Initial revision

From-SVN: r102074

libjava/classpath/java/util/TimerTask.java

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+/* TimerTask.java -- Task that can be run at a later time if given to a Timer.
+   Copyright (C) 2000 Free Software Foundation, Inc.
+
+This file is part of GNU Classpath.
+
+GNU Classpath is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2, or (at your option)
+any later version.
+
+GNU Classpath is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Classpath; see the file COPYING.  If not, write to the
+Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301 USA.
+
+Linking this library statically or dynamically with other modules is
+making a combined work based on this library.  Thus, the terms and
+conditions of the GNU General Public License cover the whole
+combination.
+
+As a special exception, the copyright holders of this library give you
+permission to link this library with independent modules to produce an
+executable, regardless of the license terms of these independent
+modules, and to copy and distribute the resulting executable under
+terms of your choice, provided that you also meet, for each linked
+independent module, the terms and conditions of the license of that
+module.  An independent module is a module which is not derived from
+or based on this library.  If you modify this library, you may extend
+this exception to your version of the library, but you are not
+obligated to do so.  If you do not wish to do so, delete this
+exception statement from your version. */
+
+package java.util;
+
+/**
+ * Task that can be run at a later time if given to a Timer.
+ * The TimerTask must implement a run method that will be called by the
+ * Timer when the task is scheduled for execution. The task can check when
+ * it should have been scheduled and cancel itself when no longer needed.
+ * <p>
+ * Example:
+ * <pre>
+ *  Timer timer = new Timer();
+ *  TimerTask task = new TimerTask() {
+ *      public void run() {
+ *      if (this.scheduledExecutionTime() &lt; System.currentTimeMillis() + 500)
+ *          // Do something
+ *      else
+ *          // Complain: We are more then half a second late!
+ *      if (someStopCondition)
+ *          this.cancel(); // This was our last execution
+ *  };
+ *  timer.scheduleAtFixedRate(task, 1000, 1000); // schedule every second
+ * </pre>
+ * <p>
+ * Note that a TimerTask object is a one shot object and can only given once
+ * to a Timer. (The Timer will use the TimerTask object for bookkeeping,
+ * in this implementation).
+ * <p>
+ * This class also implements <code>Runnable</code> to make it possible to
+ * give a TimerTask directly as a target to a <code>Thread</code>.
+ *
+ * @see Timer
+ * @since 1.3
+ * @author Mark Wielaard (mark@klomp.org)
+ */
+public abstract class TimerTask implements Runnable
+{
+  /**
+   * If positive the next time this task should be run.
+   * If negative this TimerTask is canceled or executed for the last time.
+   */
+  long scheduled;
+
+  /**
+   * If positive the last time this task was run.
+   * If negative this TimerTask has not yet been scheduled.
+   */
+  long lastExecutionTime;
+
+  /**
+   * If positive the number of milliseconds between runs of this task.
+   * If -1 this task doesn't have to be run more then once.
+   */
+  long period;
+
+  /**
+   * If true the next time this task should be run is relative to
+   * the last scheduled time, otherwise it can drift in time.
+   */
+  boolean fixed;
+
+  /**
+   * Creates a TimerTask and marks it as not yet scheduled.
+   */
+  protected TimerTask()
+  {
+    this.scheduled = 0;
+    this.lastExecutionTime = -1;
+  }
+
+  /**
+   * Marks the task as canceled and prevents any further execution.
+   * Returns true if the task was scheduled for any execution in the future
+   * and this cancel operation prevents that execution from happening.
+   * <p>
+   * A task that has been canceled can never be scheduled again.
+   * <p>
+   * In this implementation the TimerTask it is possible that the Timer does
+   * keep a reference to the TimerTask until the first time the TimerTask
+   * is actually scheduled. But the reference will disappear immediatly when
+   * cancel is called from within the TimerTask run method.
+   */
+  public boolean cancel()
+  {
+    boolean prevented_execution = (this.scheduled >= 0);
+    this.scheduled = -1;
+    return prevented_execution;
+  }
+
+  /**
+   * Method that is called when this task is scheduled for execution.
+   */
+  public abstract void run();
+
+  /**
+   * Returns the last time this task was scheduled or (when called by the
+   * task from the run method) the time the current execution of the task
+   * was scheduled. When the task has not yet run the return value is
+   * undefined.
+   * <p>
+   * Can be used (when the task is scheduled at fixed rate) to see the
+   * difference between the requested schedule time and the actual time
+   * that can be found with <code>System.currentTimeMillis()</code>.
+   */
+  public long scheduledExecutionTime()
+  {
+    return lastExecutionTime;
+  }
+}