Package mondrian.util

Class ObjectFactory<V>

java.lang.Object

mondrian.util.ObjectFactory<V>

Direct Known Subclasses:

ExpCompiler.Factory, ObjectFactory.Singleton

public abstract class ObjectFactory<V>
extends Object

Concrete derived classes of the generic ObjectFactory class are used to produce an implementation of an interface (a normal interface implementation or a Proxy). In general, a factory should produce a default implementation for general application use as well as particular implementations used during testing. During testing of application code and during normal execution, the application code uses one of the ObjectFactory's methods for producing implementation instances - the same method is used both for test and non-test modes. There are two ways of modifying the implementation returned to the application code.

The first is for the application to use Properties. The ObjectFactory implementation looks for a given property (by default the name of the property is the class name of the interfaceClass object) and if found uses it as the classname to create.

A second approach is to use a ThreadLocal; if the ThreadLocal is non-empty then use it as the class name.

When to use a Factory?

Everyone has an opinion. For me, there are two criteria: enabling unit testing and providing end-user/developer-customizer overriding.

If a method has side-effects, either its result depends upon a side-effect or calling it causes a side-effect, then the Object hosting the method is a candidate for having a factory. Why? Well, consider the case where a method returns the value of a System property and the System property is determined only once and set to a static final variable:

      class OneValue {
          private static final boolean propValue;
          static {
              propValue = Boolean.getBoolean("com.app.info.value");
          }
          .....
          public boolean hasInfo() {
              return propValue;
          }
      }
 

In this case, only one value is ever returned. If you have a module, a client of the above code, that uses the value returned by a call to the hasInfo() method, how do you write a unit test of your module that tests both possible return values? You can not, its value is based upon a side-effect, an external value that can not be controlled by the unit test.

If the OneValue class was an interface and there was a factory, then the unit test could arrange that its own version of the OneValue interface was returned and in one test arrange that true was returned and in a second test, arrange that false was returned.

The above is a trivial example of code that disallows clients of the code from being properly tested.

Another example might be a module that directly initializes a JMS queue and receives JMS message from the JMS queue. This code can not be tested without having a live JMS queue. On the other hand, if one defines an interface allowing one to wrap access to the JMS queue and accesses the implementation via a factory, then unit tests can be create that use a mock JMS queue.

With regards to providing end-user/developer-customizer overriding, its generally good to have a flexible application framework. Experimental or just different implementations can be developed and tested without having to touch a lot of the application code itself.

There is, of course, a trade-off between the use of a factory and the size or simplicity of the object being created.

What are the requirements for a template ObjectFactory?

First, every implementation must support the writing of unit tests. What this means it that test cases can override what the factory produces. The test cases can all use the same produced Object or each can request an Object targeted to its particular test. All this without changing the default behavior of the factory.

Next, it should be possible to create a factory from the template that is intended to deliver the same Object each time it is called, a different, new Object each time it is called, or, based on the calling environment (parameters, properties, ThreadLocal, etc.) one of a set of Objects. These are possible default behaviors, but, again, they can be overridden for test purposes.

While a factory has a default behavior in an application, it must be possible for every factory's behavior in that application to be globally overridden. What that means is if the application designer has dictated a default, the application user should be able to change the default. An example of this is overriding what Object is returned based upon a System property value.

Lastly, every factory is a singleton - if an interface with an implementation whose creation is mediated by a factory, then there is a single factory that does that creating. This does not mean that such a factory always return the same value, rather that there is only one instance of the factory itself.

The following is an example class that generates a factory singleton. In this case, the factory extends the ObjectFactory rather than the ObjectFactory.Singleton:

      public final class FooFactory extends ObjectFactory {
          // The single instance of the factory
          private static final FooFactory factory;
          static {
              factory = new FooFactory();
          }
          public static FooFactory instance() {
              return factory;
          }
          ..........
          private FooFactory() {
              super(Foo.class);
          }
          ..........
      }

 

There are multiple ways of creating derived classes that have support for unit testing. A very simple way is to use ThreadLocals.

          private static final ThreadLocal ClassName = new ThreadLocal();
          private static String getThreadLocalClassName() {
              return (String) ClassName.get();
          }
          public static void setThreadLocalClassName(String className) {
              ClassName.set(className);
          }
          public static void clearThreadLocalClassName() {
              ClassName.set(null);
          }
          ..........
          protected String getClassName() {
              return getThreadLocalClassName();
          }

 

Here, the unit test will call the setThreadLocalClassName method setting it with the class name of a specialized implementation of the template interface. In the finally clause of the unit test, it is very important that there be a call to the clearThreadLocalClassName method so that other tests, etc. do not get an instance of the test-specific specialized implementation.

