Tiny mocking framework (less that 100 LOC) that I've used to demonstrate dynamic proxies.

TinyMock.java

/*
 * Copyright © 2012 Philipp Eichhorn
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package de.fips.util.tinymock;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.HashMap;
import java.util.Map;

/**
 * <p>Mockito-ish API for simple Mocks of no argument methods, like getters.</p>
 * <p>Since this implementation uses the {@link Proxy Dynamic-Proxy} API, it will only work with interfaces.</p>
 * <p><pre>
 * IDeepThought deepThought = TinyMock.mock(IDeepThought.class);
 * TinyMock.doReturn("42").when(deepThought).getAnswerToTheUltimateQuestionOfLifeTheUniverseAndEverything();
 * </pre></p>
 * 
 * @author Philipp Eichhorn
 */
final class TinyMock {

  private TinyMock() {
    super();
  }

  /** */
  @SuppressWarnings("unchecked")
  public static <INTERFACE_TYPE, TYPE extends INTERFACE_TYPE> TYPE mock(final Class<INTERFACE_TYPE> interfaceType) {
    return (TYPE) Proxy.newProxyInstance(TinyMock.class.getClassLoader(), new Class[] { interfaceType }, new Stubs(interfaceType));
  }

  /** */
  public static OngoingStubbing doReturn(final Object returnValue) {
    return new OngoingStubbing(returnValue);
  }

  /** */
  public final static class OngoingStubbing {
    private final Object _returnValue;

    private OngoingStubbing(final Object returnValue) {
      _returnValue = returnValue;
    }

    /** */
    @SuppressWarnings("unchecked")
    public <INTERFACE_TYPE> INTERFACE_TYPE when(final INTERFACE_TYPE mock) {
      final InvocationHandler invocationHandler = Proxy.getInvocationHandler(mock);
      if (!(invocationHandler instanceof Stubs)) {
        throw new IllegalArgumentException("The given object is not a mock.");
      }
      final Stubs stubs = (Stubs) invocationHandler;
      return (INTERFACE_TYPE) Proxy.newProxyInstance(TinyMock.class.getClassLoader(), new Class[] { stubs.getInterfaceType() }, new Stubbing(stubs, _returnValue));
    }
  }

  private static class Stubbing implements InvocationHandler {
    private final Stubs _stubs;
    private final Object _returnValue;

    private Stubbing(final Stubs stubs, final Object returnValue) {
      _stubs = stubs;
      _returnValue = returnValue;
    }

    public final Object invoke(final Object proxy, final Method method, final Object[] args) throws Throwable {
      _stubs.addStub(method.getName(), _returnValue);
      return null;
    }
  }

  private static class Stubs implements InvocationHandler {
    private final Map<String, Object> _stubs = new HashMap<String, Object>();
    private final Class<?> _interfaceType;

    private Stubs(final Class<?> interfaceType) {
      _interfaceType = interfaceType;
    }

    public Class<?> getInterfaceType() {
      return _interfaceType;
    }

    public void addStub(final String methodName, final Object returnValue) {
      _stubs.put(methodName, returnValue);
    }

    public final Object invoke(final Object proxy, final Method method, final Object[] args) throws Throwable {
      final Object object = _stubs.get(method.getName());
      if ((object != null) && (args == null)) {
        return object;
      } else {
        return null;
      }
    }
  }
}