JAVA 线程状态中可能存在的一些误区

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/