Java实现基于Redis的分布式锁
我这里不实现JDK的java.util.concurrent.locks.Lock接口，而是自定义一个，因为JDK的有个newCondition()方法我这里暂时没实现。这个Lock提供了5个lock方法的变体，可以自行选择使用哪一个来获取锁，我的想法是最好用带超时返回的那几个方法，因为不这样的话，假如redis挂了，线程永远都在那死循环了(关于这里，应该还可以进一步优化，如果redis挂了，Jedis的操作肯定会抛异常之类的，可以定义个机制让redis挂了的时候通知使用这个lock的用户，或者说是线程)
Java代码 下载
[*] package cc.lixiaohui.lock;
[*]
[*] import java.util.concurrent.TimeUnit;
[*]
[*] public interface Lock {
[*]
[*] /**
[*] * 阻塞性的获取锁, 不响应中断
[*] */
[*] void lock();
[*]
[*] /**
[*] * 阻塞性的获取锁, 响应中断
[*] *
[*] * @throws InterruptedException
[*] */
[*] void lockInterruptibly() throws InterruptedException;
[*]
[*] /**
[*] * 尝试获取锁, 获取不到立即返回, 不阻塞
[*] */
[*] boolean tryLock();
[*]
[*] /**
[*] * 超时自动返回的阻塞性的获取锁, 不响应中断
[*] *
[*] * @param time
[*] * @param unit
[*] * @return {@code true} 若成功获取到锁, {@code false} 若在指定时间内未获取到锁
[*] *
[*] */
[*] boolean tryLock(long time, TimeUnit unit);
[*]
[*] /**
[*] * 超时自动返回的阻塞性的获取锁, 响应中断
[*] *
[*] * @param time
[*] * @param unit
[*] * @return {@code true} 若成功获取到锁, {@code false} 若在指定时间内未获取到锁
[*] * @throws InterruptedException 在尝试获取锁的当前线程被中断
[*] */
[*] boolean tryLockInterruptibly(long time, TimeUnit unit) throws InterruptedException;
[*]
[*] /**
[*] * 释放锁
[*] */
[*] void unlock();
[*]
[*] }
看其抽象实现：
Java代码 下载
[*] package cc.lixiaohui.lock;
[*]
[*] import java.util.concurrent.TimeUnit;
[*]
[*] /**
[*] * 锁的骨架实现, 真正的获取锁的步骤由子类去实现.
[*] *
[*] * @author lixiaohui
[*] *
[*] */
[*] public abstract class AbstractLock implements Lock {
[*]
[*] /**
[*] *
[*] * 这里需不需要保证可见性值得讨论, 因为是分布式的锁,
[*] * 1.同一个jvm的多个线程使用不同的锁对象其实也是可以的, 这种情况下不需要保证可见性
[*] * 2.同一个jvm的多个线程使用同一个锁对象, 那可见性就必须要保证了.
[*] *
[*] */
[*] protected volatile boolean locked;
[*]
[*] /**
[*] * 当前jvm内持有该锁的线程(if have one)
[*] */
[*] private Thread exclusiveOwnerThread;
[*]
[*] public void lock() {
[*] try {
[*] lock(false, 0, null, false);
[*] } catch (InterruptedException e) {
[*] // TODO ignore
[*] }
[*] }
[*]
[*] public void lockInterruptibly() throws InterruptedException {
[*] lock(false, 0, null, true);
[*] }
[*]
[*] public boolean tryLock(long time, TimeUnit unit) {
[*] try {
[*] return lock(true, time, unit, false);
[*] } catch (InterruptedException e) {
[*] // TODO ignore
[*] }
[*] return false;
[*] }
[*]
[*] public boolean tryLockInterruptibly(long time, TimeUnit unit) throws InterruptedException {
[*] return lock(true, time, unit, true);
[*] }
[*]
[*] public void unlock() {
[*] // TODO 检查当前线程是否持有锁
[*] if (Thread.currentThread() != getExclusiveOwnerThread()) {
[*] throw new IllegalMonitorStateException("current thread does not hold the lock");
[*] }
[*]
[*] unlock0();
[*] setExclusiveOwnerThread(null);
[*] }
[*]
[*] protected void setExclusiveOwnerThread(Thread thread) {
[*] exclusiveOwnerThread = thread;
[*] }
[*]
[*] protected final Thread getExclusiveOwnerThread() {
[*] return exclusiveOwnerThread;
[*] }
[*]
[*] protected abstract void unlock0();
[*]
[*] /**
[*] * 阻塞式获取锁的实现
[*] *
[*] * @param useTimeout
[*] * @param time
[*] * @param unit
[*] * @param interrupt 是否响应中断
[*] * @return
[*] * @throws InterruptedException
[*] */
[*] protected abstract boolean lock(boolean useTimeout, long time, TimeUnit unit, boolean interrupt) throws InterruptedException;
[*]
[*] }
基于Redis的最终实现，关键的获取锁，释放锁的代码在这个类的lock方法和unlock0方法里，大家可以只看这两个方法然后完全自己写一个：
Java代码 下载
[*] package cc.lixiaohui.lock;
[*]
[*] import java.util.concurrent.TimeUnit;
[*]
[*] import redis.clients.jedis.Jedis;
[*]
[*] /**
[*] *
[*] * 基于Redis的SETNX操作实现的分布式锁
[*] *
[*] * 获取锁时最好用lock(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
[*] *
[*] *
[*] * @author lixiaohui
[*] *
[*] */
[*] public class RedisBasedDistributedLock extends AbstractLock {
[*]
[*] private Jedis jedis;
[*]
[*] // 锁的名字
[*] protected String lockKey;
