Shulou(Shulou.com)06/02 Report--

What is the difference between Synchronized and ReentrantLock? I believe many inexperienced people don't know what to do about it. Therefore, this article summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

Similarities:

These two synchronization methods have many similarities, they are both locked synchronization, and both are blocking synchronization, that is, when a thread acquires an object lock and enters the synchronization block, other threads accessing the synchronization block must block and wait outside the synchronization block, and the cost of thread blocking and waking up is relatively high (the operating system needs to switch back and forth between user mode and kernel mode, the cost is very high. However, it can be improved by optimizing the lock.

Functional differences:

The biggest difference between the two approaches is that for Synchronized, it is a keyword of the java language, a mutex at the native syntax level, and requires jvm implementation. ReentrantLock is an API level mutex provided after JDK 1.5, which requires lock () and unlock () methods to cooperate with try/finally statement blocks to complete.

Convenience: it is obvious that the use of Synchronized is more convenient and simple, and the compiler ensures the locking and release of the lock, while ReenTrantLock needs manual declaration to add the lock and release the lock, in order to avoid forgetting to manually release the lock caused by deadlock, it is best to declare the release lock in finally.

Fine granularity and flexibility of locks: it is clear that ReenTrantLock is better than Synchronized

Differences in performance:

Before Synchronized optimization, the performance of synchronized is much worse than ReenTrantLock, but since Synchronized introduced a bias lock, lightweight lock (spin lock), the performance of the two is almost the same, in the case of both methods are available, the official even recommended to use synchronized, in fact, I feel that the optimization of synchronized draws lessons from the CAS technology in ReenTrantLock. It is an attempt to solve the locking problem in the user mode to avoid thread blocking in the kernel state.

1.Synchronized

When Synchronized is compiled, two bytecode instructions, monitorenter and monitorexit, are formed before and after the synchronization block. When executing the monitorenter instruction, you first try to acquire the object lock. If the object is not locked, or the current thread already owns that object lock, add 1 to the lock calculator, accordingly, the lock calculator is subtracted by 1 when the monitorexit instruction is executed, and when the calculator is 0, the lock is released. If the acquisition of the object lock fails, the current thread blocks until the object lock is released by another thread.

Public class SynDemo {

Public static void main (String [] arg) {

Runnable t1=new MyThread ()

New Thread (T1, "T1") .start ()

New Thread (T1, "T2") .start ()

}

}

Class MyThread implements Runnable {

@ Override

Public void run () {

Synchronized (this) {

For (int iTuno Bandi)

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