1.Redisson的分布式锁简单总结
Redisson分布式锁包括：可重入锁、公平锁、联锁、红锁、读写锁。
(1)可重入锁RedissonLock
非公平锁，最基础的分布式锁，最常用的锁。
(2)公平锁RedissonFairLock
各个客户端尝试获取锁时会排队，按照队列的顺序先后获取锁。
(3)联锁MultiLock
可以一次性加多把锁，从而实现一次性锁多个资源。
(4)红锁RedLock
RedLock相当于一把锁。虽然利用了MultiLock包裹了多个小锁，但这些小锁并不对应多个资源，而是每个小锁的key对应一个Redis实例。只要大多数的Redis实例加锁成功，就可以认为RedLock加锁成功。RedLock的健壮性要比其他普通锁要好。
但是RedLock也有一些场景无法保证正确性，当然RedLock只要求部署主库。比如客户端A尝试向5个Master实例加锁，但仅仅在3个Maste中加锁成功。不幸的是此时3个Master中有1个Master突然宕机了，而且锁key还没同步到该宕机Master的Slave上，此时Salve切换为Master。于是在这5个Master中，由于其中有一个是新切换过来的Master，所以只有2个Master是有客户端A加锁的数据，另外3个Master是没有锁的。但继续不幸的是，此时客户端B来加锁，那么客户端B就很有可能成功在没有锁数据的3个Master上加到锁，从而满足了过半数加锁的要求，最后也完成了加锁，依然发生重复加锁。
(5)读写锁之读锁RedissonReadLock和写锁RedissonWriteLock
不同客户端线程的四种加锁情况：
情况一：先加读锁再加读锁，不互斥
情况二：先加读锁再加写锁，互斥
情况三：先加写锁再加读锁，互斥
情况四：先加写锁再加写锁，互斥
同一个客户端线程的四种加锁情况：
情况一：先加读锁再加读锁，不互斥
情况二：先加读锁再加写锁，互斥
情况三：先加写锁再加读锁，不互斥
情况四：先加写锁再加写锁，不互斥
2.Redisson的Semaphore简介
(1)Redisson的Semaphore原理图
Semaphore也是Redisson支持的一种同步组件。Semaphore作为一个锁机制，可以允许多个线程同时获取一把锁。任何一个线程释放锁之后，其他等待的线程就可以尝试继续获取锁。
(2)Redisson的Semaphore使用演示
public class RedissonDemo {
public static void main(String[] args) throws Exception {
//连接3主3从的Redis CLuster
Config config = new Config();
...
//Semaphore
RedissonClient redisson = Redisson.create(config);
final RSemaphore semaphore = redisson.getSemaphore("semaphore");
semaphore.trySetPermits(3);
for (int i = 0; i < 10; i++) {
new Thread(new Runnable() {
public void run() {
try {
System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]尝试获取Semaphore锁");
semaphore.acquire();
System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]成功获取到了Semaphore锁，开始工作");
Thread.sleep(3000);
semaphore.release();
System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]释放Semaphore锁");
} catch (Exception e) {
e.printStackTrace();
}
}
}).start();
}
}
}3.Redisson的Semaphore源码剖析
(1)Semaphore的初始化
public class Redisson implements RedissonClient {
//Redis的连接管理器，封装了一个Config实例
protected final ConnectionManager connectionManager;
//Redis的命令执行器，封装了一个ConnectionManager实例
protected final CommandAsyncExecutor commandExecutor;
...
protected Redisson(Config config) {
this.config = config;
Config configCopy = new Config(config);
//初始化Redis的连接管理器
connectionManager = ConfigSupport.createConnectionManager(configCopy);
...
//初始化Redis的命令执行器
commandExecutor = new CommandSyncService(connectionManager, objectBuilder);
...
}
@Override
public RSemaphore getSemaphore(String name) {
return new RedissonSemaphore(commandExecutor, name);
}
...
}
public class RedissonSemaphore extends RedissonExpirable implements RSemaphore {
private final SemaphorePubSub semaphorePubSub;
final CommandAsyncExecutor commandExecutor;
public RedissonSemaphore(CommandAsyncExecutor commandExecutor, String name) {
super(commandExecutor, name);
this.commandExecutor = commandExecutor;
this.semaphorePubSub = commandExecutor.getConnectionManager().getSubscribeService().getSemaphorePubSub();
}
...
}(2)Semaphore设置允许获取的锁数量
public class RedissonSemaphore extends RedissonExpirable implements RSemaphore {
...
