Java Technology and the JVM

Official References

Interesting Common References

Terminology

Java Programming Language

Java 是一门面向对象的编程语言并有以下特性:

  • 平台独立、跨操作系: Java 程序被编译为字节码,运行在 JVM 中,JVM 支持多种操作系统,因此一套程序可以运行在不同的操作系统上。
  • 面向对象: 封装、抽象、继承、多态
  • 自动垃圾收集: 由 JVM 进行内存管理,垃圾回收
  • 丰富的标准库: Java 提供了很多标准库如 IO、网络、日期操作等等。

Java Runtime Edition(JRE)

JRE 包含 Java Virtual Machine(JVM),Java platform core classes,和 supporting Java platform libraries。

Java Development Kit

JDK 是一组用于开发 Java 应用程序的工具(如javacjavapjavajarjps…)。使用 JDK,可以编译 Java 程序并在 JVM 中运行,还提供了用于打包和分发应用程序的一组工具。

JDK 和 JRE 共享 Java 应用程序编程接口(Java API 是开发人员用于创建 Java 应用程序的预打包库的集合。 Java API 通过提供工具类来完成许多常见的编程任务(包括字符串操作,日期/时间处理,网络和实现数据结构(例如,列表,映射,堆栈和队列)),使开发更容易。

JDK = JRE + A collection of tools for developing Java applications

Java Virtual Machine

Java SE Platform at a Glance

有帮助的截图

Java Garbage Collection

Tools and Commands Reference

Mac 各版本的 JDK 默认安装路径/Library/Java/JavaVirtualMachines//usr/libexec/java_home -V查看已安装JDK。

JVM 相关命令在/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands可以找到(当然在 JDK 的默认安装目录也能找到),有很多命令和工具。

可以使用info或者man查阅当前版本JDK Tools命令的详细介绍,例如查看java命令可以使用info java或者man java。也可以通过 Oracle 官网查看:

下面介绍一些比较有用的命令。

java

运行已经编译好的 java 程序,Java Options Details

$ java
Usage: java [-options] class [args...]
           (to execute a class)
   or  java [-options] -jar jarfile [args...]
           (to execute a jar file)

# 需要在 src/META-INF/MANIFEST.MF 指定 Main-Class: com.xxx.YourMainApp,然后打成 executable jar
# 否则会出现 Can't execute jar- file: “no main manifest attribute” 的问题
$ java -jar yourapp.jar

# 如果不是一个 executable jar 包,则可以通过下面的方式指定主类
$ java -cp .:yourapp.jar com.xxx.YourMainApp

# 为 jvm 分配可用堆内存
$ java -cp .:yourapp.jar -Xms16g -Xmx16g com.xxx.YourMainApp

# 注意 [options] 必须在 class/-jar 前面,否则会被当作[args...]
$ java -Xms16g -Xmx16g -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -jar rockfs-server-1.0-SNAPSHOT.jar

jar

压缩/解压缩 jar 文件

# 解压缩到当前目录,没有解压缩到指定文件夹的参数,可以将jar包copy到一个空文件夹中,然后解压缩
$ jar -xvf test.jar

# 压缩 
$ jar -cvf test.jar *

jps

查看虚拟机进程状况:显示指定系统内所有的 Hotspot 虚拟机进程

# usage
jps [options] [hostid]

# jps 可以通过RMI协议查询开启了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
# 常用 options;
# -q 只输出LVMID,省略主类名称
# -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,如果进程执行的是Jar包,输出Jar路径
# -v 输出虚拟机进程启动时的JVM参数

jstat

虚拟机统计信息监控工具,用于收集运行中的 Hotspot 虚拟机各方面的运行数据

$ jstat
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.

# 查看可用的 options
$ jstat -options
-class
-compiler
-gc
-gccapacity
-gccause
-gcmetacapacity
-gcnew
-gcnewcapacity
-gcold
-gcoldcapacity
-gcutil
-printcompilation

查看 pid=29732 gc 情况,interval=500ms, samples=5000。

$ jstat -gcutil 29732 500ms 5000
# 具体每列的含义可以通过`info jstat`或者`man jstat`查阅
S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT   
0.00  20.29  12.03   9.84  97.26  92.48     89    0.486     3    0.209    0.695
0.00  20.29  12.03   9.84  97.26  92.48     89    0.486     3    0.209    0.695
0.00  20.29  12.03   9.84  97.26  92.48     89    0.486     3    0.209    0.695
0.00  20.29  12.14   9.84  97.26  92.48     89    0.486     3    0.209    0.695
0.00  20.29  12.14   9.84  97.26  92.48     89    0.486     3    0.209    0.695
0.00  20.29  12.14   9.84  97.26  92.48     89    0.486     3    0.209    0.695

