Calling a Groovy function from Java

Question

How do you call a function defined in a Groovy script file from Java?

Example groovy script:

def hello_world() {    println "Hello, world!" } 

I've looked at the GroovyShell, GroovyClassLoader, and GroovyScriptEngine.

Answer 1

The simplest way is to compile the script into a java class file and just call it directly. Example:

// Script.groovy def hello_world() {     println "Hello, World!" }  // Main.java public class Main {     public static void main(String[] args) {         Script script = new Script();         script.hello_world();     } }  $ groovyc Script.groovy $ javac -classpath .:$GROOVY_HOME/embeddable/groovy-all-1.7.5.jar Main.java $ java -classpath .:$GROOVY_HOME/embeddable/groovy-all-1.7.5.jar Main Hello, World! 

Answer 2

Assuming you have a file called test.groovy, which contains (as in your example):

def hello_world() {    println "Hello, world!" } 

Then you can create a file Runner.java like this:

import groovy.lang.GroovyShell ; import groovy.lang.GroovyClassLoader ; import groovy.util.GroovyScriptEngine ; import java.io.File ;  class Runner {   static void runWithGroovyShell() throws Exception {     new GroovyShell().parse( new File( "test.groovy" ) ).invokeMethod( "hello_world", null ) ;   }    static void runWithGroovyClassLoader() throws Exception {     // Declaring a class to conform to a java interface class would get rid of     // a lot of the reflection here     Class scriptClass = new GroovyClassLoader().parseClass( new File( "test.groovy" ) ) ;     Object scriptInstance = scriptClass.newInstance() ;     scriptClass.getDeclaredMethod( "hello_world", new Class[] {} ).invoke( scriptInstance, new Object[] {} ) ;   }    static void runWithGroovyScriptEngine() throws Exception {     // Declaring a class to conform to a java interface class would get rid of     // a lot of the reflection here     Class scriptClass = new GroovyScriptEngine( "." ).loadScriptByName( "test.groovy" ) ;     Object scriptInstance = scriptClass.newInstance() ;     scriptClass.getDeclaredMethod( "hello_world", new Class[] {} ).invoke( scriptInstance, new Object[] {} ) ;   }    public static void main( String[] args ) throws Exception {     runWithGroovyShell() ;     runWithGroovyClassLoader() ;     runWithGroovyScriptEngine() ;   } } 

compile it with:

$ javac -cp groovy-all-1.7.5.jar Runner.java  Note: Runner.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. 

(Note: The warnings are left as an exercise to the reader) ;-)

Then, you can run this Runner.class with:

$ java -cp .:groovy-all-1.7.5.jar Runner Hello, world! Hello, world! Hello, world!