@Override
public boolean trySetPermits(int permits) {
return get(trySetPermitsAsync(permits));
}
@Override
public RFuture<Boolean> trySetPermitsAsync(int permits) {
RFuture<Boolean> future = commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
//执行命令"get semaphore"，获取到当前的数值
"local value = redis.call('get', KEYS[1]); " +
"if (value == false) then " +
//然后执行命令"set semaphore 3"
//设置这个信号量允许客户端同时获取锁的总数量为3
"redis.call('set', KEYS[1], ARGV[1]); " +
"redis.call('publish', KEYS[2], ARGV[1]); " +
"return 1;" +
"end;" +
"return 0;",
Arrays.asList(getRawName(), getChannelName()),
permits
);
if (log.isDebugEnabled()) {
future.onComplete((r, e) -> {
if (r) {
log.debug("permits set, permits: {}, name: {}", permits, getName());
} else {
log.debug("unable to set permits, permits: {}, name: {}", permits, getName());
}
});
}
return future;
}
...
}首先执行命令"get semaphore"，获取到当前的数值。然后执行命令"set semaphore 3"，也就是设置这个信号量允许客户端同时获取锁的总数量为3。
(3)客户端尝试获取Semaphore的锁
public class RedissonSemaphore extends RedissonExpirable implements RSemaphore {
...
private final SemaphorePubSub semaphorePubSub;
final CommandAsyncExecutor commandExecutor;
public RedissonSemaphore(CommandAsyncExecutor commandExecutor, String name) {
super(commandExecutor, name);
this.commandExecutor = commandExecutor;
this.semaphorePubSub = commandExecutor.getConnectionManager().getSubscribeService().getSemaphorePubSub();
}
@Override
public void acquire() throws InterruptedException {
acquire(1);
}
@Override
public void acquire(int permits) throws InterruptedException {
if (tryAcquire(permits)) {
return;
}
CompletableFuture<RedissonLockEntry> future = subscribe();
commandExecutor.syncSubscriptionInterrupted(future);
try {
while (true) {
if (tryAcquire(permits)) {
return;
}
//获取Redisson的Semaphore失败，于是便调用本地JDK的Semaphore的acquire()方法，此时当前线程会被阻塞
//之后如果Redisson的Semaphore释放了锁，那么当前客户端便会通过监听订阅事件释放本地JDK的Semaphore，唤醒被阻塞的线程，继续执行while循环
//注意：getLatch()返回的是JDK的Semaphore = "new Semaphore(0)" ==> (state - permits)
//首先调用CommandAsyncService.getNow()方法
//然后调用RedissonLockEntry.getLatch()方法
//接着调用JDK的Semaphore的acquire()方法
commandExecutor.getNow(future).getLatch().acquire();
}
} finally {
unsubscribe(commandExecutor.getNow(future));
}
}
@Override
public boolean tryAcquire(int permits) {
//异步转同步
return get(tryAcquireAsync(permits));
}
@Override
public RFuture<Boolean> tryAcquireAsync(int permits) {
if (permits < 0) {
throw new IllegalArgumentException("Permits amount can't be negative");
}
if (permits == 0) {
return RedissonPromise.newSucceededFuture(true);
}
return commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
//执行命令"get semaphore"，获取到当前值
"local value = redis.call('get', KEYS[1]); "+
//如果semaphore的当前值不是false，且大于客户端线程申请获取锁的数量
"if (value ~= false and tonumber(value) >= tonumber(ARGV[1])) then " +
//执行"decrby semaphore 1"，将信号量允许获取锁的总数量递减1
"local val = redis.call('decrby', KEYS[1], ARGV[1]); " +
"return 1; " +
"end; " +
//如果semaphore的值变为0，那么客户端就无法获取锁了，此时返回false
"return 0;",
Collections.<Object>singletonList(getRawName()),
permits//ARGV[1]默认是1
);
}
...
}
public class CommandAsyncService implements CommandAsyncExecutor {
...
@Override
public <V> V getNow(CompletableFuture<V> future) {
try {
return future.getNow(null);
} catch (Exception e) {
return null;
}
}
...
}
public class RedissonLockEntry implements PubSubEntry<RedissonLockEntry> {
private final Semaphore latch;
...
public RedissonLockEntry(CompletableFuture<RedissonLockEntry> promise) {
super();
this.latch = new Semaphore(0);
this.promise = promise;
}
public Semaphore getLatch() {
return latch;
}
...
}执行命令"get semaphore"，获取到semaphore的当前值。如果semaphore的当前值不是false，且大于客户端线程申请获取锁的数量。那么就执行"decrby semaphore 1"，将信号量允许获取锁的总数量递减1。
如果semaphore的值变为0，那么客户端就无法获取锁了，此时tryAcquire()方法返回false。表示获取semaphore的锁失败了，于是当前客户端线程便会通过本地JDK的Semaphore进行阻塞。
当客户端后续收到一个订阅事件把本地JDK的Semaphore进行释放后，便会唤醒阻塞线程继续while循环。在while循环中，会不断尝试获取这个semaphore的锁，如此循环往复，直到成功获取。
(4)客户端释放Semaphore的锁
public class RedissonSemaphore extends RedissonExpirable implements RSemaphore {
...
