java线程waiting与blocked的区别

java线程中含有waiting与blocked两种状态，两种状态有着明显的区别，本文从jdk源码中两种状态的设计角度和状态进入角度来分析两者的区别。

源码分析

    /*
     * 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,

处于 blocked状态的线程等待获取监视器锁以期进入同步代码块/方法中，文档中的在调用Object.wait()方法后重入同步代码块/方法不是很理解。

    /**
     * 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,

文档介绍了导致线程进入Wating状态的原因以及如何退出Wating状态。

区别

线程可以通过notify,join,LockSupport.park方式进入wating状态，进入wating状态的线程等待唤醒(notify或notifyAll)才有机会获取cpu的时间片段来继续执行。

线程的 blocked状态往往是无法进入同步方法/代码块来完成的。这是因为无法获取到与同步方法/代码块相关联的锁。

与wating状态相关联的是等待队列，与blocked状态相关的是同步队列，一个线程由等待队列迁移到同步队列时，线程状态将会由wating转化为blocked。可以这样说，blocked状态是处于wating状态的线程重新焕发生命力的必由之路。