Promise


1   // $Id: Promise.java,v 1.9 2005/04/25 08:35:44 belaban Exp $
2   
3   package org.jgroups.util;
4   
5   import org.jgroups.TimeoutException;
6   
7   
8   /**
9    * Allows a thread to submit an asynchronous request and to wait for the result. The caller may choose to check
10   * for the result at a later time, or immediately and it may block or not. Both the caller and responder have to
11   * know the promise.
12   * @author Bela Ban
13   */
14  public class Promise {
15      Object   result=null;
16      boolean hasResult=false;
17  
18  
19      /**
20       * Blocks until a result is available, or timeout milliseconds have elapsed
21       * @param timeout
22       * @return An object
23       * @throws TimeoutException. If a timeout occurred (implies that timeout > 0)
24       */
25      public Object   getResultWithTimeout(long timeout) throws TimeoutException {
26          synchronized(this) {
27              try {
28                  return _getResultWithTimeout(timeout);
29              }
30              finally {
31                  notifyAll();
32              }
33          }
34      }
35  
36  
37      /**
38       * Blocks until a result is available, or timeout milliseconds have elapsed. Needs to be called with
39       * a lock held on 'this'
40       * @param timeout
41       * @return An object
42       * @throws TimeoutException. If a timeout occurred (implies that timeout > 0)
43       */
44      private Object   _getResultWithTimeout(long timeout) throws TimeoutException {
45          Object    ret=null;
46          long    time_to_wait=timeout, start;
47          boolean timeout_occurred=false;
48  
49          start=System.currentTimeMillis();
50          while(hasResult == false) {
51              if(timeout <= 0) {
52                  doWait();
53              }
54              else {
55                  if(time_to_wait <= 0) {
56                      timeout_occurred=true;
57                      break; // terminate the while loop
58                  }
59                  else {
60                      doWait(time_to_wait);
61                      time_to_wait=timeout - (System.currentTimeMillis() - start);
62                  }
63              }
64          }
65  
66          ret=result;
67          result=null;
68          hasResult=false;
69          if(timeout_occurred)
70              throw new TimeoutException();
71          else
72              return ret;
73      }
74  
75      public Object   getResult() {
76          try {
77              return getResultWithTimeout(0);
78          }
79          catch(TimeoutException e) {
80              return null;
81          }
82      }
83  
84      /**
85       * Returns the result, but never throws a TimeoutException; returns null instead
86       * @param timeout
87       * @return
88       */
89      public Object   getResult(long timeout) {
90          try {
91              return getResultWithTimeout(timeout);
92          }
93          catch(TimeoutException e) {
94              return null;
95          }
96      }
97  
98  
99      void doWait() {
100         try {wait();} catch(InterruptedException   e) {}
101     }
102 
103     void doWait(long timeout) {
104         try {wait(timeout);} catch(InterruptedException   e) {}
105     }
106 
107 
108 
109 
110     /**
111      * Checks whether result is available. Does not block.
112      */
113     public boolean hasResult() {
114         synchronized(this) {
115             return hasResult;
116         }
117     }
118 
119     /**
120      * Sets the result and notifies any threads
121      * waiting for it
122      */
123     public void setResult(Object   obj) {
124         synchronized(this) {
125             result=obj;
126             hasResult=true;
127             notifyAll();
128         }
129     }
130 
131 
132     /**
133      * Causes all waiting threads to return
134      */
135     public void reset() {
136         synchronized(this) {
137             result=null;
138             hasResult=false;
139             notifyAll();
140         }
141     }
142 
143 
144     public String   toString() {
145         return "hasResult=" + hasResult + ",result=" + result;
146     }
147 
148 
149 }
A to Z: JavaDoc & Examples Daily Java News & Articles Open Source Projects Open Source Codes Free Computer Books Remove Frame
Popular Tags