方案一:数据库乐观锁
乐观锁通常实现基于数据版本(version)的记录机制实现的,比如有一张红包表(t_bonus),有一个字段(left_count)记录礼物的剩余个数,用户每领取一个奖品,对应的left_count减1,在并发的情况下如何要保证left_count不为负数,乐观锁的实现方式为在红包表上添加一个版本号字段(version),默认为0。
异常实现流程
-- 可能会发生的异常情况
-- 线程1查询,当前left_count为1,则有记录
select * from t_bonus where id = and left_count > -- 线程2查询,当前left_count为1,也有记录
select * from t_bonus where id = and left_count > -- 线程1完成领取记录,修改left_count为0,
update t_bonus set left_count = left_count - where id = -- 线程2完成领取记录,修改left_count为-,产生脏数据
update t_bonus set left_count = left_count - where id =
通过乐观锁实现
-- 添加版本号控制字段
ALTER TABLE table ADD COLUMN version INT DEFAULT '' NOT NULL AFTER t_bonus; -- 线程1查询,当前left_count为1,则有记录,当前版本号为1234
select left_count, version from t_bonus where id = and left_count > -- 线程2查询,当前left_count为1,有记录,当前版本号为1234
select left_count, version from t_bonus where id = and left_count > -- 线程1,更新完成后当前的version为1235,update状态为1,更新成功
update t_bonus set version = , left_count = left_count- where id = and version = -- 线程2,更新由于当前的version为1235,udpate状态为0,更新失败,再针对相关业务做异常处理
update t_bonus set version = , left_count = left_count- where id = and version =
方案二:基于Redis的分布式锁
SETNX命令(SET if Not eXists)
语法:SETNX key value
功能:原子性操作,当且仅当 key 不存在,将 key 的值设为 value ,并返回1;若给定的 key 已经存在,则 SETNX 不做任何动作,并返回0。
Expire命令
语法:expire(key, expireTime)
功能:key设置过期时间
GETSET命令
语法:GETSET key value
功能:将给定 key 的值设为 value ,并返回 key 的旧值 (old value),当 key 存在但不是字符串类型时,返回一个错误,当key不存在时,返回nil。
GET命令
语法:GET key
功能:返回 key 所关联的字符串值,如果 key 不存在那么返回特殊值 nil 。
DEL命令
语法:DEL key [KEY …]
功能:删除给定的一个或多个 key ,不存在的 key 会被忽略。
第一种:使用redis的setnx()、expire()方法,用于分布式锁
- setnx(lockkey, 1) 如果返回0,则说明占位失败;如果返回1,则说明占位成功
- expire()命令对lockkey设置超时时间,为的是避免死锁问题。
- 执行完业务代码后,可以通过delete命令删除key。
这个方案其实是可以解决日常工作中的需求的,但从技术方案的探讨上来说,可能还有一些可以完善的地方。比如,如果在第一步setnx执行成功后,在expire()命令执行成功前,发生了宕机的现象,那么就依然会出现死锁的问题
第二种:使用redis的setnx()、get()、getset()方法,用于分布式锁,解决死锁问题
- setnx(lockkey, 当前时间+过期超时时间) ,如果返回1,则获取锁成功;如果返回0则没有获取到锁,转向2。
- get(lockkey)获取值oldExpireTime ,并将这个value值与当前的系统时间进行比较,如果小于当前系统时间,则认为这个锁已经超时,可以允许别的请求重新获取,转向3。
- 计算newExpireTime=当前时间+过期超时时间,然后getset(lockkey, newExpireTime) 会返回当前lockkey的值currentExpireTime。
- 判断currentExpireTime与oldExpireTime 是否相等,如果相等,说明当前getset设置成功,获取到了锁。如果不相等,说明这个锁又被别的请求获取走了,那么当前请求可以直接返回失败,或者继续重试。
- 在获取到锁之后,当前线程可以开始自己的业务处理,当处理完毕后,比较自己的处理时间和对于锁设置的超时时间,如果小于锁设置的超时时间,则直接执行delete释放锁;如果大于锁设置的超时时间,则不需要再锁进行处理。
import cn.com.tpig.cache.redis.RedisService;
import cn.com.tpig.utils.SpringUtils; /**
* Created by IDEA
* User: shma1664
* Date: 2016-08-16 14:01
* Desc: redis分布式锁
*/
public final class RedisLockUtil { private static final int defaultExpire = ; private RedisLockUtil() {
//
} /**
* 加锁
* @param key redis key
* @param expire 过期时间,单位秒
* @return true:加锁成功,false,加锁失败
*/
