Basic use and understanding of synchronized

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Preface

Contents of this chapter

`Shared issues, synchronized

1. Problems with sharing

1. There is no problem with a program running multiple threads
2. The problem lies in multiple thread access共享资源

2.1 多个线程读`共享资源`其实也没有问题
2.2 在多个线程对`共享资源`读写操作时发生过指令交错，就会出现问题

3. 共享资源If there are pairs of multi-threaded read and write operations in a block of code , this block of code is called临界区

2. synchronized

语法：

		synchronized(对象){
    
    
            //临界区
        }

Synchronized actually 对象锁guarantees 临界区内代码的原子性that the code in the critical section is indivisible to the outside world and will not be interrupted by thread switching.

2.1 synchronized object-oriented writing method

public class App {
    
    
    public static void main(String[] args) throws Exception {
    
    
        Room room = new Room();
        Thread t1 = new Thread(() -> {
    
    
            for (int i = 1; i <= 5000; i++) {
    
    
                room.inc();
            }
        }, "t1");
        Thread t2 = new Thread(() -> {
    
    
            for (int i = 1; i <= 5000; i++) {
    
    
                room.dec();
            }
        }, "t2");
        t1.start();
        t2.start();
        t1.join();//等待t1线程执行完毕
        t2.join();//等待t2线程执行完毕
        System.out.println(room.getCount());
    }
}

class Room {
    
    
    private int count = 0;

    public void inc() {
    
    
        synchronized (this) {
    
    
            count++;
        }
    }
    public void dec() {
    
    
        synchronized (this) {
    
    
            count--;
        }
    }
    public int getCount() {
    
    
        synchronized (this) {
    
    
            return count;
        }
    }
}

2.2 Synchronized on methods

成员方法上的写法: What is locked is the current this object

class Room{
    public synchronized void inc(){
        //代码
    }
}

等价于

class Room {
    public void inc() {
        synchronized (this) {
            //代码
        }
    }
}

静态方法上的写法: What is locked is the class object XXX.class

class Room {
    public static synchronized void inc() {
        //代码
    }
}

等价于

class Room {
    public static void inc(){
        synchronized (Room.class){
            //代码
        }
    }
}

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