详细的info jstat介绍,不同操作系统不同版本的 JDK 会有差异,下面是OSX jdk1.8.0_231的信息

File: *manpages*,  Node: jstat,  Up: (dir)

jstat(1)                                                              jstat(1)

NAME
       jstat - Java Virtual Machine statistics monitoring tool

SYNOPSIS
       jstat  [ generalOption | outputOptions vmid [ interval [ s|ms ] [ count
       ] ] ]

DESCRIPTION
       The jstat tool displays  performance  statistics  for  an  instrumented
       HotSpot Java virtual machine (JVM). The target JVM is identified by its
       virtual machine identifier, or vmid option described below.

       NOTE - This utility is unsupported and may or may not be  available  in
       future versions of the J2SE SDK.  It is currently not available on Win-
       dows 98 and Windows ME platforms.

   VIRTUAL MACHINE IDENTIFIER
       The syntax of the vmid string largely corresponds to the  syntax  of  a
       URI:

              [protocol:][//]lvmid[@hostname][:port][/servername]

       protocol       The  communications protocol. If the protocol is omitted
                      and a hostname is not specified, the default protocol is
                      a  platform  specific  optimized  local protocol. If the
                      protocol is omitted and a hostname  is  specified,  then
                      the default protocol is rmi.

       lvmid          The local virtual machine identifier for the target JVM.
                      The lvmid is a  platform-specific  value  that  uniquely
                      identifies  a  JVM  on  a  system. The lvmid is the only
                      required component of a virtual machine identifier.  The
                      lvmid  is  typically, but not necessarily, the operating
                      system's process identifier for the target JVM  process.
                      You  can  use  the  jps  command to determine the lvmid.
                      Also, you can determine lvmid on Unix platforms with the
                      ps  command,  and  on Windows with the Windows Task Man-
                      ager.

       hostname       A hostname or IP address indicating the target host.  If
                      hostname  is  omitted, then the target host is the local
                      host.

       port           The default  port  for  communicating  with  the  remote
                      server. If the hostname is omitted or the protocol spec-
                      ifies  an  optimized,  local  protocol,  then  port   is
                      ignored.  Otherwise,  treatment of the port parameter is
                      implementation specific. For the default  rmi  protocol,
                      the  port  indicates the port number for the rmiregistry
                      on the remote host. If port  is  omitted,  and  protocol
                      indicates  rmi, then the default rmiregistry port (1099)
                      is used.

       servername     The treatment of this parameter depends  on  implementa-
                      tion.  For  the  optimized local protocol, this field is
                      ignored. For the rmi protocol, it represents the name of
                      the RMI remote object on the remote host.

PARAMETERS
       generalOption  A  single  general command-line option (-help, -options,
                      or -version)

       outputOptions  One or more  output  options,  consisting  of  a  single
                      statOption, plus any of the -t, -h, and -J options.

       vmid           Virtual machine identifier, a string indicating the tar-
                      get Java virtual machine (JVM). The general syntax is

              [protocol:][//]lvmid[@hostname[:port]/servername

       The syntax of the vmid string largely corresponds to the  syntax  of  a
       URI.  The  vmid can vary from a simple integer representing a local JVM
       to a more complex construction specifying  a  communications  protocol,
       port  number,  and  other  implementation-specific  values. See Virtual
       Machine Identifier below for details.

       interval[s|ms] Sampling interval in the specified units, seconds (s) or
                      milliseconds (ms). Default units are milliseconds.  Must
                      be a positive integer.  If specified, jstat will produce
                      its output at each interval.

       count          Number of samples to display. Default value is infinity;
                      that is, jstat displays statistics until the target  JVM
                      terminates  or the jstat command is terminated.  Must be
                      a positive integer.

OPTIONS
       The jstat command supports two types of options,  general  options  and
       output  options.   General  options cause jstat to display simple usage
       and version information. Output options determine the content and  for-
       mat of the statistical output.

       NOTE-  All  options,  and  their functionality are subject to change or
       removal in future releases.

   GENERAL OPTIONS
       If you specify one of the general options, you cannot specify any other
       option or parameter.

       -help          Display help message.

       -version       Display version information.

       -options       Display  list  of  statistics  options.  See  the Output
                      Options section below.

   OUTPUT OPTIONS
       If you do not specify a general option, then  you  can  specify  output
       options.   Output  options  determine the content and format of jstat's
       output, and consist of a single statOption, plus any of the other  out-
       put options (-h, -t, and -J).  The statOption must come first.

