1 |
tim |
1.1 |
/* |
2 |
dl |
1.12 |
* Written by Doug Lea with assistance from members of JCP JSR-166 |
3 |
dl |
1.22 |
* Expert Group and released to the public domain, as explained at |
4 |
jsr166 |
1.35 |
* http://creativecommons.org/publicdomain/zero/1.0/ |
5 |
tim |
1.1 |
*/ |
6 |
|
|
|
7 |
|
|
package java.util.concurrent; |
8 |
|
|
|
9 |
|
|
/** |
10 |
jsr166 |
1.37 |
* A {@code Future} represents the result of an asynchronous |
11 |
dl |
1.12 |
* computation. Methods are provided to check if the computation is |
12 |
|
|
* complete, to wait for its completion, and to retrieve the result of |
13 |
|
|
* the computation. The result can only be retrieved using method |
14 |
jsr166 |
1.37 |
* {@code get} when the computation has completed, blocking if |
15 |
dl |
1.17 |
* necessary until it is ready. Cancellation is performed by the |
16 |
jsr166 |
1.37 |
* {@code cancel} method. Additional methods are provided to |
17 |
dl |
1.17 |
* determine if the task completed normally or was cancelled. Once a |
18 |
|
|
* computation has completed, the computation cannot be cancelled. |
19 |
jsr166 |
1.37 |
* If you would like to use a {@code Future} for the sake |
20 |
dl |
1.21 |
* of cancellability but not provide a usable result, you can |
21 |
jsr166 |
1.34 |
* declare types of the form {@code Future<?>} and |
22 |
jsr166 |
1.37 |
* return {@code null} as a result of the underlying task. |
23 |
tim |
1.1 |
* |
24 |
jsr166 |
1.41 |
* <p><b>Sample Usage</b> (Note that the following classes are all |
25 |
jsr166 |
1.38 |
* made-up.) |
26 |
|
|
* |
27 |
jsr166 |
1.40 |
* <pre> {@code |
28 |
dl |
1.10 |
* interface ArchiveSearcher { String search(String target); } |
29 |
tim |
1.1 |
* class App { |
30 |
tim |
1.18 |
* ExecutorService executor = ... |
31 |
dl |
1.5 |
* ArchiveSearcher searcher = ... |
32 |
jsr166 |
1.41 |
* void showSearch(String target) throws InterruptedException { |
33 |
|
|
* Callable<String> task = () -> searcher.search(target); |
34 |
|
|
* Future<String> future = executor.submit(task); |
35 |
dl |
1.5 |
* displayOtherThings(); // do other things while searching |
36 |
tim |
1.1 |
* try { |
37 |
dl |
1.5 |
* displayText(future.get()); // use future |
38 |
tim |
1.8 |
* } catch (ExecutionException ex) { cleanup(); return; } |
39 |
tim |
1.1 |
* } |
40 |
jsr166 |
1.34 |
* }}</pre> |
41 |
tim |
1.1 |
* |
42 |
jsr166 |
1.37 |
* The {@link FutureTask} class is an implementation of {@code Future} that |
43 |
|
|
* implements {@code Runnable}, and so may be executed by an {@code Executor}. |
44 |
|
|
* For example, the above construction with {@code submit} could be replaced by: |
45 |
jsr166 |
1.40 |
* <pre> {@code |
46 |
jsr166 |
1.41 |
* FutureTask<String> future = new FutureTask<>(task); |
47 |
jsr166 |
1.36 |
* executor.execute(future);}</pre> |
48 |
brian |
1.29 |
* |
49 |
jsr166 |
1.31 |
* <p>Memory consistency effects: Actions taken by the asynchronous computation |
50 |
|
|
* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a> |
51 |
|
|
* actions following the corresponding {@code Future.get()} in another thread. |
52 |
dl |
1.30 |
* |
53 |
tim |
1.1 |
* @see FutureTask |
54 |
|
|
* @see Executor |
55 |
dl |
1.12 |
* @since 1.5 |
56 |
dl |
1.4 |
* @author Doug Lea |
57 |
jsr166 |
1.37 |
* @param <V> The result type returned by this Future's {@code get} method |
58 |
tim |
1.1 |
*/ |
59 |
dl |
1.17 |
public interface Future<V> { |
60 |
|
|
|
61 |
|
|
/** |
62 |
dl |
1.42 |
* Attempts to cancel execution of this task. This method has no |
63 |
|
|
* effect if the task is already completed or cancelled, or could |
64 |
|
|