The following is an example unit test that uses the factory's ThreadLocal to override the implementation that is returned.

      interface Boo {
          boolean getValue();
          .......
      }
      class NormalBooImpl implements Boo {
          public boolean getValue() { ... }
          .......
      }
      class MyCode {
          private Boo boo;
          MyCode() {
              boo = BooFactory.instance().getObject();
          }
          .......
          int getValue() {
              if (boo.getValue()) {
                  return 1;
              } else {
                  return 0;
              }

          }
      }

      class MyCodeTest {
          private static boolean testValue;
          static class BooTest1 implements Boo {
              public boolean getValue() {
                  return MyTest.testValue;
              }
              .....
          }
          static class BooTest2 implements
                      java.lang.reflect.InvocationHandler {
              private final Boo boo;
              public BooTest2() {
                  // remove test class name
                  BooFactory.clearThreadLocalClassName();
                  // get default Boo implementation
                  this.boo = BooFactory.instance().getObject();
              }
              public Object invoke(Object proxy, Method method, Object[] args)
                  throws Throwable {
                  if (method.getName().equals("getValue")) [
                      return new Boolean(MyTest.testValue);
                  } else {
                      return method.invoke(this.boo, args);
                  }
              }
          }
          public void test1() {
              try {
                  // Factory will creates test class
                  BooFactory.setThreadLocalClassName("MyTest.BooTest1");

                  MyTest.testValue = true;
                  MyCode myCode = new MyCode();
                  int value = myCode.getValue();
                  assertTrue("Value not 1", (value == 1));

                  MyTest.testValue = false;
                  myCode = new MyCode();
                  value = myCode.getValue();
                  assertTrue("Value not 0", (value == 0));
              } finally {
                  BooFactory.clearThreadLocalClassName();
              }
          }
          public void test2() {
              try {
                  // Use InvocationHandler and Factory Proxy capability
                  BooFactory.setThreadLocalClassName("MyTest.BooTest2");

                  MyTest.testValue = true;
                  MyCode myCode = new MyCode();
                  int value = myCode.getValue();
                  assertTrue("Value not 1", (value == 1));

                  MyTest.testValue = false;
                  myCode = new MyCode();
                  value = myCode.getValue();
                  assertTrue("Value not 0", (value == 0));
              } finally {
                  BooFactory.clearThreadLocalClassName();
              }
          }
      }

 

While this is a very simple example, it shows how using such factories can aid in creating testable code. The MyCode method is a client of the Boo implementation. How to test the two different code branches the method can take? Because the Boo object is generated by a factory, one can override what object the factory returns.

Since:

Feb 01 2007

Author:

Richard M. Emberson

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static interface	ObjectFactory.Context	This is for testing only.
static class	ObjectFactory.Singleton<T>	Implementation of ObjectFactory that returns only a single instance of the Object.

Constructor Summary

Constructors

Modifier	Constructor	Description
protected	ObjectFactory(Class<V> interfaceClass)	Creates a new factory object.

Method Summary

Modifier and Type	Method	Description
protected String	getClassName()	Returns the name of a class to use to create an object.
protected String	getClassName(Properties props)	Returns the name of a class to use to create an object.
protected abstract V	getDefault(Class[] parameterTypes, Object[] parameterValues)	For most uses (other than testing) this is the method that derived classes implement that return the desired object.
protected final V	getObject()	Constructs an object where the System Properties can be used to look up a class name.
protected final V	getObject(Class[] parameterTypes, Object[] parameterValues)	Constructs an object where the parameterTypes and parameterValues are constructor parameters and System Properties are used to look up a class name.
protected V	getObject(String className, Class[] parameterTypes, Object[] parameterValues)	Creates an instance with the given className, parameterTypes and parameterValues or throw a CreationException.
protected final V	getObject(Properties props)	Constructs an object where the Properties parameter can be used to look up a class name.
protected V	getObject(Properties props, Class[] parameterTypes, Object[] parameterValues)	Constructs an object where the parameterTypes and parameterValues are constructor parameters and Properties parameter is used to look up a class name.
protected abstract org.eigenbase.util.property.StringProperty	getStringProperty()	Return the StringProperty associated with this factory.
Object	removeContext()	Gets the current override values in the opaque context object and clears those values within the Factory.
void	restoreContext(Object context)	Restores the context object resetting override values.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

ObjectFactory

protected ObjectFactory(Class<V> interfaceClass)

Creates a new factory object. The interfaceClass parameter is used to cast the object generated to type right type.

