BLOCKED和WAITING的区别

BLOCKED和WAITING两种状态从后果上来看，都是线程暂停，不会占用CPU资源，不过还是有一些区别的

BLOCKED

期待Monitor锁的阻塞线程的线程状态，处于阻塞状态的线程正在期待Monitor锁进入synchronized Block或者Method，或者在调用Object.wait后从新进入同步块/办法。简略的说，就是线程期待synchronized模式的锁时的状态

上面这段代码中，t1在期待t0的锁开释（synchronized代码块执行实现），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.");//outputt0 acquire lock successt0 state: TIMED_WAITINGt1 state: BLOCKEDdone.t1 acquire lock success

WAITING

期待中的线程状态，上面几个办法的调用会导致线程进入WAITING状态：

  • Object.wait()
  • Thread.join()
  • LockSupport.park()

WAITING状态中的线程在期待其余线程执行某些操作，比方在某个对象上调用Object.wait()的线程正在期待另一个线程在该对象上调用Object.notify()或Object.notifyAll()。为Thread.join()的线程正在期待指定的线程进行。

上面这段代码中，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.");//outputt0 acquire lock successt0 state: WAITINGdone.

区别

JAVA中除了synchronized Block/Method的锁，还提供了JUC下的锁实现，juc.lock下的锁性能更弱小。比方反对中断，反对重入/非重入，偏心/非偏心等；然而juc下的锁和synchronized的实现可是不太一样的

比方上面这段代码，同样是期待锁，可是和synchronized期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.");//outputt0 acquire lock successt0 state: TIMED_WAITINGt1 state: WAITINGdone.

同样是加锁，在JUC的锁实现下线程状态不太一样，所以在察看线程状态时，不止是BLOCKED的状态才是期待锁，WAITING/TIMEWAITING的状态依然可能是期待锁的状态

不过JUC下的锁实现，让线程暂停/期待的外围办法还是LockSupport.park，jstack对于PARKING模式的WAITING会有标注，所以在线程stack时还是能一眼看进去的：

//这里显示了期待类型"Thread-0" #11 prio=5 os_prio=31 tid=0x00007f9308110000 nid=0x5c03 waiting on condition [0x0000700007fc3000]   java.lang.Thread.State: WAITING (parking)//这里尽管是WAITING，但还是标注了是parking类型的        at sun.misc.Unsafe.park(Native Method)

而synchronized模式的锁在jstack下的输入会有所区别：

//这里显示了期待类型为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

所以在察看线程状态时，须要留神Object.wait()这种WAITING和juc下锁导致的WAITING的区别

RUNNABLE真的是RUNNABLE吗？

上面是一段jstack输入的例子，该线程当初正在执行socketRead0办法(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: RUNNABLEat 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，在操作系统或过程角度来看，该线程还是WAITING的状态；SocketInputStream是一个BIO的实现，当没有收到数据（或者说没有筹备好可读的数据）时会产生阻塞，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工夫片，内核在承受到数据后会完结这个阻塞

参考

  • https://blog.fastthread.io/2018/09/02/threads-stuck-in-java-net-socketinputstream-socketread0/