4><a id="_installing_the_java_plugin_in_logstash"></a>Installing the Java plugin in Logstash<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h4>
<p>After you have packaged your Java plugin as a Ruby gem, you can install it in
Logstash with this command:</p>
<div class="pre_wrapper lang-shell">
<pre class="programlisting prettyprint lang-shell">bin/logstash-plugin install --no-verify --local /path/to/javaPlugin.gem</pre>
</div>
<p>For Windows platforms: Substitute backslashes for forward slashes as appropriate in the command.</p>
<h3><a id="_running_logstash_with_the_java_input_plugin"></a>Running Logstash with the Java input plugin<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h3>
<p>The following is a minimal Logstash configuration that can be used to test that
the Java input plugin is correctly installed and functioning.</p>
<div class="pre_wrapper lang-java">
<pre class="programlisting prettyprint lang-java">input {
java_input_example {}
}
output {
stdout { codec => rubydebug }
}</pre>
</div>
<p>Copy the above Logstash configuration to a file such as <code class="literal">java_input.conf</code>.
Start Logstash with:</p>
<div class="pre_wrapper lang-shell">
<pre class="programlisting prettyprint lang-shell">bin/logstash -f /path/to/java_input.conf</pre>
</div>
<div class="note admon">
<div class="icon"></div>
<div class="admon_content">
<p>The Java execution engine, the default execution engine since Logstash 7.0, is required
as Java plugins are not supported in the Ruby execution engine.</p>
</div>
</div>
<p>The expected Logstash output (excluding initialization) with the configuration above is:</p>
<div class="pre_wrapper lang-txt">
<pre class="programlisting prettyprint lang-txt">{
"@version" => "1",
"message" => "message 1 of 3 ",
"@timestamp" => yyyy-MM-ddThh:mm:ss.SSSZ
}
{
"@version" => "1",
"message" => "message 2 of 3 ",
"@timestamp" => yyyy-MM-ddThh:mm:ss.SSSZ
}
{
"@version" => "1",
"message" => "message 3 of 3 ",
"@timestamp" => yyyy-MM-ddThh:mm:ss.SSSZ
}</pre>
</div>
<h3><a id="_feedback"></a>Feedback<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/logstash/edit/7.3/docs/static/contributing-java-plugin.asciidoc">edit</a></h3>
<p>If you have any feedback on Java plugin support in Logstash, please comment on our
<a href="https://github.com/elastic/logstash/issues/9215" class="ulink" target="_top">main Github issue</a> or post in the
<a href="https://discuss.elastic.co/c/logstash" class="ulink" target="_top">Logstash forum</a>.</p>
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