java線程阻塞問題排查方法

  我開發的worker，每隔幾個月線上都會阻塞一次，一直都沒查出問題。今天終于了了這個心結。把解決過程總結下和大家分享。

    首先用jstack指令打出這個程序的全部線程堆棧。拿到線程dump檔案之後，搜尋自己的worker名字。

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

    很快便找到了線程在哪一行被阻塞。但是就憑這麼點資訊，并不能查出問題的真正原因，這裡推薦一個工具，叫tda.bat。同僚給我的，網上應該有下載下傳。把這個dump檔案導入到tda中。找到阻塞的線程。阻塞的線程是紅色的。

   之是以說這個軟體好，是因為當你找到blocked的線程後，界面的下方，會打出阻塞的更詳細的線程堆棧。截取這個堆棧的部分資訊。

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

    這個才是真正有用的堆棧！它告訴了我程式是在執行SQL的時候，SQL發生死鎖，于是線程被阻塞。它還提供了更有用的資訊，那就是到底是哪個SQL導緻的死鎖。堆棧的倒數第三行訓示了導緻死鎖的SQL。

    但是一定要用這個工具才能找到具體的原因嗎？答案當然是NO！

    告訴大家怎麼不通過工具找到阻塞的真正原因！

    剛剛通過“BLOCKED”關鍵字搜到了線程堆棧，找到它的線程名“DefaultQuartzScheduler_Worker-10”。OK，然後，把最後的10改成1，也就是“DefaultQuartzScheduler_Worker-1”，然後再拿這個關鍵字搜尋整個程序堆棧。

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