How to dynamically create a Groovy Closure from a String in Java

问题

I would like to know how to create a Closure object at run-time from within a Java application, where the content of the Closure is not known ahead of time. I have found a solution but I doubt that it is optimal.

Background: I have written some Groovy code that parses a Domain Specific Language. The parsing code is statically compiled and included in a Java application. In the parser implementation I have classes acting as delegates for specific sections of the DSL. These classes are invoked using the following pattern:

class DslDelegate {
  private Configuration configuration

  def section(@DelegatesTo(SectionDelegate) Closure cl) {
    cl.delegate = new SectionDelegate(configuration)
    cl.resolveStrategy = Closure.DELEGATE_FIRST
    cl()
  }
}

I wish to call such a method directly from Java code. I am able to create a new DslDelegate object and then invoke the section() method. However I need to create and pass an argument that is an instance of Closure. I want the content to be initialised from a String object.

My Solution: The following Java code (utility) is working but I am asking for improvements. Surely this can be done in a cleaner or more efficient manner?

/**
 * Build a Groovy Closure dynamically
 *
 * @param strings
 *            an array of strings for the text of the Closure
 * @return a Groovy Closure comprising the specified text from {@code strings}
 * @throws IOException
 */
public Closure<?> buildClosure(String... strings) throws IOException {
    Closure<?> closure = null;

    // Create a method returning a closure
    StringBuilder sb = new StringBuilder("def closure() { { script -> ");
    sb.append(String.join("\n", strings));
    sb.append(" } }");

    // Create an anonymous class for the method
    GroovyClassLoader loader = new GroovyClassLoader();
    Class<?> groovyClass = loader.parseClass(sb.toString());

    try {
        // Create an instance of the class
        GroovyObject groovyObject = (GroovyObject) groovyClass.newInstance();

        // Invoke the object's method and thus obtain the closure
        closure = (Closure<?>) groovyObject.invokeMethod("closure", null);
    } catch (InstantiationException | IllegalAccessException e) {
        throw new RuntimeException(e);
    } finally {
        loader.close();
    }

    return closure;
}

回答1:

You can use GroovyShell to create a Closure from strings:

public Closure<?> buildClosure(String... strings) {
    String scriptText = "{ script -> " + String.join("\n", strings) + " }";
    return (Closure<?>) new GroovyShell().evaluate(scriptText);
}

回答2:

Thanks to @hzpz I've soved the similar task, but I'd made it more beautiful and easy-to-use. In my case the closure might accept any arguments, so I put arguments list to closures code. Lets say the closure dynamically created in the String and looks like this:

script1 = 'out,a,b,c-> out.println "a=${a}; b=${b}; c=${c}"; return a+b+c;'

Now, create new method in the String class

String.metaClass.toClosure = {
   return (Closure) new GroovyShell().evaluate("{${delegate}}")
}

Now I can call a closure from String or file or from anything else.

println script1.toClosure()(out,1,2,3)

println (new File('/folder/script1.groovy')).getText('UTF-8').toClosure()(out,1,2,3)

来源：https://stackoverflow.com/questions/36771508/how-to-dynamically-create-a-groovy-closure-from-a-string-in-java

标签

java

groovy

closures