Lock


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17  
18  package org.apache.james.util;
19  
20  import java.util.Hashtable  ;
21  
22  /**
23   * Provides Lock functionality
24   *
25   */
26  public class Lock {
27      /**
28       * An internal hash table of keys to locks
29       */
30      private Hashtable   locks = new Hashtable  ();
31  
32      /**
33       * Check to see if the object is locked
34       *
35       * @param key the Object on which to check the lock
36       * @return true if the object is locked, false otherwise
37       */
38      public boolean isLocked(final Object   key) {
39          return (locks.get(key) != null);
40      }
41  
42      /**
43       * Check to see if we can lock on a given object.
44       *
45       * @param key the Object on which to lock
46       * @return true if the calling thread can lock, false otherwise
47       */
48      public boolean canI(final Object   key) {
49          Object   o = locks.get( key );
50  
51          if (null == o || o == this.getCallerId()) {
52              return true;
53          }
54  
55          return false;
56      }
57  
58      /**
59       * Lock on a given object.
60       *
61       * @param key the Object on which to lock
62       * @return true if the locking was successful, false otherwise
63       */
64      public boolean lock(final Object   key) {
65          Object   theLock;
66  
67          synchronized(this) {
68              theLock = locks.get(key);
69  
70              if (null == theLock) {
71                  locks.put(key, getCallerId());
72                  return true;
73              } else if (getCallerId() == theLock) {
74                  return true;
75              } else {
76                  return false;
77              }
78          }
79      }
80  
81      /**
82       * Release the lock on a given object.
83       *
84       * @param key the Object on which the lock is held
85       * @return true if the unlocking was successful, false otherwise
86       */
87      public boolean unlock(final Object   key) {
88          Object   theLock;
89          synchronized (this) {
90              theLock = locks.get(key);
91  
92              if (null == theLock) {
93                  return true;
94              } else if (getCallerId() == theLock) {
95                  locks.remove(key);
96                  return true;
97              } else {
98                  return false;
99              }
100         }
101     }
102 
103     /**
104      * Private helper method to abstract away caller ID.
105      *
106      * @return the id of the caller (i.e. the Thread reference)
107      */
108     private Object   getCallerId() {
109         return Thread.currentThread();
110     }
111 }
112
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