思维导图
概述
前面介绍了虚拟机的内存分配和回收，大致有了一些理论基础，接下来我们从实践的角度去了解虚拟机内存管理。定位问题的时候，知识、经验是关键基础，数据是依据，工具是运用知识处理数据的手段。
这里我们说的数据主要包括
- 运行日志
- 异常堆栈
- GC日志
- 线程快照(thread dump / javacore文件)
- 堆转储快照(heapdump/hprof文件)等
命令行工具
Java安装目录 bin目录下有很多官方提供的命令行程序，今天我们俩了解下虚拟机性能监控与故障处理工具
jps 虚拟机进程状况工具
概述
Java Virtual Machine Process Status Tool
不仅名字像unix主机的ps命令，功能也相似，可以列举出正在运行的虚拟机进程，并显示虚拟机执行主类（Main Class,main函数所在的类）名称以及这些进程的本地虚拟机唯一ID（Local Virtual Machine Identifier ，LVMID）。.
使用频率最高的JDK命令行工具，以为内其他的JDK工具大多需要输入它查询到LVMID来确定要监控的是哪一个虚拟机进程。 对于本地虚拟机进程来讲，LVMID和操作系统的进程ID(Process Identifer ,PID) 一致。
语法及使用
usage: jps [-help] jps [-q] [-mlvV] [<hostid>] Definitions: <hostid>: <hostname>[:<port>]
jps可以通过RMI协议查看开启了RMI服务的远程虚拟机进程状态。，hostid为RMI注册表中注册的主机名。
jps的其他常用选项
| 选项 | 作用 |
| -q | 只输出LVMID，省略主类的名称 |
| -m | 输出虚拟机进程启动时传递给主类main函数的参数 |
| -l | 输出主类的全名，如果执行的是jar包，输出jar路径 |
| -v | 输出虚拟机进程启动时jvm参数 |
jstat 虚拟机统计信息监视工具
概述
jstat (JVM Statistics Monitoring Tool)用于监视虚拟机各种运行状态信息的命令行工具。 它可以显示本地或者远程虚拟机进程中的类装载、内存、垃圾会搜、JIT编译等运行数据。
在没有图形化界面的服务器上，它将是运行期定位虚拟机性能问题的首选工具
语法及使用
Usage: jstat -help|-options jstat -<option> [-t] [-h<lines>] <vmid> [<interval> [<count>]] Definitions: <option> An option reported by the -options option <vmid> Virtual Machine Identifier. A vmid takes the following form: <lvmid>[@<hostname>[:<port>]] Where <lvmid> is the local vm identifier for the target Java virtual machine, typically a process id; <hostname> is the name of the host running the target Java virtual machine; and <port> is the port number for the rmiregistry on the target host. See the jvmstat documentation for a more complete description of the Virtual Machine Identifier. <lines> Number of samples between header lines. <interval> Sampling interval. The following forms are allowed: <n>["ms"|"s"] Where <n> is an integer and the suffix specifies the units as milliseconds("ms") or seconds("s"). The default units are "ms". <count> Number of samples to take before terminating. -J<flag> Pass <flag> directly to the runtime system.
- 对于命令格式中的VMID和LVMID，如果是本地虚拟机进程，两者一致。如果是远程虚拟机进程，则VMID的格式为
[protocol:][//]lvmind[@hostname[:port]/servername] - interval : 查询间隔
- count：查询次数 如果省略interval和count，则只查询一次
假设需要250毫秒查询一次进程3245的垃圾收集状况，一共查询20次
jstat -gc 3245 250 20
- 选项option代表用户希望查询的虚拟机信息，主要分为3类：类装载、垃圾收集、运行期编译状况。
jstat的主要选项
使用举例
E:\Program Files\Java\jdk1.8.0_161\bin>jstat -gcutil 12888 1000 10 Warning: Unresolved Symbol: sun.gc.metaspace.capacity substituted NaN Warning: Unresolved Symbol: sun.gc.metaspace.used substituted NaN Warning: Unresolved Symbol: sun.gc.metaspace.capacity substituted NaN Warning: Unresolved Symbol: sun.gc.compressedclassspace.capacity substituted NaN Warning: Unresolved Symbol: sun.gc.compressedclassspace.used substituted NaN Warning: Unresolved Symbol: sun.gc.compressedclassspace.capacity substituted NaN S0 S1 E O M CCS YGC YGCT FGC FGCT GCT 0.00 91.47 46.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215 0.00 91.47 48.39 9.12 - - 9 0.215 0 0.000 0.215
- S0: Survivor0是空的
- S1:Survivor1使用了91.47的空间
- E：Eden 使用了 48.39%的空间
- O: 老年代使用了 9.12%的空间
- M： 因为我这里是使用了JDK8 ，没有了永久代的概念，这里是元数据区
- YGC: Minor GC表示程序启动以来，发生了9次Minor GC
- YGCT：Young GC Time 耗时为0.215秒
- FGC：Full GC 表示程序启动以来，发生了0次Minor GC
- FGCT：Full GC Time 耗时为0秒
- GCT：表示GC Time ，所有的GC总耗时 0.215秒
纯文本状态下监视虚拟机的变化确实不如后面提到的VisualVM等可视化的建施工局直接用图形的方式展现直观，但直接在主机上使用jstat命令仍然是一种常用的监控方式。
jinfo Java配置信息工具
概述
语法及使用
Usage: jinfo [option] <pid> (to connect to running process) jinfo [option] <executable <core> (to connect to a core file) jinfo [option] [server_id@]<remote server IP or hostname> (to connect to remote debug server) where <option> is one of: -flag <name> to print the value of the named VM flag -flag [+|-]<name> to enable or disable the named VM flag -flag <name>=<value> to set the named VM flag to the given value -flags to print VM flags -sysprops to print Java system properties <no option> to print both of the above -h | -help to print this help message
使用举例
E:\Program Files\Java\jdk1.8.0_161\bin>jinfo -flag MaxPermSize 12888 -XX:MaxPermSize=85983232
jmap Java 内存映射工具
概述
语法及使用
