Explore the internal implementation mechanism of object methods wait and notify in Java

In-depth understanding of object methods in Java: the underlying implementation principles of wait and notify require specific code examples

Object methods in Javawait and notify are the key methods used to implement inter-thread communication, and their underlying implementation principles involve the monitor mechanism of the Java virtual machine. This article will delve into the underlying implementation principles of these two methods and provide specific code examples.

First, let’s understand the basic uses of wait and notify. The function of the wait method is to cause the current thread to release the object's lock and enter the waiting state until other threads call the object's notify method to wake it up. The notify method is used to wake up a thread waiting on the object and make it re-enter the runnable state.

Below we use specific code examples to illustrate the use and underlying implementation principles of the wait and notify methods.

public class WaitNotifyExample {
    public static void main(String[] args) {
        final Object lock = new Object();

        // 线程A
        Thread threadA = new Thread(() -> {
            synchronized (lock) {
                try {
                    System.out.println("ThreadA: 开始执行");
                    lock.wait(); // 线程A进入等待状态
                    System.out.println("ThreadA: 被唤醒，继续执行");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });

        // 线程B
        Thread threadB = new Thread(() -> {
            synchronized (lock) {
                System.out.println("ThreadB: 开始执行");
                lock.notify(); // 唤醒等待的线程A
                System.out.println("ThreadB: 调用notify方法，结束");
            }
        });

        threadA.start();
        threadB.start();
    }
}

In the above code, we create an example of waiting to wake up, where thread A enters the waiting state by calling the wait method, and thread B by calling the notify Method wakes up thread A. By running the above code, we can see that thread A continues execution after being awakened from the wait state.

After understanding the basic usage, let’s explore the underlying implementation principles of wait and notify. In the Java language, each object has a monitor associated with it, which is actually part of the Object Header. When a thread calls the wait method of an object, the thread will release the object's lock and enter the waiting state, and add itself to the object's waiting queue. When other threads call the object's notify method, the JVM will select a thread from the waiting queue to wake it up. Note that the awakened thread will continue to wait to acquire the object's lock. Only after it acquires the lock can it return from the wait method to continue execution.

When executing the wait and notify methods, you must first obtain the lock of the object, otherwise an IllegalMonitorStateException exception will be thrown. Therefore, these two methods must be used within a synchronized block or method.

It should be noted that the wait method and the notify method can only operate on the same object, even different instances of the same class. In addition, the notify method can only wake up one thread in the waiting queue. If there are multiple threads waiting, the specific thread to wake up is uncertain.

To sum up, the wait and notify methods are important methods to achieve inter-thread communication in Java, and their underlying implementation principles involve the monitor of the Java virtual machine. mechanism. By properly using the wait and notify methods, we can achieve synchronization and mutual exclusion between multiple threads, thereby ensuring thread safety while improving program performance.

I hope this article will help you understand the underlying implementation principles of object methods wait and notify in Java. If you still have questions about this, you can continue to learn more about it.

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