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利用redis實(shí)現(xiàn)分布式鎖,快速解決高并發(fā)時(shí)的線程安全問(wèn)題

更新時(shí)間：2021年01月25日 10:13:28 作者：鹽城吊霸天

這篇文章主要介紹了利用redis實(shí)現(xiàn)分布式鎖,快速解決高并發(fā)時(shí)的線程安全問(wèn)題，具有很好的參考價(jià)值，希望對(duì)大家有所幫助。一起跟隨小編過(guò)來(lái)看看吧

實(shí)際工作中，經(jīng)常會(huì)遇到多線程并發(fā)時(shí)的類似搶購(gòu)的功能，本篇描述一個(gè)簡(jiǎn)單的redis分布式鎖實(shí)現(xiàn)的多線程搶票功能。

直接上代碼。首先按照慣例，給出一個(gè)錯(cuò)誤的示范：

我們可以看看，當(dāng)20個(gè)線程一起來(lái)?yè)?0張票的時(shí)候，會(huì)發(fā)生什么事。

package com.tiger.utils; 
public class TestMutilThread {
 
	// 總票量
	public static int count = 10; 
	public static void main(String[] args) {
		statrtMulti();
	}
 
	public static void statrtMulti() {
		for (int i = 1; i <= 20; i++) {
			TicketRunnable tickrunner = new TicketRunnable();
			Thread thread = new Thread(tickrunner, "Thread No: " + i);
			thread.start();
		} 
	}
 
	public static class TicketRunnable implements Runnable {
 
		@Override
		public void run() {
			System.out.println(Thread.currentThread().getName() + " start "
					+ count);
			// TODO Auto-generated method stub
			// logger.info(Thread.currentThread().getName()
			// + " really start" + count);
			if (count <= 0) {
				System.out.println(Thread.currentThread().getName()
						+ " ticket sold out ! No tickets remained!" + count);
				return;
			} else {
				count = count - 1;
				System.out.println(Thread.currentThread().getName()
						+ " bought a ticket,now remaining :" + (count));
			}
		}
	}
}

測(cè)試結(jié)果，從結(jié)果可以看到，票數(shù)在不同的線程中已經(jīng)出現(xiàn)混亂。

Thread No: 2 start 10
Thread No: 6 start 10
Thread No: 4 start 10
Thread No: 5 start 10
Thread No: 3 start 10
Thread No: 9 start 6
Thread No: 1 start 10
Thread No: 1 bought a ticket,now remaining :3
Thread No: 9 bought a ticket,now remaining :4
Thread No: 3 bought a ticket,now remaining :5
Thread No: 12 start 3
Thread No: 5 bought a ticket,now remaining :6
Thread No: 4 bought a ticket,now remaining :7
Thread No: 8 start 7
Thread No: 7 start 8
Thread No: 12 bought a ticket,now remaining :1
Thread No: 14 start 0
Thread No: 6 bought a ticket,now remaining :8
Thread No: 16 start 0
Thread No: 2 bought a ticket,now remaining :9
Thread No: 16 ticket sold out ! No tickets remained!0
Thread No: 14 ticket sold out ! No tickets remained!0
Thread No: 18 start 0
Thread No: 18 ticket sold out ! No tickets remained!0
Thread No: 7 bought a ticket,now remaining :0
Thread No: 15 start 0
Thread No: 8 bought a ticket,now remaining :1
Thread No: 13 start 2
Thread No: 19 start 0
Thread No: 11 start 3
Thread No: 11 ticket sold out ! No tickets remained!0
Thread No: 10 start 3
Thread No: 10 ticket sold out ! No tickets remained!0
Thread No: 19 ticket sold out ! No tickets remained!0
Thread No: 13 ticket sold out ! No tickets remained!0
Thread No: 20 start 0
Thread No: 20 ticket sold out ! No tickets remained!0
Thread No: 15 ticket sold out ! No tickets remained!0
Thread No: 17 start 0
Thread No: 17 ticket sold out ! No tickets remained!0

為了解決多線程時(shí)出現(xiàn)的混亂問(wèn)題，這里給出真正的測(cè)試類!!!

