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Implementing an Object Pool Pattern in .NET
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A complete Object Pool pattern implementation in .NET is a bit more complicated than the UML diagram in Figure 7-2. For example, in general, object pools should have an external factory that instantiates the individual object. Figure 7-3 shows another UML diagram of the Object Pool pattern.
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Figure 7-3. UML diagram for .NET Object Pool pattern implementation When compared to Figure 7-2, the UML diagram in Figure 7-3 appears more complicated, due mainly to the refactoring of the object pool collection functionality that is separate from the factory and the use of interfaces. The interface IObjectPool<> defines the core object pool collection functionality. IPoolableObjectFactory<> is an interface that a developer implements and uses to instantiate an object when requested by an IObjectPool<> implementation. The class TestFactory implements the IPoolableObjectFactory<> and is responsible for instantiating TestObject, which is used by Client and managed by GenericObjectPool. GenericObjectPool references an IPoolableObjectFactory<> instance. In the example, GenericObjectPool references the type TestObject directly, but it s possible to reference an interface instance.
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What is radically different from the original Object Pool pattern is the use of .NET finalizers. A .NET finalizer is like a destructor that is called before the .NET runtime disposes of an object. It s used because it makes the Object Pool pattern implementation self-managing. Let s say that you re instantiating an object that will be pooled. You ask the pool for the object, and an object is removed from the inactive pool and moved into the active pool. Then when you re done, you need to give the object back to the pool. This additional step of returning the object to the pool is the dilemma. Coders might forget to do this additional step, and an object pool will hand out objects without the caller pooling any of them. A possible alternative would be to add a finalizer to a pooled object that, when called, would resurrect the object again. Following is an example pooled object with a finalizer declaration:
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class TestPoolAllocation { IObjectPool< TestPoolAllocation> _parent; public TestPoolAllocation( IObjectPool< TestPoolAllocation> parent) { _parent = parent; } ~TestPoolAllocation() { _parent.ReturnObject( this); } } The class TestPoolAllocation has a constructor parameter parent, which represents the object pool collection manager. The parent parameter is assigned to the data member _parent. A pooled object has a reference to the object pool collection that instantiated it. When a client finishes using a pooled object, the .NET garbage collector will collect the pooled object and call the finalizer (~TestPoolAllocation). This in turn causes the pooled object to call the method _parent.ReturnObject, which creates a strong reference to the pooled object by the pool, thus resurrecting the pooled object. There is a catch in this pooled object scenario, namely that the object pool collection can t reference the pooled object when a client is using the pooled object. The garbage collector will only call the TestPoolAllocation finalizer if no references exist to the instance. Were the pool collection to have a reference, there would be no need to garbage collect the object, thus the garbage collector never calls the finalizer. The only time when the object pool collection can reference the pooled object is if the pooled object is available for reuse. Making an object poolable requires the additional steps of adding a constructor with a parameter and adding a finalizer (destructor). This might deter usage because it requires modifications of already existing types. However, to convert an existing object into a pooled object does require some changes. Consider the scenario of adding a pooled object back into the pool. Before the pooled object can be used, it has to be reset to some initialized state. Not doing that would cause objects to use an object that has the state from previous client interactions.
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