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StrategyRegistry.java | - | - | - | - |
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1 | // Copyright 2004, 2005 The Apache Software Foundation | |
2 | // | |
3 | // Licensed under the Apache License, Version 2.0 (the "License"); | |
4 | // you may not use this file except in compliance with the License. | |
5 | // You may obtain a copy of the License at | |
6 | // | |
7 | // http://www.apache.org/licenses/LICENSE-2.0 | |
8 | // | |
9 | // Unless required by applicable law or agreed to in writing, software | |
10 | // distributed under the License is distributed on an "AS IS" BASIS, | |
11 | // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | |
12 | // See the License for the specific language governing permissions and | |
13 | // limitations under the License. | |
14 | ||
15 | package org.apache.hivemind.lib.util; | |
16 | ||
17 | /** | |
18 | * An implementation of the <b>strategy </b> pattern. The strategy pattern allows new functionality | |
19 | * to be assigned to an existing class. As implemented here, this is a smart lookup between a | |
20 | * particular class (called the <em>subject class</em>) and some object instance that can provide | |
21 | * the extra functionality (called the <em>strategy</em>). The implementation of the strategy is | |
22 | * not relevant to the StrategyRegistry class. | |
23 | * <p> | |
24 | * Strategies are registered before they can be used; the registration maps a particular class to a | |
25 | * strategy instance. The strategy instance will be used when the subject class matches the | |
26 | * registered class, or the subject class inherits from the registered class. | |
27 | * <p> | |
28 | * This means that a search must be made that walks the inheritance tree (upwards from the subject | |
29 | * class) to find a registered mapping. | |
30 | * <p> | |
31 | * In addition, strategies can be registered against <em>interfaces</em>. Searching of interfaces | |
32 | * occurs after searching of classes. The exact order is: | |
33 | * <ul> | |
34 | * <li>Search for the subject class, then each super-class of the subject class (excluding | |
35 | * java.lang.Object) | |
36 | * <li>Search interfaces, starting with interfaces implemented by the subject class, continuing | |
37 | * with interfaces implemented by the super-classes, then interfaces extended by earlier interfaces | |
38 | * (the exact order is a bit fuzzy) | |
39 | * <li>Search for a match for java.lang.Object, if any | |
40 | * </ul> | |
41 | * <p> | |
42 | * The first match terminates the search. | |
43 | * <p> | |
44 | * The StrategyRegistry caches the results of search; a subsequent search for the same subject class | |
45 | * will be resolved immediately. | |
46 | * <p> | |
47 | * StrategyRegistry does a minor tweak of the "natural" inheritance. Normally, the parent class of | |
48 | * an object array (i.e., <code>Foo[]</code>) is simply <code>Object</code>, even though you | |
49 | * may assign <code>Foo[]</code> to a variable of type <code>Object[]</code>. StrategyRegistry | |
50 | * "fixes" this by searching for <code>Object[]</code> as if it was the superclass of any object | |
51 | * array. This means that the search path for <code>Foo[]</code> is <code>Foo[]</code>, | |
52 | * <code>Object[]</code>, then a couple of interfaces {@link java.lang.Cloneable}, | |
53 | * {@link java.io.Serializable}, etc. that are implicitily implemented by arrays), and then, | |
54 | * finally, <code>Object</code> | |
55 | * <p> | |
56 | * This tweak doesn't apply to arrays of primitives, since such arrays may <em>not</em> be | |
57 | * assigned to <code>Object[]</code>. | |
58 | * | |
59 | * @author Howard M. Lewis Ship | |
60 | * @see org.apache.hivemind.lib.util.StrategyRegistryImpl | |
61 | * @since 1.1 | |
62 | */ | |
63 | public interface StrategyRegistry | |
64 | { | |
65 | /** | |
66 | * Registers an adapter for a registration class. | |
67 | * | |
68 | * @throws IllegalArgumentException | |
69 | * if a strategy has already been registered for the given class. | |
70 | */ | |
71 | public void register(Class registrationClass, Object strategy); | |
72 | ||
73 | /** | |
74 | * Gets the stategy object for the specified subjectClass. | |
75 | * | |
76 | * @throws IllegalArgumentException | |
77 | * if no strategy could be found. | |
78 | */ | |
79 | public Object getStrategy(Class subjectClass); | |
80 | } |
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