In-depth understanding of thread synchronization and mutual exclusion mechanisms in Java development

In Java development, multi-threaded programming is already an inevitable situation. Multi-threading will inevitably involve thread synchronization and mutual exclusion mechanisms, so an in-depth understanding of thread synchronization and mutual exclusion mechanisms in Java development is very important for programmers.

1. Thread synchronization mechanism

In a multi-threaded environment, if multiple threads access the same resource at the same time, data competition problems may occur. Data race problems include accessing the same variable, calling the same method at the same time, etc. At this time, a thread synchronization mechanism is needed to avoid data competition problems.

The basic idea of ​​the thread synchronization mechanism is: when accessing shared resources, only one thread is allowed to perform the access operation, and other threads must wait for the current access operation to end before they can continue to access. In Java, commonly used thread synchronization mechanisms include the synchronized keyword and Lock interface.

1. synchronized keyword

The synchronized keyword is used to modify methods or code blocks. It can serialize multiple threads’ access to shared resources to ensure that they are accessed at the same time. Only one thread accesses the shared resource.

To perform synchronized synchronization operations on methods, you can use the following two methods:

public class Test {
  // synchronized 修饰方法
  public synchronized void testMethod() {
    // 对共享资源进行访问
  }

  // 在方法内部，使用 synchronized 代码块
  public void testMethod2() {
    synchronized(this){
      // 对共享资源进行访问
    }
  }
}

For an object, the synchronized lock is implemented on the Mark Word mark bit in the object header, and a thread executes it When synchronizing a method or code block, it will try to acquire the object's lock.

2. Lock interface

In Java, the Lock interface is a reentrant mutex lock that can replace the synchronized keyword and provides more flexible synchronization control. . The Lock interface provides two implementations: ReentrantLock and ReentrantReadWriteLock.

public class Test {
  // Lock接口的使用
  private Lock lock = new ReentrantLock();
  
  public void testMethod() {
    lock.lock();
    try {
      //对共享资源进行访问
    } finally {
      lock.unlock();
    }
  }
}

When using the Lock interface, you need to use the try-finally block to ensure the release of the lock. Releasing the lock in the finally block can ensure that when the lock is acquired, it does not matter whether the method is executed normally or an exception is thrown. Release the lock safely.

2. Thread mutual exclusion mechanism

The thread mutual exclusion mechanism is a way to achieve thread synchronization. When multiple threads compete for the lock of the same object, only one thread can occupy the lock. The mutual exclusion mechanism ensures that only one thread is executing a specific code segment or accessing a specific resource at the same time.

The thread mutual exclusion mechanism is usually implemented in Java through the synchronized keyword or Lock interface. When a thread acquires a lock on an object, other threads must wait for the thread to release the lock before acquiring the lock again.

1. synchronized keyword

The synchronized keyword is the most common thread mutual exclusion mechanism, which is implemented by locking the same object. .

public class Test {
  // synchronized 关键字的实现
  public synchronized void testMethod() {
    // 这里是同步代码块
  }
}

2. Lock interface

Lock interface can also be used to implement thread mutual exclusion mechanism. When using the Lock interface, you need to call the lock() method to acquire the lock, and call the unlock() method when releasing the lock.

public class Test {
  // Lock 接口的实现
  private Lock lock = new ReentrantLock();
  public void testMethod() {
    lock.lock();
    try {
      // 这里是同步代码块
    } finally {
      lock.unlock();
    }
  }
}

3. Summary

In Java development, multi-threaded programming is relatively common. Thread synchronization and mutual exclusion mechanisms ensure the correctness and safety of the program, but there are also some performance issues. When developing a program, you need to weigh the benefits and costs of using synchronization and mutual exclusion mechanisms.

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