Java综合性实验----------

/* * Copyright (c) 1999, 2008, Oracle and/or its affiliates. All rights reserved. * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ package java.util; import java.util.Date; import java.util.concurrent.atomic.AtomicInteger; /** * A facility for threads to schedule tasks for future execution in a * background thread. Tasks may be scheduled for one-time execution, or for * repeated execution at regular intervals. * * <p>Corresponding to each <tt>Timer</tt> object is a single background * thread that is used to execute all of the timer's tasks, sequentially. * Timer tasks should complete quickly. If a timer task takes excessive time * to complete, it "hogs" the timer's task execution thread. This can, in * turn, delay the execution of subsequent tasks, which may "bunch up" and * execute in rapid succession when (and if) the offending task finally * completes. * * <p>After the last live reference to a <tt>Timer</tt> object goes away * <i>and</i> all outstanding tasks have completed execution, the timer's task * execution thread terminates gracefully (and becomes subject to garbage * collection). However, this can take arbitrarily long to occur. By * default, the task execution thread does not run as a <i>daemon thread</i>, * so it is capable of keeping an application from terminating. If a caller * wants to terminate a timer's task execution thread rapidly, the caller * should invoke the timer's <tt>cancel</tt> method. * * <p>If the timer's task execution thread terminates unexpectedly, for * example, because its <tt>stop</tt> method is invoked, any further * attempt to schedule a task on the timer will result in an * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt> * method had been invoked. * * <p>This class is thread-safe: multiple threads can share a single * <tt>Timer</tt> object without the need for external synchronization. * * <p>This class does <i>not</i> offer real-time guarantees: it schedules * tasks using the <tt>Object.wait(long)</tt> method. * * <p>Java 5.0 introduced the {@code java.util.concurrent} package and * one of the concurrency utilities therein is the {@link * java.util.concurrent.ScheduledThreadPoolExecutor * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly * executing tasks at a given rate or delay. It is effectively a more * versatile replacement for the {@code Timer}/{@code TimerTask} * combination, as it allows multiple service threads, accepts various * time units, and doesn't require subclassing {@code TimerTask} (just * implement {@code Runnable}). Configuring {@code * ScheduledThreadPoolExecutor} with one thread makes it equivalent to * {@code Timer}. * * <p>Implementation note: This class scales to large numbers of concurrently * scheduled tasks (thousands should present no problem). Internally, * it uses a binary heap to represent its task queue, so the cost to schedule * a task is O(log n), where n is the number of concurrently scheduled tasks. * * <p>Implementation note: All constructors start a timer thread. * * @author Josh Bloch * @see TimerTask * @see Object#wait(long) * @since 1.3 */ public class Timer { /** * The timer task queue. This data structure is shared with the timer * thread. The timer produces tasks, via its various schedule calls, * and the timer thread consumes, executing timer tasks as appropriate, * and removing them from the queue when they're obsolete. */ private final TaskQueue queue = new TaskQueue(); /** * The timer thread. */ private final TimerThread thread = new TimerThread(queue); /** * This object causes the timer's task execution thread to exit * gracefully when there are no live references to the Timer object and no * tasks in the timer queue. It is used in preference to a finalizer on * Timer as such a finalizer would be susceptible to a subclass's * finalizer forgetting to call it. */ private final Object threadReaper = new Object() { protected void finalize() throws Throwable { synchronized(queue) { thread.newTasksMayBeScheduled = false; queue.notify(); // In case queue is empty. } } }; /** * This ID is used to generate thread names. */ private final static AtomicInteger nextSerialNumber = new AtomicInteger(0); private static int serialNumber() { return nextSerialNumber.getAndIncrement(); } /** * Creates a new timer. The associated thread does <i>not</i> * {@linkplain Thread#setDaemon run as a daemon}. */ public Timer() { this("Timer-" + serialNumber()); } /** * Creates a new timer whose associated thread may be specified to * {@linkplain Thread#setDaemon run as a daemon}. * A daemon thread is called for if the timer will be used to * schedule repeating "maintenance activities", which must be * performed as long as the application is running, but should not * prolong the lifetime of the application. * * @param isDaemon true if the associated thread should run as a daemon. */ public Timer(boolean isDaemon) { this("Timer-" + serialNumber(), isDaemon); } /** * Creates a new timer whose associated thread has the specified name. * The associated thread does <i>not</i> * {@linkplain Thread#setDaemon run as a daemon}. * * @param name the name of the associated thread * @throws NullPointerException if {@code name} is null * @since 1.5 */ public Timer(String name) { thread.setName(name); thread.start(); } /** * Creates a new timer whose associated thread has the specified name, * and may be specified to * {@linkplain Thread#setDaemon run as a daemon}. * * @param name the name of the associated thread * @param isDaemon true if the associated thread should run as a daemon * @throws NullPointerException if {@code name} is null * @since 1.5 */ public Timer(String name, boolean isDaemon) { thread.setName(name); thread.setDaemon(isDaemon); thread.start(); } /** * Schedules the specified task for execution after the specified delay. * * @param task task to be scheduled. * @param delay delay in milliseconds before task is to be executed. * @throws IllegalArgumentException if <tt>delay</tt> is negative, or * <tt>delay + System.currentTimeMillis()</tt> is negative. * @throws IllegalStateException if task was already scheduled or * cancelled, timer was cancelled, or timer thread terminated. * @throws NullPointerException if {@code task} is null */ public void schedule(TimerTask task, long delay) { if (delay < 0) throw new IllegalArgumentException("Negative delay."); sched(task, System.currentTimeMillis()+delay, 0); } /** * Schedules the specified task for execution at the specified time. If * the time is in the past, the task is scheduled for immediate execution. * * @param task task to be scheduled. * @param time time at which task is to be executed. * @throws IllegalArgumentException if <tt>time.getTime()</tt> is negative. * @throws IllegalStateException if task was already scheduled or * cancelled, timer was cancelled, or timer thread terminated. * @throws NullPointerException if {@code task} or {@code time} is null */ public void schedule(TimerTask task, Date time) { sched(task, time.getTime(), 0); } /** * Schedules the specified task for repeated <i>fixed-delay execution</i>, * beginning after the specified delay. Subsequent executions take place