Before/After Patterns in Java

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14 Before/After Patterns
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Many concurrent designs are best described as patterns A pattern encapsulates a successful and common design form, usually an object structure (also known as a micro-architecture) consisting of one or more interfaces, classes, and/or objects that obey certain static and dynamic constraints and relationships Patterns are an ideal vehicle for characterizing designs and techniques that need not be implemented in exactly the same way across different contexts, and thus cannot be usefully encapsulated as reusable components Reusable components and frameworks can play a central role in concurrent software development But much of concurrent OO programming entails the reuse, adaptation, and extension of recurring design forms and practices rather than of particular classes Unlike those in the pioneering Design Patterns book by Gamma, Helm, Johnson, and Vlissides (see Further Readings in 145), the patterns here are embedded within chapters discussing sets of related contexts and software design principles that generate the main forces and constraints resolved in the patterns Many of these patterns are minor extensions or variants of other common OO layering and composition patterns This section reviews some that are relied on heavily in subsequent chapters Others are briefly described upon first encounter
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141 Layering
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Layering policy control over mechanism is a common structuring principle in systems of all sorts Many OO layering and composition techniques rely on sandwiching some method call or body of code between a given before-action and an after-action All forms of before/after control arrange that a given ground method, say method, is intercepted so as always to execute in the sequence:
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before(); method(); after();
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Or, to ensure that after-actions are performed even if the ground methods encounter exceptions:
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before(); try { method(); } finally { after(); }
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Most examples in this book of course revolve around concurrency control For example, a synchronized method acquires a lock before executing the code inside the method, and releases the lock after the method otherwise completes But the basic ideas of before/after patterns can be illustrated in conjunction with another useful practice in OO programming, self-checking code: The fields of any object should preserve all invariants whenever the object is not engaged in a public method (see 131) Invariants should be maintained even if these methods throw any of their declared exceptions, unless these exceptions denote corruption or program failure (as may be true for RuntimeExceptions and Errors) Conformance to computable invariants can be tested dynamically by creating classes that check them both on entry to and on exit from every public method Similar techniques apply to preconditions and postconditions, but for simplicity, we'll illustrate only with invariants As an example, suppose we'd like to create water tank classes that contain a self-check on the invariant that the volume is always between zero and the capacity To do this, we can define a checkVolumeInvariant method and use it as both the before and after operation We can first define an exception to throw if the invariant fails:
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class AssertionError extends javalangError { public AssertionError() { super(); } public AssertionError(String message) { super(message); } }
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It can be disruptive to insert these checks manually inside each method Instead, one of three before/after design patterns can be used to separate the checks from the ground methods: adapter classes, subclass-based designs, and method adapter classes In all cases, the best way to set this up is to define an interface describing the basic functionality Interfaces are almost always necessary when you need to give yourself enough room to vary implementations Conversely, the lack of existing interfaces limits options when retrospectively applying before/after patterns Here is an interface describing a minor variant of the water tank class discussed in 124 Before/after techniques may be applied to check invariants around the transferWater operation
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interface Tank { float getCapacity(); float getVolume(); void transferWater(float amount) throws OverflowException, UnderflowException; }
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