Java 实现基于Redis的分布式可重入锁
如何实现可重入？首先锁信息(指redis中lockKey关联的value值) 必须得设计的能负载更多信息，之前non-reentrant时value直接就是一个超时时间，但是要实现可重入单超时时间是不够的，必须要标识锁是被谁持有的，也就是说要标识分布式环境中的线程，还要记录锁被入了多少次。
如何在分布式线程中标识唯一线程？
MAC地址 +jvm进程 + 线程ID（或者线程地址都行）,三者结合即可唯一分布式环境中的线程。下载
实现
锁的信息采用json存储，格式如下：
代码框架还是和之前实现的非重入的差不多，重点是lock方法，代码已有非常详细的注释
Java代码 下载
[*] package cc.lixiaohui.lock.redis;
[*]
[*] import java.io.IOException;
[*] import java.net.SocketAddress;
[*] import java.util.concurrent.TimeUnit;
[*]
[*] import org.slf4j.Logger;
[*] import org.slf4j.LoggerFactory;
[*]
[*] import redis.clients.jedis.Jedis;
[*] import cc.lixiaohui.lock.AbstractLock;
[*] import cc.lixiaohui.lock.Lock;
[*] import cc.lixiaohui.lock.time.nio.client.TimeClient;
[*] import cc.lixiaohui.lock.util.LockInfo;
[*]
[*] /**
[*] * 基于Redis的SETNX操作实现的分布式锁, 获取锁时最好用tryLock(long time, TimeUnit unit), 以免网路问题而导致线程一直阻塞.
[*] * SETNC操作参考资料.
[*] *
[*] * 可重入实现关键:
[*] *
[*] * 在分布式环境中如何确定一个线程? mac地址 + jvm pid + threadId (mac地址唯一, jvm
[*] * pid在单机内唯一, threadId在单jvm内唯一)
[*] * 任何一个线程从redis拿到value值后都需要能确定 该锁是否被自己持有, 因此value值要有以下特性: 保存持有锁的主机(mac), jvm
[*] * pid, 持有锁的线程ID, 重复持有锁的次数
[*] *
[*] *
[*] * redis中value设计如下(in json):
[*] *
[*] * {
[*] *expires : expire time in long
[*] *mac : mac address of lock holder's machine
[*] *pid : jvm process id
[*] *threadId : lock holder thread id
[*] *count : hold count(for use of reentrancy)
[*] * }
[*] * 由{@link LockInfo LockInfo}表示.
[*] *
[*] *
[*] * Usage Example:
[*] *
[*] *{@link Lock} lock = new {@link ReentrantLock}(jedis, "lockKey", lockExpires, timeServerAddr);
[*] *if (lock.tryLock(3, TimeUnit.SECONDS)) {
[*] * try {
[*] * // do something
[*] * } catch (Exception e) {
[*] * lock.unlock();
[*] * }
[*] *}
[*] *
[*] *
[*] *
[*] * @author lixiaohui
[*] * @date 2016年9月15日 下午2:52:38
[*] *
[*] */
[*] public class ReentrantLock extends AbstractLock {
[*]
[*] private Jedis jedis;
[*]
[*] private TimeClient timeClient;
[*]
[*] // 锁的名字
[*] protected String lockKey;
[*]
[*] // 锁的有效时长(毫秒)
[*] protected long lockExpires;
[*]
[*] private static final Logger logger = LoggerFactory.getLogger(ReentrantLock.class);
[*]
[*] public ReentrantLock(Jedis jedis, String lockKey, long lockExpires, SocketAddress timeServerAddr) throws IOException {
[*] this.jedis = jedis;
[*] this.lockKey = lockKey;
[*] this.lockExpires = lockExpires;
[*] timeClient = new TimeClient(timeServerAddr);
[*] }
[*]
[*] // 阻塞式获取锁的实现
[*] protected boolean lock(boolean useTimeout, long time, TimeUnit unit, boolean interrupt) throws InterruptedException {
[*] if (interrupt) {
[*] checkInterruption();
[*] }
[*]
[*] // 超时控制 的时间可以从本地获取, 因为这个和锁超时没有关系, 只是一段时间区间的控制
[*] long start = localTimeMillis();
[*] long timeout = unit.toMillis(time); // if !useTimeout, then it's useless
[*]
[*] // walkthrough
[*] // 1. lockKey未关联value, 直接设置lockKey, 成功获取到锁, return true
[*] // 2. lock 已过期, 用getset设置lockKey, 判断返回的旧的LockInfo
[*] // 2.1 若仍是超时的, 则成功获取到锁, return true
[*] // 2.2 若不是超时的, 则进入下一次循环重新开始 步骤1
[*] // 3. lock没过期, 判断是否是当前线程持有
[*] // 3.1 是, 则计数加 1, return true
[*] // 3.2 否, 则进入下一次循环重新开始 步骤1
