Java线程的生命周期

Posted 2021-09-01 热心邻居老王

java.lang.Thread.State 枚举类型中（内部类形式），定义了线程的几种状态，其代码为：

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;
 }

1.生命周期

一个事物从出生的那一刻开始到最终死亡中间的整个过程.在事物的漫长的生命周期过程中,总会经历不同的状态(婴儿状态/青少年状态/中年状态/老年状态...).线程也是有生命周期的,也是存在不同的状态的,状态相互之间的转换.

2.根据java.lang.Thread.State线程的几种状态可以大致描绘出java线程的生命周期

1. NEW 新建
   线程刚被创建，还没调用start方法，或者刚刚调用了start方法，调用start方法不一定"立即"改变线程状态，中间可能需要一些步骤才完成一个线程的启动。

2. RUNNABLE 可行性
   start方法调用结束,线程由NEW变成RUNNABLE，线程存活着，并尝试抢占CPU资源，或者已经抢占到CPU资源正在运行，这俩种情况的状态都显示为RUNNABLE分别为：
   1）就绪：等待分配时间片
   2）运行：抢到CPU资源（分配到时间片）后执行代码

3. BLOCKED 锁阻塞
   线程A和线程B都要执行方法test，而且方法test被加了锁，线程A先拿到了锁去执行test方法，线程B这时候需要等待线程A把锁释放。这时候线程B就是处理BLOCKED（锁池）

4. WAITING 无限期等待
   一个线程在等待另一个线程执行一个（唤醒）动作时，该线程进入Waiting状态。进入这个状态后是不能自动唤醒的，必须等待另一个线程调用notify或者notifyAll方法才能够唤醒。还有join（无参）也会让当前线程进入WAITING状态

5. TIMED_WAITING 有限期等待
   和WAITING状态类似，但是有一个时间期限，时间到了，自己也会主动醒来（sleep、join（有参））

6. TERMINATED 终止（死亡）
   run方法执行结束的线程处于这种状态

作图如下：

蓝色字体部分表示线程当前状态

需要注意的是：在一个同步代码中，释放锁对象（锁对象.wait()）该线程进入wait等待区后是无法再对锁对象进行操作的！！！所以notify（）、notifyAll（）要写在锁对象释放之前，例如锁对象调用notifyAll（）后wait等待区中所有需要该锁对象的线程被唤醒进入锁池等待该锁对象，然后才开始释放锁对象，JVM将随机从锁池中挑选一个需要该锁对象的线程进入就绪状态