概述
解析
根據源碼, 可以看到線程擁有的狀態枚舉.
// java.lang.Thread 1742
public enum State {
/**
* Thread state for a thread which has not yet started.
* 新建狀態, 線程還沒有啓動.
*/
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.
* 可運行狀態, 但是可能需要等待操作系統資源.
*/
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}.
* 阻塞狀態, 正在等待某個鎖.
*/
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.
* 等待狀態, 線程正在等待某個其他線程完成某個操作.
*/
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>
* 定時等待狀態, 線程正在定時等待某個時間, 然後甦醒.
*/
TIMED_WAITING,
/**
* Thread state for a terminated thread.
* The thread has completed execution.
* 已終止狀態, 線程已經完成執行.
*/
TERMINATED;
}
NEW狀態:
Thread t = new Thread();
System.out.println("t.state: " + t.getState());
// t.state: NEW
RUNABLE狀態:
Thread t = new Thread(() -> {
for (int i = 0; i < 10000; i++) {
System.out.println(i);
}
});
t.start();
System.out.println("t.state: " + t.getState());
// t.state: RUNNABLE
BLOCKED狀態:
public class ThreadStateStarter {
private static synchronized void func() {
for (int i = 0; i < 1000; i++) {
System.out.print("");
}
}
public static void main(String[] args) {
Thread t1 = new Thread(ThreadStateStarter::func);
Thread t2 = new Thread(ThreadStateStarter::func);
t1.start();
t2.start();
for (int i = 0; i < 1000; i++) {
System.out.println("t1.state: " + t1.getState());
System.out.println("t2.state: " + t2.getState());
}
}
}
// t1或者t2會被阻塞在鎖上, 狀態爲BLOCKED
WAITING狀態:
Object lock = new Object();
Thread t = new Thread(() -> {
System.out.print("");
try {
synchronized (lock) {
lock.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
});
t.start();
Thread.sleep(200);
System.out.println("t.state: " + t.getState());
JOIN導致的WAITING狀態:
public static void main(String[] args) throws InterruptedException {
Thread t = new Thread(() -> {
try {
Thread.sleep(2000);
System.out.println("t end.");
} catch (InterruptedException e) {
e.printStackTrace();
}
});
MyThread mt = new MyThread(t);
t.start();
mt.start();
Thread.sleep(500);
System.out.println("mt.state: " + mt.getState());
}
static class MyThread extends Thread {
private Thread joinThread;
public MyThread(Thread joinThread) {
this.joinThread = joinThread;
}
@Override
public void run() {
try {
// 等待joinThread執行完成之後, 當前線程才繼續執行.
joinThread.join();
System.out.println("mt end.");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/*
輸出:
------------------------------
mt.state: WAITING
t end.
mt end.
------------------------------
*/
不同狀態的轉換
線程的停止
有啓動則會有停止, 如何停止一個線程?可以直接關閉電源而粗暴的停止線程, 這會導致數據狀態的不一致性, 所以最好是採用讓線程緩慢執行完所有的過程而停止.
如果一個線程的任務,沒有阻塞,則可能自然而然的結束了. 但是如果線程執行過程中存在sleep,wait, join等情況, 可能會導致線程無法快速結束, 甚至如果存在死鎖, 則永久無法結束等問題.
可以採用interrrupt來中斷線程.