Parameters:

interfaceClass - the class object for the interface implemented by the objects returned by this factory

Method Details

getObject

protected final V getObject()
                     throws CreationException

Constructs an object where the System Properties can be used to look up a class name. The constructor for the object takes no parameters.

Returns:

the newly created object

Throws:

CreationException - if unable to create the object

getObject

protected final V getObject(Properties props)
                     throws CreationException

Constructs an object where the Properties parameter can be used to look up a class name. The constructor for the object takes no parameters.

Parameters:

props - the property definitions to use to determine the implementation class

Returns:

the newly created object

Throws:

CreationException - if unable to create the object

getObject

protected final V getObject(Class[] parameterTypes,
 Object[] parameterValues)
                     throws CreationException

Constructs an object where the parameterTypes and parameterValues are constructor parameters and System Properties are used to look up a class name.

Parameters:

parameterTypes - the class parameters that define the signature of the constructor to use

parameterValues - the values to use to construct the current instance of the object

Returns:

the newly created object

Throws:

CreationException - if unable to create the object

getObject

protected V getObject(Properties props,
 Class[] parameterTypes,
 Object[] parameterValues)
               throws CreationException

Constructs an object where the parameterTypes and parameterValues are constructor parameters and Properties parameter is used to look up a class name.

This returns a new instance of the Object each time its called (assuming that if the method getDefault, which derived classes implement), if called, creates a new object each time.

Parameters:

props - the property definitions to use to determine the

parameterTypes - the class parameters that define the signature of the constructor to use

parameterValues - the values to use to construct the current instance of the object

Returns:

the newly created object

Throws:

CreationException - if unable to create the object

getObject

protected V getObject(String className,
 Class[] parameterTypes,
 Object[] parameterValues)
               throws CreationException

Creates an instance with the given className, parameterTypes and parameterValues or throw a CreationException. There are two different mechanims available. The first is to uses reflection to create the instance typing the generated Object based upon the interfaceClass factory instance object. With the second the className is an class that implements the InvocationHandler interface and in this case the java.lang.reflect.Proxy class is used to generate a proxy.

Parameters:

className - the class name used to create Object instance

parameterTypes - the class parameters that define the signature of the constructor to use

parameterValues - the values to use to construct the current instance of the object

Returns:

the newly created object

Throws:

CreationException - if unable to create the object

getClassName

protected String getClassName()

Returns the name of a class to use to create an object. The default implementation returns null but derived classes can return a class name.

This method is the primary mechanism for supporting Unit testing. A derived class can have, as an example, this method return the value of a ThreadLocal. For testing it return a class name while for normal use it returns null.

Returns:

null or a class name

getClassName

protected String getClassName(Properties props)

Returns the name of a class to use to create an object. The factory's StringProperty is gotten and if it has a non-null value, then that is returned. Otherwise, the StringProperty's name (path) is used as the name to probe the Properties object for a value. This method is allowed to return null.

Returns:

null or a class name

getStringProperty

protected abstract org.eigenbase.util.property.StringProperty getStringProperty()

Return the StringProperty associated with this factory.

Returns:

the StringProperty

getDefault

protected abstract V getDefault(Class[] parameterTypes,
 Object[] parameterValues)
                         throws CreationException

For most uses (other than testing) this is the method that derived classes implement that return the desired object.

Parameters:

parameterTypes - the class parameters that define the signature of the constructor to use

parameterValues - the values to use to construct the current instance of the object

Returns:

the newly created object

Throws:

CreationException - if unable to create the object

removeContext

public Object removeContext()

Gets the current override values in the opaque context object and clears those values within the Factory.

This is used in testing.

Returns:

the test Context object.

restoreContext

public void restoreContext(Object context)

Restores the context object resetting override values.

This is used in testing.

Parameters:

context - the context object to be restored.