Mocking Your Objects with Impunity

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Mocking Your Objects with Impunity

2
Goal of Presentation

• Show how Mocking Objects can greatly improve
  unit testing by facilitating tests that are
  easier to write and less dependant upon objects
  outside the test domain

3
Topics of Discussion

• What is Object Mocking?
• Why Mock Objects?
• What tools are available to support Object Mockery

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What is Object Mocking?

• Creating a mock implementation of an object that
  can be used as a stand-in in testing
• A mock objects allow you to set expectations
• The mock object lets you verify that the
  expectations were met

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Why engage in Object Mocking?

• The real object has behaviour that is hard to
  cause or is non-deterministic
• The real object is difficult to set up
• The real object is slow
• The real object has (or is ) a UI
• Test needs to query the object but queries are
  not available
• Real objects do not exist

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What tools are available to support Obect Mockery

• The MockObjects framework
• MockMaker
• EasyMockt

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MockObjects Framework

• Open source framework available at
  MockObjects.org
• Provides some utility tools for creating mock
  objects
• Provides mock implementations of common libraries
  such as IO, Servlets and JDBC

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MockMaker Overview

• Open source tool available at mockmaker.org
• Passive code generation tool
• Generates mock objects that depend on the
  MockObjects framework
• Has a command line and gui interface
• Mock objects can only be generated from an
  interface

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MockMaker Features

• Generated MockObject has
• Methods to set expected minimal method call
• Methods to set expected parameter values
• Methods to set return value from method
• Method to verify everything was as expected

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MockMaker Drawbacks

• Interface being used is not directly visible
• If interface changes MockObject must be modified
  or regenerated
• No method to throw exceptions

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EasyMock Overview

• Open source tool available at easymock.org
• Uses reflection to dynamically create proxies
  that implement the interface at run time
• EasyMock generates two objects the EasyMock Mock
  Object and the EasyMock Mock Control

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EasyMock Features

• Allows the expected parameter value to be set for
  each method
• Allows you to set up the value to be returned by
  each method
• Allows the number of times the method is to be
  invoked to be specified
• Allows you to specify an exception the a method
  should throw
• Will verify that everything went as expected

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EasyMock Drawbacks

• EasyMock generates objects on the fly so you
  cant add any behaviour
• EasyMock uses the equals method to compare
  parameters

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Test Domain

• We need to test a Copier class
• The Copier constructor takes a Reader and a
  Writer
• The Copier copy method reads from the Reader and
  Writes to the Writer until the Reader returns
  null

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Interfaces

• public interface Reader
• public String readln()
• public interface Writer
• public void writeln(String line)

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Copier

• public class Copier
• private Reader reader
• private Writer writer
• public Copier(Reader aReader, Writer aWriter)
  
• this.reader aReader
• this.writer aWriter
• public void copy()
• String line
• while ( ( line reader.readln() ) !
  null )
• writer.writeln(line)

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Run MockMaker

• java mockmaker.MockMaker Reader MockReader.java
• java mockmaker.MockMaker Writer MockWriter.java

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MockReader.java

• import mockmaker.ReturnValues
• import com.mockobjects.
• import Reader
• public class MockReader implements Reader
• private ExpectationCounter myReadlnCalls new
  ExpectationCounter("Reader ReadlnCalls")
• private ReturnValues myActualReadlnReturnValues
  new ReturnValues(false)
• public void setExpectedReadlnCalls(int calls)
• myReadlnCalls.setExpected(calls)
• public String readln()
• myReadlnCalls.inc()
• Object nextReturnValue myActualReadlnRetur
  nValues.getNext()
• return (String) nextReturnValue
• public void setupReadln(String arg)
• myActualReadlnReturnValues.add(arg)
• public void verify()
• myReadlnCalls.verify()

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MockWriter.java

• import mockmaker.ReturnValues
• import com.mockobjects.
• import Writer
• public class MockWriter implements Writer
• private ExpectationCounter myWritelnCalls
  new ExpectationCounter("Writer WritelnCalls")
• private ExpectationList myWritelnParameter0Valu
  es new ExpectationList("Writer
  WritelnParameter0Values")
• public void setExpectedWritelnCalls(int
  calls)
• myWritelnCalls.setExpected(calls)
• public void addExpectedWritelnValues(String
  arg0)
• myWritelnParameter0Values.addExpected(arg0)
  
• public void writeln(String arg0)
• myWritelnCalls.inc()
• myWritelnParameter0Values.addActual(arg0)
• public void verify()
• myWritelnCalls.verify()
• myWritelnParameter0Values.verify()

