This is a mixin class that is resolvable with the package location and class naming convention resolution strategy.

StringMixins.groovy

package groovy.runtime.metaclass.java.lang

import groovy.json.JsonSlurper

class StringMixins {

    // Must be static & must take an instance of the receiver as the first vararg
    static def toJson(String input) {
        new JsonSlurper().parseText(input)
    }
}

StringMixinsTest.groovy

package groovy.runtime.metaclass.java.lang

import com.objectpartners.groovy.ext.CustomMetaClassCreationHandle
import org.junit.BeforeClass
import org.junit.Test

class StringMixinsTest {

    @BeforeClass
    static void init() {
        // We need to tell the GroovySystem to use our CustomMetaClassCreationHandle
        GroovySystem.getMetaClassRegistry().setMetaClassCreationHandle(new CustomMetaClassCreationHandle())
    }

    @Test
    void testToJson() {
        def json = '{ "id": "1", "name": "Dan Woods", "twitter": "@danveloper" }'.toJson()

        assert json instanceof Map
        assert json.id == "1"
    }
}