可重入锁/synchronized/deadlock

在java中，死锁可以很容易用如下代码来模拟

package jdk8;
public class Deadlock {
 
    public static void main (String args[]){
 
        final Object objA = new Object();
 
        final Object objB = new Object();
 
        new Thread() {
        public void run (){
            while (1==1) {
                System.out.println ("Thread A synchronizing on objA");
                synchronized (objA){
                    System.out.println ("Thread A synchronized on objA");
                    System.out.println ("Thread A synchronizing on objB");
                    synchronized (objB){
                        System.out.println ("Thread A synchronized on objB");
                    }
                }
            }
        }
    }.start();
 
        new Thread() {
        public void run (){
            while (1==1) {
                System.out.println ("Thread B synchronizing on objB");
                synchronized (objB){
                    System.out.println ("Thread B synchronized on objB");
                    System.out.println ("Thread B synchronizing on objA");
                    synchronized (objA){
                        System.out.println ("Thread B synchronized on objA");
                    }
                }
            }
        }
    }.start();
 
    }
 
}

另外java中的synchronized关键字和ReentrantLock都是实现了可重入锁的机制，如下

public synchronized void functionOne() {

    // do something

    functionTwo();
    synchronized(this) {
        // do more stuff
    }    
}

public synchronized void functionTwo() {
     // do even more stuff!
}

如果synchronized没有可重入锁的机制的话，当某线程访问了functionOne的时候，此线程已经得到了当前对象的monitor，如果没有可以重入锁的机制，这个时候他去访问functionTwo又会尝试获取当前对象的monitor，结果就会是死锁

可重入锁/synchronized/deadlock

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