       Output  is  formatted as a table, with columns are separated by spaces.
       A header row with titles describes the columns.  Use the -h  option  to
       set  the  frequency  at  which  the header is displayed.  Column header
       names are generally consistent between the different options.  In  gen-
       eral, if two options provide a column with the same name, then the data
       source for the two columns are the same.

       Use the -t option to display a time stamp column, labeled Timestamp  as
       the  first  column of output. The Timestamp column contains the elapsed
       time, in seconds, since startup of the target JVM.  The  resolution  of
       the time stamp is dependent on various factors and is subject to varia-
       tion due to delayed thread scheduling on heavily loaded systems.

       Use the interval and count parameters to determine how  frequently  and
       how many times, respectively, jstat displays its output.

       NOTE-  You  are  advised  not  to write scripts to parse jstat's output
       since the format may change in future releases. If you choose to  write
       scripts  that  parse  jstat  output,  expect  to modify them for future
       releases of this tool.

       -statOption    Determines the statistics information  that  jstat  dis-
                      plays.  The following table lists the available options.
                      Use the -options general option to display the  list  of
                      options for a particular platform installation.

      +-----------------+------------------------------------------------------+
      |     Option      |                     Displays...                      |
      +-----------------+------------------------------------------------------+
      |class            | Statistics on the behavior of the class loader       |
      |compiler         | Statistics  on  the behavior of the HotSpot Just-In- |
      |                 | Time compiler                                        |
      |gc               | Statistics on the behavior of the garbage  collected |
      |                 | heap                                                 |
      |gccapacity       | Statistics  of the capacities of the generations and |
      |                 | their corresponding spaces.                          |
      |gccause          | Summary of garbage collection  statistics  (same  as |
      |                 | -gcutil), with the cause of the last and current (if |
      |                 | applicable) garbage collection events.               |
      |gcnew            | Statistics of the behavior of the new generation.    |
      |gcnewcapacity    | Statistics of the sizes of the new  generations  and |
      |                 | its corresponding spaces.                            |
      |gcold            | Statistics  of the behavior of the old and permanent |
      |                 | generations.                                         |
      |gcoldcapacity    | Statistics of the sizes of the old generation.       |
      |gcpermcapacity   | Statistics of the sizes of the permanent generation. |
      |gcutil           | Summary of garbage collection statistics.            |
      |printcompilation | Summary of garbage collection statistics.            |
      +-----------------+------------------------------------------------------+
       -h n           Display a column header every n samples  (output  rows),
                      where n is a positive integer. Default value is 0, which
                      displays the column header above the first row of  data.

       -t n           Display  a  timestamp column as the first column of out-
                      put. The timestamp is the the time since the start  time
                      of the target JVM.

       -JjavaOption   Pass  javaOption  to  the java application launcher. For
                      example,  -J-Xms48m  sets  the  startup  memory  to   48
                      megabytes.  For a complete list of options, see java(1).

   STATOPTIONS AND OUTPUT
       The following tables summarize the columns that jstat outputs for  each
       statOption.

       class  Class Loader Statistics

         +---------+--------------------------------------------------------+
         | Column  |                      Description                       |
         +---------+--------------------------------------------------------+
         |Loaded   | Number of classes loaded                               |
         |Bytes    | Number of Kbytes loaded                                |
         |Unloaded | Number of classes unloaded                             |
         |Bytes    | Number of Kbytes unloaded                              |
         |Time     | Time spent performing class load and unload operations |
         +---------+--------------------------------------------------------+
       compiler
              HotSpot Just-In-Time Compiler Statistics

       +-------------+-------------------------------------------------------+
       |   Column    |                      Description                      |
       +-------------+-------------------------------------------------------+
       |Compiled     | Humber of compilation tasks performed                 |
       |Failed       | Number of compilation tasks that failed               |
       |Invalid      | Number of compilation tasks that were invalidated     |
       |Time         | Time spent performing compilation tasks               |
       |FailedType   | Compile type of the last failed compilation           |
       |FailedMethod | Class name and method for the last failed compilation |
       +-------------+-------------------------------------------------------+
       gc     Garbage-collected heap statistics

                +-------+-------------------------------------------+
                |Column |                Description                |
                +-------+-------------------------------------------+
                |SOC    | Current survivor space 0 capacity (KB).   |
                |S1C    | Current survivor space 1 capacity (KB).   |
                |S0U    | Survivor space 0 utilization (KB).        |
                |S1U    | Survivor space 1 utilization (KB).        |
                |EC     | Current eden space capacity (KB).         |
                |EU     | Eden space utilization (KB).              |
                |OC     | Current old space capacity (KB).          |
                |OU     | Old space utilization (KB).               |
                |PC     | Current permanent space capacity (KB).    |
                |PU     | Permanent space utilization (KB).         |
                |YGC    | Number of young generation GC Events.     |
                |YGCT   | Young generation garbage collection time. |
                |FGC    | Number of full GC events.                 |
                |FGCT   | Full garbage collection time.             |
                |GCT    | Total garbage collection time.            |
                +-------+-------------------------------------------+
       gccapacity
              Memory Pool Generation and Space Capacities

