The use and precautions of volatile keyword in Java

This article mainly introduces the relevant information about the use of java volatile keyword and precautions. When a variable is declared as volatile, the java memory model ensures that all threads using the variable can see the same and consistent value. . , friends in need can refer to

java volatile keyword usage and precautions

What is the volatile keyword

## The #volatile keyword plays an important role in multi-threaded programs. When multiple threads operate on the same variable, each thread will have a local cache copy of that variable. Therefore, when a thread modifies the value of this variable, it actually modifies the variable value in its local cache. Instead of the variable value in main memory, other threads operating this variable do not know that the value of this variable has been changed. In order to avoid this situation, we can declare this variable with the volatile keyword. After declaring this variable with volatile, the variable will not be saved in the local cache, but in the main memory. When a thread modifies its value, it will The updated value will be updated to the main memory, and then other threads can access the updated value. When a variable is declared volatile, the Java memory model ensures that all threads using the variable see the same, consistent value.

Use the volatile keyword

First, create the VolatileData class, the code is as follows:

public class VolatileData {

  //声明为volatile类型
  private volatile int counter = 0;

  /**
  * 返回counter变量的值
  * @return
  */
  public int getCounter() {
    return counter;
  }

  /**
  * 自增counter变量的值
  */
  public void increaseCounter() {
    ++counter;
  }
}

Next create VolatileThread class, the code is as follows:

public class VolatileThread extends Thread {
  private final VolatileData volatileData;

  public VolatileThread(VolatileData volatileData) {
    this.volatileData = volatileData;
  }

  /**
  * 调用VolatileData类中的两个方法，进行取值和自增操作
  */
  @Override
  public void run() {
    int oldValue = volatileData.getCounter();
    System.out.println("[Thread " + Thread.currentThread().getId() + "]: Old value = " + oldValue);
    volatileData.increaseCounter();
    int newValue = volatileData.getCounter();
    System.out.println("[Thread " + Thread.currentThread().getId() + "]: New value = " + newValue);
  }
 }

Finally, create the VolatileMain class to test the above program, the code is as follows:

public class VolatileMain {

  private final static int TOTAL_THREADS = 2;

  public static void main(String[] args) throws InterruptedException {
    VolatileData volatileData = new VolatileData();

    Thread[] threads = new Thread[TOTAL_THREADS];
    for(int i = 0; i < TOTAL_THREADS; ++i)
      threads[i] = new VolatileThread(volatileData);

    //开始读取变量值的操作
    for(int i = 0; i < TOTAL_THREADS; ++i)
      threads[i].start();

    //等待所有线程操作终止
    for(int i = 0; i < TOTAL_THREADS; ++i)
      threads[i].join();
  }
}

In the VolatileMain class, two threads are used to access volatile variables. The output is as follows:

[Thread 10]: Old value = 0
[Thread 11]: Old value = 0
[Thread 10]: New value = 1
[Thread 11]: New value = 2

As can be seen from the output, first, both threads output Then, after the increaseCounter method is called, both threads access and output the latest volatile variable value.

happens-before relationship

When using the volatile keyword, I have to mention the happens-before relationship of the java memory model. The happens-before relationship is an important aspect of Java's memory model. It is built between two different events so that all changes to the object by the first event can be seen and reflected by the second event. For example, when one thread writes to a volatile variable and another thread subsequently accesses the variable, a happens-before relationship is established. Therefore, all changes to volatile variables are visible to other threads.

Things to note

When using the volatile keyword in a program, we must pay attention to the following points:

1. The volatile keyword does not eliminate the need for synchronized operations between atoms, since memory consistency errors are still possible

2. Using atomic variables is more efficient than using synchronized synchronized code, but in order Avoiding memory consistency errors requires extra effort

3. volatile keyword cannot replace synchronized synchronized code blocks and methods

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