destroyjavavm线程 jvm 线程状态

ps：java线程状态，很基础的问题，但是却很少人能回答正确。昨天面试，被问到这个问题，按照网上的资料巴拉巴拉的讲了一遍，面试官让我回去的时候再看看源码。今天一看，果然网上70%的文章都是错的。

首先说明，这里说的java线程的状态，指的是JVM的线程状态，不能反映操作系统的线程状态。所以jvm线程状态与操作系统线程状态不是一致的。

Thread的内部枚举类Thread.State定义了java线程的六种线程状态，分别是：NEW,RUNNABLE,RUNNABLE,WAITING,TIMED_WAITING,TERMINATED;

下面我来详细解释一下什么时候会出现各种状态：

• NEW 新建状态

/**
 * Thread state for a thread which has not yet started.
 * 
 * 还未调用start方法的时候线程处于这个状态
 */
  NEW,

• RUNNABLE 可运行状态

/**
 * 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.
 * 
 * 表示jvm中正在执行，但是在操作系统中可能处于watting状态。即在操作系统的
 * 线程状态可能处于watting状态，也有可能处于running状态。
 */
 RUNNABLE,

• BLOCKED 阻塞状态：

/**
 * 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}.
 * 
 * 1.想要获取对象的同步锁，但是该同步锁被其他线程占用；
 * 2.在调用无参的wait方法之后(notify,notifyAll方法已调用)的状态。
 * 简而言之：当因为获取不到锁而无法进入同步块时，线程处于 BLOCKED 状态。
 * 
 * 总结：Blocked是在等待线程获取锁，是被动的阻塞；waiting或者timed_waiting是在等待其它
 * 线程发来通知（调用Thread类的notify()/notifyAll()方法），收到通知后就可能进入
 * runnable状态或者进入重新获取锁的竞争，如果竞争失败进入Blocked状态。
 */
 BLOCKED,

• WAITING 等待状态

/**
 * 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.
 * 
 * 1.调用无参的wait方法，等待另一个线程调用对象的notify,notifyAll；
 * 2.调用无参的join方法，等待另一个线程执行结束； 
 * 3.调用LockSupport.park方法； 
 */
 WAITING,

• TIMED_WAITING 定时状态

/**
 * 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>
 * 
 * 1.调用Sleep方法后；
 * 2.调用有参的wait方法后；
 * 3.调用有参的join方法后；
 * 4.调用LockSupport.parkNanos和LockSupport.parkUntil方法；
 */
 TIMED_WAITING,

• TERMINATED 终止状态

/**
  * Thread state for a terminated thread.
  * The thread has completed execution.
  * 线程已经执行完成，或者发生异常，或者调用Thread.stop方法
  */
  TERMINATED;

附图：

JVM层面与OS层面线程状态的关系