1 /*
2 * Copyright (c) 1999, 2004, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.util;
27
28 /**
29 * A task that can be scheduled for one-time or repeated execution by a Timer.
30 *
31 * @author Josh Bloch
32 * @see Timer
33 * @since 1.3
34 */
35
36 public abstract class TimerTask implements Runnable {
37 /**
38 * This object is used to control access to the TimerTask internals.
39 */
40 final Object lock = new Object();
41
42 /**
43 * The state of this task, chosen from the constants below.
44 */
45 int state = VIRGIN;
46
47 /**
48 * This task has not yet been scheduled.
49 */
50 static final int VIRGIN = 0;
51
52 /**
53 * This task is scheduled for execution. If it is a non-repeating task,
54 * it has not yet been executed.
55 */
56 static final int SCHEDULED = 1;
57
58 /**
59 * This non-repeating task has already executed (or is currently
60 * executing) and has not been cancelled.
61 */
62 static final int EXECUTED = 2;
63
64 /**
65 * This task has been cancelled (with a call to TimerTask.cancel).
66 */
67 static final int CANCELLED = 3;
68
69 /**
70 * Next execution time for this task in the format returned by
71 * System.currentTimeMillis, assuming this task is scheduled for execution.
72 * For repeating tasks, this field is updated prior to each task execution.
73 */
74 long nextExecutionTime;
75
76 /**
77 * Period in milliseconds for repeating tasks. A positive value indicates
78 * fixed-rate execution. A negative value indicates fixed-delay execution.
79 * A value of 0 indicates a non-repeating task.
80 */
81 long period = 0;
82
83 /**
84 * Creates a new timer task.
85 */
86 protected TimerTask() {
87 }
88
89 /**
90 * The action to be performed by this timer task.
91 */
92 public abstract void run();
93
94 /**
95 * Cancels this timer task. If the task has been scheduled for one-time
96 * execution and has not yet run, or has not yet been scheduled, it will
97 * never run. If the task has been scheduled for repeated execution, it
98 * will never run again. (If the task is running when this call occurs,
99 * the task will run to completion, but will never run again.)
100 *
101 * <p>Note that calling this method from within the <tt>run</tt> method of
102 * a repeating timer task absolutely guarantees that the timer task will
103 * not run again.
104 *
105 * <p>This method may be called repeatedly; the second and subsequent
106 * calls have no effect.
107 *
108 * @return true if this task is scheduled for one-time execution and has
109 * not yet run, or this task is scheduled for repeated execution.
110 * Returns false if the task was scheduled for one-time execution
111 * and has already run, or if the task was never scheduled, or if
112 * the task was already cancelled. (Loosely speaking, this method
113 * returns <tt>true</tt> if it prevents one or more scheduled
114 * executions from taking place.)
115 */
116 public boolean cancel() {
117 synchronized(lock) {
118 boolean result = (state == SCHEDULED);
119 state = CANCELLED;
120 return result;
121 }
122 }
123
124 /**
125 * Returns the <i>scheduled</i> execution time of the most recent
126 * <i>actual</i> execution of this task. (If this method is invoked
127 * while task execution is in progress, the return value is the scheduled
128 * execution time of the ongoing task execution.)
129 *
130 * <p>This method is typically invoked from within a task's run method, to
131 * determine whether the current execution of the task is sufficiently
132 * timely to warrant performing the scheduled activity:
133 * <pre>{@code
134 * public void run() {
135 * if (System.currentTimeMillis() - scheduledExecutionTime() >=
136 * MAX_TARDINESS)
137 * return; // Too late; skip this execution.
138 * // Perform the task
139 * }
140 * }</pre>
141 * This method is typically <i>not</i> used in conjunction with
142 * <i>fixed-delay execution</i> repeating tasks, as their scheduled
143 * execution times are allowed to drift over time, and so are not terribly
144 * significant.
145 *
146 * @return the time at which the most recent execution of this task was
147 * scheduled to occur, in the format returned by Date.getTime().
148 * The return value is undefined if the task has yet to commence
149 * its first execution.
150 * @see Date#getTime()
151 */
152 public long scheduledExecutionTime() {
153 synchronized(lock) {
154 return (period < 0 ? nextExecutionTime + period
155 : nextExecutionTime - period);
156 }
157 }
158 }