synchronized实现原理

Posted 2020-11-19 zhangfengshi

在Java语言中存在两种内建的synchronized语法：synchronized语句、synchronized方法。

• synchronized语句：当源代码被编译成字节码的时候，会在同步块的入口位置和退出位置分别插入monitorenter和monitorexit字节码指令;
• synchronized方法：在Class文件的方法表中将该方法的access_flags字段中的synchronized标志位置1。这个在specification中没有明确说明。

在Java虚拟机的specification中，有关于monitorenter和monitorexit字节码指令的详细描述：http://docs.oracle.com/Javase/specs/jvms/se7/html/jvms-6.html#jvms-6.5.monitorenter 。

monitorenter

The objectref must be of type reference.
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.

每个对象都有一个锁，也就是监视器（monitor）。当monitor被占有时就表示它被锁定。线程执行monitorenter指令时尝试获取对象所对应的monitor的所有权，过程如下：

• 如果monitor的进入数为0，则该线程进入monitor，然后将进入数设置为1，该线程即为monitor的所有者;
• 如果线程已经拥有了该monitor，只是重新进入，则进入monitor的进入数加1;
• 如果其他线程已经占用了monitor，则该线程进入阻塞状态，直到monitor的进入数为0，再重新尝试获取monitor的所有权。

monitorexit

The objectref must be of type reference.
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.

执行monitorexit的线程必须是相应的monitor的所有者。
指令执行时，monitor的进入数减1，如果减1后进入数为0，那线程退出monitor，不再是这个monitor的所有者。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。

在JDK1.6及其之前的版本中monitorenter和monitorexit字节码依赖于底层的操作系统的Mutex Lock来实现的，但是由于使用Mutex Lock需要将当前线程挂起并从用户态切换到内核态来执行，这种切换的代价是非常昂贵的。然而在现实中的大部分情况下，同步方法是运行在单线程环境（无锁竞争环境）。如果每次都调用Mutex Lock将严重的影响程序的性能。因此在JDK 1.6之后的版本中对锁的实现做了大量的优化，这些优化在很大程度上减少或避免了Mutex Lock的使用。