线程的生命周期

进程和线程

• 进程: 操作系统资源管理的基本单位(Windows系统进入任务管理器,选择进程,即可看到一个个的进程)
• 线程: CPU 调度的基本单位

多线程优点

1. 资源利用率更好,利用 CPU 空闲时间处理其他任务
2. 某些情况下程序设计更简单
3. 提高接口响应速度

线程的生命周期

线程的 6 个状态

public enum State {
    /**
     * Thread state for a thread which has not yet started.
     */
    NEW,

    /**
     * Thread state for a runnable thread.  A thread in the runnable
     * state is executing in the Java virtual machine but it may
     * be waiting for other resources from the operating system
     * such as processor.
     */
    RUNNABLE,

    /**
     * Thread state for a thread blocked waiting for a monitor lock.
     * A thread in the blocked state is waiting for a monitor lock
     * to enter a synchronized block/method or
     * reenter a synchronized block/method after calling
     * {@link Object#wait() Object.wait}.
     */
    BLOCKED,

    /**
     * Thread state for a waiting thread.
     * A thread is in the waiting state due to calling one of the
     * following methods:
     * <ul>
     *   <li>{@link Object#wait() Object.wait} with no timeout</li>
     *   <li>{@link #join() Thread.join} with no timeout</li>
     *   <li>{@link LockSupport#park() LockSupport.park}</li>
     * </ul>
     *
     * <p>A thread in the waiting state is waiting for another thread to
     * perform a particular action.
     *
     * For example, a thread that has called <tt>Object.wait()</tt>
     * on an object is waiting for another thread to call
     * <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
     * that object. A thread that has called <tt>Thread.join()</tt>
     * is waiting for a specified thread to terminate.
     */
    WAITING,

    /**
     * Thread state for a waiting thread with a specified waiting time.
     * A thread is in the timed waiting state due to calling one of
     * the following methods with a specified positive waiting time:
     * <ul>
     *   <li>{@link #sleep Thread.sleep}</li>
     *   <li>{@link Object#wait(long) Object.wait} with timeout</li>
     *   <li>{@link #join(long) Thread.join} with timeout</li>
     *   <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
     *   <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
     * </ul>
     */
    TIMED_WAITING,

    /**
     * Thread state for a terminated thread.
     * The thread has completed execution.
     */
    TERMINATED;
}

• 初始状态(NEW)
  新创建了一个线程对象,但还没有调用 start() 方法

• 可运行状态(RUNNABLE)
  可运行状态的线程位于可运行线程池中,等待被线程调度程序选中,被选中后,获得 CPU 时间片,然后真正执行.
  也就是说,可运行状态的线程可能在运行,也可能不在运行,虽然源码中没有进一步细分为两个状态,不过可以参照 linux 的进程状态,进一步将可运行状态细分为就绪状态(READY)和运行中状态(RUNNING)

• 阻塞状态(BLOCKED)
  当前线程进入 synchronized block/method 时,若 monitor lock 此时被其他线程持有,则当前线程被阻塞,等待其他线程释放锁

• 等待状态(WAITING)
  无限期等待其他线程唤醒/终止

• 超时等待状态(TIMED_WAITING)
  等待其他线程唤醒/终止,但不用无限期等待,即使其他线程没有操作,达到指定的时间后,也会自动唤醒

• 终止状态(TERMINATED)
  线程执行终止,即 run() 方法执行完成.这个线程对象也许还存活着,但它已经不是一个单独执行的线程,线程一旦终止了,不能复生

线程的状态转化

参考