Sonntag, 22. November 2009

xUnit test implementation pattern for mocking methods in a SUT object

Simple test implementation pattern for mock a method in the SUT object. The pattern is interesting when  you would refactor a method in legacy system. Before you can start with the refactoring stuff you must write a test for the method with a high coverage (Refactoring details see Martin Fowlers great book "Refactoring - Improving the Design of Existing Code").

The idea is to test only the legacy method you would like to refactor (to get quick a high coverage),  there for we want mock all method calls which will be invoked from this method. Normal mocking frameworks like EasyMock or JMock for Java can create mock objects for interfaces and classes. This pattern shows how this tools can be used for mock an internal method in the SUT object.

The idea, overwrite all method which should be mocked in the SUT object and delegate the calls to a mock object of the same type as the SUT object.

Example setup code with JMock as mocking framework:
...
@Before 
public void setUp(){
   ...
   // Create mock object for the SUT class, this mock will be used to mock methods in the SUT
   mockFrameworkCommandProvider = context.mock(FrameworkCommandProvider.class);
  
   // Create the SUT object of type FrameworkCommandProvider
   frameworkCommandProvider = new FrameworkCommandProvider(mockFramework) {
   
    /** 
     * Mock the SUT Method getBundleFromToken(...)
     * All method calls will delegate to a mock 
     * of the same type then the SUT object 
     */
     @Override
     protected AbstractBundle getBundleFromToken(CommandInterpreter intp, String token, boolean error) {
         // delegate the call to a mock object.
         return mockFrameworkCommandProvider.getBundleFromToken(intp, token, error);
     }
   };
}
...

The code with a example test which expected a mock method call:
...
@Test 
public void _bundle_OneArgumentsAndBundleNotFoundByToken() throws Exception { 
 final String firstToken = "myBundleName[1.0.0]";
 // Setup mock context
 context.checking(new Expectations() {{
  // one argument
  one (mockCommandInterpreter).nextArgument(); will(returnValue(firstToken));
  // next calles return null
  allowing (mockCommandInterpreter).nextArgument(); will(returnValue(null));
   
  // getBundleFromToken returns null
  one (mockFrameworkCommandProvider).getBundleFromToken(mockCommandInterpreter, firstToken, true);
  will(returnValue(null));
 }});
 
 // invoke the _bundle() method
 frameworkCommandProvider._bundle(mockCommandInterpreter);
}
... 

Downsides:
  • Much code for the test setup to overwrite all the methods which should be mocked in the SUT object
  • Only protected, package and public methods can be mocked by this approach
  • Private methods can't be mock with this pattern
  • In the test code it's not clear which SUT methods are mocked and which not

Here the whole example test code:
package org.eclipse.osgi.framework.internal.core;

import static org.junit.Assert.*;

import org.eclipse.osgi.framework.console.CommandInterpreter;
import org.jmock.Expectations;
import org.jmock.Mockery;
import org.jmock.integration.junit4.JMock;
import org.jmock.integration.junit4.JUnit4Mockery;
import org.jmock.lib.legacy.ClassImposteriser;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;

@RunWith(JMock.class)
public class FrameworkCommandProviderTest {

 // Create mock context and activate mocking for java classes.
 Mockery context = new JUnit4Mockery() {{
     setImposteriser(ClassImposteriser.INSTANCE);
    }};
 
    InternalSystemBundle mockInternalSystemBundle;
 
 Framework mockFramework;
 
 CommandInterpreter mockCommandInterpreter;
 
 FrameworkCommandProvider mockFrameworkCommandProvider;
 
 FrameworkCommandProvider frameworkCommandProvider;
 
 @Before 
 public void setUp(){
  
  // Create mock object for system bundle
  mockInternalSystemBundle = context.mock(InternalSystemBundle.class);
  // Mocking the getContext Method, returns always null
  context.checking(new Expectations() {{
   allowing (mockInternalSystemBundle).getContext(); will(returnValue(null));
  }});
  
  // Create mock object for framework
  mockFramework = context.mock(Framework.class);
  // Set the mock object system bundle into the framework mock
  mockFramework.systemBundle = mockInternalSystemBundle;
  
  // Create mock object for command interpreter
  mockCommandInterpreter = context.mock(CommandInterpreter.class);
  
  // Create mock object for the SUT class, this mock will be used to mock methods in the SUT
  mockFrameworkCommandProvider = context.mock(FrameworkCommandProvider.class);
  
  // Create the SUT object of type FrameworkCommandProvider
  frameworkCommandProvider = new FrameworkCommandProvider(mockFramework) {
   
   /** 
    * Mock the SUT Method getBundleFromToken(...)
    * All method calls will delegate to a mock 
    * of the same type then the SUT object 
    */
   @Override
   protected AbstractBundle getBundleFromToken(CommandInterpreter intp, String token, boolean error) {
    // delegate the call to a mock object.
    return mockFrameworkCommandProvider.getBundleFromToken(intp, token, error);
   }
  };
  
 }
 
 @Test 
 public void _bundle_OneArgumentsAndBundleNotFoundByToken() throws Exception { 
  final String firstToken = "myBundleName[1.0.0]";
  // Setup mock context
  context.checking(new Expectations() {{
   // one argument
   one (mockCommandInterpreter).nextArgument(); will(returnValue(firstToken));
   // next calles return null
   allowing (mockCommandInterpreter).nextArgument(); will(returnValue(null));
   
   // getBundleFromToken returns null
   one (mockFrameworkCommandProvider).getBundleFromToken(mockCommandInterpreter, firstToken, true);
   will(returnValue(null));
  }});
  
  // invoke the _bundle() method
  frameworkCommandProvider._bundle(mockCommandInterpreter);
 }

}

Run the Example Test:
To run the test you need the equinox osgi implementation as SUT, you can get the equinox source from the eclipse CVS ("pserver:dev.eclipse.org/cvsroot/rt/org.eclipse.equinox/framework/bundles/org.eclipse.osgi/").

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