               +-------+---------------------------------------------+
               |Column |                 Description                 |
               +-------+---------------------------------------------+
               |NGCMN  | Minimum new generation capacity (KB).       |
               |NGCMX  | Maximum new generation capacity (KB).       |
               |NGC    | Current new generation capacity (KB).       |
               |S0C    | Current survivor space 0 capacity (KB).     |
               |S1C    | Current survivor space 1 capacity (KB).     |
               |EC     | Current eden space capacity (KB).           |
               |OGCMN  | Minimum old generation capacity (KB).       |
               |OGCMX  | Maximum old generation capacity (KB).       |
               |OGC    | Current old generation capacity (KB).       |
               |OC     | Current old space capacity (KB).            |
               |PGCMN  | Minimum permanent generation capacity (KB). |
               |PGCMX  | Maximum Permanent generation capacity (KB). |
               |PGC    | Current Permanent generation capacity (KB). |
               |PC     | Current Permanent space capacity (KB).      |
               |VGC    | Number of Young generation GC Events.       |
               |FGC    | Number of Full GC Events.                   |
               +-------+---------------------------------------------+
       gccause
              This option displays the same summary of garbage collection sta-
              tistics as the -gcutil option, but includes the  causes  of  the
              last  garbage  collection  event and (if applicable) the current
              garbage collection event. In addition to the columns listed  for
              -gcutil, this option adds the following columns:

                   +-------+--------------------------------------+
                   |Column |             Description              |
                   +-------+--------------------------------------+
                   |LGCC   | Cause of last Garbage Collection.    |
                   |GCC    | Cause of current Garbage Collection. |
                   +-------+--------------------------------------+
       gcnew  New Generation Statistics

                +-------+-------------------------------------------+
                |Column |                Description                |
                +-------+-------------------------------------------+
                |SOC    | Current survivor space 0 capacity (KB).   |
                |S1C    | Current survivor space 1 capacity (KB).   |
                |S0U    | Survivor space 0 utilization (KB).        |
                |S1U    | Survivor space 1 utilization (KB).        |
                |TT     | Tenuring threshold.                       |
                |MTT    | Maximum tenuring threshold.               |
                |DSS    | Desired survivor size (KB).               |
                |EC     | Current eden space capacity (KB).         |
                |EU     | Eden space utilization (KB).              |
                |VGC    | Number of young generation GC events.     |
                |VGCT   | Young generation garbage collection time. |
                +-------+-------------------------------------------+
       gcnewcapacity
              New Generation Space Size Statistics

                 +-------+-----------------------------------------+
                 |Column |               Description               |
                 +-------+-----------------------------------------+
                 |NGCMN  | Minimum new generation capacity (KB).   |
                 |NGCMX  | Maximum new generation capacity (KB).   |
                 |NGC    | Current new generation capacity (KB).   |
                 |S0CMX  | Maximum survivor space 0 capacity (KB). |
                 |S0C    | Current survivor space 0 capacity (KB). |
                 |S1CMX  | Maximum survivor space 1 capacity (KB). |
                 |S1C    | Current survivor space 1 capacity (KB). |
                 |ECMX   | Maximum eden space capacity (KB).       |
                 |EC     | Current eden space capacity (KB).       |
                 |YGC    | Number of young generation GC events.   |
                 |FGC    | Number of Full GC Events.               |
                 +-------+-----------------------------------------+
       gcold  Old and Permanent Generation Statistics

                  +-------+----------------------------------------+
                  |Column |              Description               |
                  +-------+----------------------------------------+
                  |PC     | Current permanent space capacity (KB). |
                  |PU     | Permanent space utilization (KB).      |
                  |OC     | Current old space capacity (KB).       |
                  |OU     | Old space utilization (KB).            |
                  |YGC    | Number of young generation GC events.  |
                  |FGC    | Number of Full GC events.              |
                  |FGCT   | Full garbage collection time.          |
                  |GCT    | Total garbage collection time.         |
                  +-------+----------------------------------------+
       gcoldcapacity
              Old Generation Statistics