[*] // note: 每次进入循环都检查 : 1.是否超时, 若是则return false; 2.是否检查中断(interrupt)被中断,
[*] // 若需检查中断且被中断, 则抛InterruptedException
[*] while (useTimeout ? !isTimeout(start, timeout) : true) {
[*] if (interrupt) {
[*] checkInterruption();
[*] }
[*]
[*] long lockExpireTime = serverTimeMillis() + lockExpires + 1;// 锁超时时间
[*] String newLockInfoJson = LockInfo.newForCurrThread(lockExpireTime).toString();
[*] if (jedis.setnx(lockKey, newLockInfoJson) == 1) { // 条件能成立的唯一情况就是redis中lockKey还未关联value
[*] // TODO 成功获取到锁, 设置相关标识
[*] logger.debug("{} get lock(new), lockInfo: {}", Thread.currentThread().getName(), newLockInfoJson);
[*] locked = true;
[*] return true;
[*] }
[*]
[*] // value已有值, 但不能说明锁被持有, 因为锁可能expired了
[*] String currLockInfoJson = jedis.get(lockKey);
[*] // 若这瞬间锁被delete了
[*] if (currLockInfoJson == null) {
[*] continue;
[*] }
[*]
[*] LockInfo currLockInfo = LockInfo.fromString(currLockInfoJson);
[*] // 竞争条件只可能出现在锁超时的情况, 因为如果没有超时, 线程发现锁并不是被自己持有, 线程就不会去动value
[*] if (isTimeExpired(currLockInfo.getExpires())) {
[*] // 锁超时了
[*] LockInfo oldLockInfo = LockInfo.fromString(jedis.getSet(lockKey, newLockInfoJson));
[*] if (oldLockInfo != null && isTimeExpired(oldLockInfo.getExpires())) {
[*] // TODO 成功获取到锁, 设置相关标识
[*] logger.debug("{} get lock(new), lockInfo: {}", Thread.currentThread().getName(), newLockInfoJson);
[*] locked = true;
[*] return true;
[*] }
[*] } else {
[*] // 锁未超时, 不会有竞争情况
[*] if (isHeldByCurrentThread(currLockInfo)) { // 当前线程持有
[*] // TODO 成功获取到锁, 设置相关标识
[*] currLockInfo.setExpires(serverTimeMillis() + lockExpires + 1); // 设置新的锁超时时间
[*] currLockInfo.incCount();
[*] jedis.set(lockKey, currLockInfo.toString());
[*] logger.debug("{} get lock(inc), lockInfo: {}", Thread.currentThread().getName(), currLockInfo);
[*] locked = true;
[*] return true;
[*] }
[*] }
[*] }
[*] locked = false;
[*] return false;
[*] }
[*]
[*] public boolean tryLock() {
[*] long lockExpireTime = serverTimeMillis() + lockExpires + 1;
[*] String newLockInfo = LockInfo.newForCurrThread(lockExpireTime).toString();
[*]
[*] if (jedis.setnx(lockKey, newLockInfo) == 1) {
[*] locked = true;
[*] return true;
[*] }
[*]
[*] String currLockInfoJson = jedis.get(lockKey);
[*] if (currLockInfoJson == null) {
[*] // 再一次尝试获取
[*] if (jedis.setnx(lockKey, newLockInfo) == 1) {
[*] locked = true;
[*] return true;
[*] } else {
[*] locked = false;
[*] return false;
[*] }
[*] }
[*]
[*] LockInfo currLockInfo = LockInfo.fromString(currLockInfoJson);
[*]
[*] if (isTimeExpired(currLockInfo.getExpires())) {
[*] LockInfo oldLockInfo = LockInfo.fromString(jedis.getSet(lockKey, newLockInfo));
[*] if (oldLockInfo != null && isTimeExpired(oldLockInfo.getExpires())) {
[*] locked = true;
[*] return true;
[*] }
[*] } else {
[*] if (isHeldByCurrentThread(currLockInfo)) {
[*] currLockInfo.setExpires(serverTimeMillis() + lockExpires + 1);
[*] currLockInfo.incCount();
[*] jedis.set(lockKey, currLockInfo.toString());
[*] locked = true;
[*] return true;
[*] }
[*] }
[*] locked = false;
[*] return false;
[*] }
[*]
[*] /**
[*] * Queries if this lock is held by any thread.
