java 锁！

问题：如何实现死锁。

关键：

1 两个线程ta、tb

2 两个对象a、b

3 ta拥有a的锁，同时在这个锁定的过程中，需要b的锁；tb拥有b的锁，同时在这个锁定的过程中，需要a的锁；

关键的实现难点是3， —— 所以说，死锁也不是那么容易出现的吧。。

实现方式synchronized、Lock 等等

死锁例子1 采用了不同类的两个对象。原理是：两个线程尝试进入同一个需要对象锁的方法

package basic.thread;

public class DL {

    
    public static void main(String[] args) {
        dla a = new dla();
        a.start();
        
        dlb b = new dlb();
        b.start();
    }
}


class dla extends Thread{
    
    @Override
    public void run() {
        System.out.println("dla-------------");
        Res.getInstace().aa();
    }
}

class dlb extends Thread{
    
    @Override
    public void run() {
        System.out.println("dlb-----------");
        Res2.getInstace().bb();
        //super.run();
    }
}

class Res {
    private static Res res = new Res();;
    
    public static Res getInstace() {
        return res;
    }
    
    private Res() {
        
    }
    public synchronized void aa() {
        System.out.println("Res.aa()");
        try {
            Thread.sleep(1000);
            Res2.getInstace().bb();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}


class Res2 {
    private static Res2 res = new Res2();;
    
    public static Res2 getInstace() {
        return res;
    }
    
    private Res2() {
        
    }

    public synchronized void bb() {
        System.out.println("Res2.bb()");
        try {
            Thread.sleep(1000);
            Res.getInstace().aa();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

死锁例子2 原理是：两个线程尝试进入同一个需要对象锁的方法。这就说明的，只要是一个线程ta正在执行synchronized的方法ma（没有返回，这一点相当关键），下次别的线程进入同一个类的同一方法也好，别的synchronized方法也好，都是需要等待ta在ma的返回，就是要等ta在ma执行完了后才行，简单说就是不能进入。。—— 说起来好拗口。。。

package basic.thread;

public class DL {

    
    public static void main(String[] args) {
        dla a = new dla();
        a.start();
        
        dlb b = new dlb();
        b.start();
    }
}


class dla extends Thread{
    
    @Override
    public void run() {
        System.out.println("dla-------------");
        Res.getInstace().aa();
    }
}

class dlb extends Thread{
    
    @Override
    public void run() {
        System.out.println("dlb-----------");
        Res2.getInstace().bb();
        //super.run();
    }
}

class Res {
    private static Res res = new Res();;
    
    public static Res getInstace() {
        return res;
    }
    
    private Res() {
        
    }
    public synchronized void aa() {
        System.out.println("Res.aa()");
            try {
                Thread.sleep(1000);
                Res2.getInstace().bbb();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
    }
    

    public synchronized void aaa() {
        System.out.println("Res.aaa()");
    }
}


class Res2 {
    private static Res2 res = new Res2();;
    
    public static Res2 getInstace() {
        return res;
    }
    
    private Res2() {
        
    }

    public synchronized void bb() {
            System.out.println("Res2.bb() + " + Thread.currentThread().getName());
            try {
                Thread.sleep(1000);
                Res.getInstace().aaa();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
    }

    public synchronized void bbb() {

        System.out.println("Res2.bbb()");
    }
}

死锁例子3 采用了同一个类的两个对象。（自旋锁？这个例子相当不好，因为可能出现死循环！看来还是两个不同类来实现死锁比较容易）

package basic.thread;

public class DL {    
    public static void main(String[] args) {
        Res res = new Res();
        Res another = new Res();
        res.setAnother(another);
        another.setAnother(res);
        dlb b1 = new dlb(res);
        dlb b2 = new dlb(another);
        b1.start();
        b2.start();
    }
}

class dlb extends Thread{
    Res res;
    public dlb(Res res) {
        this.res = res;
    }
    @Override
    public void run() {
        System.out.println("dlb-----------");
        res.aa();
    }
}

class Res {
    
    Res another;
    public void setAnother(Res another) {
        this.another = another;
    }
    
    public synchronized void aa() {
        System.out.println("Res.aa()");
        try {
            Thread.sleep(1000);
            another.aa();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

lock trylock

上例中把 synchronized 改成lock，则发生死锁，一样的效果。但是改成trylock ，则不会发生死锁！

package basic.thread;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class DL {
    