* not be cancelled for some other reason. Otherwise, if this |
65 |
|
|
* task has not started when {@code cancel} is called, this task |
66 |
|
|
* should never run. If the task has already started, then the |
67 |
|
|
* {@code mayInterruptIfRunning} parameter determines whether the |
68 |
|
|
* thread executing this task (when known by the implementation) |
69 |
|
|
* is interrupted in an attempt to stop the task. |
70 |
|
|
* |
71 |
|
|
* <p>The return value from this method does not necessarily |
72 |
|
|
* indicate whether the task is now cancelled; use {@link |
73 |
|
|
* #isCancelled}. |
74 |
|
|
* |
75 |
|
|
* @param mayInterruptIfRunning {@code true} if the thread |
76 |
|
|
* executing this task should be interrupted (if the thread is |
77 |
|
|
* known to the implementation); otherwise, in-progress tasks are |
78 |
|
|
* allowed to complete |
79 |
jsr166 |
1.37 |
* @return {@code false} if the task could not be cancelled, |
80 |
dl |
1.42 |
* typically because it has already completed; {@code true} |
81 |
|
|
* otherwise. If two or more threads cause a task to be cancelled, |
82 |
|
|
* then at least one of them returns {@code true}. Implementations |
83 |
|
|
* may provide stronger guarantees. |
84 |
dl |
1.17 |
*/ |
85 |
|
|
boolean cancel(boolean mayInterruptIfRunning); |
86 |
|
|
|
87 |
|
|
/** |
88 |
jsr166 |
1.37 |
* Returns {@code true} if this task was cancelled before it completed |
89 |
dl |
1.17 |
* normally. |
90 |
|
|
* |
91 |
jsr166 |
1.37 |
* @return {@code true} if this task was cancelled before it completed |
92 |
dl |
1.17 |
*/ |
93 |
|
|
boolean isCancelled(); |
94 |
|
|
|
95 |
|
|
/** |
96 |
jsr166 |
1.37 |
* Returns {@code true} if this task completed. |
97 |
dl |
1.17 |
* |
98 |
|
|
* Completion may be due to normal termination, an exception, or |
99 |
|
|
* cancellation -- in all of these cases, this method will return |
100 |
jsr166 |
1.37 |
* {@code true}. |
101 |
jsr166 |
1.24 |
* |
102 |
jsr166 |
1.37 |
* @return {@code true} if this task completed |
103 |
dl |
1.17 |
*/ |
104 |
|
|
boolean isDone(); |
105 |
tim |
1.1 |
|
106 |
|
|
/** |
107 |
dl |
1.23 |
* Waits if necessary for the computation to complete, and then |
108 |
tim |
1.1 |
* retrieves its result. |
109 |
|
|
* |
110 |
dl |
1.23 |
* @return the computed result |
111 |
|
|
* @throws CancellationException if the computation was cancelled |
112 |
|
|
* @throws ExecutionException if the computation threw an |
113 |
dl |
1.4 |
* exception |
114 |
dl |
1.23 |
* @throws InterruptedException if the current thread was interrupted |
115 |
dl |
1.4 |
* while waiting |
116 |
tim |
1.1 |
*/ |
117 |
|
|
V get() throws InterruptedException, ExecutionException; |
118 |
|
|
|
119 |
|
|
/** |
120 |
|
|
* Waits if necessary for at most the given time for the computation |
121 |
dl |
1.23 |
* to complete, and then retrieves its result, if available. |
122 |
tim |
1.1 |
* |
123 |
|
|
* @param timeout the maximum time to wait |
124 |
dl |
1.14 |
* @param unit the time unit of the timeout argument |
125 |
dl |
1.23 |
* @return the computed result |
126 |
|
|
* @throws CancellationException if the computation was cancelled |
127 |
|
|
* @throws ExecutionException if the computation threw an |
128 |
dl |
1.4 |
* exception |
129 |
dl |
1.23 |
* @throws InterruptedException if the current thread was interrupted |
130 |
dl |
1.4 |
* while waiting |
131 |
tim |
1.1 |
* @throws TimeoutException if the wait timed out |
132 |
|
|
*/ |
133 |
dl |
1.14 |
V get(long timeout, TimeUnit unit) |
134 |
tim |
1.1 |
throws InterruptedException, ExecutionException, TimeoutException; |
135 |
|
|
} |