                  +-------+---------------------------------------+
                  |Column |              Description              |
                  +-------+---------------------------------------+
                  |OGCMN  | Minimum old generation capacity (KB). |
                  |OGCMV  | Maximum old generation capacity (KB). |
                  |OGC    | Current old generation capacity (KB). |
                  |OC     | Current old space capacity (KB).      |
                  |YGC    | Number of young generation GC events. |
                  |FGC    | Number of Full GC events.             |
                  |FGCT   | Full garbage collection time.         |
                  |GCT    | Total garbage collection time.        |
                  +-------+---------------------------------------+
       gcpermcapacity
              Permanent Generation Statistics

               +-------+---------------------------------------------+
               |Column |                 Description                 |
               +-------+---------------------------------------------+
               |PGCMN  | Minimum permanent generation capacity (KB). |
               |PGCMX  | Maximum permanent generation capacity (KB). |
               |PGC    | Current permanent generation capacity (KB). |
               |PC     | Current permanent space capacity (KB).      |
               |YGC    | Number of young generation GC events.       |
               |FGC    | Number of Full GC events.                   |
               |FGCT   | Full garbage collection time.               |
               |GCT    | Total garbage collection time.              |
               +-------+---------------------------------------------+
       gcutil Summary of Garbage Collection Statistics

                +-------+-------------------------------------------+
                |Column |                Description                |
                +-------+-------------------------------------------+
                |S0     | Survivor  space  0  utilization as a per- |
                |       | centage of the space's current  capacity. |
                |S1     | Survivor  space  1  utilization as a per- |
                |       | centage of the space's current  capacity. |
                |E      | Eden space utilization as a percentage of |
                |       | the space's current capacity.             |
                |O      | Old space utilization as a percentage  of |
                |       | the space's current capacity.             |
                |P      | Permanent space utilization as a percent- |
                |       | age of the space's current capacity.      |
                |YGC    | Number of young generation GC events.     |
                |YGCT   | Young generation garbage collection time. |
                |FGC    | Number of Full GC events.                 |
                |FGCT   | Full garbage collection time.             |
                |GCT    | Total garbage collection time.            |
                +-------+-------------------------------------------+
       printcompilation
              HotSpot Compiler Method Statistics

              +---------+---------------------------------------------+
              | Column  |                 Description                 |
              +---------+---------------------------------------------+
              |Compiled | Number of compilation tasks performed.      |
              |Size     | Number of bytes of bytecode for the method. |
              |Type     | Compilation type.                           |
              |Method   | Class name and method name identifying  the |
              |         | compiled   method.   Class  name  uses  "/" |
              |         | instead  of  "."  as  namespace  separator. |
              |         | Method  name is the method within the given |
              |         | class. The format for these two  fields  is |
              |         | consistent with the HotSpot - XX:+PrintCom- |
              |         | plation option.                             |
              +---------+---------------------------------------------+

EXAMPLES
       This section presents some examples of monitoring a local  JVM  with  a
       lvmid of 21891.

   Using the gcutil option
       This  example  attaches  to lvmid 21891 and takes 7 samples at 250 mil-
       lisecond intervals and displays the output as specified by the  -gcutil
       option.

       jstat -gcutil 21891 250 7
         S0     S1     E      O      P     YGC    YGCT    FGC    FGCT     GCT
        12.44   0.00  27.20   9.49  96.70    78    0.176     5    0.495    0.672
        12.44   0.00  62.16   9.49  96.70    78    0.176     5    0.495    0.672
        12.44   0.00  83.97   9.49  96.70    78    0.176     5    0.495    0.672
         0.00   7.74   0.00   9.51  96.70    79    0.177     5    0.495    0.673
         0.00   7.74  23.37   9.51  96.70    79    0.177     5    0.495    0.673
         0.00   7.74  43.82   9.51  96.70    79    0.177     5    0.495    0.673
         0.00   7.74  58.11   9.51  96.71    79    0.177     5    0.495    0.673

       The  output  of  this  example shows that a young generation collection
       occurred between the 3rd and 4th sample. The collection took 0.001 sec-
       onds and promoted objects from the eden space (E) to the old space (O),
       resulting in an increase of old space utilization from 9.49% to  9.51%.
       Before  the  collection,  the  survivor  space was 12.44% utilized, but
       after this collection it is only 7.74% utilized.

