java - lock与synchronize的解锁顺序问题？

Question

在学习lock与synchronize的区别时，看到这样一句话：“ lock更灵活，可以自由定义多把锁的枷锁解锁顺序（synchronized要按照先加的后解顺序）”。请问这里：

1.lock的自由定义多把锁的枷锁解锁顺序怎么理解？
2.synchronized要按照先加的后解顺序怎么理解？

谢谢各位！

Answer 1

Lock is an interface, and the most commonly used implementation is ReentrantLock. One of its flexibility is that it can set the fair parameter.

ReentrantLock with synchronized and fair=false cannot determine the locking order. In other words, threads A, B, and C all lock the object. The first time they try to lock is A, then B, and finally C. Then when A unlocks the object, it cannot be determined whether B or C will lock the object next.

If you use ReentrantLock with fair=true, the situation is determined: when A unlocks the object, since B tries to lock the object before C, B must lock it next, and only when B unlocks it is C's turn .

Answer 2

new lock1
new lock2

lock1.lock();
lock2.lock();
...
lock2.unlock();
lock1.unlock();

LockThe locking and unlocking is implemented at the java semantic level, and there is no necessary relationship between locks

synchronized(obj1){
    synchronized(obj2){
        ...
    }
}

synchronized加解锁是由JVM来实现，在执行完synchronized块后自行解锁，所有会按照synchronized’s nesting sequence is unlocked.