public static boolean lock(String key, int expire) { RedisService redisService = SpringUtils.getBean(RedisService.class);
long status = redisService.setnx(key, ""); if(status == ) {
redisService.expire(key, expire);
return true;
} return false;
} public static boolean lock(String key) {
return lock2(key, defaultExpire);
} /**
* 加锁
* @param key redis key
* @param expire 过期时间,单位秒
* @return true:加锁成功,false,加锁失败
*/
public static boolean lock2(String key, int expire) { RedisService redisService = SpringUtils.getBean(RedisService.class); long value = System.currentTimeMillis() + expire;
long status = redisService.setnx(key, String.valueOf(value)); if(status == ) {
return true;
}
long oldExpireTime = Long.parseLong(redisService.get(key, ""));
if(oldExpireTime < System.currentTimeMillis()) {
//超时
long newExpireTime = System.currentTimeMillis() + expire;
long currentExpireTime = Long.parseLong(redisService.getSet(key, String.valueOf(newExpireTime)));
if(currentExpireTime == oldExpireTime) {
return true;
}
}
return false;
} public static void unLock1(String key) {
RedisService redisService = SpringUtils.getBean(RedisService.class);
redisService.del(key);
} public static void unLock2(String key) {
RedisService redisService = SpringUtils.getBean(RedisService.class);
long oldExpireTime = Long.parseLong(redisService.get(key, ""));
if(oldExpireTime > System.currentTimeMillis()) {
redisService.del(key);
}
} } public void drawRedPacket(long userId) {
String key = "draw.redpacket.userid:" + userId; boolean lock = RedisLockUtil.lock2(key, );
if(lock) {
try {
//领取操作
} finally {
//释放锁
RedisLockUtil.unLock(key);
}
} else {
new RuntimeException("重复领取奖励");
}
}
Spring AOP基于注解方式和SpEL实现开箱即用的redis分布式锁策略
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target; /**
* RUNTIME
* 定义注解
* 编译器将把注释记录在类文件中,在运行时 VM 将保留注释,因此可以反射性地读取。
* @author shma1664
*
*/
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface RedisLockable { String[] key() default ""; long expiration() default ;
}
import javax.annotation.Resource; import java.lang.reflect.Method; import com.autohome.api.dealer.util.cache.RedisClient;
import com.google.common.base.Joiner;
import org.aspectj.lang.ProceedingJoinPoint;
import org.aspectj.lang.Signature;
import org.aspectj.lang.annotation.Around;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.Pointcut;
import org.aspectj.lang.reflect.MethodSignature;
import org.springframework.expression.EvaluationContext;
import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;
import org.springframework.stereotype.Component; /**
* Created by IDEA
* User: mashaohua
* Date: 2016-09-28 18:08
* Desc:
*/
@Aspect
@Component
public class RedisLockAop { @Resource
private RedisClient redisClient; @Pointcut("execution(* com.autohome.api.dealer.tuan.service.*.*(..))")