   Repeating the column header string
       This example attaches to lvmid 21891 and takes samples at 250 millisec-
       ond  intervals  and displays the output as specified by -gcutil option.
       In addition, it uses the -h3 option to output the column  header  after
       every 3 lines of data.

       jstat -gcnew -h3 21891 250
        S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT
         64.0   64.0    0.0   31.7 31  31   32.0    512.0    178.6    249    0.203
         64.0   64.0    0.0   31.7 31  31   32.0    512.0    355.5    249    0.203
         64.0   64.0   35.4    0.0  2  31   32.0    512.0     21.9    250    0.204
        S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT
         64.0   64.0   35.4    0.0  2  31   32.0    512.0    245.9    250    0.204
         64.0   64.0   35.4    0.0  2  31   32.0    512.0    421.1    250    0.204
         64.0   64.0    0.0   19.0 31  31   32.0    512.0     84.4    251    0.204
        S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT
         64.0   64.0    0.0   19.0 31  31   32.0    512.0    306.7    251    0.204

       In  addition to showing the repeating header string, this example shows
       that between the 2nd and 3rd samples, a young GC occurred. Its duration
       was  0.001 seconds. The collection found enough live data that the sur-
       vivor space 0 utilization (S0U) would would have exceeded  the  desired
       survivor Size (DSS). As a result, objects were promoted to the old gen-
       eration (not visible in this output), and the tenuring  threshold  (TT)
       was lowered from 31 to 2.

       Another collection occurs between the 5th and 6th samples. This collec-
       tion found very few survivors and returned the  tenuring  threshold  to
       31.

   Including a time stamp for each sample
       This  example  attaches  to lvmid 21891 and takes 3 samples at 250 mil-
       lisecond intervals. The -t option is used to generate a time stamp  for
       each sample in the first column.

       jstat -gcoldcapacity -t 21891 250 3
       Timestamp  OGCMN    OGCMX    OGC      OC       YGC   FGC   FGCT    GCT
       150.1      1408.0   60544.0  11696.0  11696.0  194   80    2.874   3.799
       150.4      1408.0   60544.0  13820.0  13820.0  194   81    2.938   3.863
       150.7      1408.0   60544.0  13820.0  13820.0  194   81    2.938   3.863

   Monitor instrumentation for a remote JVM
       This  example attaches to lvmid 40496 on the system named remote.domain
       using the -gcutil option, with samples taken every second indefinitely.

       jstat -gcutil [email protected] 1000
        ... output omitted

       The  lvmid  is combined with the name of the remote host to construct a
       vmid of [email protected] This vmid results in the use  of  the  rmi
       protocol  to  communicate  to  the  default jstatd server on the remote
       host.  The  jstatd  server  is  located  using   the   rmiregistry   on
       remote.domain  that  is  bound  to  the  default rmiregistry port (port
       1099).

SEE ALSO
       java(1) jps(1) jstatd(1) rmiregistry(1)

                                 13 June 2004                         jstat(1)

jstatd

虚拟机统计信息监控工具守护进程,可以方便的建立远程 RMI 服务器,这样就可以通过jvisualvm分析远程服务器上的 jvm 统计信息了。

# 详细的说明可以阅读下面命令返回的信息
$ info jstatd

# 首先给 jstatd 访问权限
$ vim ./jstatd.all.policy
grant codebase "file:${java.home}/../lib/tools.jar" {
           permission java.security.AllPermission;
       }; 

# 启动 jstatd,这样 jvisualvm 就可以通过 jstatd 所在的服务器 host 和 port 获取 jvm 统计信息了
$ jstatd -J-Djava.security.policy=./jstatd.all.policy -J-Djava.rmi.server.hostname=192.168.51.82 -p 2020 &

jvisualvm添加一个远程服务器,图形界面右键远程添加远程主机(H)...主机名192.168.51.82,点高级设置端口(P):改为2020,确认即可。

当使用jstatd + jvisualvm监控远程 JVM 的时候,你会发现无法获得 JVM 的 Cpu、Thread、MBean 等信息,它会提示你使用JMX连接。参考连接

JMX(Java Management Extensions)是管理 Java 的一些扩展。这种机制可以方便的管理、监控正在运行中的 Java 程序。常用于管理线程,内存,日志 Level,服务重启,系统环境等。jdk6+,Java 程序启动时都会在 JVM 内部启动一个 JMX Agent,JMX Agent 会启动一个 MBean Server 组件,把 MBeans(Java 平台标准的 MBean + 你自己创建的 MBean)注册到它里面,然后暴露给 JMX Client 管理。简单来说就是每个 Java 程序都可以通过 JMX 来被 JMX Client 管理,而且这一切都是自动发生的。而 VisualVm 就是一个JMX Client。

