Synchronized lock and its expansion

One 、 preface

In concurrent programming ,synchronized The lock is simple to use , Inter process synchronization is widely used . The principle and lock upgrade process are explored below .

Two 、 How to use

1、 Decorated instance method

When the instance method is synchronized When decorating , All threads calling this method through the current instance share a lock , When different objects call this method, the threads do not affect each other .

public class A {    public synchronized void func() {        }}

When using synchronized Lock modifier instance method , The lock is added to the instance of the current class , How many instances and how many locks can be added .

2、 Decorated code block

Decorated code blocks are less granular than decorated methods . When the instance method code block is synchronized When decorating , All threads calling this method through the current instance share a lock , When different objects call this method, the threads do not affect each other .

public class B {    public void func() {        synchronized (this) {                    }    }}

When using synchronized Lock modifier code block , The lock is added to the instance of the current class , How many instances and how many locks can be added .

3、 Modified static method

When static methods are synchronized When decorating , Whole JVM All threads calling this method are bound by the same lock .

public class C {    public static synchronized void func() {        }}

When using synchronized Lock decorated static methods , The lock is added to the of the current class Class object On , Add at most one lock .

Don't use it unnecessarily synchronized Modified static method

3、 ... and 、 Lock upgrade

Java 8 What is used synchronized The lock is optimized , There is Biased locking 、 Lightweight lock 、 Heavyweight lock Equal state .

（ One ） Biased locking

There is no lock competition between threads , At most one thread has the need to acquire a lock , Common scenarios are Single threaded program .

1、 Recognition method

The identification to judge whether the lock is biased is to check whether there is and only one thread calling this method .

In multithreaded programming , The lock decorated method is only called by a single thread and almost does not exist , Therefore, the bias lock is more chicken ribs ： If you can make it clear that a single thread calls the target method , Using lockless programming is more appropriate .

2、 Performance comparison

The performance difference between no lock and bias lock is very close , Almost negligible .

（ Two ） Lightweight lock

There are locks between threads Pseudo competitive behavior , That is, there will never be two threads applying for locks at the same time , Although each thread has the need to use locks , But it's alternating locks .

1、 Recognition method

When two or more threads call a lock decorated method , Then at least it's a lightweight lock .

2、 Performance comparison

Lightweight lock because there are no two threads competing with each other at the same time , So there are no threads Blocking - Wake up the Context switch of , Therefore, the performance is much higher than that of heavyweight lock .

（ 3、 ... and ） Heavyweight lock

There are locks between threads Substantial competition Behavior , There is a need to obtain locks between threads , But time cannot be staggered , Blocking wait of mutex .

1、 Recognition method

When it is certain that at least two or more threads call the lock decorated method , Thread calls are random , Then the probability is heavyweight lock .

2、 Performance comparison

Heavyweight locks involve thread blocking - Wake up context switching , Cause comparison with unlocked state , It's a lot less efficient .

Four 、 Other contents

（ One ） Nature of lock

1、 Fairness

synchronized The lock is Not fair lock , No, FIFO The queue mechanism ensures that threads competing for locks have a chance to obtain locks .

2、 Reentrant

synchronized The lock is Reentrant lock , Reentrant means that nested calls do not cause deadlock problems .

3、 Happy （ sad ） Watch the lock

synchronized Lock is a pessimistic lock , Achieve synchronization between threads by locking .

（ Two ） Understand heavyweight locks

In multithreaded environment , If you use synchronized lock , Then the probability will be upgraded to heavyweight lock . Biased locks and lightweight locks are not intentional , It's hard to exist , More importantly, comparison helps understand the performance of heavyweight locks .

Although heavyweight locks will have a great impact on performance , However, it is still an effective means to solve the synchronization between threads .

1、 Suggestions for choosing locks

When the execution time of the method or code block decorated by the lock is relatively long Long when , Select thread based blocking - The method of wake-up and context switching for thread synchronization is relatively efficient .

When the execution time of the method or code block decorated by the lock is relatively long short when , Other alternative locks shall be selected , Such as spin lock .

（ 3、 ... and ） understand synchronized lock

In the actual multi-threaded scenario development ,synchronized There is a high probability that the lock will be upgraded to the heavyweight lock , Because of its one-way upgrade , Heavyweight state synchronized Locks may adversely affect the concurrency of actual business , Manual selection of other locks may be more appropriate .

synchronized Locks can only be used to solve synchronization between different threads in the same process , For distributed projects, thread synchronization across cities depends on distributed locks ,synchronized The more meaning of lock is to understand the process of lock .

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