public void pointcut(){} @Around("pointcut()")
public Object doAround(ProceedingJoinPoint point) throws Throwable{
Signature signature = point.getSignature();
MethodSignature methodSignature = (MethodSignature) signature;
Method method = methodSignature.getMethod();
String targetName = point.getTarget().getClass().getName();
String methodName = point.getSignature().getName();
Object[] arguments = point.getArgs(); if (method != null && method.isAnnotationPresent(RedisLockable.class)) {
RedisLockable redisLock = method.getAnnotation(RedisLockable.class);
long expire = redisLock.expiration();
String redisKey = getLockKey(targetName, methodName, redisLock.key(), arguments);
boolean isLock = RedisLockUtil.lock2(redisKey, expire);
if(!isLock) {
try {
return point.proceed();
} finally {
unLock2(redisKey);
}
} else {
throw new RuntimeException("您的操作太频繁,请稍后再试");
}
} return point.proceed();
} private String getLockKey(String targetName, String methodName, String[] keys, Object[] arguments) { StringBuilder sb = new StringBuilder();
sb.append("lock.").append(targetName).append(".").append(methodName); if(keys != null) {
String keyStr = Joiner.on(".").skipNulls().join(keys);
String[] parameters = ReflectParamNames.getNames(targetName, methodName);
ExpressionParser parser = new SpelExpressionParser();
Expression expression = parser.parseExpression(keyStr);
EvaluationContext context = new StandardEvaluationContext();
int length = parameters.length;
if (length > ) {
for (int i = ; i < length; i++) {
context.setVariable(parameters[i], arguments[i]);
}
}
String keysValue = expression.getValue(context, String.class);
sb.append("#").append(keysValue);
}
return sb.toString();
}
<!-- https://mvnrepository.com/artifact/javassist/javassist -->
<dependency>
<groupId>org.javassist</groupId>
<artifactId>javassist</artifactId>
<version>3.18.1-GA</version>
</dependency>
import javassist.*;
import javassist.bytecode.CodeAttribute;
import javassist.bytecode.LocalVariableAttribute;
import javassist.bytecode.MethodInfo;
import org.apache.log4j.Logger; /**
* Created by IDEA
* User: mashaohua
* Date: 2016-09-28 18:39
* Desc:
*/
public class ReflectParamNames {
private static Logger log = Logger.getLogger(ReflectParamNames.class);
private static ClassPool pool = ClassPool.getDefault(); static{
ClassClassPath classPath = new ClassClassPath(ReflectParamNames.class);
pool.insertClassPath(classPath);
} public static String[] getNames(String className,String methodName) {
CtClass cc = null;
try {
cc = pool.get(className);
CtMethod cm = cc.getDeclaredMethod(methodName);
// 使用javaassist的反射方法获取方法的参数名
MethodInfo methodInfo = cm.getMethodInfo();
CodeAttribute codeAttribute = methodInfo.getCodeAttribute();
LocalVariableAttribute attr = (LocalVariableAttribute) codeAttribute.getAttribute(LocalVariableAttribute.tag);
if (attr == null) return new String[0]; int begin = 0; String[] paramNames = new String[cm.getParameterTypes().length];
int count = 0;
int pos = Modifier.isStatic(cm.getModifiers()) ? 0 : 1; for (int i = 0; i < attr.tableLength(); i++){
// 为什么 加这个判断,发现在windows 跟linux执行时,参数顺序不一致,通过观察,实际的参数是从this后面开始的
if (attr.variableName(i).equals("this")){
begin = i;
break;
}
} for (int i = begin+1; i <= begin+paramNames.length; i++){
paramNames[count] = attr.variableName(i);
count++;
}
return paramNames;
} catch (Exception e) {
e.printStackTrace();
}finally{
try {
if(cc != null) cc.detach();
} catch (Exception e2) {
log.error(e2.getMessage());
} }