    public static void main(String[] args) {
        Res res = new Res();
        Res another = new Res();
        res.setAnother(another);
        another.setAnother(res);
        dlb b1 = new dlb(res);
        dlb b2 = new dlb(another);
        b1.start();
        b2.start();
    }
}


class dlb extends Thread{
    Res res;
    public dlb(Res res) {
        this.res = res;
    }
    @Override
    public void run() {
        System.out.println("dlb-----------");
        res.aa();
    }
}

class Res {
    
    Res another;
    public void setAnother(Res another) {
        this.another = another;
    }
    ReentrantLock lock = new ReentrantLock();
    
    public void aa() {
        lock.lock();
        System.out.println("Res.aa()");
        try {
            Thread.sleep(1000);
            another.aa();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        lock.unlock();
    }

}

lock时候堆栈为：

"Thread-1" prio=6 tid=0x022eec00 nid=0x1e74 waiting on condition [0x0487f000..0x0487fae8]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x22e75958> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:7
47)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:778)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1114)
        at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:186)
        at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:262)
        at basic.thread.Res.aa(DL.java:57)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.dlb.run(DL.java:44)

"Thread-0" prio=6 tid=0x022ee400 nid=0x2c50 waiting on condition [0x047ef000..0x047efb68]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x22e75a50> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:7
47)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:778)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1114)
        at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:186)
        at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:262)
        at basic.thread.Res.aa(DL.java:57)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.dlb.run(DL.java:44)

----

Found one Java-level deadlock:
=============================
"Thread-1":
waiting for ownable synchronizer 0x22e75958, (a java.util.concurrent.locks.ReentrantLock$NonfairSync), —— 注意这里的NonfairSync，非公平同步锁。
which is held by "Thread-0"
"Thread-0":
waiting for ownable synchronizer 0x22e75a50, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
which is held by "Thread-1"

这个时候的死锁，其实跟synchronized时候的锁是不同的，但是效果是一样的。

打印：

dlb-----------
Res.aa()
dlb-----------
Res.aa()

之后就一直‘卡住’了

tryLock的时候，打印堆栈发现：

C:UsersAdministrator>jstack 16640
2014-07-03 15:48:34
Full thread dump Java HotSpot(TM) Client VM (11.2-b01 mixed mode, sharing):

"DestroyJavaVM" prio=6 tid=0x01edc800 nid=0x3a54 waiting on condition [0x00000000..0x01eafd20]
   java.lang.Thread.State: RUNNABLE

"Thread-1" prio=6 tid=0x01f93800 nid=0x35a8 waiting on condition [0x0480f000..0x0480fce8]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at basic.thread.Res.aa(DL.java:60)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.dlb.run(DL.java:44)

"Thread-0" prio=6 tid=0x01f90c00 nid=0x2abc waiting on condition [0x0477f000..0x0477fd68]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at basic.thread.Res.aa(DL.java:60)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.Res.aa(DL.java:61)
        at basic.thread.dlb.run(DL.java:44)

"Low Memory Detector" daemon prio=6 tid=0x01f68000 nid=0x48d0 runnable [0x00000000..0x00000000]
   java.lang.Thread.State: RUNNABLE

"CompilerThread0" daemon prio=10 tid=0x01f63c00 nid=0x3a0c waiting on condition [0x00000000..0x045cf714]
   java.lang.Thread.State: RUNNABLE

"Attach Listener" daemon prio=10 tid=0x01f95c00 nid=0x3728 waiting on condition [0x00000000..0x00000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=10 tid=0x01f95000 nid=0x365c runnable [0x00000000..0x00000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=8 tid=0x01f1b800 nid=0x258c in Object.wait() [0x01cdf000..0x01cdfc68]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x22960b28> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:116)
        - locked <0x22960b28> (a java.lang.ref.ReferenceQueue$Lock)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:132)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

"Reference Handler" daemon prio=10 tid=0x01f17000 nid=0x4584 in Object.wait() [0x003cf000..0x003cfce8]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x22960a30> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(Object.java:485)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
        - locked <0x22960a30> (a java.lang.ref.Reference$Lock)

