1 /* 2 * Copyright (c) 1999, 2013, 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 */ TimerTask()86 protected TimerTask() { 87 } 88 89 /** 90 * The action to be performed by this timer task. 91 */ run()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 */ cancel()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 */ scheduledExecutionTime()152 public long scheduledExecutionTime() { 153 synchronized(lock) { 154 return (period < 0 ? nextExecutionTime + period 155 : nextExecutionTime - period); 156 } 157 } 158 } 159