return new String[0];
}
}
在需要使用分布式锁的地方添加注解
/**
* 抽奖接口
* 添加redis分布式锁保证一个订单只有一个请求处理,防止用户刷礼物,支持SpEL表达式
* redisLockKey:lock.com.autohome.api.dealer.tuan.service.impl.drawBonus#orderId
* @param orderId 订单id
* @return 抽中的奖品信息
*/
@RedisLockable(key = {"#orderId"}, expiration = 120)
@Override
public BonusConvertBean drawBonus(Integer orderId) throws BonusException{
// 业务逻辑
}
第三种方案:基于Zookeeper的分布式锁
利用节点名称的唯一性来实现独占锁
ZooKeeper机制规定同一个目录下只能有一个唯一的文件名,zookeeper上的一个znode看作是一把锁,通过createznode的方式来实现。所有客户端都去创建/lock/${lock_name}_lock节点,最终成功创建的那个客户端也即拥有了这把锁,创建失败的可以选择监听继续等待,还是放弃抛出异常实现独占锁。
package com.shma.example.zookeeper.lock; import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock; import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat; /**
* Created by IDEA
* User: mashaohua
* Date: 2016-09-30 16:09
* Desc:
*/
public class ZookeeperLock implements Lock, Watcher {
private ZooKeeper zk;
private String root = "/locks";//根
private String lockName;//竞争资源的标志
private String myZnode;//当前锁
private int sessionTimeout = 30000;
private List<Exception> exception = new ArrayList<Exception>(); /**
* 创建分布式锁,使用前请确认config配置的zookeeper服务可用
* @param config 127.0.0.1:2181
* @param lockName 竞争资源标志,lockName中不能包含单词lock
*/
public ZookeeperLock(String config, String lockName){
this.lockName = lockName;
// 创建一个与服务器的连接
try {
zk = new ZooKeeper(config, sessionTimeout, this);
Stat stat = zk.exists(root, false);
if(stat == null){
// 创建根节点
zk.create(root, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
}
} catch (IOException e) {
exception.add(e);
} catch (KeeperException e) {
exception.add(e);
} catch (InterruptedException e) {
exception.add(e);
}
} @Override
public void lock() {
if(exception.size() > 0){
throw new LockException(exception.get(0));
}
if(!tryLock()) {
throw new LockException("您的操作太频繁,请稍后再试");
}
} @Override
public void lockInterruptibly() throws InterruptedException {
this.lock();
} @Override
public boolean tryLock() {
try {
myZnode = zk.create(root + "/" + lockName, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
return true;
} catch (KeeperException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
return false;
} @Override
public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return tryLock();
} @Override
public void unlock() {
try {
zk.delete(myZnode, -1);
myZnode = null;
zk.close();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (KeeperException e) {
e.printStackTrace();
}
} @Override
public Condition newCondition() {
return null;
} @Override
public void process(WatchedEvent watchedEvent) {
//
} } ZookeeperLock lock = null;
try {
lock = new ZookeeperLock("127.0.0.1:2182","test1");
lock.lock();
//业务逻辑处理
} catch (LockException e) {
throw e;
} finally {
if(lock != null)
lock.unlock();
}
利用临时顺序节点控制时序实现
/lock已经预先存在,所有客户端在它下面创建临时顺序编号目录节点,和选master一样,编号最小的获得锁,用完删除,依次方便。
算法思路:对于加锁操作,可以让所有客户端都去/lock目录下创建临时顺序节点,如果创建的客户端发现自身创建节点序列号是/lock/目录下最小的节点,则获得锁。否则,监视比自己创建节点的序列号小的节点(比自己创建的节点小的最大节点),进入等待。
对于解锁操作,只需要将自身创建的节点删除即可。
package com.shma.example.zookeeper.lock; import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock; import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat; /**
* Created by IDEA
* User: mashaohua
* Date: 2016-09-30 16:09
* Desc:
*/
public class DistributedLock implements Lock, Watcher{
private ZooKeeper zk;
private String root = "/locks";//根
private String lockName;//竞争资源的标志
private String waitNode;//等待前一个锁
private String myZnode;//当前锁
private CountDownLatch latch;//计数器
