分布式锁
分布式锁是用来控制分布式系统对共享资源进行有序的操作,在分布式系统中,如果不同的系统或是同一个系统的不同主机之间共享了一个或一组资源,那么访问这些资源的时候,往往需要互斥来防止彼此干扰来保证一致性,在这种情况下,便需要使用到分布式锁。 实现原理包含:
- 互斥性:保证同一时间只有一个客户端可以拿到锁,也就是可以对共享资源进行操作
- 安全性:只有加锁的服务才能有解锁权限
- 避免死锁:通过expire给锁设置过期时间,避免了线程长期占用锁而导致死锁
- 保证加锁与解锁操作是原子性操作:通过setnx加锁,当key不存在时,进行set操作,若key已经存在,则不做任何操作
在业务中的应用
1.创建分布式锁工具类
import java.util.Collections;
import redis.clients.jedis.Jedis;
public class RedisLock {
private static final String LOCK_SUCCESS = "OK";
private static final String SET_IF_NOT_EXIST = "NX";//当key不存在时,进行set操作,若key已经存在,则不做任何操作
private static final String SET_WITH_EXPIRE_TIME = "PX";//加一个过期的设置,单位毫秒,具体时间由第五个参数决定
private static final Long RELEASE_SUCCESS = 1L;
/**
* 尝试获取分布式锁
* @param jedis Redis客户端
* @param lockKey 锁
* @param requestId 请求标识
* @param expireTime 超期时间
* @return 是否获取成功
*/
public static boolean tryGetDistributedLock(Jedis jedis, String lockKey, String requestId, int expireTime) {
String result = jedis.set(lockKey, requestId, SET_IF_NOT_EXIST, SET_WITH_EXPIRE_TIME, expireTime);
if (LOCK_SUCCESS.equals(result)) {
return true;
}
return false;
}
/**
* 释放分布式锁
* @param jedis Redis客户端
* @param lockKey 锁
* @param requestId 请求标识
* @return 是否释放成功
*/
//参数KEYS[1]赋值为lockKey,ARGV[1]赋值为requestId,首先获取锁对应的value值,检查是否与requestId相等,如果相等则删除锁(解锁)
public static boolean releaseDistributedLock(Jedis jedis, String lockKey, String requestId) {
String script = "if redis.call('get', KEYS[1]) == ARGV[1] then return redis.call('del', KEYS[1]) else return 0 end";
Object result = jedis.eval(script, Collections.singletonList(lockKey), Collections.singletonList(requestId));
if (RELEASE_SUCCESS.equals(result)) {
return true;
}
return false;
}
}
2.创建Redis操作工具类
import java.util.List;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.JSONArray;
import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisPool;
@Service
public class RedisService {
@Autowired
private JedisPool jedisPool;
/**
* 获取对象
*/
public <T> T get(String key,Class<T> clazz){
Jedis jedis = null;
try {
jedis = jedisPool.getResource();
String sval = jedis.get(key);
//将String转换为Bean
T t = stringToBean(sval,clazz);
return t;
}finally {
if(jedis != null) {
jedis.close();
}
}
}
/**
* 设置对象
*/
public <T> boolean set(String key,T value){
Jedis jedis = null;
try {
jedis = jedisPool.getResource();
//将Bean转换为String
String s = beanToString(value);
if(s == null || s.length() <= 0) {
return false;
}
jedis.set(key, s);
return true;
}finally {
if(jedis != null) {
jedis.close();
}
}
}
/**
* 减少值
*/
public <T> Long decr(String key){
Jedis jedis = null;
try {
jedis = jedisPool.getResource();
//返回value减1后的值
return jedis.decr(key);
}finally {
if(jedis != null) {
jedis.close();
}
}
}
/**
* 将字符串转换为Bean对象
*/
@SuppressWarnings("unchecked")
public static <T> T stringToBean(String str,Class<T> clazz) {
if(str == null || str.length() == 0 || clazz == null) {
return null;
}
if(clazz == int.class || clazz == Integer.class) {
return ((T) Integer.valueOf(str));
}else if(clazz == String.class) {
return (T) str;
}else if(clazz == long.class || clazz == Long.class) {
return (T) Long.valueOf(str);
}else if(clazz == List.class) {
return JSON.toJavaObject(JSONArray.parseArray(str), clazz);
}else {
return JSON.toJavaObject(JSON.parseObject(str), clazz);
}
}
/**
* 将Bean对象转换为字符串类型
*/
public static <T> String beanToString(T value) {
if(value == null){
return null;
}
Class<?> clazz = value.getClass();
if(clazz == int.class || clazz == Integer.class) {
return ""+value;
}else if(clazz == String.class) {
return (String)value;
}else if(clazz == long.class || clazz == Long.class) {
return ""+value;
}else {
return JSON.toJSONString(value);
}
}
}
3.秒杀业务调用
import java.util.HashMap;
import java.util.Map;
import java.util.UUID;
import javax.servlet.http.HttpServletRequest;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;
import redis.clients.jedis.Jedis;
import redis.clients.jedis.JedisPool;
import cn.com.app.redis.RedisLock;
import cn.com.app.redis.RedisService;
@Controller
public class RedisLockController implements InitializingBean {
@Autowired
private RedisService redisService;
@Autowired
private JedisPool jedisPool;
//内存标记,减少redis访问
private HashMap<String, Boolean> localOverMap = new HashMap<String, Boolean>();
/**
* 系统初始化的时把商品库存加入到缓存中
*/
@Override
public void afterPropertiesSet() throws Exception {
//设置默认商品库存
redisService.set("goodsStock", "10");
//添加内存标记
localOverMap.put("goodsStock", false);
}
/**
* 请求秒杀,redis分布式锁
*/
@RequestMapping(value="/miaoshalock")
@ResponseBody
public String miaoshalock(HttpServletRequest request,@RequestParam("userid")String userid){
boolean over = localOverMap.get("goodsStock");
if(over) {
System.out.println("秒杀结束");
return "秒杀结束";
}
Jedis jedis = null;
try {
jedis = jedisPool.getResource();
String requestId = UUID.randomUUID().toString();
boolean isOk = RedisLock.tryGetDistributedLock(jedis,"LOCK:goodsStock",requestId,10000);//获取锁
if(isOk){
try{
int count = redisService.get("goodsStock",Integer.class);
if(count <= 0){
localOverMap.put("goodsStock", true);
System.out.println("库存不足");
return "库存不足";
}
String orderInfo = redisService.get("order"+userid,String.class);
if(orderInfo != null){
System.out.println("重复下单");
return "重复下单";
}
long stock = redisService.decr("goodsStock");
System.out.println("秒杀成功,剩余库存:"+stock);
redisService.set("order"+userid, userid+"_"+UUID.randomUUID().toString());
/**
* 数据库操作减少库存,下订单,在一个事务中
*/
}finally{
RedisLock.releaseDistributedLock(jedis,"LOCK:goodsStock",requestId);//释放锁
}
}else{
System.out.println("该线程未拿到锁");
return "秒杀失败";
}
}finally {
if(jedis != null) {
jedis.close();
}
}
return "秒杀成功";
}
}