参考资料
- (豆瓣链接)Java核心技术 卷I 基础知识 第10版 英文版 的 14.3 Thread States(线程状态) 以下简称为 Java 核心技术
-
java.lang.Thread.State里的相关javadoc
线程的六种状态
Java 核心技术 的 14.3 中讲述了线程状态的相关知识。线程的状态共有以下六种(在 java.lang.Thread.State 中也可以看到这六种状态的定义)
- NEW
java.lang.Thread.State里相关的javadoc如下
Thread state for a thread which has not yet started.
- RUNNABLE
java.lang.Thread.State里相关的javadoc如下
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.
- BLOCKED
java.lang.Thread.State里相关的javadoc如下
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}.
- WAITING
java.lang.Thread.State里相关的javadoc如下
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.
- TIMED_WAITING
java.lang.Thread.State里相关的javadoc如下
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>
- TERMINATED
java.lang.Thread.State里相关的javadoc如下
Thread state for a terminated thread.
The thread has completed execution.
如何看到六种状态的线程
由于线程共有六种状态，那么下文就分别举六个例子来进行说明。
我在写例子的时候，把代码放在 com.coder.rising 包(这些例子的代码是原创的，但是包名借用了 码农翻身
的名字)下了，在下面的六个例子中，包名部分(也就是 package com.coder.rising; 这行代码)都略去了。
- NEW
程序如下
public class ShowNew {
public static void main(String[] args) {
System.out.println(new Thread().getState());
}
}
运行结果如下
- RUNNABLE
程序如下
public class ShowRunnable {
public static void main(String[] args) {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
while (true) {
Thread.yield();
}
}
});
// 将 t 设置为后台线程, 这样在主线程结束后, t 也会被中止
t.setDaemon(true);
t.start();
System.out.println(t.getState());
}
}
运行结果如下
- BLOCKED
程序如下
public class ShowBlocked {
public static void main(String[] args) throws Exception {
Runnable r = new Runnable() {
@Override
public void run() {
synchronized (Object.class) {
while (true) {
Thread.yield();
}
}
}
};
Thread t1 = new Thread(r);
// 将 t1 设置为后台线程, 这样在主线程结束后, t1 也会被中止
t1.setDaemon(true);
t1.start();
// 等待 t1 运行结束(实际上 t1 里面是个死循环, 所以超时之后, 主线程就不再等了)
// t1.join(100) 返回时, t1 很可能已经在执行同步语句块了
t1.join(100);
Thread t2 = new Thread(r);
// 将 t2 设置为后台线程, 这样在主线程结束后, t2 也会被中止
t2.setDaemon(true);
t2.start();
// 等待 t2 运行结束(实际上 t2 里面是个死循环, 所以超时之后, 主线程就不再等了)
// t2.join(100) 返回时, t2 很可能已经处于 BLOCKED 状态了
t2.join(100);
System.out.println(t2.getState());
}
}
运行结果如下
- WAITING
程序如下
public class ShowWaiting {
public static void main(String[] args) throws Exception {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
// 进入同步语句块时, 会持有 Object.class 上的锁
synchronized (Object.class) {
try {
Object.class.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
});
// 将 t 设置为后台线程, 这样在主线程结束后, t 也会被中止
t.setDaemon(true);
t.start();
// 等待 t 运行结束(实际上 t 很可能会在 100ms 内变为 WAITING 状态, 主线程等待 100ms 左右就继续执行后面的代码了)
t.join(100);
System.out.println(t.getState());
}
}
运行结果如下
- TIMED_WAITING
程序如下
public class ShowTimedWaiting {
public static void main(String[] args) throws Exception {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
// 进入同步语句块时, 会持有 Object.class 上的锁
synchronized (Object.class) {
try {
Object.class.wait(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
});
// 将 t 设置为后台线程, 这样在主线程结束后, t 也会被中止
t.setDaemon(true);
t.start();
// 等待 t 运行结束(实际上 t 很可能会在 100ms 内变为 TIMED_WAITING 状态, 主线程等待 100ms 左右就继续执行后面的代码了)
t.join(100);
System.out.println(t.getState());
}
}
运行结果如下
- TERMINATED
程序如下
public class ShowTerminated {
public static void main(String[] args) throws Exception {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("Hello world");
}
});
t.start();
t.join();
System.out.println(t.getState());
}
}
运行结果如下
最后附一张各个状态之间的转换关系图。图片来源是https://github.com/Snailclimb/JavaGuide/blob/master/docs/java/Multithread/JavaConcurrencyBasicsCommonInterviewQuestionsSummary.md ， 而这篇文章指出，该图出自《Java 并发编程艺术》4.1.4 节