private int sessionTimeout = 30000;
private List<Exception> exception = new ArrayList<Exception>(); /**
* 创建分布式锁,使用前请确认config配置的zookeeper服务可用
* @param config 127.0.0.1:2181
* @param lockName 竞争资源标志,lockName中不能包含单词lock
*/
public DistributedLock(String config, String lockName){
this.lockName = lockName;
// 创建一个与服务器的连接
try {
zk = new ZooKeeper(config, sessionTimeout, this);
Stat stat = zk.exists(root, false);
if(stat == null){
// 创建根节点
zk.create(root, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE,CreateMode.PERSISTENT);
}
} catch (IOException e) {
exception.add(e);
} catch (KeeperException e) {
exception.add(e);
} catch (InterruptedException e) {
exception.add(e);
}
} /**
* zookeeper节点的监视器
*/
public void process(WatchedEvent event) {
if(this.latch != null) {
this.latch.countDown();
}
} public void lock() {
if(exception.size() > 0){
throw new LockException(exception.get(0));
}
try {
if(this.tryLock()){
System.out.println("Thread " + Thread.currentThread().getId() + " " +myZnode + " get lock true");
return;
}
else{
waitForLock(waitNode, sessionTimeout);//等待锁
}
} catch (KeeperException e) {
throw new LockException(e);
} catch (InterruptedException e) {
throw new LockException(e);
}
} public boolean tryLock() {
try {
String splitStr = "_lock_";
if(lockName.contains(splitStr))
throw new LockException("lockName can not contains \\u000B");
//创建临时子节点
myZnode = zk.create(root + "/" + lockName + splitStr, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE,CreateMode.EPHEMERAL_SEQUENTIAL);
System.out.println(myZnode + " is created ");
//取出所有子节点
List<String> subNodes = zk.getChildren(root, false);
//取出所有lockName的锁
List<String> lockObjNodes = new ArrayList<String>();
for (String node : subNodes) {
String _node = node.split(splitStr)[0];
if(_node.equals(lockName)){
lockObjNodes.add(node);
}
}
Collections.sort(lockObjNodes);
System.out.println(myZnode + "==" + lockObjNodes.get(0));
if(myZnode.equals(root+"/"+lockObjNodes.get(0))){
//如果是最小的节点,则表示取得锁
return true;
}
//如果不是最小的节点,找到比自己小1的节点
String subMyZnode = myZnode.substring(myZnode.lastIndexOf("/") + 1);
waitNode = lockObjNodes.get(Collections.binarySearch(lockObjNodes, subMyZnode) - 1);
} catch (KeeperException e) {
throw new LockException(e);
} catch (InterruptedException e) {
throw new LockException(e);
}
return false;
} public boolean tryLock(long time, TimeUnit unit) {
try {
if(this.tryLock()){
return true;
}
return waitForLock(waitNode,time);
} catch (Exception e) {
e.printStackTrace();
}
return false;
} private boolean waitForLock(String lower, long waitTime) throws InterruptedException, KeeperException {
Stat stat = zk.exists(root + "/" + lower,true);
//判断比自己小一个数的节点是否存在,如果不存在则无需等待锁,同时注册监听
if(stat != null){
System.out.println("Thread " + Thread.currentThread().getId() + " waiting for " + root + "/" + lower);
this.latch = new CountDownLatch(1);
this.latch.await(waitTime, TimeUnit.MILLISECONDS);
this.latch = null;
}
return true;
} public void unlock() {
try {
System.out.println("unlock " + myZnode);
zk.delete(myZnode,-1);
myZnode = null;
zk.close();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (KeeperException e) {
e.printStackTrace();
}
} public void lockInterruptibly() throws InterruptedException {
this.lock();
} public Condition newCondition() {
return null;
} public class LockException extends RuntimeException {
private static final long serialVersionUID = 1L;
public LockException(String e){
super(e);
}
public LockException(Exception e){
super(e);
}
} }