实现计划框架
在上一节，我们学习了如何使用计划框架，并将它与 Java 定时器框架进行了比较。下面，我将向您展示如何实现这个框架。除了 清单 3 中展示的 ScheduleIterator 接口，构成这个框架的还有另外两个类 —— Scheduler 和 SchedulerTask 。这些类实际上在内部使用 Timer 和 SchedulerTask，因为计划其实就是一系列的单次定时器。
清单 5 和 6 显示了这两个类的源代码：
清单 5. Scheduler
package org.tiling.scheduling;
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
public class Scheduler {
class SchedulerTimerTask extends TimerTask {
private SchedulerTask schedulerTask;
private ScheduleIterator iterator;
public SchedulerTimerTask(SchedulerTask schedulerTask,
ScheduleIterator iterator) {
this.schedulerTask = schedulerTask;
this.iterator = iterator;
}
public void run() {
schedulerTask.run();
reschedule(schedulerTask, iterator);
}
}
private final Timer timer = new Timer();
public Scheduler() {
}
public void cancel() {
timer.cancel();
}
public void schedule(SchedulerTask schedulerTask,
ScheduleIterator iterator) {
Date time = iterator.next();
if (time == null) {
schedulerTask.cancel();
} else {
synchronized(schedulerTask.lock) {
if (schedulerTask.state != SchedulerTask.VIRGIN) {
throw new IllegalStateException("Task already
scheduled " + "or cancelled");
}
schedulerTask.state = SchedulerTask.SCHEDULED;
schedulerTask.timerTask =
new SchedulerTimerTask(schedulerTask, iterator);
timer.schedule(schedulerTask.timerTask, time);
}
}
}
private void reschedule(SchedulerTask schedulerTask,
ScheduleIterator iterator) {
Date time = iterator.next();
if (time == null) {
schedulerTask.cancel();
} else {
synchronized(schedulerTask.lock) {
if (schedulerTask.state != SchedulerTask.CANCELLED) {
schedulerTask.timerTask =
new SchedulerTimerTask(schedulerTask, iterator);
timer.schedule(schedulerTask.timerTask, time);
}
}
}
}
}
清单 6 显示了 SchedulerTask 类的源代码：
package org.tiling.scheduling;
import java.util.TimerTask;
public abstract class SchedulerTask implements Runnable {
final Object lock = new Object();
int state = VIRGIN;
static final int VIRGIN = 0;
static final int SCHEDULED = 1;
static final int CANCELLED = 2;
TimerTask timerTask;
protected SchedulerTask() {
}
public abstract void run();
public boolean cancel() {
synchronized(lock) {
if (timerTask != null) {
timerTask.cancel();
}
boolean result = (state == SCHEDULED);
state = CANCELLED;
return result;
}
}
public long scheduledExecutionTime() {
synchronized(lock) {
return timerTask == null ? 0 : timerTask.scheduledExecutionTime();
}
}
}
就像煮蛋计时器，Scheduler 的每一个实例都拥有 Timer 的一个实例，用于提供底层计划。
