redis scan命令导致redis连接耗尽,线程上锁的解决

使用redis scan方法无法获取connection，导致线程锁死。

0、关键字

redis

springboot

redistemplate

scan

try-with-resource

1、异常现象

应用部署后，功能正常使用，但约数小时左右，部分功能接口异常，接口请求无响应。

2、异常排查

查看堆栈信息，jstask pid。首先找到java进程pid；输出堆栈信息至log文件，jstask 30 > stask.log，看到与redis相关的日志，线程状态为waiting。

"pool-13-thread-6" prio=10 tid=0x00007f754800e800 nid=0x71b5 waiting on condition [0x00007f758f0ee000]
  java.lang.Thread.State: WAITING (parking)
  at sun.misc.Unsafe.park(Native Method)
  - parking to wait for <0x0000000779b75f40> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
  at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
  at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2043)
  at org.apache.commons.pool2.impl.LinkedBlockingDeque.takeFirst(LinkedBlockingDeque.java:583)
  at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:442)
  at org.apache.commons.pool2.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:363)
  at redis.clients.util.Pool.getResource(Pool.java:49)
  at redis.clients.jedis.JedisPool.getResource(JedisPool.java:99)
  at org.reborndb.reborn.RoundRobinJedisPool.getResource(RoundRobinJedisPool.java:300)
  at com.le.smartconnect.adapter.spring.RebornConnectionFactory.getConnection(RebornConnectionFactory.java:43)
  at org.springframework.data.redis.core.RedisConnectionUtils.doGetConnection(RedisConnectionUtils.java:128)
  at org.springframework.data.redis.core.RedisConnectionUtils.getConnection(RedisConnectionUtils.java:91)
  at org.springframework.data.redis.core.RedisConnectionUtils.getConnection(RedisConnectionUtils.java:78)
  at xxx.run(xxx.java:80)
  at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
  at java.util.concurrent.FutureTask.run(FutureTask.java:262)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)

  Locked ownable synchronizers:
- <0x000000074f529b08> (a java.util.concurrent.ThreadPoolExecutor$Worker)

也就是说，redis连接获取不到，线程一直在等待可用的redis连接。大概率是应用中有功能模块获取到连接，并没有释放。找到一个功能使用了scan，具体如下：

public void releaseCallbackMessage() throws Exception {
  Cursor> cursor = RedisCacheUtils.scan(key) 

  if (cursor == null) {
    logger.info("通过scan(H key, ScanOptions options)方法获取匹配键值对记录为空");
    return;
  }
  while (cursor.hasNext()) {
    // 遍历缓存
    Map.Entry entry = cursor.next();
    String key = String.valueOf(entry.getKey());
    }
  }
}

查看scan源码，发现其使用过程中，并未主动释放connection，而get/set操作均会主动释放connection

public Cursor> scan(K key, ScanOptions options) {

  byte[] rawKey = rawKey(key);
  return template.executeWithStickyConnection(
     (RedisCallback>>) connection -> new ConvertingCursor<>(connection.hScan(rawKey, options),
        new Converter, Entry>() {

         @Override
         public Entry convert(final Entry source) {

           return new Entry() {

            @Override
            public HK getKey() {
              return deserializeHashKey(source.getKey());
            }

            @Override
            public HV getValue() {
              return deserializeHashValue(source.getValue());
            }

            @Override
            public HV setValue(HV value) {
              throw new UnsupportedOperationException("Values cannot be set when scanning through entries.");
            }
           };
         }
        }));
}

get操作源码finally中有releaseConnection操作。

@Nullable
public  T execute(RedisCallback action, boolean exposeConnection, boolean pipeline) {

  Assert.isTrue(initialized, "template not initialized; call afterPropertiesSet() before using it");
  Assert.notNull(action, "Callback object must not be null");

  RedisConnectionFactory factory = getRequiredConnectionFactory();
  RedisConnection conn = null;
  try {

   if (enableTransactionSupport) {
     // only bind resources in case of potential transaction synchronization
     conn = RedisConnectionUtils.bindConnection(factory, enableTransactionSupport);
   } else {
     conn = RedisConnectionUtils.getConnection(factory);
   }

   boolean existingConnection = TransactionSynchronizationManager.hasResource(factory);

   RedisConnection connToUse = preProcessConnection(conn, existingConnection);

   boolean pipelineStatus = connToUse.isPipelined();
   if (pipeline && !pipelineStatus) {
     connToUse.openPipeline();
   }

   RedisConnection connToExpose = (exposeConnection ? connToUse : createRedisConnectionProxy(connToUse));
   T result = action.doInRedis(connToExpose);

   // close pipeline
   if (pipeline && !pipelineStatus) {
     connToUse.closePipeline();
   }

   // TODO: any other connection processing?
   return postProcessResult(result, connToUse, existingConnection);
  } finally {
   RedisConnectionUtils.releaseConnection(conn, factory);
  }
}

3、解决方式

scan操作后，主动关闭游标，使用try(resource) catch(exception)方式编码。

1、redis scan操作记住需要主动关闭cursor，即cursor.close;

