Java基础悲观锁&乐观锁

水深无声 2023-02-25 02:06 44阅读 0赞

### 前言 ###

悲观锁与乐观锁 只是2种不同的概念. 在JDK内各有不同的实现.

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### 解析 ###

*  悲观锁  
    悲观锁. 即. 认为: 我们每次修改数据时, 都会有其他人来修改数据. 所以, 我们在进行更新数据时, 都会加锁进行控制.

即 执行类似如下流程.

1- 锁定
    2- 修改数据
    3- 解除锁定

Java 中并发包的锁主要有2种: `synchronized关键字`与`RentrantLock对象`. 其中之一写法:

public void update(OptimisticLockObject object, int value) throws InterruptedException {
    			synchronized (object){
    				object.updateValue(value);
    			}
    }

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*  乐观锁  
    即认为: 在修改数据时, 并无其他线程对相同数据进行修改. 但是乐观锁会在修改前记录下数据数值，修改时进行比对. 我们经常使用的`CAS`和原子类(也是用的`CAS`). 都是乐观锁的实现. (注: CAS 即 Compare And Swap. 比较并更新.)

乐观锁的执行流程为:

1- 获取数据Value 和Version
    2- 检测Value和Version是否正确. 
    3- 匹配成功即更新;
    4- 匹配不成功即失败.

JDK中乐观锁实现为原子类和CAS. 手写一个CAS的流程并不困难.

public void update(){
    	int value = getValue();
    	int version = getVersion();
    	// 更新
    	if(value==getValue() && version == getVersion()){
    		// update
    	}else{
    		// wrong & 重复更新
    	}
    }

--------------------

### Codes ###

import java.util.Random;
    
    /**
     * 乐观锁 & 悲观锁.
     * 
     * */
    public class OptimisticPessimisticLock {
    	static class OptimisticLockObject{
    		int value;
    		int version;
    		
    		public OptimisticLockObject() {
    			init();
    		}
    		public void init() {
    			value = 0;
    			version = 1;
    		}
    		
    		public int getVersion() {
    			return this.version;
    		}
    		
    		public boolean checkVersion(int version) {
    			return this.version==version;
    		}
    		
    		public void updateValue(int value) {
    			this.value = value;
    			version++;
    		} 
    	}
    	
    	
    	static class OptimisticThread extends Thread {
    		public OptimisticLockObject object;
    		public OptimisticThread(OptimisticLockObject object) {
    			this.object = object;
    		}
    		public void run() {
    			try {
    				update(object,new Random().nextInt(10));
    			} catch (InterruptedException e) {
    				e.printStackTrace();
    			}
    		}
    		
    		public void update(OptimisticLockObject object, int value) throws InterruptedException {
    			int lastVersion = object.version;
    			Thread.sleep(2);
    			System.out.println(this.currentThread().getName()+" value:"+object.value+" | "+" lastVersion:"+lastVersion);
    			if(object.checkVersion(lastVersion)) {
    				object.updateValue(value);
    				System.out.println(this.currentThread().getName()+" "+object.value+":"+object.version);
    				System.out.println("OptimisticLock OK .Update OK.");
    			}else {
    				System.out.println(this.currentThread().getName()+" "+object.value+":"+object.version);
    				System.out.println("OptimisticLock Wrong. Update Wrong. Version Changed.");
    			}
    		}
    	}
    	
    	
    	static class PessimisticThread extends Thread {
    		public OptimisticLockObject object;
    		public PessimisticThread(OptimisticLockObject object) {
    			this.object = object;
    		}
    		public void run() {
    			try {
    				update(object,new Random().nextInt(10));
    			} catch (InterruptedException e) {
    				e.printStackTrace();
    			}
    		}
    		
    		public void update(OptimisticLockObject object, int value) throws InterruptedException {
    			synchronized (object){
    				int lastVersion = object.version;
    				Thread.sleep(2);
    				System.out.println(this.currentThread().getName()+" value:"+object.value+" | "+" lastVersion:"+lastVersion);
    				if(object.checkVersion(lastVersion)) {
    					object.updateValue(value);
    					System.out.println(this.currentThread().getName()+" "+object.value+":"+object.version);
    					System.out.println("Pessimistic Lock OK .Update OK.");
    				}else {
    					System.out.println(this.currentThread().getName()+" "+object.value+":"+object.version);
    					System.out.println("Pessimistic Lock Wrong. Update Wrong. Version Changed.");
    				}
    			}
    			
    		}
    	}
    	
    	// 乐观锁 Case
    	public static void optimisticLockCase() {
    		OptimisticLockObject obj = new OptimisticLockObject();
    		Thread threadA = new OptimisticThread(obj);
    		Thread threadB = new OptimisticThread(obj);
    		threadA.start();
    		threadB.start();
    	}
    	
    	// 悲观锁 Case
    	public static void pessimisticLockCase() {
    		OptimisticLockObject obj = new OptimisticLockObject();
    		Thread threadA = new PessimisticThread(obj);
    		Thread threadB = new PessimisticThread(obj);
    		threadA.start();
    		threadB.start();
    	}
    	
    	public static void main(String[]args) throws InterruptedException {
    //		optimisticLockCase();
    		pessimisticLockCase();
    		// 放在前面2个线程没有执行完毕. 停顿10秒钟.
    		Thread.sleep(10* 1000);
    		
    	}
    	
    }

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### Others ###

*  排它锁(独占锁) & 共享锁
 *  读锁 / 写锁

上面2项说的是一个东西.

--------------------

### Reference ###

\[1\]. [什么是乐观锁，什么是悲观锁][Link 1]  
\[2\]. [面试官：如何实现一个乐观锁（小白都能看得懂的代码）][Link 2]

[Link 1]: https://www.jianshu.com/p/d2ac26ca6525
[Link 2]: https://blog.csdn.net/SDDDLLL/article/details/105893956