java线程阻塞问题排查方法

    我开发的worker，每隔几个月线上都会阻塞一次，一直都没查出问题。今天终于了了这个心结。把解决过程总结下和大家分享。

    首先用jstack命令打出这个进程的全部线程堆栈。拿到线程dump文件之后，搜索自己的worker名字。

"DefaultQuartzScheduler_Worker-10" prio=10 tid=0x00007f55cd54d800 nid=0x3e2e waiting for monitor entry [0x00007f51ab8f7000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at com.jd.chat.worker.service.impl.NewPopAccountSyncServiceImpl.addAccounts(NewPopAccountSyncServiceImpl.java:86)
	- waiting to lock <0x0000000782359268> (a com.jd.chat.worker.service.impl.NewPopAccountSyncServiceImpl)
	at com.jd.chat.worker.service.timer.AccountIncSyncTimer.run(AccountIncSyncTimer.java:114)
	at com.jd.chat.worker.service.timer.AbstractTimer.start(AbstractTimer.java:44)
	at com.jd.chat.worker.service.timer.AbstractTimer.doJob(AbstractTimer.java:49)
	at com.jd.chat.worker.web.context.StartAppListener$TimerJob.execute(StartAppListener.java:188)
	at org.quartz.core.JobRunShell.run(JobRunShell.java:202)
	at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:573)
	- locked <0x0000000783641c68> (a java.lang.Object)

    很快便找到了线程在哪一行被阻塞。但是就凭这么点信息，并不能查出问题的真正原因，这里推荐一个工具，叫tda.bat。同事给我的，网上应该有下载。把这个dump文件导入到tda中。找到阻塞的线程。阻塞的线程是红色的。

   之所以说这个软件好，是因为当你找到blocked的线程后，界面的下方，会打出阻塞的更详细的线程堆栈。截取这个堆栈的部分信息。

at org.mariadb.jdbc.MySQLPreparedStatement.execute(MySQLPreparedStatement.java:141)
	at org.apache.commons.dbcp.DelegatingPreparedStatement.execute(DelegatingPreparedStatement.java:172)
	at org.apache.commons.dbcp.DelegatingPreparedStatement.execute(DelegatingPreparedStatement.java:172)
	at com.ibatis.sqlmap.engine.execution.SqlExecutor.executeUpdate(SqlExecutor.java:80)
	at com.ibatis.sqlmap.engine.mapping.statement.MappedStatement.sqlExecuteUpdate(MappedStatement.java:216)
	at com.ibatis.sqlmap.engine.mapping.statement.MappedStatement.executeUpdate(MappedStatement.java:94)
	at com.ibatis.sqlmap.engine.impl.SqlMapExecutorDelegate.update(SqlMapExecutorDelegate.java:457)
	at com.ibatis.sqlmap.engine.impl.SqlMapSessionImpl.update(SqlMapSessionImpl.java:90)
	at org.springframework.orm.ibatis.SqlMapClientTemplate$9.doInSqlMapClient(SqlMapClientTemplate.java:380)
	at org.springframework.orm.ibatis.SqlMapClientTemplate$9.doInSqlMapClient(SqlMapClientTemplate.java:1)
	at org.springframework.orm.ibatis.SqlMapClientTemplate.execute(SqlMapClientTemplate.java:200)
	at org.springframework.orm.ibatis.SqlMapClientTemplate.update(SqlMapClientTemplate.java:378)
	at com.jd.im.data.dataresource.ImSqlMapClientTemplate.retriedWithoutAnyInterventionUpdate(ImSqlMapClientTemplate.java:169)
	at com.jd.im.data.dataresource.ImSqlMapClientTemplate.update(ImSqlMapClientTemplate.java:137)
	at com.jd.chat.dao.impl.WriteDaoImpl.update(WriteDaoImpl.java:21)
	at com.jd.chat.zone.service.impl.GroupServiceImpl.updateRoute(GroupServiceImpl.java:766)
	at com.jd.chat.worker.service.impl.NewPopAccountSyncServiceImpl.addAccounts(NewPopAccountSyncServiceImpl.java:267)
	- locked <0x0000000782359268> (a com.jd.chat.worker.service.impl.NewPopAccountSyncServiceImpl)

    这个才是真正有用的堆栈！它告诉了我程序是在执行SQL的时候，SQL发生死锁，于是线程被阻塞。它还提供了更有用的信息，那就是到底是哪个SQL导致的死锁。堆栈的倒数第三行指示了导致死锁的SQL。

    但是一定要用这个工具才能找到具体的原因吗？答案当然是NO！

    告诉大家怎么不通过工具找到阻塞的真正原因！

    刚刚通过“BLOCKED”关键字搜到了线程堆栈，找到它的线程名“DefaultQuartzScheduler_Worker-10”。OK，然后，把最后的10改成1，也就是“DefaultQuartzScheduler_Worker-1”，然后再拿这个关键字搜索整个进程堆栈。

"DefaultQuartzScheduler_Worker-1" prio=10 tid=0x00007f55cd2aa000 nid=0x3e25 runnable [0x00007f51b02c0000]
   java.lang.Thread.State: RUNNABLE
	at java.net.SocketInputStream.socketRead0(Native Method)
	at java.net.SocketInputStream.read(SocketInputStream.java:129)
	at java.io.BufferedInputStream.fill(BufferedInputStream.java:218)
	at java.io.BufferedInputStream.read1(BufferedInputStream.java:258)
	at java.io.BufferedInputStream.read(BufferedInputStream.java:317)
	- locked <0x0000000791370d50> (a java.io.BufferedInputStream)
	at org.mariadb.jdbc.internal.common.packet.buffer.ReadUtil.readFully(ReadUtil.java:82)
	at org.mariadb.jdbc.internal.common.packet.buffer.ReadUtil.readFully(ReadUtil.java:92)
	at org.mariadb.jdbc.internal.common.packet.RawPacket.nextPacket(RawPacket.java:77)
	at org.mariadb.jdbc.internal.common.packet.SyncPacketFetcher.getRawPacket(SyncPacketFetcher.java:67)
	at org.mariadb.jdbc.internal.mysql.MySQLProtocol.getResult(MySQLProtocol.java:891)
	at org.mariadb.jdbc.internal.mysql.MySQLProtocol.executeQuery(MySQLProtocol.java:982)
	at org.mariadb.jdbc.MySQLStatement.execute(MySQLStatement.java:280)
	- locked <0x0000000791370678> (a org.mariadb.jdbc.internal.mysql.MySQLProtocol)
	at org.mariadb.jdbc.MySQLPreparedStatement.execute(MySQLPreparedStatement.java:141)

               贴出这个进程堆栈的一部分。这个进程堆栈其实也就是刚刚tda软件界面下方展示的导致线程阻塞的真正的堆栈！这个线程是runnable状态的，可惜mysql是锁死的。也就是说阻塞在了mysql里。

    感觉这是一个由张三的命案牵出李四的命案的故事。