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<Map.Entry<Object, Object>> 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<Object, Object> 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<Entry<HK, HV>> scan(K key, ScanOptions options) {
byte[] rawKey = rawKey(key);
return template.executeWithStickyConnection(
(RedisCallback<Cursor<Entry<HK, HV>>>) connection -> new ConvertingCursor<>(connection.hScan(rawKey, options),
new Converter<Entry<byte[], byte[]>, Entry<HK, HV>>() {
@Override
public Entry<HK, HV> convert(final Entry<byte[], byte[]> source) {
return new Entry<HK, HV>() {
@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> T execute(RedisCallback<T> 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<Map.Entry<Object, Object>> 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<Object, Object> 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.<init>(PrintWriter.java:116)
at java.io.PrintWriter.<init>(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。
以上这篇redis scan命令导致redis连接耗尽,线程上锁的解决就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持。
