ThreadLocalCyclicDependencyGuard


1   /*****************************************************************************
2    * Copyright (c) PicoContainer Organization. All rights reserved.            *
3    * ------------------------------------------------------------------------- *
4    * The software in this package is published under the terms of the BSD      *
5    * style license a copy of which has been included with this distribution in *
6    * the LICENSE.txt file.                                                     *
7    *                                                                           *
8    * Original code by Joerg Schaible                                           *
9    *****************************************************************************/
10  
11  package org.picocontainer.defaults;
12  
13  /**
14   * Abstract utility class to detect recursion cycles.
15   * Derive from this class and implement {@link ThreadLocalCyclicDependencyGuard#run}. 
16   * The method will be called by  {@link ThreadLocalCyclicDependencyGuard#observe}. Select
17   * an appropriate guard for your scope. Any {@link ObjectReference} can be 
18   * used as long as it is initialized with  <code>Boolean.FALSE</code>.
19   * 
20   * @author J&ouml;rg Schaible
21   * @since 1.1
22   */
23  public abstract class ThreadLocalCyclicDependencyGuard extends ThreadLocal   implements CyclicDependencyGuard {
24  
25      /**
26       * {@inheritDoc}
27       * @see java.lang.ThreadLocal#initialValue()
28       */
29      protected Object   initialValue() {
30          return Boolean.FALSE;
31      }
32      /**
33       * Derive from this class and implement this function with the functionality 
34       * to observe for a dependency cycle.
35       * 
36       * @return a value, if the functionality result in an expression, 
37       *      otherwise just return <code>null</code>
38       */
39      public abstract Object   run();
40      
41      /**
42       * Call the observing function. The provided guard will hold the {@link Boolean} value.
43       * If the guard is already <code>Boolean.TRUE</code> a {@link CyclicDependencyException} 
44       * will be  thrown.
45       * 
46       * @param stackFrame the current stack frame
47       * @return the result of the <code>run</code> method
48       */
49      public final Object   observe(Class   stackFrame) {
50          if (Boolean.TRUE.equals(get())) {
51              throw new CyclicDependencyException(stackFrame);
52          }
53          Object   result = null;
54          try {
55              set(Boolean.TRUE);
56              result = run();
57          } catch (final CyclicDependencyException e) {
58              e.push(stackFrame);
59              throw e;
60          } finally {
61              set(Boolean.FALSE);
62          }
63          return result;
64      }
65  }
66
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