要启用 JMX,需要在 Java 进程启动的时候指定几个参数

  • com.sun.management.jmxremote.port: 指定 JMX 暴露的端口。
  • com.sun.management.jmxremote.rmi.port: 指定 RMI Connector 端口,可以和com.sun.management.jmxremote.port保持一致。
  • com.sun.management.jmxremote.ssl: 指定是否使用 SSL,在开发环境下可以是false,但是在生产环境下强烈建议为true
  • com.sun.management.jmxremote.authenticate: 指定是否需要密码才能够创建 JMX 连接。

启动我们要被监控的 Java 程序

$ java \
-Dcom.sun.management.jmxremote.authenticate=false \
-Dcom.sun.management.jmxremote.ssl=false \
-Dcom.sun.management.jmxremote.port=1100 \
-Dcom.sun.management.jmxremote.rmi.port=1100 \
-Djava.rmi.server.hostname=192.168.51.82 \
-XX:+PrintGCDetails \
-XX:+PrintGCDateStamps  \
-Xloggc:/var/log/rockfs/gc.log  \
-XX:+UseConcMarkSweepGC \
-XX:+UseParNewGC \
-XX:MaxTenuringThreshold=15 \
-XX:+ExplicitGCInvokesConcurrent \
-XX:+CMSParallelRemarkEnabled \
-XX:+HeapDumpOnOutOfMemoryError \
-XX:HeapDumpPath=/var/log/rockfs \
-Xmx3g \
-Xms3g \
-jar target/rockfs-server-1.0-SNAPSHOT.jar \
>rockfs.log 2>&1 &

然后在jvisualvm界面jstatd已经建立的远程服务器上右键,添加 JMX 连接...,输入192.168.51.82:1100不要求SSL连接(N)钩上,确定即可。

jinfo

Java 配置信息工具:显示虚拟机配置信息,比如给 main 方法的提交参数等

JVM 在发生 crash 的时候一般会生成两个文件hs_err_pid3742.logcore.37423742pidhs_err_pid3742.log通过 JVM 参数-XX:ErrorFile=./hs_err_pid<pid>.log配置,core.3742是 core dump 文件,它的命名规则通过/proc/sys/kernel/core_pattern的值控制,因此你生成的 core dump 文件不一定和我的命名规则一样。注意,需要保证ulimit -c unlimited才会在系统崩溃的时候生成core.xxx,默认ulimit -c0,不生成。除了系统崩溃时生成 core dump,在系统卡住或者 cpu 使用率很高的时候也可以手动触发kill -3 pid吓唬下 JVM 生成 core dump 文件。下面的<core>即指的 core dump 文件。

$ jinfo
Usage:
    jinfo [option] <pid>
        (to connect to running process)
    jinfo [option] <executable> <core>
        (to connect to a core file)
    jinfo [option] [[email protected]]<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

查看一个 core dump 的 vm flags

$ jinfo -flags $JAVA_HOME/bin/java core.3742

Attaching to core core.3742 from executable /usr/java/jdk1.8.0_181-amd64/bin/java, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.181-b13
Non-default VM flags: -XX:CICompilerCount=3 -XX:InitialHeapSize=526385152 -XX:MaxHeapSize=8392802304 -XX:MaxNewSize=2797600768 -XX:MinHeapDeltaBytes=524288 -XX:NewSize=175112192 -XX:OldSize=351272960 -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps -XX:+UseParallelGC 
Command line:  

jstack

Java 堆栈跟踪工具:显示虚拟机的线程堆栈信息

$ jstack
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] [[email protected]]<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

attach core dump 文件

$ jstack $JAVA_HOME/bin/java core.3742

Attaching to core core.3742 from executable /usr/java/jdk1.8.0_181-amd64/bin/java, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.181-b13
Deadlock Detection:

No deadlocks found.