@Override
public void release() {
release(1);
}
@Override
public void release(int permits) {
get(releaseAsync(permits));
}
@Override
public RFuture<Void> releaseAsync(int permits) {
if (permits < 0) {
throw new IllegalArgumentException("Permits amount can't be negative");
}
if (permits == 0) {
return RedissonPromise.newSucceededFuture(null);
}
RFuture<Void> future = commandExecutor.evalWriteAsync(getRawName(), StringCodec.INSTANCE, RedisCommands.EVAL_VOID,
//执行命令"incrby semaphore 1"
"local value = redis.call('incrby', KEYS[1], ARGV[1]); " +
"redis.call('publish', KEYS[2], value); ",
Arrays.asList(getRawName(), getChannelName()),
permits
);
if (log.isDebugEnabled()) {
future.onComplete((o, e) -> {
if (e == null) {
log.debug("released, permits: {}, name: {}", permits, getName());
}
});
}
return future;
}
...
}
//订阅semaphore不为0的事件，semaphore不为0时会触发执行这里的监听回调
public class SemaphorePubSub extends PublishSubscribe<RedissonLockEntry> {
public SemaphorePubSub(PublishSubscribeService service) {
super(service);
}
@Override
protected RedissonLockEntry createEntry(CompletableFuture<RedissonLockEntry> newPromise) {
return new RedissonLockEntry(newPromise);
}
@Override
protected void onMessage(RedissonLockEntry value, Long message) {
Runnable runnableToExecute = value.getListeners().poll();
if (runnableToExecute != null) {
runnableToExecute.run();
}
//将客户端本地JDK的Semaphore进行释放
value.getLatch().release(Math.min(value.acquired(), message.intValue()));
}
}
//订阅锁被释放的事件，锁被释放为0时会触发执行这里的监听回调
public class LockPubSub extends PublishSubscribe<RedissonLockEntry> {
public static final Long UNLOCK_MESSAGE = 0L;
public static final Long READ_UNLOCK_MESSAGE = 1L;
public LockPubSub(PublishSubscribeService service) {
super(service);
}
@Override
protected RedissonLockEntry createEntry(CompletableFuture<RedissonLockEntry> newPromise) {
return new RedissonLockEntry(newPromise);
}
@Override
protected void onMessage(RedissonLockEntry value, Long message) {
if (message.equals(UNLOCK_MESSAGE)) {
Runnable runnableToExecute = value.getListeners().poll();
if (runnableToExecute != null) {
runnableToExecute.run();
}
value.getLatch().release();
} else if (message.equals(READ_UNLOCK_MESSAGE)) {
while (true) {
Runnable runnableToExecute = value.getListeners().poll();
if (runnableToExecute == null) {
break;
}
runnableToExecute.run();
}
//将客户端本地JDK的Semaphore进行释放
value.getLatch().release(value.getLatch().getQueueLength());
}
}
}客户端释放Semaphore的锁时，会执行命令"incrby semaphore 1"。每当客户端释放掉permits个锁，就会将信号量的值累加permits，这样Semaphore信号量的值就不再是0了。然后通过publish命令发布一个事件，之后订阅了该事件的其他客户端都会对getLatch()返回的本地JDK的Semaphore进行加1。于是其他客户端正在被本地JDK的Semaphore进行阻塞的线程，就会被唤醒继续执行。此时其他客户端就可以尝试获取到这个信号量的锁，然后再次将这个Semaphore的值递减1。
4.Redisson的CountDownLatch简介
(1)Redisson的CountDownLatch原理图解
CountDownLatch的基本原理：要求必须有n个线程来进行countDown，才能让执行await的线程继续执行。如果没有达到指定数量的线程来countDown，会导致执行await的线程阻塞。
(2)Redisson的CountDownLatch使用演示
public class RedissonDemo {
public static void main(String[] args) throws Exception {
//连接3主3从的Redis CLuster
Config config = new Config();
...
