今天学习了《深入理解java虚拟机》的故障处理工具，都是一些小命令，不复杂，只不过有些命令在prod环境不要使用，jmap这种，在动辄几G的内存的pod中，生成一次文件可能就导致了环境停止，这时如果还有大量请求进来，这个pod估计在生成dump文件之前就崩溃了。
jps命令查看java进程
buxuesongdeMacBook-Pro:~ buxuesong$ jps -l
1121 /Users/buxuesong/Documents/git_code/account/account-api/target/account-api-1.3.0-SNAPSHOT.jar
1131 sun.tools.jps.Jps
buxuesongdeMacBook-Pro:~ buxuesong$ ps -ef|grep 1121
501 1121 1120 0 11:01下午 ttys000 0:24.41 /usr/bin/java -jar /Users/buxuesong/Documents/git_code/account/account-api/target/account-api-1.3.0-SNAPSHOT.jar
501 1138 1125 0 11:02下午 ttys001 0:00.00 grep 1121
buxuesongdeMacBook-Pro:~ buxuesong$
jstat命令用于监视虚拟机各种运行状态信息
buxuesongdeMacBook-Pro:~ buxuesong$ jstat -gcutil 1121
S0 S1 E O M CCS YGC YGCT FGC FGCT GCT
0.00 96.78 63.52 23.52 95.12 92.69 11 0.101 2 0.211 0.311
//1121命令下，2个Survivor区（S0、S1，表示Survivor0、Survivor1）,S0为空，S1占用96.78%，新生代Eden区（E，表示Eden）使用了63.52%
//老年代（O，表示Old）和永久代（M）则分别使用了23.52%和95.12%的空间
//CCS，压缩使用比例92.69%
//程序运行以来共发生Minor GC（YGC，表示Young GC）11次，总耗时0.101秒；
//发生Full GC（FGC，表示Full GC）2次，总耗时（FGCT，表示Full GCTime）为0.211秒；所有GC总耗时（GCT，表示GC Time）为0.311秒。
jinfo命令实时查看和调整虚拟机各项参数
buxuesongdeMacBook-Pro:~ buxuesong$ jinfo -flag CMSInitiatingOccupancyFraction 1835
-XX:CMSInitiatingOccupancyFraction=-1
jmap（Memory Map for Java）命令用于生成堆转储快照（一般称为heapdump或dump文件）
buxuesongdeMacBook-Pro:~ buxuesong$ jmap -dump:format=b,file=account.dump 1868
Dumping heap to /Users/buxuesong/account.dump ...
Heap dump file created
jhat（JVM Heap Analysis Tool）命令与jmap搭配使用，来分析jmap生成的堆转储快照。运行完毕后通过http://localhost:7000来访问查看dump内容
buxuesongdeMacBook-Pro:~ buxuesong$ jhat account.dump
Reading from account.dump...
Dump file created Wed Nov 25 23:37:23 CST 2020
Snapshot read, resolving...
Resolving 402521 objects...
Chasing references, expect 80 dots................................................................................
Eliminating duplicate references................................................................................
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.
jstack（Stack Trace for Java）命令用于生成虚拟机当前时刻的线程快照（一般称为threaddump或者javacore文件）
buxuesongdeMacBook-Pro:~ buxuesong$ jstack -l 1939
2020-11-25 23:47:43
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.201-b09 mixed mode):
"Attach Listener" #34 daemon prio=9 os_prio=31 tid=0x00007fbe7a496800 nid=0x9d03 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"Druid-ConnectionPool-Destroy-205735257" #33 daemon prio=5 os_prio=31 tid=0x00007fbe77f6f800 nid=0x9f03 waiting on condition [0x0000700012133000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at com.alibaba.druid.pool.DruidDataSource$DestroyConnectionThread.run(DruidDataSource.java:2032)
Locked ownable synchronizers:
- None
"Druid-ConnectionPool-Create-205735257" #32 daemon prio=5 os_prio=31 tid=0x00007fbe77a01800 nid=0xa003 waiting on condition [0x0000700012030000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00000007aa5174d0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:1948)
Locked ownable synchronizers:
- None
"mysql-cj-abandoned-connection-cleanup" #31 daemon prio=5 os_prio=31 tid=0x00007fbe7a3cf000 nid=0x6703 in Object.wait() [0x0000700011f2d000]
java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000797d2a1f0> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
- locked <0x0000000797d2a1f0> (a java.lang.ref.ReferenceQueue$Lock)
at com.mysql.cj.jdbc.AbandonedConnectionCleanupThread.run(AbandonedConnectionCleanupThread.java:85)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Locked ownable synchronizers:
- <0x0000000797d2cce0> (a java.util.concurrent.ThreadPoolExecutor$Worker)
"DestroyJavaVM" #30 prio=5 os_prio=31 tid=0x00007fbe7c87f800 nid=0x1003 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
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