Thread 28312: (state = BLOCKED)
 - sun.misc.Unsafe.park(boolean, long) @bci=0 (Compiled frame; information may be imprecise)
 - java.util.concurrent.locks.LockSupport.park(java.lang.Object) @bci=14, line=175 (Interpreted frame)
 - java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt() @bci=1, line=836 (Interpreted frame)
 - java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(java.util.concurrent.locks.AbstractQueuedSynchronizer$Node, int) @bci=67, line=870 (Interpreted frame)
 - java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(int) @bci=17, line=1199 (Compiled frame)
 - java.util.concurrent.locks.ReentrantLock$NonfairSync.lock() @bci=21, line=209 (Compiled frame)
 - java.util.concurrent.locks.ReentrantLock.lock() @bci=4, line=285 (Compiled frame)
 - com.rich.rockfs.server.Rockfs.getWritableRocksDB(com.rich.rockfs.beans.DBInfo) @bci=85, line=97 (Compiled frame)
 - com.rich.rockfs.server.RockServerImpl.batchPut(com.rich.rockfs.beans.MultiplePutRequest, io.grpc.stub.StreamObserver) @bci=15, line=128 (Compiled frame)
 - com.rich.rockfs.server.RocksServerGrpc$MethodHandlers.invoke(java.lang.Object, io.grpc.stub.StreamObserver) @bci=90, line=472 (Interpreted frame)
 - io.grpc.stub.ServerCalls$UnaryServerCallHandler$UnaryServerCallListener.onHalfClose() @bci=53, line=171 (Compiled frame)
 - io.grpc.internal.ServerCallImpl$ServerStreamListenerImpl.halfClosed() @bci=15, line=283 (Compiled frame)
 - io.grpc.internal.ServerImpl$JumpToApplicationThreadServerStreamListener$1HalfClosed.runInContext() @bci=7, line=761 (Compiled frame)
 - io.grpc.internal.ContextRunnable.run() @bci=9, line=37 (Compiled frame)
 - io.grpc.internal.SerializingExecutor.run() @bci=18, line=123 (Compiled frame)
 - java.util.concurrent.ThreadPoolExecutor.runWorker(java.util.concurrent.ThreadPoolExecutor$Worker) @bci=95, line=1149 (Interpreted frame)
 - java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=5, line=624 (Interpreted frame)
 - java.lang.Thread.run() @bci=11, line=748 (Interpreted frame)

...

jmap

Java 内存映像工具:生成虚拟机的内存转储快照(heapdump文件)

# 查看使用方法
$ jmap
Usage:
    jmap [option] <pid>
        (to connect to running process)
    jmap [option] <executable> <core>
        (to connect to a core file)
    jmap [option] [[email protected]]<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

查看指定进程堆内存分配情况

$ jmap -heap 8357
Attaching to process ID 8357, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.181-b13

using thread-local object allocation.
Parallel GC with 6 thread(s)

Heap Configuration:
 MinHeapFreeRatio         = 0
 MaxHeapFreeRatio         = 100
 MaxHeapSize              = 8392802304 (8004.0MB)
 NewSize                  = 175112192 (167.0MB)
 MaxNewSize               = 2797600768 (2668.0MB)
 OldSize                  = 351272960 (335.0MB)
 NewRatio                 = 2
 SurvivorRatio            = 8
 MetaspaceSize            = 21807104 (20.796875MB)
 CompressedClassSpaceSize = 1073741824 (1024.0MB)
 MaxMetaspaceSize         = 17592186044415 MB
 G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
 capacity = 132120576 (126.0MB)
 used     = 42286136 (40.32720184326172MB)
 free     = 89834440 (85.67279815673828MB)
 32.00571574862041% used
From Space:
 capacity = 21495808 (20.5MB)
 used     = 0 (0.0MB)
 free     = 21495808 (20.5MB)
 0.0% used
To Space:
 capacity = 21495808 (20.5MB)
 used     = 0 (0.0MB)
 free     = 21495808 (20.5MB)
 0.0% used
PS Old Generation
 capacity = 351272960 (335.0MB)
 used     = 0 (0.0MB)
 free     = 351272960 (335.0MB)
 0.0% used

4277 interned Strings occupying 340832 bytes.

分析 core dump

$ jmap -heap $JAVA_HOME/bin/java core.3742

从 core dump 中抽取 hprof 类型的 heap dump 文件

$ jmap -dump:format=b,file=3742.bin $JAVA_HOME/bin/java core.3742

jhat

虚拟机堆转储快照分析工具:用于分析 heap dump 文件,它会建立一个 HTTP/HTML 服务器,让用户可以在浏览器上查看分析结果

一般不会直接使用 jhat 去分析 heap dump 文件,因为它的功能比较简陋。一般都会把转储文件拷贝到其他节点,使用更强大的 JProfiler/VisualVM/MAT 进行分析。

jvisualvm

$ jvisualvm

jvisualvm默认没有安装Visual GC插件,需要手动安装,打开jvisualvm后,工具->插件->可用插件->Visual GC->安装,这样就可以以可视化的方式查看整个GC的过程

jmc

使用 Java Mission Control 监控和管理 Java 程序,它使用 JVM 的常规自适应动态收集数据,能够最小化额外的性能开销,并且消除了观察器效应的问题,JMC由客户端和一些插件组成。

# 启动
$ jmc

# 覆盖jmc.ini文件中指定的那些选项
$ jmc -vmarg [args]

# 附加到jmc.ini文件中指定的选项
$ jmc --launcher.appendVmargs -vmarg [args]

默认连接本地 JVM 应用,如果要连接远程 JVM 应用,类似jvisualvm需要保证远程 JVM 应用启用了 JMX。

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