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public Object testRedisWatch() { try { stringRedisTemplate.execute(new SessionCallback() { @Override public Object execute(RedisOperations operations) throws DataAccessException { operations.watch("testRedisWatch"); operations.multi(); operations.opsForValue().set("testRedisWatch", "0"); try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } Object rs = operations.exec(); System.out.println(rs); return rs; } }); } catch (Exception e) { e.printStackTrace(); } return null; } 初始值：
127.0.0.1:6389> get testRedisWatch
"initial"
考虑两种情况：
A
代码执行：
operations.watch("testRedisWatch"); operations.multi(); operations.opsForValue().set("testRedisWatch", "0"); try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } 代码执行：
Object rs = operations.exec();
输出：
[]此时的值为：
127.0.0.1:6389> get testRedisWatch
"0"
表明watch与exec之间没有其它客户端改变值的情况下，rs输出非空对象
B
代码执行：
operations.watch("testRedisWatch"); operations.multi(); operations.opsForValue().set("testRedisWatch", "0"); try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } 客户端执行： 127.0.0.1:6389> set testRedisWatch another
OK
代码执行：
Object rs = operations.exec();
输出：
null此时的最终值为：
127.0.0.1:6389> get testRedisWatch
"another"
代码中的
operations.opsForValue().set("testRedisWatch", "0");失效了 表明watch与exec之间有其它客户端改变值的情况下，rs输出空对象，改值失败
参考：
本文主要讲述如何在java里头使用redis进行cas操作。其实呢，redis不像memcached那样显示地支持cas操作，不过它有事务的概念。
Redis通过使用WATCH, MULTI, and EXEC组成的事务来实现乐观锁(注意没有用DISCARD),Redis事务没有回滚操作。在SpringDataRedis当中通过RedisTemplate的SessionCallback中来支持(否则事务不生效)。discard的话不需要自己代码处理，callback返回null，成的话，返回非null，依据这个来判断事务是否成功(没有抛异常)。
@Test public void cas() throws InterruptedException, ExecutionException { String key = "test-cas-1"; ValueOperations strOps = redisTemplate.opsForValue(); strOps.set(key, "hello"); ExecutorService pool = Executors.newCachedThreadPool(); List 输出
set:hello2 rs:nullset:hello3 rs:[]set:hello1 rs:nullset:hello4 rs:nullset:hello0 rs:null
我：其中1个拿到了锁，其余4个挂了
查看该值
127.0.0.1:6379> get test-cas-1"\"hello3\""
没有在SessionCallback里头执行watch、multi、exec，而是自己单独写
template.setEnableTransactionSupport(true);
这个应该是支持数据库的事务成功才执行的意思。
/** * Gets a Redis connection. Is aware of and will return any existing corresponding connections bound to the current * thread, for example when using a transaction manager. Will create a new Connection otherwise, if * { @code allowCreate} is true. * * @param factory connection factory for creating the connection * @param allowCreate whether a new (unbound) connection should be created when no connection can be found for the * current thread * @param bind binds the connection to the thread, in case one was created * @param enableTransactionSupport * @return an active Redis connection */ public static RedisConnection doGetConnection(RedisConnectionFactory factory, boolean allowCreate, boolean bind, boolean enableTransactionSupport) { Assert.notNull(factory, "No RedisConnectionFactory specified"); RedisConnectionHolder connHolder = (RedisConnectionHolder) TransactionSynchronizationManager.getResource(factory); if (connHolder != null) { if (enableTransactionSupport) { potentiallyRegisterTransactionSynchronisation(connHolder, factory); } return connHolder.getConnection(); } if (!allowCreate) { throw new IllegalArgumentException("No connection found and allowCreate = false"); } if (log.isDebugEnabled()) { log.debug("Opening RedisConnection"); } RedisConnection conn = factory.getConnection(); if (bind) { RedisConnection connectionToBind = conn; if (enableTransactionSupport && isActualNonReadonlyTransactionActive()) { connectionToBind = createConnectionProxy(conn, factory); } connHolder = new RedisConnectionHolder(connectionToBind); TransactionSynchronizationManager.bindResource(factory, connHolder); if (enableTransactionSupport) { potentiallyRegisterTransactionSynchronisation(connHolder, factory); } return connHolder.getConnection(); } return conn; } 不要跟本文的乐观锁说的事务混淆在一起。
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