synchronized简单原理开启掘金成长之旅！这是我参与「掘金日新计划 · 12 月更文挑战」的第25天，点击查看活

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public class SyncTest {
    public void syncBlock(){
        synchronized (this){
            System.out.println("hello block");
        }
    }
    public synchronized void syncMethod(){
        System.out.println("hello method");
    }
}

反编译如上代码，如下图：

synchronized修饰代码块时

monitorenter

首先我们来看一下JVM规范中对于monitorenter的描述： docs.oracle.com/javase/spec…

Each object is associated with a monitor. A monitor is locked if and only if it has an owner. The thread that executes monitorenter attempts to gain ownership of the monitor associated with objectref， as follows: • If the entry count of the monitor associated with objectref is zero， the thread enters the monitor and sets its entry count to one. The thread is then the owner of the monitor. • If the thread already owns the monitor associated with objectref， it reenters the monitor， incrementing its entry count. • If another thread already owns the monitor associated with objectref， the thread blocks until the monitor’s entry count is zero， then tries again to gain ownership.

翻译过来：每一个对象都会和一个监视器monitor关联。监视器被占用时会被锁住，其他线程无法来获取该monitor。当JVM执行某个线程的某个方法内部的monitorenter时，它会尝试去获取当前对象对应的monitor的所有权。其过程如下：

若monior的进入数为0，线程可以进入monitor，并将monitor的进入数置为1。当前线程成为 monitor的owner（所有者）
若线程已拥有monitor的所有权，允许它重入monitor，则进入monitor的进入数加1
若其他线程已经占有monitor的所有权，那么当前尝试获取monitor的所有权的线程会被阻塞，直到monitor的进入数变为0，才能重新尝试获取monitor的所有权。

monitorenter小结: synchronized的锁对象会关联一个monitor,这个monitor不是我们主动创建的,是JVM的线程执行到这个同步代码块,发现锁对象没有monitor就会创建monitor,monitor内部有两个重要的成员变量owner:拥有这把锁的线程,recursions会记录线程拥有锁的次数,当一个线程拥有monitor后其他线程只能等待

monitorexit

首先我们来看一下JVM规范中对于monitorexit的描述： docs.oracle.com/javase/spec…

The thread that executes monitorexit must be the owner of the monitor associated with the instance referenced by objectref. The thread decrements the entry count of the monitor associated with objectref. If as a result the value of the entry count is zero， the thread exits the monitor and is no longer its owner. Other threads that are blocking to enter the monitor are allowed to attempt to do so.

翻译过来：

1.能执行monitorexit指令的线程一定是拥有当前对象的monitor的所有权的线程。

执行monitorexit时会将monitor的进入数减1。当monitor的进入数减为0时，当前线程退出 monitor，不再拥有monitor的所有权，此时其他被这个monitor阻塞的线程可以尝试去获取这个 monitor的所有权

monitorexit释放锁。 monitorexit插入在方法结束处和异常处，JVM保证每个monitorenter必须有对应的monitorexit。

总结：synchronized在修饰代码块时，是通过monitorenter 和 monitorexit来保证并发安全。

[]("synchronized 修饰方法的的情况")synchronized 修饰方法的的情况

synchronized 修饰的方法并没有 monitorenter 指令和 monitorexit 指令，取得代之的确实是 ACC_SYNCHRONIZED 标识，该标识指明了该方法是一个同步方法。JVM 通过该 ACC_SYNCHRONIZED 访问标志来辨别一个方法是否声明为同步方法，从而执行相应的同步调用。

不过两者的本质都是对对象监视器 monitor 的获取。