BLOCKED 和 WAITING 的区别
BLOCKED 和 WAITING 两种状态从结果上来看,都是线程暂停,不会占用 CPU 资源,不过还是有一些区别的
BLOCKED
等待 Monitor 锁的阻塞线程的线程状态,处于阻塞状态的线程正在等待 Monitor 锁进入 synchronized Block 或者 Method ,或者在调用 Object.wait 后重新进入同步块/方法。简单的说,就是线程等待 synchronized 形式的锁时的状态
下面这段代码中, t1 在等待 t0 的锁释放(synchronized代码块执行完成),那么此时 t1 的状态就是 BLOCKED
Object lock = new Object();
Thread t0 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock){
System.out.println("t0 acquire lock success");
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
});
t0.start();
Thread.sleep(100);
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock){
System.out.println("t1 acquire lock success");
}
}
});
t1.start();
Thread.sleep(100);
System.out.println("t0 state: "+t0.getState());
System.out.println("t1 state: "+t1.getState());
System.out.println("done.");
//output
t0 acquire lock success
t0 state: TIMED_WAITING
t1 state: BLOCKED
done.
t1 acquire lock success
WAITING
等待中的线程状态,下面几个方法的调用会导致线程进入 WAITING 状态:
- Object.wait()
- Thread.join()
- LockSupport.park()
WAITING 状态中的线程在等待其他线程执行某些操作,比如在某个对象上调用 Object.wait() 的线程正在等待另一个线程在该对象上调用 Object.notify() 或 Object.notifyAll()。为 Thread.join() 的线程正在等待指定的线程停止。
下面这段代码中,t0 在通过 synchronized 获取了 lock 对象的锁之后,进行了 wait 操作,导致 t0 进入 WAITING 状态:
Object lock = new Object();
Thread t0 = new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock){
System.out.println("t0 acquire lock success");
try {
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
});
t0.start();
Thread.sleep(100);
System.out.println("t0 state: "+t0.getState());
System.out.println("done.");
//output
t0 acquire lock success
t0 state: WAITING
done.
区别
JAVA 中除了 synchronized Block/Method 的锁,还提供了 JUC 下的锁实现, juc.lock 下的锁功能更强大。比如支持中断,支持重入/非重入,公平/非公平等;但是 juc 下的锁和 synchronized 的实现可是不太一样的
比如下面这段代码,同样是等待锁,可是和synchronized等待锁的状态还不一样:
ReentrantLock reentrantLock = new ReentrantLock();
Thread t0 = new Thread(new Runnable() {
@Override
public void run() {
reentrantLock.lock();
System.out.println("t0 acquire lock success");
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
t0.start();
Thread.sleep(100);
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
reentrantLock.lock();
System.out.println("t1 acquire lock success");
}
});
t1.start();
Thread.sleep(100);
System.out.println("t0 state: "+t0.getState());
System.out.println("t1 state: "+t1.getState());
System.out.println("done.");
//output
t0 acquire lock success
t0 state: TIMED_WAITING
t1 state: WAITING
done.
同样是加锁,在 JUC 的锁实现下线程状态不太一样,所以在观察线程状态时,不止是 BLOCKED 的状态才是等待锁, WAITING/TIMEWAITING 的状态仍然可能是等待锁的状态
不过 JUC 下的锁实现,让线程暂停/等待的核心方法还是 LockSupport.park , jstack 对于 PARKING 形式的 WAITING 会有标注,所以在线程 stack 时还是能一眼看出来的:
//这里显示了等待类型
"Thread-0" #11 prio=5 os_prio=31 tid=0x00007f9308110000 nid=0x5c03 waiting on condition [0x0000700007fc3000]
java.lang.Thread.State: WAITING (parking)//这里虽然是WAITING,但还是标注了是parking类型的
at sun.misc.Unsafe.park(Native Method)
而 synchronized 形式的锁在 jstack 下的输出会有所区别:
//这里显示了等待类型为monitor
"Thread-1" #12 prio=5 os_prio=31 tid=0x00007f833d919800 nid=0x5a03 waiting for monitor entry [0x00007000035af000]
java.lang.Thread.State: BLOCKED (on object monitor)//这里是BLOCKED状态,同时显示了monitor的归属
所以在观察线程状态时,需要注意Object.wait()这种WAITING和juc下锁导致的WAITING的区别
RUNNABLE 真的是 RUNNABLE 吗?
下面是一段 jstack 输出的例子,该线程现在正在执行 socketRead0 方法(Native),并且是 RUNNABLE 状态
"RMI TCP Connection(2)-192.xxx.xx.xx" daemon prio=6 tid=0x000000000a3e8800 nid=0x158e50 runnable [0x000000000adbe000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(Unknown Source)
at java.net.SocketInputStream.read(Unknown Source)
at java.io.BufferedInputStream.fill(Unknown Source)
at java.io.BufferedInputStream.read(Unknown Source)
- locked (0x00000007ad784010) (a java.io.BufferedInputStream)
at java.io.FilterInputStream.read(Unknown Source)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(Unknown Source)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(Unknown Source)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
但其实这里的 RUNNABLE 只是 JAVA 层面的线程状态,在操作系统或进程角度来看,该线程还是 WAITING 的状态; SocketInputStream 是一个 BIO 的实现,当没有收到数据(或者说没有准备好可读的数据)时会发生阻塞,可这个阻塞在JAVA线程状态里是 RUNNABLE 的状态,不过他并不会占用用户态的 CPU 时间片,内核在接受到数据后会结束这个阻塞
参考
https://blog.fastthread.io/2018/09/02/threads-stuck-in-java-net-socketinputstream-socketread0/
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