"VM Thread" prio=10 tid=0x01f15800 nid=0x4388 runnable

"VM Periodic Task Thread" prio=10 tid=0x01f7f000 nid=0x4154 waiting on condition

JNI global references: 596


———— 这个时候并没有发生死锁，但是，有两个线程确确实实是阻塞了。 就效果来说一样的，但是原理不同。

而且后台打印不同：

dlb-----------
Res.aa()
dlb-----------
Res.aa()
Res.aa()
Res.aa()
Res.aa()
Res.aa()
Res.aa()
Res.aa()
Res.aa()

... 
一直有打印，每隔一秒钟一次———— 死循环！！！！！
。。
说明，线程有一个轮询机制，一直在等待另一个线程释放锁,但是它又不释放，如是就一直“卡”在了那里。 —— 卡，其实就相当于了‘锁’！

好吧，其实上面的例子有问题，因为，里面出现了一个死循环。。 而且，没有判断trylock 的返回值。

使用这个例子吧，但是，惊人的发现每次的结果不一样！：有时候很快就两个end了，有时候一个end，有时候根本没有end——每次的tryLock都能返回true ，太奇怪了吧，如上例一样一直打印

package basic.thread;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class DL {

    
    public static void main(String[] args) {
        Res res = new Res();
        Res another = new Res();
        res.setAnother(another);
        another.setAnother(res);
        dlb b1 = new dlb(res);
        dlb b2 = new dlb(another);
        b1.start();
        b2.start();
    }
}

class dlb extends Thread{
    Res res;
    public dlb(Res res) {
        this.res = res;
    }
    @Override
    public void run() {
        System.out.println("dlb-----------");
        res.aa();
    }
}

class Res {
    
    Res another;
    public void setAnother(Res another) {
        this.another = another;
    }
    ReentrantLock lock = new ReentrantLock();
    
    public void aa() {
        boolean boo = lock.tryLock();
        if(boo){
            System.out.println("Res.aa()aaaaaaaa");
            try {
                Thread.sleep(1000);
                another.aa();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            lock.unlock();
        } else {
            System.out.println("Res.aa() end !! ");
        }
    }
}

好吧，其实上面的例子都不太好。

总结：

1 一般使用lock即可。

2 使用tryLock的话，需要判断其返回值，否则就达不到锁的效果。应该如下，注意 unlock 必须要放在if里面

        boolean tryLock = lock.tryLock();
        System.out.println(" trylock : " + tryLock);
        if(tryLock) {
               do sth.
            lock.unlock(); ---- unlock 必须要放在if里面，否则如果tryLock为false的话，报错java.lang.IllegalMonitorStateException。 因为如果false，则lock 的holder 并非当前线程。。。  很明显的道理吧！
        }

3 tryLock方法无阻塞，立即返回(true/false)，所以，使用tryLock是不会造成死锁的！！！但是，正因为如此，可能到不到我们想要的目的，比如，有时候时候我就是想锁定某个资源，否则就不能继续操作。 ———— 所以说tryLock适合无阻塞的场景！

4 java-tryLock可轮询可中断可定时的锁 —— 好奇怪的名字。。

参考http://aguang520.iteye.com/blog/815575 http://zfzaizheli.iteye.com/blog/1394823 （表示看不懂。。）等

5 注意到 ReentrantLock 的 lock() 返回值为void、而tryLock返回boolean。

6 synchronized不能和lock等混用。 synchronized的用法见我另外一篇博客

7 ReentrantLock 是接口Lock的实现，其功能、底层原理远比synchronized复杂，

8 synchronized 可以加在方法前，代码块前。而且可以是静态的！

9 ReentrantReadWriteLock 的用法一般是：

　　ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
　　ReadLock readLock = lock.readLock(); —— 用来对get，即查询操作加锁
　　WriteLock writeLock = lock.writeLock();—— 用来对add/update/delete，即新增修改删除操作加锁

—— 问题是，有必要分的这么细吗？直接一个ReentrantLock 不就行了吗？

参见 http://www.cnblogs.com/sunwei2012/archive/2010/10/09/1846358.html

http://www.cnblogs.com/GnagWang/archive/2011/02/27/1966606.html ———— 写得非常非常非常详细。。

http://www.cnblogs.com/violinn/archive/2013/05/21/multiThread1.html 非常非常非常详细而且全面的关于java锁、同步、多线程的介绍

另外注意区别FileLock 类中也有 lock 、tryLock 方法，不过，当然，用法不同。

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原文地址：https://www.cnblogs.com/FlyAway2013/p/3840689.html