Thread状态
一、Thread中对状态定义的枚举
/**
* 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>
* 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>
* <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;
}```
###二、状态解释
<p>*NEW*：线程被创建，还没有调用start()方法。
<p>*RUNNABLE*：线程调用start()方法后的状态，这个时候可能还没有真正的运行，有可能还在等待系统资 源，可能还没获得CPU。
<p>*BLOCKED*： 线程处于阻塞状态，等待锁释放。等待进入synchronized方法或者代码块时会进入这种状态。
<p>*WAITING*：等待状态，当线程调用Thread.sleep()方法、或者调用Object.wait()方法、或者有其他线程调用Thread.join方法、或者线程调用LockSupport.park()方法时会处于这种状态。
<p>*TIMED_WAITING*：和*WAITING*状态类似，但是会指定等待的时间。调用Thread.sleep(long)、Object.wait(long)、Thread.join(long)、LockSupport.parkNanos(long)、LockSupport.parkUntil(Object blocker, long deadline)的时候会进入这个状态
<p>*TERMINATED*：进程结束
###三、引申--synchronized和ReentrantLock实现“阻塞”时的线程状态的区别
小伙伴们去看ReentrantLock的源码就知道，ReentrantLock是通过LockSupport.park实现阻塞的。所以synchronized实现阻塞后，等待的线程状态是*BLOCKED*状态，而通过ReentrantLock实现阻塞后，现成的状态是*WAITING*状态。看下面一个栗子：
class TestObject {
public void testMethod(){
synchronized (TestObject.class){
try {
Thread.sleep(1000000000000L);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
class SynchronizedThread extends Thread{
@Override
public void run() {
TestObject to = new TestObject();
to.testMethod();
}
}
public class Test {
public static void main(String[] args) {
SynchronizedThread st1 = new SynchronizedThread();
SynchronizedThread st2 = new SynchronizedThread();
st1.setName("1111111111");
st2.setName("2222222222");
st1.start();
try {
Thread.sleep(1000L);
} catch (InterruptedException e) {
e.printStackTrace();
}
st2.start();
}
}
运行后通过jstack命令查看线程状态：
可以看到线程命是‘2222222222’的线程状态是*BLOCKED*
修改一下代码：
class TestObject {
ReentrantLock lock;
public TestObject(ReentrantLock lock){
this.lock = lock;
}
public void testMethod(){
lock.lock();
try {
Thread.sleep(1000000000000L);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
class SynchronizedThread extends Thread{
ReentrantLock lock;
public SynchronizedThread(ReentrantLock lock){
this.lock = lock;
}
@Override
public void run() {
TestObject to = new TestObject(lock);
to.testMethod();
}
}
public class Test {
ReentrantLock lock = new ReentrantLock();
public static void main(String[] args) {
ReentrantLock lock = new ReentrantLock();
SynchronizedThread st1 = new SynchronizedThread(lock);
SynchronizedThread st2 = new SynchronizedThread(lock);
st1.setName("1111111111");
st2.setName("2222222222");
st1.start();
try {
Thread.sleep(1000L);
} catch (InterruptedException e) {
e.printStackTrace();
}
st2.start();
}
}
查看线程状态：
可以看到线程命是‘2222222222’的线程状态是*WAITING*
