Java定时器Timer源码详解

Java中的Timer类是用于在后台线程中调度任务执行的工具，可以安排任务在指定延迟后执行或定期重复执行。以下从源码角度分析其实现原理。

Timer类的基本结构

Timer类位于java.util包中，核心成员变量包括：

TaskQueue：优先级队列，存储待执行任务
TimerThread：执行任务的线程
nextSerialNumber：生成线程名称的序列号

public class Timer {
    private final TaskQueue queue = new TaskQueue();
    private final TimerThread thread = new TimerThread(queue);
    private static final AtomicInteger nextSerialNumber = new AtomicInteger(0);
}

任务队列TaskQueue

TaskQueue是Timer的核心数据结构，本质是基于数组的优先级堆（最小堆），确保每次取出执行时间最近的任务：

class TaskQueue {
    private TimerTask[] queue = new TimerTask[128];
    private int size = 0;
    
    void add(TimerTask task) {
        if (size + 1 == queue.length)
            queue = Arrays.copyOf(queue, 2*queue.length);
        queue[++size] = task;
        fixUp(size);
    }
    
    TimerTask getMin() {
        return queue[1];
    }
}

堆排序通过fixUp()和fixDown()方法维护，确保父节点始终比子节点小。

TimerThread工作原理

TimerThread是实际执行任务的线程，核心逻辑在run()方法中：

private void mainLoop() {
    while (true) {
        synchronized(queue) {
            while (queue.isEmpty() && newTasksMayBeScheduled)
                queue.wait();
            
            TimerTask task = queue.getMin();
            long currentTime = System.currentTimeMillis();
            long executionTime = task.nextExecutionTime;
            
            if (task.cancelled) {
                queue.removeMin();
                continue;
            }
            
            if (executionTime <= currentTime) {
                task.run();
                if (task.period == 0) {
                    queue.removeMin();
                } else {
                    reschedulePeriodicTask(task);
                }
            } else {
                queue.wait(executionTime - currentTime);
            }
        }
    }
}

任务调度过程

定时任务通过schedule()方法添加：

private void sched(TimerTask task, long time, long period) {
    synchronized(queue) {
        task.nextExecutionTime = time;
        task.period = period;
        task.state = TimerTask.SCHEDULED;
        queue.add(task);
        if (queue.getMin() == task)
            queue.notify();
    }
}

对于固定延迟任务(schedule())，执行间隔计算方式为：

nextTime = previousExecutionTime + period

对于固定速率任务(scheduleAtFixedRate())，计算方式为：

nextTime = initialExecutionTime + n*period

关键注意事项

Timer存在单线程执行的局限性，一个任务延迟会影响后续任务。推荐替代方案：

ScheduledThreadPoolExecutor：线程池实现
Quartz：企业级调度框架
Spring的@Scheduled注解

典型使用示例

创建一次性任务：

Timer timer = new Timer();
timer.schedule(new TimerTask() {
    public void run() {
        System.out.println("Task executed");
    }
}, 2000);  // 2秒后执行

创建周期性任务：

timer.scheduleAtFixedRate(new TimerTask() {
    public void run() {
        System.out.println("Periodic task");
    }
}, 0, 1000);  // 立即开始，每秒执行

源码设计要点

线程安全通过synchronized块实现
使用wait/notify机制实现任务唤醒
最小堆确保高效获取最近任务
任务状态机管理（VIRGIN、SCHEDULED、EXECUTED、CANCELLED）

通过分析源码可见，Timer实现简洁但存在单点故障风险，适合简单场景，复杂场景应考虑更现代的调度方案。

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Java Timer源码深度解析