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Java how to use the volatile keyword, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain for you in detail, people with this need can come to learn, I hope you can gain something.

The purpose of the volatile keyword is to make variables visible across multiple threads.

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We see that the thread cannot stop when it enters the loop. Although we set the isRuning property equal to false, the loop still does not stop. What is the reason for this? If we want to know the cause of this problem, we need to understand the memory structure of threads. Here we just take a brief look, and in later articles we will focus on the memory model of threads in Java.

In fact, in Java, multithreading has its own memory space, and the memory space of each thread is out of sync with that of other threads, that is to say, different threads cannot interact directly with each other. They interact through main memory. When multithreaded execution, it will first copy the attributes in the main memory to its own thread memory, and then perform specific logical operations, and then synchronize the processing results to the main memory when the processing is finished. If the two threads execute asynchronously, they will copy the data in the main memory into their own memory space for processing, and when the processing is finished, they will synchronize their processing data to the main memory. This is also the root cause of thread safety problems in the development of multithreaded programs. It is the problem that occurs when multiple threads synchronize data from their own thread memory to the main memory.

The reason why the loop does not end in the above code is that when the thread starts, it will copy the isRuning property to its own thread memory, and then execute the loop. At this time, we set the isRuning property to false, but this sets the value of the main memory property, the isRuning property in the thread is still true, and the thread will not take the value in the main memory, so the program will always execute the loop.

So how can we solve this problem? some people may want to use the synchronized synchronization method, but according to our above analysis, this is not the problem caused by multiple threads accessing instance variables at the same time, but the problem that thread memory is out of sync with main memory. Obviously, synchronized can not solve the above problems. So what should I do?

Rest assured that a keyword is also provided in Java to solve the thread safety caused by problems like the above, which is the volatile keyword. This is what I said at the beginning of the article. The purpose of the volatile keyword is to make variables visible across multiple threads. It's kind of puzzling to see its description. Let's make it simple that variables modified with the volatile keyword can only be stored in main memory, or threads can only take values in main memory when using volatile data. It can also be understood that when each thread copies main memory data to its own thread memory, the variables modified by the volatile keyword cannot be copied. This ensures that no matter how many threads there are, if they need variables modified with the volatile keyword, they can only go to the main memory to take the value. This is exactly what we want, so in the above code we only need to add a volatile keyword to solve the problem of endless loops.

We see that the cycle is over because of what we just said. In actual development, we often use the volatile keyword in multithreading.

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