真正的測(cè)試類，這里啟動(dòng)20個(gè)線程，來(lái)?yè)?0張票。

RedisTemplate 是用來(lái)實(shí)現(xiàn)redis操作的，由spring進(jìn)行集成。這里是使用到了RedisTemplate，所以我以構(gòu)造器的形式在外部將RedisTemplate傳入到測(cè)試類中。

MultiTestLock 是用來(lái)實(shí)現(xiàn)加鎖的工具類。

總票數(shù)使用volatile關(guān)鍵字，實(shí)現(xiàn)多線程時(shí)變量在系統(tǒng)內(nèi)存中的可見(jiàn)性，這點(diǎn)可以去了解下volatile關(guān)鍵字的作用。

TicketRunnable用于模擬搶票功能。

其中由于lock與unlock之間存在if判斷，為保證線程安全，這里使用synchronized來(lái)保證。

測(cè)試類：

package com.tiger.utils; 
import java.io.Serializable; 
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.data.redis.core.RedisTemplate; 
public class MultiConsumer {
	Logger logger=LoggerFactory.getLogger(MultiTestLock.class);	
	private RedisTemplate<Serializable, Serializable> redisTemplate;	
	public MultiTestLock lock;
	//總票量
	public volatile static int count = 10;
 
	public void statrtMulti() {
		lock = new MultiTestLock(redisTemplate);
		for (int i = 1; i <= 20; i++) {
			TicketRunnable tickrunner = new TicketRunnable();
			Thread thread = new Thread(tickrunner, "Thread No: " + i);
			thread.start();
			} 
	}
 
	public class TicketRunnable implements Runnable {
 
		@Override
		public void run() {
			logger.info(Thread.currentThread().getName() + " start "
					+ count);
			// TODO Auto-generated method stub
			if (count > 0) {
//				logger.info(Thread.currentThread().getName()
//						+ " really start" + count);
				lock.lock();
				synchronized (this) {
					if(count<=0){
						logger.info(Thread.currentThread().getName()
								+ " ticket sold out ! No tickets remained!" + count);
						lock.unlock();
						return;
					}else{
						count=count-1;
						logger.info(Thread.currentThread().getName()
								+ " bought a ticket,now remaining :" + (count));
					}
				}
				lock.unlock();
			}else{
				logger.info(Thread.currentThread().getName()
						+ " ticket sold out !" + count);
			}
		}
	}
 
	public RedisTemplate<Serializable, Serializable> getRedisTemplate() {
		return redisTemplate;
	}
 
	public void setRedisTemplate(
			RedisTemplate<Serializable, Serializable> redisTemplate) {
		this.redisTemplate = redisTemplate;
	}
 
	public MultiConsumer(RedisTemplate<Serializable, Serializable> redisTemplate) {
		super();
		this.redisTemplate = redisTemplate;
	}
}

Lock工具類：

我們知道為保證線程安全，程序中執(zhí)行的操作必須時(shí)原子的。redis后續(xù)的版本中可以使用set key同時(shí)設(shè)置expire超時(shí)時(shí)間。

想起上次去電信翼支付面試時(shí)，面試官問(wèn)過(guò)一個(gè)問(wèn)題：分布式鎖如何防止死鎖，問(wèn)題關(guān)鍵在于我們?cè)诜植际街羞M(jìn)行加鎖操作時(shí)成功了，但是后續(xù)業(yè)務(wù)操作完畢執(zhí)行解鎖時(shí)出現(xiàn)失敗。導(dǎo)致分布式鎖無(wú)法釋放。出現(xiàn)死鎖，后續(xù)的加鎖無(wú)法正常進(jìn)行。所以這里設(shè)置expire超時(shí)時(shí)間的目的就是防止出現(xiàn)解鎖失敗的情況，這樣，即使解鎖失敗了，分布式鎖依然會(huì)在超時(shí)時(shí)間過(guò)了之后自動(dòng)釋放。

具體在代碼中也有注釋，也可以作為參考。

package com.tiger.utils; 
import java.io.Serializable;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Random;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock; 
import javax.sound.midi.MidiDevice.Info; 
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.dao.DataAccessException;
import org.springframework.data.redis.core.RedisOperations;
import org.springframework.data.redis.core.RedisTemplate;
import org.springframework.data.redis.core.SessionCallback;
import org.springframework.data.redis.core.script.RedisScript; 
 
public class MultiTestLock implements Lock {	
	Logger logger=LoggerFactory.getLogger(MultiTestLock.class);	
	private RedisTemplate<Serializable, Serializable> redisTemplate;	
	public MultiTestLock(RedisTemplate<Serializable, Serializable> redisTemplate) {
		super();
		this.redisTemplate = redisTemplate;
	}
 
