How to use function defined in a groovy script using the "evaluate" method from Java

Question

I just discover Groovy call from Java and have problem with this case :

I have a groovy file : "test.groovy"

a = 1.0
def mul2( x ) { 2.0 * x }

And I want to use it from Java code like this

GroovyShell gs = new GroovyShell();
gs.parse( new File( ".../test.groovy" ) ).run();

System.out.printf( "a = %s%n", gs.evaluate("a") ); // ok
System.out.printf( "mul2(a) = %s%n", gs.evaluate( "mul2(a)" ) ); // error

The error is :

groovy.lang.MissingMethodException: No signature of method: Script1.mul2() is applicable for argument types: (BigDecimal) values: [1.0]

What I have to do to have access to function defined in groovy script, using evaluate() method ?

I need to use "evaluate" method because I want finally to evaluate something like Math.sin( a * mul2(Math.Pi) ).

Now I have 4 solutions (the forth is what I searched for) :

use closure as in answer of 'Szymon Stepniak'
use import static as in answer of 'daggett'
extends the script that contains Java functions with the script that evaluate the expression :

...the class (in Java, not Groovy)...

  public static abstract class ScriptClass extends Script
  {
    double mul2( double x )
    {
      return x * 2;
    }
  }

...the code...

  CompilerConfiguration config = new CompilerConfiguration();
  config.setScriptBaseClass(ScriptClass.class.getName());

  GroovyShell gs = new GroovyShell(config);

  System.out.printf( "result = %s%n", gs.evaluate("mul2(5.05)") );

That works but the code is in Java, not what I want, but I note it here for ones need to do that

And finally extends groovy script :

the groovy file :

double mul2( x ) { x * 2 } 
a=mul2(3.33)

the java code that use it

GroovyClassLoader gcl = new GroovyClassLoader();
Class<?> r = gcl.parseClass( resourceToFile("/testx.groovy") );
CompilerConfiguration config = new CompilerConfiguration();
config.setScriptBaseClass(r.getName());
GroovyShell gs = new GroovyShell(gcl, config);    

System.out.printf( "mul2(5.05) = %s%n", gs.evaluate("mul2(5.05)") );

// WARNING : call super.run() in evaluate expression to have access to variables defined in script
System.out.printf( "result = %s%n", gs.evaluate("super.run(); mul2(a) / 123.0") );

It's exactly what I wanted :-)

you mixed several problems into your code. the main thing: with gs.evaluate you are parsing a new groovy script and absolutely not connected with previously parsed scripts. — daggett
– daggett, Commented Oct 29, 2018 at 15:25

Szymon Stepniak · Accepted Answer · 2018-10-30 07:01:35Z

4

There are two things worth explaining to understand what is happening here. There are two different scopes in the script you have provided.

The variable a gets stored in GroovyShell binding object and that is why it is available in every gs.evaluate() call. Take a look at this example:

import groovy.lang.Binding;
import groovy.lang.GroovyShell;
import groovy.lang.Script;

final class ExecuteGroovyScriptExample {

    public static void main(String[] args) {
        final String script = "a = 1.0 \n" +
                "def mul2(x) { 2.0 * x }\n";

        final Binding binding = new Binding();

        final GroovyShell gs = new GroovyShell(binding);
        final Script sc = gs.parse(script);
        sc.run();

        System.out.printf("binding.getVariable(\"a\") == %s\n", binding.getVariable("a"));
    }
}

Running this example produces following output:

binding.getVariable("a") == 1.0

The second thing is that every gs.evaluate() call generates a new groovy.lang.Script class which has a completely different context. This is why calling:

gs.evaluate("mul2(a)")

throws something like this:

Exception in thread "main" groovy.lang.MissingMethodException: No signature of method: Script2.mul2() is applicable for argument types: (BigDecimal) values: [1.0]

because the script class that gets generated from gs.evaluate("mul2(a)") invocation does not contain mul2(x) method. The class that gets generated by this call looks something like this:

class Script2 extends groovy.lang.Script {
    void run() {
        mul2(a)
    }
}

However, the script class returned from gs.parse(script) contains mul2(x) method, so you can invoke it, but not as gs.evaluate() call, but Script.invokeMethod(name, args) instead. Something like this:

import groovy.lang.GroovyShell;
import groovy.lang.Script;

final class ExecuteGroovyScriptExample {

    public static void main(String[] args) {
        final String script = "a = 1.0 \n" +
                "def mul2(x) { 2.0 * x }\n";

        final GroovyShell gs = new GroovyShell();
        final Script sc = gs.parse(script);
        sc.run();

        System.out.printf("mul2(a) = %s%n", sc.invokeMethod("mul2", gs.evaluate("a")));
    }
}

This example produces following output:

mul2(a) = 2.00

Take a look how mul2(x) method got invoked. Firstly, we store script returned by gs.parse(script) in sc variable and it allows us to invoke method defined in this script by following call:

sc.invokeMethod("mul2", gs.evaluate("a"));

In this example we take value of a variable simply by gs.evaluate("a"), but you can also use binding object from the first example as well. And keep in mind that if a variable was defined like:

def a = 1.0

or

@groovy.transform.Field
def a = 1.0

it would not get stored in the binding object anymore and in the first case it defines script's local variable a and in the second case it defines script class field a.

Alternatively, if you want to execute following invocation:

gs.evaluate("mul2(a)")

or even

gs.evaluate("Math.sin( a * mul2(Math.PI))")

you would have to modify input Groovy script file and replace function mul2(x) definition with a closure in the same scope as the a variable, so it gets stored in the binding object:

a = 1.0
mul2 = { x -> 2.0 * x }

edited Oct 30, 2018 at 7:01

answered Oct 29, 2018 at 15:35

Szymon Stepniak

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3 Comments

Didier Over a year ago

Thanks a lot, but as I explain at the last line of my post, I can do that. I want to use function defined in my groovy file in an expression. Perhaps I have to extend the first script class and evaluate my expression in it. But I don't know how to do that...

Szymon Stepniak Over a year ago

Alternatively you can replace mul2(x) with a closure like mul2 = { x -> 2.0 * x } in the Groovy script file and then following invocation gs.evaluate("Math.sin( a * mul2(Math.PI))") will be evaluated correctly.

Didier Over a year ago

I have edited the initial post, I want to evaluate complex expression as if this expression is at the end of groovy file. And it seems be possible if I use first groovy file to create Script (may be abstract) and extend it in a new script that evaluate the complex expression. I have succeeded if Script is in java, I work to have script in Groovy...

daggett · Accepted Answer · 2018-10-29 17:05:02Z

0

let you have /my/groovy/classes/Test.groovy:

static mul2( x ) { 2.0 * x }
def    mul3( x ) { 3.0 * x }

then you can use class loader to load it as a class and use this class in expressions:

GroovyShell gs = new GroovyShell();
gs.getClassLoader().addClasspath("/my/groovy/classes");

System.out.println( gs.evaluate( "import static Test.*; mul2(5)" ) );
System.out.println( gs.evaluate( "new Test().mul3(6)" ) );

answered Oct 29, 2018 at 17:05

daggett

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