Jedis returnResource使用注意事项

在线上环境发现了一个工作线程异常终止，看日志先是一些SocketTimeoutException，然后突然有一个ClassCastException

redis.clients.jedis.exceptions.JedisConnectionException: java.net.SocketTimeoutException: Read timed out...java.lang.ClassCastException: [B cannot be cast to java.lang.Long        at redis.clients.jedis.Connection.getIntegerReply(Connection.java:208)        at redis.clients.jedis.Jedis.sismember(Jedis.java:1307)

经过在本地人工模拟网络异常的情境，最终复现了线上的这一异常。又经过深入分析（提出假设-->验证假设），最终找出了导致这一问题的原因。见如下示例代码：

JedisPool pool = ...;Jedis jedis = pool.getResource();String value = jedis.get("foo");System.out.println("Make SocketTimeoutException");System.in.read(); //等待制造SocketTimeoutExceptiontry {    value = jedis.get("foo");    System.out.println(value);} catch (JedisConnectionException e) {    e.printStackTrace();}System.out.println("Recover from SocketTimeoutException");System.in.read();  //等待恢复Thread.sleep(5000); // 继续休眠一段时间 等待网络完全恢复boolean isMember = jedis.sismember("urls", "baidu.com");

以及日志输出：

barMake SocketTimeoutExceptionredis.clients.jedis.exceptions.JedisConnectionException: java.net.SocketTimeoutException: Read timed outRecover from SocketTimeoutException    at redis.clients.util.RedisInputStream.ensureFill(RedisInputStream.java:210)    at redis.clients.util.RedisInputStream.readByte(RedisInputStream.java:47)    at redis.clients.jedis.Protocol.process(Protocol.java:131)    at redis.clients.jedis.Protocol.read(Protocol.java:196)    at redis.clients.jedis.Connection.readProtocolWithCheckingBroken(Connection.java:283)    at redis.clients.jedis.Connection.getBinaryBulkReply(Connection.java:202)    at redis.clients.jedis.Connection.getBulkReply(Connection.java:191)    at redis.clients.jedis.Jedis.get(Jedis.java:101)    at com.tcl.recipevideohunter.JedisTest.main(JedisTest.java:23)Caused by: java.net.SocketTimeoutException: Read timed out    at java.net.SocketInputStream.socketRead0(Native Method)    at java.net.SocketInputStream.read(SocketInputStream.java:152)    at java.net.SocketInputStream.read(SocketInputStream.java:122)    at java.net.SocketInputStream.read(SocketInputStream.java:108)    at redis.clients.util.RedisInputStream.ensureFill(RedisInputStream.java:204)    ... 8 moreException in thread "main" java.lang.ClassCastException: [B cannot be cast to java.lang.Long    at redis.clients.jedis.Connection.getIntegerReply(Connection.java:208)    at redis.clients.jedis.Jedis.sismember(Jedis.java:1307)    at com.tcl.recipevideohunter.JedisTest.main(JedisTest.java:32)

分析：

等执行第二遍的get("foo")时，网络超时，并未实际发送 get foo 命令，等执行sismember时，网络已恢复正常，并且是同一个jedis实例，于是将之前的get foo命令（已在输出流缓存中）一并发送。

执行顺序如下所示：

127.0.0.1:9379> get foo"bar"127.0.0.1:9379> sismember urls baidu.com(integer) 1

故在上述示例代码中最后的sismember得到的结果是get foo的结果，即一个字符串，而sismember需要的是一个Long型，故导致了ClassCastException。

为什么线上会出现这一问题呢？原因是其执行redis的逻辑类似这样：

while(true){        Jedis jedis = null;    try {        jedis = pool.getResource();        //some redis operation here.    } catch (Exception e) {       logger.error(e);    } finally {        pool.returnResource(jedis);    }}

因若是网络异常的话，pool.returnResource(jedis)仍能成功执行，即能将其返回到池中（这时jedis并不为空）。等网络恢复后，并是多线程环境，导致后续其他某个线程获得了同一个Jedis实例（pool.getResource()），

