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1 /* |
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2 * Copyright 1999-2004 Sun Microsystems, Inc. All Rights Reserved. |
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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4 * |
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5 * This code is free software; you can redistribute it and/or modify it |
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6 * under the terms of the GNU General Public License version 2 only, as |
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7 * published by the Free Software Foundation. Sun designates this |
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8 * particular file as subject to the "Classpath" exception as provided |
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9 * by Sun in the LICENSE file that accompanied this code. |
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10 * |
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11 * This code is distributed in the hope that it will be useful, but WITHOUT |
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12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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14 * version 2 for more details (a copy is included in the LICENSE file that |
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15 * accompanied this code). |
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16 * |
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17 * You should have received a copy of the GNU General Public License version |
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18 * 2 along with this work; if not, write to the Free Software Foundation, |
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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20 * |
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21 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
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22 * CA 95054 USA or visit www.sun.com if you need additional information or |
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23 * have any questions. |
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24 */ |
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25 |
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26 package java.util; |
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27 |
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28 /** |
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29 * A task that can be scheduled for one-time or repeated execution by a Timer. |
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30 * |
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31 * @author Josh Bloch |
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32 * @see Timer |
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33 * @since 1.3 |
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34 */ |
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35 |
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36 public abstract class TimerTask implements Runnable { |
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37 /** |
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38 * This object is used to control access to the TimerTask internals. |
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39 */ |
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40 final Object lock = new Object(); |
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41 |
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42 /** |
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43 * The state of this task, chosen from the constants below. |
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44 */ |
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45 int state = VIRGIN; |
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46 |
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47 /** |
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48 * This task has not yet been scheduled. |
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49 */ |
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50 static final int VIRGIN = 0; |
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51 |
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52 /** |
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53 * This task is scheduled for execution. If it is a non-repeating task, |
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54 * it has not yet been executed. |
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55 */ |
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56 static final int SCHEDULED = 1; |
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57 |
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58 /** |
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59 * This non-repeating task has already executed (or is currently |
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60 * executing) and has not been cancelled. |
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61 */ |
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62 static final int EXECUTED = 2; |
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63 |
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64 /** |
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65 * This task has been cancelled (with a call to TimerTask.cancel). |
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66 */ |
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67 static final int CANCELLED = 3; |
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68 |
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69 /** |
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70 * Next execution time for this task in the format returned by |
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71 * System.currentTimeMillis, assuming this task is scheduled for execution. |
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72 * For repeating tasks, this field is updated prior to each task execution. |
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73 */ |
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74 long nextExecutionTime; |
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75 |
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76 /** |
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77 * Period in milliseconds for repeating tasks. A positive value indicates |
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78 * fixed-rate execution. A negative value indicates fixed-delay execution. |
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79 * A value of 0 indicates a non-repeating task. |
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80 */ |
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81 long period = 0; |
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82 |
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83 /** |
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84 * Creates a new timer task. |
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85 */ |
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86 protected TimerTask() { |
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87 } |
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88 |
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89 /** |
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90 * The action to be performed by this timer task. |
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91 */ |
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92 public abstract void run(); |
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93 |
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94 /** |
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95 * Cancels this timer task. If the task has been scheduled for one-time |
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96 * execution and has not yet run, or has not yet been scheduled, it will |
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97 * never run. If the task has been scheduled for repeated execution, it |
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98 * will never run again. (If the task is running when this call occurs, |
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99 * the task will run to completion, but will never run again.) |
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100 * |
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101 * <p>Note that calling this method from within the <tt>run</tt> method of |
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102 * a repeating timer task absolutely guarantees that the timer task will |
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103 * not run again. |
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104 * |
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105 * <p>This method may be called repeatedly; the second and subsequent |
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106 * calls have no effect. |
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107 * |
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108 * @return true if this task is scheduled for one-time execution and has |
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109 * not yet run, or this task is scheduled for repeated execution. |
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110 * Returns false if the task was scheduled for one-time execution |
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111 * and has already run, or if the task was never scheduled, or if |
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112 * the task was already cancelled. (Loosely speaking, this method |
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113 * returns <tt>true</tt> if it prevents one or more scheduled |
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114 * executions from taking place.) |
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115 */ |
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116 public boolean cancel() { |
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117 synchronized(lock) { |
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118 boolean result = (state == SCHEDULED); |
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119 state = CANCELLED; |
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120 return result; |
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121 } |
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122 } |
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123 |
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124 /** |
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125 * Returns the <i>scheduled</i> execution time of the most recent |
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126 * <i>actual</i> execution of this task. (If this method is invoked |
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127 * while task execution is in progress, the return value is the scheduled |
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128 * execution time of the ongoing task execution.) |
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129 * |
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130 * <p>This method is typically invoked from within a task's run method, to |
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131 * determine whether the current execution of the task is sufficiently |
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132 * timely to warrant performing the scheduled activity: |
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133 * <pre> |
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134 * public void run() { |
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135 * if (System.currentTimeMillis() - scheduledExecutionTime() >= |
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136 * MAX_TARDINESS) |
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137 * return; // Too late; skip this execution. |
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138 * // Perform the task |
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139 * } |
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140 * </pre> |
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141 * This method is typically <i>not</i> used in conjunction with |
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142 * <i>fixed-delay execution</i> repeating tasks, as their scheduled |
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143 * execution times are allowed to drift over time, and so are not terribly |
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144 * significant. |
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145 * |
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146 * @return the time at which the most recent execution of this task was |
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147 * scheduled to occur, in the format returned by Date.getTime(). |
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148 * The return value is undefined if the task has yet to commence |
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149 * its first execution. |
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150 * @see Date#getTime() |
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151 */ |
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152 public long scheduledExecutionTime() { |
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153 synchronized(lock) { |
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154 return (period < 0 ? nextExecutionTime + period |
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155 : nextExecutionTime - period); |
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156 } |
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157 } |
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158 } |