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Unit test using MockMaker

• public void testCopy()
• MockReader mockReader new MockReader()
• mockReader.setupReadln("ABC")
• mockReader.setupReadln(null)
• mockReader.setExpectedReadlnCalls(2)
• MockWriter mockWriter new MockWriter()
• mockWriter.addExpectedWritelnValues("DEF")
  
• mockWriter.setExpectedWritelnCalls(1)
• Copier copier new Copier(mockReader,
  mockWriter)
• copier.copy()
• mockReader.verify()
• mockWriter.verify()

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Unit test results

• 
  
• Errors logged for the 'CopierTest' test
• No errors.
• 
  
• Failures logged for the 'CopierTest' test
• Total failures 1
• Test case 'testCopy(CopierTest)' failed with
  'Writer WritelnParameter0Values Writer
  WritelnParameter0Values added item does not match
• Expected DEF
• Received ABC'
• at CopierTest.testCopy(CopierTest.java40)
  
• 
  
• Summary of 'CopierTest' test
• Result Failed
• Run 1
• Failures 1
• Errors 0
• Elapsed time 0.07

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Unit Test with EasyMock

• public void testCopy()
• MockControl mockReaderControl
  EasyMock.controlFor(Reader.class)
• Reader mockReader (Reader)
  mockReaderControl.getMock()
• mockReader.readln()
• mockReaderControl.setReturnValue("ABC",
  1)
• mockReaderControl.setReturnValue(null,
  1)
• mockReaderControl.activate()
• MockControl mockWriterControl
  EasyMock.controlFor(Writer.class)
• Writer mockWriter (Writer)
  mockWriterControl.getMock()
• mockWriter.writeln("DEF")
• mockWriterControl.setVoidCallable(1)
• mockWriterControl.activate()
• Copier copier new Copier(mockReader,
  mockWriter)
• copier.copy()
• mockReaderControl.verify()
• mockWriterControl.verify()

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Results of Unit Test with Easy Mock

• 
  
• Errors logged for the 'CopierTest' test
• No errors.
• 
  
• Failures logged for the 'CopierTest' test
• Total failures 1
• Test case 'testCopy(CopierTest)' failed with
  'EasyMock for interface Writer
• Unexpected method call writeln("ABC")'
• at CopierTest.testCopy(CopierTest.java60)
  
• 
  
• Summary of 'CopierTest' test
• Result Failed
• Run 1
• Failures 1
• Errors 0
• Elapsed time 0.181

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Throwing an exception

• public void testCopy()
• MockControl mockReaderControl
  EasyMock.controlFor(Reader.class)
• Reader mockReader (Reader)
  mockReaderControl.getMock()
• mockReader.readln()
• mockReaderControl.setThrowable(new
  Error("Test Exception"), 1)
• mockReaderControl.activate()
• MockControl mockWriterControl
  EasyMock.controlFor(Writer.class)
• Writer mockWriter (Writer)
  mockWriterControl.getMock()
• mockWriter.writeln("DEF")
• mockWriterControl.setVoidCallable(1)
• mockWriterControl.activate()
• Copier copier new Copier(mockReader,
  mockWriter)
• try
• copier.copy()
• fail("copier.copy() should have
  thrown an exception")
• catch ( Error err )
• assertEquals("err.getMessage()
  returned wrong text", "Test Exception",
  err.getMessage())
• mockReaderControl.verify()

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Results of Unit Test with Exception

• 
  
• Errors logged for the 'CopierTest' test
• No errors.
• 
  
• Failures logged for the 'CopierTest' test
• Total failures 1
• Test case 'testCopy(CopierTest)' failed with
  'EasyMock for interface Writer Expectation
  failure on verify
• method call writeln("DEF") calls expected 1,
  received 0'
• at CopierTest.testCopy(CopierTest.java87)
  
• 
  
• Summary of 'CopierTest' test
• Result Failed
• Run 1
• Failures 1
• Errors 0
• Elapsed time 0.17

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What this means

• Object Mocking is an invaluable technique for
  improving the overall quality of your unit tests.

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Resources

• www.mockobjects.com
• www.mockmaker.org
• www.easymock.com
• EasyMock Dynamic Mock Objects for JUnit see
  www.xp2002.org/atti/TammoFreese--EasyMock.pdf
• Automatically Generating System Mock Objects
  -http//www.xpuniverse.com/2001/pdfs/Testing04.pdf
  
• Test flexibly with AspectJ and mock objects
  www-106.ibm.com/developerworks/java/library/j-aspe
  ctj2/?openl007,tgr
• MockDoclet http//joe.truemesh.com/mockdoclet