若该线程中的jedis操作返回类型与该jedis实例在网络异常期间第一条未执行成功的jedis操作的返回类型不匹配（如一个是get，一个是sismember），则就会出现ClassCastException异常。

这还算幸运的，若返回的是同一类型的话（如lpop("queue_order_pay_failed"),lpop("queue_order_pay_success")）,那我真不敢想象。

如在上述示例代码中的sismember前插入一get("nonexist-key")(redis中不存在该key,即应该返回空).

value = jedis.get("nonexist-key");System.out.println(value);boolean isMember = jedis.sismember("urls", "baidu.com");System.out.println(isMember);

实际的日志输出为:

barException in thread "main" java.lang.NullPointerException    at redis.clients.jedis.Jedis.sismember(Jedis.java:1307)    at com.tcl.recipevideohunter.JedisTest.main(JedisTest.java:37)

分析:

get("nonexist-key")得到是之前的get("foo")的结果, 而sismember得到的是get("nonexist-key")的结果,而get("nonexist-key")返回为空,于是这时是报空指针异常了.

解决方法：不能不管什么情况都一律使用returnResource。更健壮可靠以及优雅的处理方式如下所示：

while(true){    Jedis jedis = null;    boolean broken = false;    try {        jedis = jedisPool.getResource();        return jedisAction.action(jedis); //模板方法    } catch (JedisException e) {        broken = handleJedisException(e);        throw e;    } finally {        closeResource(jedis, broken);    }}/** * Handle jedisException, write log and return whether the connection is broken. */protected boolean handleJedisException(JedisException jedisException) {    if (jedisException instanceof JedisConnectionException) {        logger.error("Redis connection " + jedisPool.getAddress() + " lost.", jedisException);    } else if (jedisException instanceof JedisDataException) {        if ((jedisException.getMessage() != null) && (jedisException.getMessage().indexOf("READONLY") != -1)) {            logger.error("Redis connection " + jedisPool.getAddress() + " are read-only slave.", jedisException);        } else {            // dataException, isBroken=false            return false;        }    } else {        logger.error("Jedis exception happen.", jedisException);    }    return true;}/** * Return jedis connection to the pool, call different return methods depends on the conectionBroken status. */protected void closeResource(Jedis jedis, boolean conectionBroken) {    try {        if (conectionBroken) {            jedisPool.returnBrokenResource(jedis);        } else {            jedisPool.returnResource(jedis);        }    } catch (Exception e) {        logger.error("return back jedis failed, will fore close the jedis.", e);        JedisUtils.destroyJedis(jedis);    }}

该段代码来自：https://github.com/springside/springside4/blob/master/modules/redis/src/main/java/org/springside/modules/nosql/redis/JedisTemplate.java

补充：

Ubuntu本地模拟访问redis网络超时：

sudo iptables -A INPUT -p tcp --dport 6379 -j DROP

恢复网络：

sudo iptables -F

补充：

若jedis操作逻辑类似下面所示的话，

Jedis jedis = null;try {    jedis = jedisSentinelPool.getResource();    return jedis.get(key);}catch(JedisConnectionException e) {    jedisSentinelPool.returnBrokenResource(jedis);    logger.error("", e);    throw e;}catch (Exception e) {    logger.error("", e);    throw e;}finally {    jedisSentinelPool.returnResource(jedis);}

若一旦发生了JedisConnectionException，如网络异常，会先执行returnBrokenResource，这时jedis已被destroy了。然后进入了finally，再一次执行returnResource，这时会报错：

redis.clients.jedis.exceptions.JedisException: Could not return the resource to the pool    at redis.clients.util.Pool.returnResourceObject(Pool.java:65)    at redis.clients.jedis.JedisSentinelPool.returnResource(JedisSentinelPool.java:221)

临时解决方法：

jedisSentinelPool.returnBrokenResource(jedis);jedis=null; //这时不会实际执行returnResource中的相关动作了

但不建议这样处理，更严谨的释放资源方法见前文所述。