[*] *
[*] * @return {@code true} if any thread holds this lock and {@code false}
[*] * otherwise
[*] */
[*] public boolean isLocked() {
[*] // walkthrough
[*] // 1. lockKey未关联value, return false
[*] // 2. 若 lock 已过期, return false, 否则 return true
[*] if (!locked) { // 本地locked为false, 肯定没加锁
[*] return false;
[*] }
[*] String json = jedis.get(lockKey);
[*] if (json == null) {
[*] return false;
[*] }
[*] if (isTimeExpired(LockInfo.fromString(json).getExpires())) {
[*] return false;
[*] }
[*] return true;
[*] }
[*]
[*] @Override
[*] protected void unlock0() {
[*] // walkthrough
[*] // 1. 若锁过期, return
[*] // 2. 判断自己是否是锁的owner
[*] // 2.1 是, 若 count = 1, 则删除lockKey; 若 count > 1, 则计数减 1, return
[*] // 2.2 否, 则抛异常 IllegalMonitorStateException, reutrn
[*] // done, return
[*] LockInfo currLockInfo = LockInfo.fromString(jedis.get(lockKey));
[*] if (isTimeExpired(currLockInfo.getExpires())) {
[*] return;
[*] }
[*]
[*] if (isHeldByCurrentThread(currLockInfo)) {
[*] if (currLockInfo.getCount() == 1) {
[*] jedis.del(lockKey);
[*] logger.debug("{} unlock(del), lockInfo: null", Thread.currentThread().getName());
[*] } else {
[*] currLockInfo.decCount(); // 持有锁计数减1
[*] String json = currLockInfo.toString();
[*] jedis.set(lockKey, json);
[*] logger.debug("{} unlock(dec), lockInfo: {}", Thread.currentThread().getName(), json);
[*] }
[*] } else {
[*] throw new IllegalMonitorStateException(String.format("current thread[%s] does not holds the lock", Thread.currentThread().toString()));
[*] }
[*]
[*] }
[*]
[*] public void release() {
[*] jedis.close();
[*] timeClient.close();
[*] }
[*]
[*] public boolean isHeldByCurrentThread() {
[*] return isHeldByCurrentThread(LockInfo.fromString(jedis.get(lockKey)));
[*] }
[*]
[*] // ------------------- utility methods ------------------------
[*]
[*] private boolean isHeldByCurrentThread(LockInfo lockInfo) {
[*] return lockInfo.isCurrentThread();
[*] }
[*]
[*] private void checkInterruption() throws InterruptedException {
[*] if (Thread.currentThread().isInterrupted()) {
[*] throw new InterruptedException();
[*] }
[*] }
[*]
[*] private boolean isTimeExpired(long time) {
[*] return time < serverTimeMillis();
[*] }
[*]
[*] private boolean isTimeout(long start, long timeout) {
[*] // 这里拿本地的时间来比较
[*] return start + timeout < System.currentTimeMillis();
[*] }
[*]
[*] private long serverTimeMillis() {
[*] return timeClient.currentTimeMillis();
[*] }
[*]
[*] private long localTimeMillis() {
[*] return System.currentTimeMillis();
[*] }
[*]
[*] }
测试
5个线程，每个线程都是不同的jedis连接，模拟分布式环境，线程的任务就是不断的去尝试重入地获取锁，重入的次数为随机但在0-5之间。
代码
Java代码 下载
[*] package cc.lixiaohui.DistributedLock.DistributedLock;
[*]
[*] import java.io.IOException;
[*] import java.net.InetSocketAddress;
[*] import java.net.SocketAddress;
[*] import java.util.ArrayList;
[*] import java.util.List;
[*] import java.util.Random;
[*] import java.util.concurrent.TimeUnit;
[*]
[*] import org.junit.Test;
[*]
[*] import redis.clients.jedis.Jedis;
[*] import cc.lixiaohui.lock.redis.ReentrantLock;
[*]
[*] /**
[*] * @author lixiaohui
[*] * @date 2016年9月28日 下午8:41:36
[*] *
[*] */
[*] public class ReentrantTest {
[*]
[*] final int EXPIRES = 10 * 1000;
[*]
[*] final String LOCK_KEY = "lock.lock";
[*]
[*] final SocketAddress TIME_SERVER_ADDR = new InetSocketAddress("localhost", 9999);
[*]
[*] @Test
[*] public void test() throws Exception {
[*] // 创建5个线程不停地去重入(随机次数n, 0
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