Java线程状态之blocked和waiting的区别
⼀、引⼦
synchronized 会阻塞线程，AQS 也会阻塞线程。那么这两种情况，阻塞后，线程的状态是什么，是 waiting 还是 blocked。虽然好像知道，但不能确定。在⽹上搜索后，经过指引，到 Thread.State 这个内部枚举类型。
/**
* A thread state.  A thread can be in one of the following states:
* <ul>
* <li>{@link #NEW}<br>
*    A thread that has not yet started is in this state.
*    </li>
* <li>{@link #RUNNABLE}<br>
*    A thread executing in the Java virtual machine is in this state.
*    </li>
* <li>{@link #BLOCKED}<br>
exited*    A thread that is blocked waiting for a monitor lock
*    is in this state.
*    </li>
* <li>{@link #WAITING}<br>
*    A thread that is waiting indefinitely for another thread to
*    perform a particular action is in this state.
*    </li>
* <li>{@link #TIMED_WAITING}<br>
*    A thread that is waiting for another thread to perform an action
*    for up to a specified waiting time is in this state.
*    </li>
* <li>{@link #TERMINATED}<br>
*    A thread that has exited is in this state.
*    </li>
* </ul>
*
* <p>
* A thread can be in only one state at a given point in time.
* These states are virtual machine states which do not reflect
* any operating system thread states.
*
* @since  1.5
* @see #getState
*/
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>
For example, a thread that has called Object.wait() on an object is waiting for another thread to ify() or
我们知道，AQS 内部就是依赖 LockSupport.park 阻塞线程，所以在 AQS 中被阻塞的线程处于 waiting 状态。
四、总结
blocked 和 waiting 是 Java 线程的两种阻塞状态。
因为争⽤ synchronized 的 monitor 对象⽽发⽣阻塞的线程处于 blocked 状态。
⽽ AQS 中的阻塞线程处于 waiting 状态。
两种状态的区别：
两种状态对应的场景的区别，源码中的注释已经讲的很清楚了。
但既然都是阻塞，还要分成这两种，除了场景不同外，肯定还有底层更深层次的原因。
个⼈认为更加本质的区别是，blocked 状态指的是进⾏系统调⽤，通过操作系统挂起线程后，线程的状态。⽽ waiting 状态则不需要进⾏系统调⽤，是⼀种 JVM 层⾯的线程阻塞后的状态。由于转换到 blocked 状态需要进⾏系统调⽤，所以到这个状态的转换操作⽐较重。
⾄于系统调⽤为什么⽐较重，可以参考 为什么系统调⽤⽐普通的函数调⽤更耗时？⽤户态和内核态切换的代价在哪？

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