1. 线程状态的含义

线程在不同的时期具有不同的状态，线程状态是线程实例的一个非常重要的属性，我们可以通过状态看到当前线程究竟是在被创建，运行，阻塞还是已经中断，或是处于别的状态，以便更好地对于代码进行调试。

我们可以在获取线程实例之后调用getState()方法获知线程当前状态，并且将其打印出来：

System.out.println(Thread.currentThread().getState());

总的来说，线程具有一下的一系列状态，这一系列状态贯穿了一个线程的生命周期，从被创建，到执行，到被销毁，线程具有不同的转态

线程的状态是一个枚举类型 Thread.State

public class ThreadState {public static void main(String[] args) {for (Thread.State state : Thread.State.values()) {System.out.println(state);}}
}

• NEW：交代了任务但是没有start
• RUNNABLE：就绪/运行；排在等待队列中也是属于该状态，即可被调度的状态，是否开始调度，则看调度器
• TIMED_WAITED：给定时间的阻塞
• BLOCKED：等待锁的阻塞
• WAITING：未指定时间的阻塞
• TERMINATED：终止

Thread.State具体源码

    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:* 
*   
{@link Object#wait() Object.wait} with no timeout
*   
{@link #join() Thread.join} with no timeout
*   
{@link LockSupport#park() LockSupport.park}
* 
** 
A thread in the waiting state is waiting for another thread to* perform a particular action.** For example, a thread that has called {@code Object.wait()}* on an object is waiting for another thread to call* {@code Object.notify()} or {@code Object.notifyAll()} on* that object. A thread that has called {@code Thread.join()}* 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:* 
*   
{@link #sleep Thread.sleep}
*   
{@link Object#wait(long) Object.wait} with timeout
*   
{@link #join(long) Thread.join} with timeout
*   
{@link LockSupport#parkNanos LockSupport.parkNanos}
*   
{@link LockSupport#parkUntil LockSupport.parkUntil}
* 
*/TIMED_WAITING,/*** Thread state for a terminated thread.* The thread has completed execution.*/TERMINATED;}

2. isAlive()判断线程是否存活

public class ThreadStateTransfer {public static void main(String[] args) throws InterruptedException {Thread t = new Thread(() -> {for (int i = 0; i < 1000_0000; i++) {}}, "MyThread");System.out.println(t.getName() + ": " + t.getState());;t.start();while (t.isAlive()) {System.out.println(t.getName() + ": " + t.getState());;}System.out.println(t.getName() + ": " + t.getState());;}
}