Adding properties from Java to groovy objects

I want to be able to add properties to an instance of a string object using the metaprogramming capabilities of groovy from Java

To do this from groovy is simple:

class GroovyClass {
    def getDynamicString() {
        def myString = "hello"
        myString.MetaClass.dynamicProperty = "there"
        return myString
    }
}

This is my Spock test case:

def "should add dynamic property"() {
    GroovyClass groovyClass = new GroovyClass()

    when:
    def theString = groovyClass.getDynamicString()
    then:
    theString == "hello"
    theString.dynamicProperty == "there"
}

But I want to do the same thing from Java and run it through the same tests As far as I know, groovy adds additional properties and methods (such as getmetaclass) to JDK classes like string, but I'm not sure how and when it does this I found the following solution to work, but initializing the groovy shell and writing the metaprogramming code as a string seems cumbersome

public String getDynamicString()
{
    String myString = "hello";
    groovyshell shell = new groovyshell();
    shell.setVariable("myString",myString);
    shell.setVariable("theDynamicPropertyValue","there");
    shell.evaluate("myString.MetaClass.dynamicProperty = theDynamicPropertyValue");
    return myString;
}

Is there any way to avoid using shell Do this in the case of evaluate – that is, by calling groovy library methods directly from Java?

Solution

So, in view of this groovy script:

def a = 'A String'

Decorator.decorate( a,'foo','bar' )

assert a       == 'A String'
assert a.class == String
assert a.foo   == 'bar'

Then we can write the Java decorator class like this:

import org.codehaus.groovy.runtime.HandleMetaClass ;
import org.codehaus.groovy.runtime.InvokerHelper ;

public class Decorator {
  public static void decorate( Object o,String name,Object value ) {
    HandleMetaClass hmc = new HandleMetaClass( InvokerHelper.getMetaClass( o ),o ) ;
    hmc.setProperty( name,value ) ;
  }
}

Then, if we compile the Java class and run the groovy script, all assertions should pass