[*]
[*] // 锁的有效时长(毫秒)
[*] protected long lockExpires;
[*]
[*] public RedisBasedDistributedLock(Jedis jedis, String lockKey, long lockExpires) {
[*] this.jedis = jedis;
[*] this.lockKey = lockKey;
[*] this.lockExpires = lockExpires;
[*] }
[*]
[*] // 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();
[*] }
[*]
[*] long start = System.currentTimeMillis();
[*] long timeout = unit.toMillis(time); // if !useTimeout, then it's useless
[*]
[*] while (useTimeout ? isTimeout(start, timeout) : true) {
[*] if (interrupt) {
[*] checkInterruption();
[*] }
[*]
[*] long lockExpireTime = System.currentTimeMillis() + lockExpires + 1;//锁超时时间
[*] String stringOfLockExpireTime = String.valueOf(lockExpireTime);
[*]
[*] if (jedis.setnx(lockKey, stringOfLockExpireTime) == 1) { // 获取到锁
[*] // TODO 成功获取到锁, 设置相关标识
[*] locked = true;
[*] setExclusiveOwnerThread(Thread.currentThread());
[*] return true;
[*] }
[*]
[*] String value = jedis.get(lockKey);
[*] if (value != null && isTimeExpired(value)) { // lock is expired
[*] // 假设多个线程(非单jvm)同时走到这里
[*] String oldValue = jedis.getSet(lockKey, stringOfLockExpireTime); // getset is atomic
[*] // 但是走到这里时每个线程拿到的oldValue肯定不可能一样(因为getset是原子性的)
[*] // 加入拿到的oldValue依然是expired的，那么就说明拿到锁了
[*] if (oldValue != null && isTimeExpired(oldValue)) {
[*] // TODO 成功获取到锁, 设置相关标识
[*] locked = true;
[*] setExclusiveOwnerThread(Thread.currentThread());
[*] return true;
[*] }
[*] } else {
[*] // TODO lock is not expired, enter next loop retrying
[*] }
[*] }
[*] return false;
[*] }
[*]
[*] public boolean tryLock() {
[*] long lockExpireTime = System.currentTimeMillis() + lockExpires + 1;//锁超时时间
[*] String stringOfLockExpireTime = String.valueOf(lockExpireTime);
[*]
[*] if (jedis.setnx(lockKey, stringOfLockExpireTime) == 1) { // 获取到锁
[*] // TODO 成功获取到锁, 设置相关标识
[*] locked = true;
[*] setExclusiveOwnerThread(Thread.currentThread());
[*] return true;
[*] }
[*]
[*] String value = jedis.get(lockKey);
[*] if (value != null && isTimeExpired(value)) { // lock is expired
[*] // 假设多个线程(非单jvm)同时走到这里
[*] String oldValue = jedis.getSet(lockKey, stringOfLockExpireTime); // getset is atomic
[*] // 但是走到这里时每个线程拿到的oldValue肯定不可能一样(因为getset是原子性的)
[*] // 假如拿到的oldValue依然是expired的，那么就说明拿到锁了
[*] if (oldValue != null && isTimeExpired(oldValue)) {
[*] // TODO 成功获取到锁, 设置相关标识
[*] locked = true;
[*] setExclusiveOwnerThread(Thread.currentThread());
[*] return true;
[*] }
[*] } else {
[*] // TODO lock is not expired, enter next loop retrying
[*] }
[*]
[*] 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() {
[*] if (locked) {
[*] return true;
[*] } else {
[*] String value = jedis.get(lockKey);
[*] // TODO 这里其实是有问题的, 想:当get方法返回value后, 假设这个value已经是过期的了,
[*] // 而就在这瞬间, 另一个节点set了value, 这时锁是被别的线程(节点持有), 而接下来的判断
[*] // 是检测不出这种情况的.不过这个问题应该不会导致其它的问题出现, 因为这个方法的目的本来就
[*] // 不是同步控制, 它只是一种锁状态的报告.
[*] return !isTimeExpired(value);
[*] }
[*] }
[*]
[*] @Override
[*] protected void unlock0() {
[*] // TODO 判断锁是否过期
[*] String value = jedis.get(lockKey);
[*] if (!isTimeExpired(value)) {
[*] doUnlock();
[*] }
[*] }
[*]
[*] private void checkInterruption() throws InterruptedException {
[*] if(Thread.currentThread().isInterrupted()) {
[*] throw new InterruptedException();
[*] }
[*] }
[*]
[*] private boolean isTimeExpired(String value) {
[*] return Long.parseLong(value) < System.currentTimeMillis();
[*] }
[*]
[*] private boolean isTimeout(long start, long timeout) {
[*] return start + timeout > System.currentTimeMillis();
[*] }
[*]
[*] private void doUnlock() {
[*] jedis.del(lockKey);
[*] }
[*]
[*] }
如果将来还换一种实现方式（比如zookeeper之类的），到时直接继承AbstractLock并实现lock(boolean useTimeout, long time, TimeUnit unit, boolean interrupt)， unlock0()方法即可(所谓抽象嘛)
页:
[1]