【java】源碼詳解java定時任務

網上找了找，java寫定時任務的方法很多，架構也有很多提供了定時任務介面，對於初學java程式員來說，還是考慮JDK裡面的內容比較適合。

先看原理： 

JDK中，定時器任務的執行需要兩個基本的類：
    java.util.Timer;
    java.util.TimerTask;
 
要運行一個定時任務，最基本的步驟如下：
1、建立一個要執行的任務TimerTask。
2、建立一個Timer執行個體，通過Timer提供的schedule()方法，將 TimerTask加入到定時器Timer中，同時設定執行的規則即可。
 
當程式執行了Timer初始化代碼後，Timer定時任務就會按照設定去執行。
 
Timer中的schedule()方法是有多種重載格式的，以適應不同的情況。該方法的格式如下：
 void schedule(TimerTask task, Date time)
          安排在指定的時間執行指定的任務。
 void schedule(TimerTask task, Date firstTime, long period)
          安排指定的任務在指定的時間開始進行重複的固定順延強制。
 void schedule(TimerTask task, long delay)
          安排在指定延遲後執行指定的任務。
 void schedule(TimerTask task, long delay, long period)
          安排指定的任務從指定的延遲後開始進行重複的固定順延強制。
 
Timer是安全執行緒的，此類可擴充到大量同時安排的任務（存在數千個都沒有問題）。其所有構造方法都啟動計時器線程。可以調用cancel() 終止此計時器，丟棄所有當前已安排的任務。purge()從此計時器的任務隊列中移除所有已取消的任務。此類不提供即時保證：它使用 Object.wait(long) 方法來安排任務。
 
TimerTask是一個抽象類別，由 Timer 安排為一次執行或重複執行的任務。它有一個抽象方法run()----計時器任務要執行的操作。因此，每個具體的任務類都必須繼承TimerTask類，並且重寫run()方法。另外它還有兩個非抽象的方法：
 boolean cancel()
          取消此計時器任務。
 long scheduledExecutionTime()

          返回此任務最近實際 執行的安排 執行時間。

再看源碼：

package nov;import java.util.Date;import java.util.Timer;import java.util.TimerTask;/** *  *  * <p> * Title: 定時任務測試代碼 /p> *  * <p> * Description: 樣本 業務類 * </p> *  * <p> * Copyright: Copyright (c) 2012 * </p> *  *  * @author dml@2012-11-29 * @version 1.0 */public class TimerTest extends TimerTask {/** * 此計時器任務要執行的操作。 */public void run() {Date executeTime = new Date(this.scheduledExecutionTime());System.out.println("本次任務執行的時間是" + executeTime);}  public static void main(String[] args) {          Timer timer = new Timer();          TimerTask task = new TimerTest();          timer.schedule(task, 500L, 1000L);  }}

執行結果：

本次任務執行的時間是Thu Nov 29 10:39:38 CST 2012本次任務執行的時間是Thu Nov 29 10:39:39 CST 2012本次任務執行的時間是Thu Nov 29 10:39:40 CST 2012本次任務執行的時間是Thu Nov 29 10:39:41 CST 2012本次任務執行的時間是Thu Nov 29 10:39:42 CST 2012本次任務執行的時間是Thu Nov 29 10:39:43 CST 2012本次任務執行的時間是Thu Nov 29 10:39:44 CST 2012本次任務執行的時間是Thu Nov 29 10:39:45 CST 2012本次任務執行的時間是Thu Nov 29 10:39:46 CST 2012本次任務執行的時間是Thu Nov 29 10:39:47 CST 2012本次任務執行的時間是Thu Nov 29 10:39:48 CST 2012本次任務執行的時間是Thu Nov 29 10:39:49 CST 2012本次任務執行的時間是Thu Nov 29 10:39:50 CST 2012本次任務執行的時間是Thu Nov 29 10:39:51 CST 2012本次任務執行的時間是Thu Nov 29 10:39:52 CST 2012本次任務執行的時間是Thu Nov 29 10:39:53 CST 2012本次任務執行的時間是Thu Nov 29 10:39:54 CST 2012本次任務執行的時間是Thu Nov 29 10:39:55 CST 2012本次任務執行的時間是Thu Nov 29 10:39:56 CST 2012本次任務執行的時間是Thu Nov 29 10:39:57 CST 2012本次任務執行的時間是Thu Nov 29 10:39:58 CST 2012本次任務執行的時間是Thu Nov 29 10:39:59 CST 2012……

分析：

上面的樣本調用的方法如下：

    /**     * Schedules the specified task for repeated <i>fixed-delay execution</i>,     * beginning after the specified delay.  Subsequent executions take place     * at approximately regular intervals separated by the specified period.     *     * <p>In fixed-delay execution, each execution is scheduled relative to     * the actual execution time of the previous execution.  If an execution     * is delayed for any reason (such as garbage collection or other     * background activity), subsequent executions will be delayed as well.     * In the long run, the frequency of execution will generally be slightly     * lower than the reciprocal of the specified period (assuming the system     * clock underlying <tt>Object.wait(long)</tt> is accurate).     *     * <p>Fixed-delay execution is appropriate for recurring activities     * that require "smoothness."  In other words, it is appropriate for     * activities where it is more important to keep the frequency accurate     * in the short run than in the long run.  This includes most animation     * tasks, such as blinking a cursor at regular intervals.  It also includes     * tasks wherein regular activity is performed in response to human     * input, such as automatically repeating a character as long as a key     * is held down.     *     * @param task   task to be scheduled.     * @param delay  delay in milliseconds before task is to be executed.     * @param period time in milliseconds between successive task executions.     * @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.     */    public void schedule(TimerTask task, long delay, long period) {        if (delay < 0)            throw new IllegalArgumentException("Negative delay.");        if (period <= 0)            throw new IllegalArgumentException("Non-positive period.");        sched(task, System.currentTimeMillis()+delay, -period);    }

三個參數：任務  延時時間  連續周期

時間設定合理後調用sched方法：

    /**     * Schedule the specified timer task for execution at the specified     * time with the specified period, in milliseconds.  If period is     * positive, the task is scheduled for repeated execution; if period is     * zero, the task is scheduled for one-time execution. Time is specified     * in Date.getTime() format.  This method checks timer state, task state,     * and initial execution time, but not period.     *     * @throws IllegalArgumentException if <tt>time()</tt> is negative.     * @throws IllegalStateException if task was already scheduled or     *         cancelled, timer was cancelled, or timer thread terminated.     */    private void sched(TimerTask task, long time, long period) {        if (time < 0)            throw new IllegalArgumentException("Illegal execution time.");        synchronized(queue) {            if (!thread.newTasksMayBeScheduled)                throw new IllegalStateException("Timer already cancelled.");            synchronized(task.lock) {                if (task.state != TimerTask.VIRGIN)                    throw new IllegalStateException(                        "Task already scheduled or cancelled");                task.nextExecutionTime = time;                task.period = period;                task.state = TimerTask.SCHEDULED;            }            queue.add(task);            if (queue.getMin() == task)                queue.notify();        }    }

synchronized修飾，保證安全執行緒...Timer類還有很多其他的方法，有空再一個個試試。

p.s.經常看JDK裡面的代碼，能協助養成良好的代碼注釋風格，這點很重要。

dml@2012.11.29