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.
The GA version of the Java plugin API is available in the 7.2
and later branches of the Logstash codebase.
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:
-
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 asoutput { java_output_example => { .... } }
-
The value of the
name
property must match the name of the class excluding casing and underscores.
-
The
-
The class must implement the
co.elastic.logstash.api.Output
interface. -
Java plugins may not be created in the
org.logstash
orco.elastic.logstash
packages to prevent potential clashes with classes in Logstash itself.
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.
-
The
version
value will be automatically read from theVERSION
file in the root of your plugin’s codebase. -
pluginInfo.pluginType
should be set to one ofinput
,filter
,codec
, oroutput
. -
pluginInfo.pluginName
must match the name specified on the@LogstashPlugin
annotation on the main plugin class. The Gradle packaging task will validate that and return an error if they do not match.
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.