//CountDownLatch
final RedissonClient redisson = Redisson.create(config);
RCountDownLatch latch = redisson.getCountDownLatch("myCountDownLatch");
//1.设置可以countDown的数量为3
latch.trySetCount(3);
System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]设置了必须有3个线程执行countDown，进入等待中。。。");
for (int i = 0; i < 3; i++) {
new Thread(new Runnable() {
public void run() {
try {
System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]在做一些操作，请耐心等待。。。。。。");
Thread.sleep(3000);
RCountDownLatch localLatch = redisson.getCountDownLatch("myCountDownLatch");
localLatch.countDown();
System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]执行countDown操作");
} catch (Exception e) {
e.printStackTrace();
}
}
}).start();
}
latch.await();
System.out.println(new Date() + "：线程[" + Thread.currentThread().getName() + "]收到通知，有3个线程都执行了countDown操作，可以继续往下执行");
}
}5.Redisson的CountDownLatch源码剖析
(1)CountDownLatch的初始
public class Redisson implements RedissonClient {
//Redis的连接管理器，封装了一个Config实例
protected final ConnectionManager connectionManager;
//Redis的命令执行器，封装了一个ConnectionManager实例
protected final CommandAsyncExecutor commandExecutor;
...
protected Redisson(Config config) {
this.config = config;
Config configCopy = new Config(config);
//初始化Redis的连接管理器
connectionManager = ConfigSupport.createConnectionManager(configCopy);
...
//初始化Redis的命令执行器
commandExecutor = new CommandSyncService(connectionManager, objectBuilder);
...
}
@Override
public RCountDownLatch getCountDownLatch(String name) {
return new RedissonCountDownLatch(commandExecutor, name);
}
...
}
public class RedissonCountDownLatch extends RedissonObject implements RCountDownLatch {
...
private final CountDownLatchPubSub pubSub;
private final String id;
protected RedissonCountDownLatch(CommandAsyncExecutor commandExecutor, String name) {
super(commandExecutor, name);
this.id = commandExecutor.getConnectionManager().getId();
this.pubSub = commandExecutor.getConnectionManager().getSubscribeService().getCountDownLatchPubSub();
}
...
}(2)trySetCount()方法设置countDown的数量
trySetCount()方法的工作就是执行命令"set myCountDownLatch 3"。
public class RedissonCountDownLatch extends RedissonObject implements RCountDownLatch {
...
@Override
public boolean trySetCount(long count) {
return get(trySetCountAsync(count));
}
@Override
public RFuture<Boolean> trySetCountAsync(long count) {
return commandExecutor.evalWriteAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
"if redis.call('exists', KEYS[1]) == 0 then " +
"redis.call('set', KEYS[1], ARGV[2]); " +
"redis.call('publish', KEYS[2], ARGV[1]); " +
"return 1 " +
"else " +
"return 0 " +
"end",
Arrays.asList(getRawName(), getChannelName()),
CountDownLatchPubSub.NEW_COUNT_MESSAGE,
count
);
}
...
}(3)awati()方法进行阻塞等待
public class RedissonCountDownLatch extends RedissonObject implements RCountDownLatch {
...
@Override
public void await() throws InterruptedException {
if (getCount() == 0) {
return;
}
CompletableFuture<RedissonCountDownLatchEntry> future = subscribe();
try {
commandExecutor.syncSubscriptionInterrupted(future);
while (getCount() > 0) {
// waiting for open state
//获取countDown的数量还大于0，就先阻塞线程，然后再等待唤醒，执行while循环
//其中getLatch()返回的是JDK的semaphore = "new Semaphore(0)" ==> (state - permits)
commandExecutor.getNow(future).getLatch().await();
}
} finally {
unsubscribe(commandExecutor.getNow(future));
}
}
@Override
public long getCount() {
return get(getCountAsync());
}
@Override
public RFuture<Long> getCountAsync() {
//执行命令"get myCountDownLatch"
return commandExecutor.writeAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.GET_LONG, getRawName());
}
...
}在while循环中，首先会执行命令"get myCountDownLatch"去获取countDown值。如果该值不大于0，就退出循环不阻塞线程。如果该值大于0，则说明还没有指定数量的线程去执行countDown操作，于是就会先阻塞线程，然后再等待唤醒来继续循环。
(4)countDown()方法对countDown的数量递减
public class RedissonCountDownLatch extends RedissonObject implements RCountDownLatch {
...
@Override
public void countDown() {
get(countDownAsync());
}
@Override
public RFuture<Void> countDownAsync() {
return commandExecutor.evalWriteNoRetryAsync(getRawName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
"local v = redis.call('decr', KEYS[1]);" +
"if v <= 0 then redis.call('del', KEYS[1]) end;" +
"if v == 0 then redis.call('publish', KEYS[2], ARGV[1]) end;",
Arrays.<Object>asList(getRawName(), getChannelName()),
CountDownLatchPubSub.ZERO_COUNT_MESSAGE
);
}
...
}countDownAsync()方法会执行decr命令，将countDown的数量进行递减1。如果这个值已经小于等于0，就执行del命令删除掉该CoutDownLatch。如果是这个值为0，还会发布一条消息：
publish redisson_countdownlatch__channel__{anyCountDownLatch} 0文章转载自：东阳马生架构
原文链接：分布式锁—6.Redisson的同步器组件 - 东阳马生架构 - 博客园
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