分布式锁一般有三种实现方式：
- 基于数据库乐观锁；
- 基于Redis的分布式锁；
- 基于ZooKeeper的分布式锁。
本篇博客将介绍第二种方式，基于Redis实现分布式锁。 本篇博客将详细介绍如何正确地实现Redis分布式锁。
实现分布式锁需要满足哪些条件呢？我们至少要确保锁的实现同时满足以下条件：
- 高性能：
- 互斥性：在任意时刻，只有一个客户端能持有锁，同一个方法在同一时间只能被一台机器上的一个线程执行。
- 避免发生死锁：即使有一个客户端在持有锁的期间崩溃而没有主动解锁，也能保证后续其他客户端能加锁；另外这把锁是一把可重入锁。
- 容错性：只要大部分的Redis节点正常运行，客户端就可以加锁和解锁。
Redis通常可以使用setnx来实现分布式锁。
首先我们要通过Maven引入Jedis开源组件，在pom.xml文件加入下面的代码：
<dependency>
<groupId>redis.clients</groupId>
<artifactId>jedis</artifactId>
<version>2.9.0</version>
</dependency>
- 加锁
public void lock(){
while(true){
ret = tryLock();
if(ret){ //获取到了锁
return;
}
sleep(100);
}
}
public boolean tryLock(){
Long result = jedis.setnx(lockKey, requestId);
if (result == 1) {
return true;
}
return false;
}
- 解锁
public void unlock(){
jedis.del(lockKey);
}
setnx来创建一个key，如果key不存在则创建成功返回1，如果key已经存在则返回0。依照上述来判定是否获取到了锁，获取到锁的执行业务逻辑，完毕后删除lock_key，来实现释放锁。
- 加锁
public boolean tryLock(long leaseTime, TimeUnit unit) {
String resp = jedis.set(resourceName, getLockName(Thread.currentThread().getId()), "NX", "PX", unit.toMillis(leaseTime));
if ("OK".equals(resp)) {
return true;
}
return false;
}
protected String getLockName(long threadId) {
return String.format("%s:%d", id, threadId);
}
- 解锁
public void unlock() {
String holder = jedis.get(resourceName);
if (getLockName(Thread.currentThread().getId()).equals(holder)) { //release
jedis.del(resourceName);
}
}
- 加锁
import java.util.concurrent.TimeUnit;
/**
* Distributed Lock
* @author Ricky Fung
*/
public interface DLock {
void lock();
boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException;
boolean isLocked();
boolean isHeldByCurrentThread();
void forceUnlock();
void unlock();
}
RedisLock
import pandora.distlock.DLock;
import pandora.redis.RedisTemplate;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;
/**
* 参考：[Distributed locks with Redis](https://redis.io/topics/distlock)
* @author Ricky Fung
*/
public class RedisLock implements DLock {
private String id; //唯一标识id
private String resourceName; //资源名称
private RedisTemplate redisTemplate;
private LuaScript luaScript = new LuaScript();
private final List<String> keys = new ArrayList<>(1);
public RedisLock(String id, String resourceName, RedisTemplate redisTemplate) {
this.id = id;
this.resourceName = resourceName;
this.redisTemplate = redisTemplate;
this.keys.add(resourceName);
}
@Override
public void lock() {
}
@Override
public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException {
long time = unit.toMillis(waitTime);
long current = System.currentTimeMillis();
boolean res = tryAcquire(leaseTime, unit);
// lock acquired
if (res) {
return true;
}
time -= (System.currentTimeMillis() - current);
if (time <= 0) {
return false;
}
while (true) {
long currentTime = System.currentTimeMillis();
res = tryAcquire(leaseTime, unit);
// lock acquired
if (res) {
return true;
}
time -= (System.currentTimeMillis() - currentTime);
if (time <= 0) {
return false;
}
}
}
@Override
public boolean isLocked() {
return redisTemplate.exists(resourceName);
}
@Override
public boolean isHeldByCurrentThread() {
List<String> args = new ArrayList<>(1);
args.add(getLockValue());
Long update = (Long) redisTemplate.eval(luaScript.buildHeldByCurrentThreadScript(), keys,
args);
if (update==1) {
return true;
}
return false;
}
@Override
public void forceUnlock() {
redisTemplate.del(resourceName);
}
@Override
public void unlock() {
List<String> args = new ArrayList<>(1);
args.add(getLockValue());
Long update = (Long) redisTemplate.eval(luaScript.buildUnLockScript(), keys, args);
}
private boolean tryAcquire(long leaseTime, TimeUnit unit) {
List<String> args = new ArrayList<>(4);
args.add(getLockValue());
args.add("NX");
args.add("PX");
args.add(String.valueOf(unit.toMillis(leaseTime)));
Long update = (Long) redisTemplate.eval(luaScript.buildLockScript(), keys, args);
if (update==1) {
return true;
}
return false;
}
protected String getLockValue() {
return String.format("%s:%d", id, Thread.currentThread().getId());
}
}
LuaScript
/**
* @author Ricky Fung
*/
public class LuaScript {
/**
* SET resource_name my_random_value NX PX 30000
* @return
*/
public String buildLockScript() {
StringBuilder sb = new StringBuilder(120);
sb.append("if redis.call('set', KEYS[1], ARGV[1], ARGV[2], ARGV[3], tonumber(ARGV[4])) == 1 then ")
.append("\t return 1 ")
.append("end ")
.append("if redis.call('get', KEYS[1]) == ARGV[1] then ")
.append("\t return 1 ")
.append("else ")
.append("\t return 0 ")
.append("end");
return sb.toString();
}
public String buildUnLockScript() {
StringBuilder sb = new StringBuilder(120);
sb.append("if redis.call('get', KEYS[1]) == ARGV[1] then ")
.append("\t return redis.call('del', KEYS[1]) ")
.append("else ")
.append("\t return 0 ")
.append("end");
return sb.toString();
}
public String buildHeldByCurrentThreadScript() {
StringBuilder sb = new StringBuilder(120);
sb.append("if redis.call('get', KEYS[1]) == ARGV[1] then ")
.append("\t return 1 ")
.append("else ")
.append("\t return 0 ")
.append("end");
return sb.toString();
}
}
/**
* @author Ricky Fung
*/
public class Panda {
private final RedisTemplate redisTemplate;
public Panda(RedisTemplate redisTemplate) {
this.redisTemplate = redisTemplate;
}
public DLock getLock(String name) {
return new RedisLock(UUIDUtils.getUUID(), name, redisTemplate);
}
}
Panda panda = new Panda(redisTemplate);
@Test
public void testReentrantLock() throws InterruptedException {
String res = "res1";
DLock lock = panda.getLock(res);
try {
boolean success = lock.tryLock(5, 30, TimeUnit.SECONDS);
System.out.println("key:"+res+" value:"+redisTemplate.get(res));
if (success) {
System.out.println("re-entry:"+lock.tryLock(5, 30, TimeUnit.SECONDS));
System.out.println("YES");
} else {
System.out.println("NO");
}
} finally {
lock.unlock();
}
System.out.println("key:"+res+" value:"+redisTemplate.get(res));
}