Advanced concurrent programming series 9 (lock interface analysis)

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1. Test you

A lot of friends are getting started java In concurrent programming , When it comes to thread safety control , It must be right synchronized Key words are very familiar .synchronized Keyword means synchronous locking , I'm using , You have to wait until I'm finished .

For example Advanced Concurrent Programming Series 7 （ Introduction to lock ） This article , We are right. add_i Variable increment operation , Synchronous locking is the same , Give Way add_i Variables at the same time , It will only be operated by one thread , So thread safety .

/**
* addI Method , Realization add_i Variables operate on themselves 
*/
public synchronized static void addI(){
        add_i ++;
}

So to speak , You may have questions ： since synchronized Keywords can be locked , Ensure thread safety . that java Why did the designer of juc The lock interface is provided in the package Lock, as well as Lock How about the implementation of the interface ？ It's not just Yoga 、 He Shengliang ？ Not good. , Do you want to repeat the story of Zhou Yu and Kong Ming ！

It's not , Although there are synchronized keyword , We said there would be more Lock Interfaces and implementations are not redundant , So, let's talk about it together .

The reason why there is synchronized keyword , We need to provide Lock Interface , It is mainly based on the following factors ：

• synchronized Synchronous locking is internally controlled by jdk control , Fair lock . Practical application , Sometimes we need fair locks , But it can't . One sentence summary ： inflexible

• synchronized Synchronous locking is internally controlled by jdk control , Non interruptible . It's more domineering to use , Once the thread that acquired the lock is not finished , The outside can't be manipulated , Non interruptible lock acquisition thread . One sentence summary ： inflexible

• synchronized Synchronous locking is internally controlled by jdk control , Unable to get lock status , It's a thread A After getting the lock , Then the thread that needs the same lock BCD Can only be blocked waiting , Not flexible enough to use . For example, you can't thread A After getting the lock , Threads B To try to get the lock , If the lock cannot be acquired , Threads B You can do something else , You don't have to wait here . One sentence summary ： inflexible

You can see , To sum up, we are interested in synchronized The impression of keywords is ： inflexible . Here we need to pay attention to , Some friends are talking about synchronized keyword , And Lock When the lock is different , From the perspective of performance , In fact, from jdk1.6 in the future ,jvm Lock optimization is implemented internally , There is almost no difference in performance between the two . The main difference is still synchronized inflexible .

Then let's take a look at , in the light of synchronized Inflexible places ,Lock What solutions does the interface provide ：

• synchronized Lock is unfair lock .Lock Interface common implementation class ReentrantLock, When constructing a lock object , We can specify to construct fair locks , Or unfair lock , Implement on-demand construction

• synchronized A lock is a non interruptible lock .Lock The interface provides lockInterruptibly() Method , Support interrupt operation

• synchronized Lock cannot be locked .Lock The interface provides tryLock() Method , Used to return the lock state . If the thread A Lock has been acquired , So thread B adopt tryLock Attempt to acquire lock , You don't need to be blocked

• in addition Lock Implementation class of interface , Through the read-write lock ReentrantReadWriteLock Read lock in class ReadLock, The shared lock is implemented . The so-called shared lock read lock , That is, everyone is reading operation , Don't be too outspoken , You can read , I can read , Reading is the real reading .

Through the above about Lock Description of the interface , Are you more and more in love with Lock Lock the interface , Yes, I think it is . So next , Let's see Lock Interface design , And basic usage .

2. Case study

2.1.Lock Interface source code

Through the analysis above , I think we'll all be curious ,Lock How the interface is designed , What capabilities does it need to provide us ？ Right , Let's try to think about it ：

• Need a lock operation , So there should be a way to lock ：lock

• When the task is completed , A lock release operation is required , So there should be a way to release the lock ：unlock

• According to our above and synchronized contrast ,Lock Need to support getting lock state , If an attempt to acquire a lock fails , No need to block threads , So there should be an attempt to acquire a lock method ：tryLock

• According to our above and synchronized contrast ,Lock Interrupt operations need to support , Therefore, there should be a lock method that supports interrupt operations ：lockInterruptibly

• The above should be what we can intuitively think of Lock Basic ability , in fact Lock Interfaces are implemented through different implementation classes , Provides more powerful capabilities . We will gradually show through the case , For the moment, you don't need to care too much about

Let's take a look ,Lock Interface source screenshot , Besides looking at the pictures , I suggest you open jdk Source code , Have a look Lock Interface source code , There are very detailed source code comments , It's a great way to learn .

2.2.Lock Interface programming templates

In your application , If it's through synchronized Keyword lock words , You must still have an impression ： Use synchronized keyword , We don't need to release the lock , Everything has jvm To control .

But if you choose a more flexible Lock Lock mode , Remember to ： Matching release lock operation , Don't just give birth to 、 If you don't care about it, it's troublesome , So what's waiting for you is deadLock（ Deadlock ） 了 .

Next, I'll give you a generic Lock Locks use templates , You need to pay attention to try{...}finally{...} Sentence block .

Here we will not expand the actual use case for the moment , Practical application cases , We'll put it in the next one to learn with you .

/**
* addI Method , Realization add_i Variables operate on themselves 
*  adopt Lock Lock 
*/
public  static void addI(){
    //  Lock 
    lock.lock();
    try{
        //  Business operations 
        add_i ++;
    }finally{
        //  Release the lock 
        lock.unlock();
    }
        
}