2、加强规范编码;

try (Cursor> cursor = RedisCacheUtils.scan(key)) {

  if (cursor == null) {
    logger.info("通过scan(H key, ScanOptions options)方法获取匹配键值对记录为空");
    return;
  }

  while (cursor.hasNext()) {
    // 遍历缓存
    Map.Entry entry = cursor.next();
    String key = String.valueOf(entry.getKey());
  }
} catch (Exception ex) {
  logger.info(ex.toString());
}

关于 try-with-resources用法需要提一点的就是，resources对象必须是实现了 java.lang.AutoCloseable接口，才会自动关闭对象。

补充知识：redis连接未释放，导致redis连接池满，从而应用服务不可用的问题定位和解决

版本提交测试验收后，跑了几天，今天测试突然跑来说平台不可用。

1. 我先是试图登录平台，发现首页可以进入，但是登录不成功。很显然是后台的问题。

2. 再看MQ中，发现消息堆积在队列中，未被消费掉，同时一点一点变化，说明很有可能是哪里有内存或连接的泄露或未释放。

3. 接着登录阿里云账号，查看redis监控，发现连接数已经达到9000多。

4. 查看日志发现大量的redis连接拒绝错误

redis.clients.jedis.exceptions.JedisConnectionException: Could not get a resource from the pool
at redis.clients.util.Pool.getResource(Pool.java:42)
at redis.clients.jedis.JedisPool.getResource(JedisPool.java:84)
at com.***(**.java:58)
at com.***(**.java:86)
at com.***(**.java:27)
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
at org.apache.log4j.Category.callAppenders(Category.java:206)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.log(Category.java:856)
at org.slf4j.impl.Log4jLoggerAdapter.error(Log4jLoggerAdapter.java:571)
at com.***(**.java:61)
at com.***(**.java:86)
at com.***(**.java:27)
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
at org.apache.log4j.Category.callAppenders(Category.java:206)
at org.apache.log4j.Category.forcedLog(Category.java:391)
at org.apache.log4j.Category.log(Category.java:856)
at org.slf4j.impl.Log4jLoggerAdapter.error(Log4jLoggerAdapter.java:571)
at com.***(**.java:61)
at com.***(**.java:86)
at com.***(**.java:27)
at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)

5. 当然后台的tomcat中也报了其他错误，比如：

Exception in thread "Thread-18" java.lang.StackOverflowError
    at java.util.Hashtable.get(Hashtable.java:367)
    at java.util.Properties.getProperty(Properties.java:969)
    at java.lang.System.getProperty(System.java:720)
    at sun.security.action.GetPropertyAction.run(GetPropertyAction.java:86)
    at sun.security.action.GetPropertyAction.run(GetPropertyAction.java:52)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.io.PrintWriter.(PrintWriter.java:116)
    at java.io.PrintWriter.(PrintWriter.java:100)
    at org.apache.log4j.DefaultThrowableRenderer.render(DefaultThrowableRenderer.java:58)
    at org.apache.log4j.spi.ThrowableInformation.getThrowableStrRep(ThrowableInformation.java:87)
    at com.aliyun.openservices.log.log4j.LoghubAppender.append(LoghubAppender.java:116)
    at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
    at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
    at org.apache.log4j.Category.callAppenders(Category.java:206)
    at org.apache.log4j.Category.forcedLog(Category.java:391)
    at org.apache.log4j.Category.log(Category.java:856)
    at org.slf4j.impl.Log4jLoggerAdapter.error(Log4jLoggerAdapter.java:571)
    at com.***(**.java:61)
    at com.***(**.java:86)
    at com.***(**.java:27)
    at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
    at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
    at org.apache.log4j.Category.callAppenders(Category.java:206)
    at org.apache.log4j.Category.forcedLog(Category.java:391)
    at org.apache.log4j.Category.log(Category.java:856)

6. 但是，还是继续查看日志，发现一直有个 [TaskId]PUPHVUNVJSSMKOTQPKHRSPOMUKKOKLPG [Message]null [Result]0 debug日志，因为只有一直不停的发，且连接不关闭才可能出现这么多连接的情况。一边对应代码，发现是页面上调用后台代码，发送给设备长连接的代码。

try{
  for(...)
  {  
    jedis = RedisManagerPool.getJedis();
    if(!"NULL".equals(value)) {
      break;
    }
  RedisUtils.return(jedis);
  }
} catch() {
  logger.error();
  RedisUtils.returnBroken(jedis);
} 
return value;

且try中没有finally块，很显然如果条件满足的话就直接break，并return value。但是RedisUtils.return(jedis)这条语句就未执行。然后进一步怀疑页面是否是定时去获取，通过F12，发现每10s钟请求一次，页面上要获取设备的上下行速率。所以会累积这么多的请求。最后，修改也比较简单，添加finally块，保证RedisUtils.return(jedis)必定会执行。

8. 接下来在另一个开发环境继续复现，我们将redisManagerPool中设置maxTotal=300，maxIdle=30。而原先是3000、300，这样有利于快速复现。

果然，一上午时间就达到了300的限制。出现了一样的问题。

9. 综上，问题定位清楚，且修复该问题。

a) 对于oss redis之类的第三方网络连接，必须要有finally块执行。否则后续很容易由于不规范的编码，出现这种连接未正常释放的问题。

b) 定位问题，还是需要有日志。如果单从代码去查，方向会比较多且很容易浪费时间。

c) 修改池大小，缩短复现时间，快速定位修改。

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