	@Override
	public void lock() {
		//這里使用while循環(huán)強(qiáng)制線程進(jìn)來(lái)之后先進(jìn)行搶鎖操作。只有搶到鎖才能進(jìn)行后續(xù)操作
		while(true){
			if(tryLock()){
				try {
					//這里讓線程睡500毫秒的目的是為了模擬業(yè)務(wù)耗時(shí)，確保業(yè)務(wù)結(jié)束時(shí)之前設(shè)置的值正好打到超時(shí)時(shí)間，
					//實(shí)際生產(chǎn)中可能有偏差，這里需要經(jīng)驗(yàn)
					Thread.sleep(500l);
//					logger.info(Thread.currentThread().getName()+" time to awake");
					return;
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}else{
				try {
					//這里設(shè)置一個(gè)隨機(jī)毫秒的sleep目的時(shí)降低while循環(huán)的頻率 
					Thread.sleep(new Random().nextInt(200)+100);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
		}
	}
 
	@Override
	public boolean tryLock() {
		//這里也可以選用transactionSupport支持事務(wù)操作
		SessionCallback<Object> sessionCallback=new SessionCallback<Object>() {
			@Override
			public Object execute(RedisOperations operations)
					throws DataAccessException {
				operations.multi();
				operations.opsForValue().setIfAbsent("secret", "answer");
				//設(shè)置超時(shí)時(shí)間要根據(jù)業(yè)務(wù)實(shí)際的可能處理時(shí)間來(lái)，是一個(gè)經(jīng)驗(yàn)值
				operations.expire("secret", 500l, TimeUnit.MILLISECONDS);
				Object object=operations.exec();
				return object;
			}
		};
		//執(zhí)行兩部操作，這里會(huì)拿到一個(gè)數(shù)組值 [true,true],分別對(duì)應(yīng)上述兩部操作的結(jié)果，如果中途出現(xiàn)第一次為false則表明第一步set值出錯(cuò)
		List<Boolean> result=(List) redisTemplate.execute(sessionCallback);
//		logger.info(Thread.currentThread().getName()+" try lock "+ result);
		if(true==result.get(0)||"true".equals(result.get(0)+"")){
			logger.info(Thread.currentThread().getName()+" try lock success");
			return true;
		}else{
			return false;
		}
	}
 
	@Override
	public boolean tryLock(long arg0, TimeUnit arg1)
			throws InterruptedException {
		// TODO Auto-generated method stub
		return false;
	}
 
	@Override
	public void unlock() {
		//unlock操作直接刪除鎖，如果執(zhí)行完還沒(méi)有達(dá)到超時(shí)時(shí)間則直接刪除，讓后續(xù)的線程進(jìn)行繼續(xù)操作。起到補(bǔ)刀的作用，確保鎖已經(jīng)超時(shí)或被刪除
		SessionCallback<Object> sessionCallback=new SessionCallback<Object>() {
			@Override
			public Object execute(RedisOperations operations)
					throws DataAccessException {
				operations.multi();
				operations.delete("secret");
				Object object=operations.exec();
				return object;
			}
		};
		Object result=redisTemplate.execute(sessionCallback);
	} 
 
	@Override
	public void lockInterruptibly() throws InterruptedException {
		// TODO Auto-generated method stub
	}
 
	@Override
	public Condition newCondition() {
		// TODO Auto-generated method stub
		return null;
	}
	
	public RedisTemplate<Serializable, Serializable> getRedisTemplate() {
		return redisTemplate;
	}
 
	public void setRedisTemplate(
			RedisTemplate<Serializable, Serializable> redisTemplate) {
		this.redisTemplate = redisTemplate;
	}
}

執(zhí)行結(jié)果

可以看到，票數(shù)穩(wěn)步減少，后續(xù)沒(méi)有搶到鎖的線程余票為0，無(wú)票可搶。

tips:

這其中也出現(xiàn)了一個(gè)問(wèn)題，redis進(jìn)行多部封裝操作時(shí)，系統(tǒng)報(bào)錯(cuò)：ERR EXEC without MULTI

后經(jīng)過(guò)查閱發(fā)現(xiàn)問(wèn)題出在：

在spring中，多次執(zhí)行MULTI命令不會(huì)報(bào)錯(cuò)，因?yàn)榈谝淮螆?zhí)行時(shí)，會(huì)將其內(nèi)部的一個(gè)isInMulti變量設(shè)為true，后續(xù)每次執(zhí)行命令是都會(huì)檢查這個(gè)變量，如果為true，則不執(zhí)行命令。

而多次執(zhí)行EXEC命令則會(huì)報(bào)開(kāi)頭說(shuō)的"ERR EXEC without MULTI"錯(cuò)誤。