Question

The problem: I’d like to be able to generically access in Java any property/field on a Java ojbect similarly to how a dynamic language (think Groovy, JavaScript) would. I won’t know at the time I’m writing this plumbing code what type of object it is or what the property/field name will be. But I will know the property/field name when I go to use it.

My current solution: So far I’ve written a simple wrapper class that uses java.beans.Introspector to grab the properties of a Bean/POJO and expose them as a Map<String, Object>. It’s crude but works for simple cases.

My question is what other methodologies are there for approaching this problem besides reflection / converting to a Map?

Before I go too much further down this path, I’d like to know if anyone knows how I could cannibalize something out of Rhino or perhaps javax.script.* which has a well thought out implementation of this concept. Or perhaps an entirely different approach that I haven’t considered.

Edit: yes I’m familiar with reflection (which I believe is what Introspector is using under the hood anyway). I was just curious if there was any other well thought out solutions.

Edit 2: It appears that the most popular answers involve 1) reflection either directly or via helper classes, and/or 2) mapping to interfaces which implement the desired class members. I’m really intrigued by the comment which talks about leveraging Groovy. Since Groovy has true duck-typing and it is a JVM language, is there a way to make a simple helper in Groovy and call it from Java? This would be really cool and probably more flexible and perform better.

Answer: I marked Mike’s answer as the best since it is a complete concept which comes the closest. I probably won’t go that route for this particular case, but it is certainly a useful approach. Anyone looking through this should be sure to read the conversations on here as there is a lot of useful info in there as well.

Thanks!

Answer 1

If you know the set of APIs that you want to expose, say you know you want access to a length method and an iterator method, you can define an interface:

public interface TheInterfaceIWant {
  int length();
  void quack();
}

and you want to be able to use this interface to access corresponding methods on instances that do not implement this interface, you can use Proxy classes : http://download.oracle.com/javase/1.4.2/docs/api/java/lang/reflect/Proxy.html

So you create a proxy

final Object aDuck = ...;
TheInterfaceIWant aDuckWrapper = (TheInterfaceIWant) Proxy.newProxyInstance(
    TheInterfaceIWant.class.getClassLoader(),
    new Class[] { TheInterfaceIWant.class },
    new InvocationHandler() {
      public Object invoke(
          Object proxy, Method method, Object[] args)
          throws Throwable {
        return aDuck.getClass().getMethod(
            method.getName(), method.getParameterTypes()).invoke(aDuck, args);
      }
    });

Then you can use the wrapper as you would the duck in a dynamically typed language.

if (aDuckWrapper.length() > 0) {
  aDuckWrapper.quack();
}

Here’s a full length runnable example that prints “Quack” four times using a wrapper:

import java.lang.reflect.*;

public class Duck {

  // The interface we use to access the duck typed object.
  public interface TheInterfaceIWant {
    int length();
    void quack();
  }

  // The underlying instance that does not implement TheInterfaceIWant!
  static final class Foo {
    public int length() { return 4; }
    public void quack() { System.out.println("Quack"); }
  }

  public static void main(String[] args) throws Exception {
    // Create an instance but cast away all useful type info.
    final Object aDuck = new Foo();

    TheInterfaceIWant aDuckWrapper = (TheInterfaceIWant) Proxy.newProxyInstance(
        TheInterfaceIWant.class.getClassLoader(),
        new Class[] { TheInterfaceIWant.class },
        new InvocationHandler() {
          public Object invoke(
              Object proxy, Method method, Object[] args)
              throws Throwable {
            return aDuck.getClass().getMethod(
                method.getName(), method.getParameterTypes()).invoke(aDuck, args);
          }
        });

    for (int n = aDuckWrapper.length(); --n >= 0;) {
      // Calling aDuck.quack() here would be invalid since its an Object.
      aDuckWrapper.quack();
    }
  }
}