OpenSearch/docs/reference/modules/scripting/native.asciidoc

[[modules-scripting-native]]
=== Native (Java) Scripts

Sometimes `painless` and <<modules-scripting-expression, expression>> aren't enough. For those times you can
implement a native script.

The best way to implement a native script is to write a plugin and install it.
The plugin {plugins}/plugin-authors.html[documentation] has more information on
how to write a plugin so that Elasticsearch will properly load it.

To register the actual script you'll need to implement `NativeScriptFactory`
to construct the script. The actual script will extend either
`AbstractExecutableScript` or `AbstractSearchScript`. The second one is likely
the most useful and has several helpful subclasses you can extend like
`AbstractLongSearchScript` and `AbstractDoubleSearchScript`.
Finally, your plugin should register the native script by implementing the
`ScriptPlugin` interface.

If you squashed the whole thing into one class it'd look like:

[source,java]
--------------------------------------------------
public class MyNativeScriptPlugin extends Plugin implements ScriptPlugin {

    @Override
    public List<NativeScriptFactory> getNativeScripts() {
        return Collections.singletonList(new MyNativeScriptFactory());
    }

    public static class MyNativeScriptFactory implements NativeScriptFactory {
        @Override
        public ExecutableScript newScript(@Nullable Map<String, Object> params) {
            return new MyNativeScript();
        }
        @Override
        public boolean needsScores() {
            return false;
        }
        @Override
        public String getName() {
            return "my_script";
        }        
    }

    public static class MyNativeScript extends AbstractDoubleSearchScript {
        @Override
        public double runAsDouble() {
            double a = (double) source().get("a");
            double b = (double) source().get("b");
            return a * b;
        }
    }
}
--------------------------------------------------

You can execute the script by specifying its `lang` as `native`, and the name
of the script as the `id`:

[source,js]
--------------------------------------------------
curl -XPOST localhost:9200/_search -d '{
  "query": {
    "function_score": {
      "query": {
        "match": {
          "body": "foo"
        }
      },
      "functions": [
        {
          "script_score": {
            "script": {
                "inline": "my_script",
                "lang" : "native"
            }
          }
        }
      ]
    }
  }
}'
--------------------------------------------------