Usage: jmap [option] <pid> (to connect to running process) jmap [option] <executable <core> (to connect to a core file) jmap [option] [server_id@]<remote server IP or hostname> (to connect to remote debug server) where <option> is one of: <none> to print same info as Solaris pmap -heap to print java heap summary -histo[:live] to print histogram of java object heap; if the "live" suboption is specified, only count live objects -clstats to print class loader statistics -finalizerinfo to print information on objects awaiting finalization -dump:<dump-options> to dump java heap in hprof binary format dump-options: live dump only live objects; if not specified, all objects in the heap are dumped. format=b binary format file=<file> dump heap to <file> Example: jmap -dump:live,format=b,file=heap.bin <pid> -F force. Use with -dump:<dump-options> <pid> or -histo to force a heap dump or histogram when <pid> does not respond. The "live" suboption is not supported in this mode. -h | -help to print this help message -J<flag> to pass <flag> directly to the runtime system
jmap的主要选项
使用举例， 对pid=12888的进程输出dump信息
E:\Program Files\Java\jdk1.8.0_161\bin>jmap -dump:format=b,file=test.bin 12888 Dumping heap to E:\Program Files\Java\jdk1.8.0_161\bin\test.bin ... Heap dump file created
jhat 虚拟机堆转储快照分析工具
概述
语法及使用
Usage: jhat [-stack <bool>] [-refs <bool>] [-port <port>] [-baseline <file>] [-debug <int>] [-version] [-h|-help] <file> -J<flag> Pass <flag> directly to the runtime system. For example, -J-mx512m to use a maximum heap size of 512MB -stack false: Turn off tracking object allocation call stack. -refs false: Turn off tracking of references to objects -port <port>: Set the port for the HTTP server. Defaults to 7000 -exclude <file>: Specify a file that lists data members that should be excluded from the reachableFrom query. -baseline <file>: Specify a baseline object dump. Objects in both heap dumps with the same ID and same class will be marked as not being "new". -debug <int>: Set debug level. 0: No debug output 1: Debug hprof file parsing 2: Debug hprof file parsing, no server -version Report version number -h|-help Print this help and exit <file> The file to read For a dump file that contains multiple heap dumps, you may specify which dump in the file by appending "#<number>" to the file name, i.e. "foo.hprof#3". All boolean options default to "true"
用法举例
E:\Program Files\Java\jdk1.8.0_161\bin>jmap -dump:format=b,file=test.bin 12888 Dumping heap to E:\Program Files\Java\jdk1.8.0_161\bin\test.bin ... File exists E:\Program Files\Java\jdk1.8.0_161\bin>jhat test.bin Reading from test.bin... Dump file created Mon Aug 06 20:49:25 CST 2018 Snapshot read, resolving... Resolving 244224 objects... Chasing references, expect 48 dots................................................ Eliminating duplicate references................................................ Snapshot resolved. Started HTTP server on port 7000 Server is ready.
浏览器输入 http://localhost:7000/
jstack Java堆栈跟踪工具
概述
语法及使用
Usage: jstack [-l] <pid> (to connect to running process) jstack -F [-m] [-l] <pid> (to connect to a hung process) jstack [-m] [-l] <executable> <core> (to connect to a core file) jstack [-m] [-l] [server_id@]<remote server IP or hostname> (to connect to a remote debug server) Options: -F to force a thread dump. Use when jstack <pid> does not respond (process is hung) -m to print both java and native frames (mixed mode) -l long listing. Prints additional information about locks -h or -help to print this help message
jstack的主要选项
| 选项 | 作用 |
| -F | 当正常输出的请求不被响应时，强制输出线程堆栈 |
| -m | 如果调用到本地方法的话，可以显示C/C++的堆栈 |
| -l | 除了堆栈信息，显示关于锁的附件信息 |
用法举例
E:\Program Files\Java\jdk1.8.0_161\bin>jstack -l 12888 2018-08-06 21:01:10 Full thread dump Java HotSpot(TM) 64-Bit Server VM (24.51-b03 mixed mode): "DestroyJavaVM" prio=6 tid=0x000000000e1ab800 nid=0x2f80 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE Locked ownable synchronizers: - None "DefaultQuartzScheduler_QuartzSchedulerThread" prio=6 tid=0x000000000e1a9800 nid=0xb5c in Object.wait() [0x000000000f59f000] java.lang.Thread.State: TIMED_WAITING (on object monitor) at java.lang.Object.wait(Native Method) at org.quartz.core.QuartzSchedulerThread.run(QuartzSchedulerThread.java:410) - locked <0x0000000762395018> (a java.lang.Object) Locked ownable synchronizers: - None ...... ....... ..........
hsdis:jit生成代码反汇编
Sun官方推荐的HotSpot虚拟机JIT编译代码的反汇编插件。了解即可。
