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A Practical Guide to jstat on JDK 8

2025-10-23

Overview

jstat is a small but very useful JDK tool for checking how JVM memory is being used, especially heap regions and class-loading activity. In JDK 1.8, it can also show different garbage collection statistics, metaspace information, and JIT compilation behavior.

The basic syntax is:

jstat [-命令选项] [vmid] [间隔时间/默认单位是ms毫秒] [查询次数]

One important note before using the examples below: the command output and field meanings here are based on JDK 1.8.

Checking the available syntax and options

If you want to see the complete command format, run jstat -help:

[root@root ~]# jstat -help
Usage: jstat -help|-options
       jstat - [-t] [-h]  [ []]

Definitions:
        An option reported by the -options option
          Virtual Machine Identifier. A vmid takes the following form:
                     [@[:]]
                Where  is the local vm identifier for the target
                Java virtual machine, typically a process id;  is
                the name of the host running the target Java virtual machine;
                and  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.
         Number of samples between header lines.
      Sampling interval. The following forms are allowed:
                    ["ms"|"s"]
                Where  is an integer and the suffix specifies the units as
                milliseconds("ms") or seconds("s"). The default units are "ms".
         Number of samples to take before terminating.
  -J      Pass  directly to the runtime system.

To see which option values are available, use jstat -options:

[root@infra2-test-k8s ~]# jstat -options
-class
-compiler
-gc
-gccapacity
-gccause
-gcmetacapacity
-gcnew
-gcnewcapacity
-gcold
-gcoldcapacity
-gcutil
-printcompilation

These options correspond to different JVM views:

  • -class: statistics for class loading behavior.
  • -compiler: statistics for the HotSpot JIT compiler.
  • -gc: overall garbage-collected heap statistics.
  • -gccapacity: capacities of the young generation, old generation, metaspace, and related spaces.
  • -gccause: similar to -gcutil, but also includes the cause of the most recent GC and the current GC when applicable.
  • -gcmetacapacity: metaspace size statistics.
  • -gcnew: new generation behavior statistics.
  • -gcnewcapacity: size statistics for the new generation and its spaces.
  • -gcold: old generation and metaspace behavior statistics.
  • -gcoldcapacity: old generation size statistics.
  • -gcutil: utilization percentages across memory regions plus GC summary data.
  • -printcompilation: HotSpot method compilation statistics.

First, find the target JVM process

To make the examples more direct, you can use jps to list Java processes and their process IDs:

[root@root ~]# jps -l
1 org.apache.catalina.startup.Bootstrap
17546 sun.tools.jps.Jps

In this case, the target JVM process ID is 1.

The jps command format is:

jps命令格式：jps [options ] [ hostid ]

[options]选项 ： -q：仅输出VM标识符，不包括classname,jar name,arguments in main method -m：输出main method的参数 -l：输出完全的包名，应用主类名，jar的完全路径名 -v：输出jvm参数 -V：输出通过flag文件传递到JVM中的参数(.hotspotrc文件或-XX:Flags=所指定的文件 -Joption：传递参数到vm,例如:-J-Xms512m

Once you have the PID, you can pass it to jstat.

jstat -class: class loading statistics

[root@root ~]# jstat -class 1
Loaded  Bytes  Unloaded  Bytes     Time
 24301 45978.0      277   411.9      44.91

Field meanings:

  • Loaded: number of classes loaded.
  • Bytes: total size of loaded classes, in KB.
  • Unloaded: number of classes unloaded.
  • Bytes: total size of unloaded classes, in KB.
  • Time: time spent on class loading and unloading.

jstat -compiler: JIT compiler statistics

[root@root ~]# jstat -compiler 1
Compiled Failed Invalid   Time   FailedType FailedMethod
   31073      3       0   182.63          1 com/mysql/jdbc/AbandonedConnectionCleanupThread run

Field meanings:

  • Compiled: number of compilation tasks completed.
  • Failed: number of compilation tasks that failed.
  • Invalid: number of compilation tasks invalidated.
  • Time: total compilation time.
  • FailedType: compile type of the last failed compilation.
  • FailedMethod: class and method of the last failed compilation.

jstat -gc: heap and GC behavior

This view is one of the most commonly used because it gives a broad picture of heap usage and GC activity.

[root@root ~]# jstat -gc 1
 S0C    S1C    S0U    S1U      EC       EU        OC         OU       MC     MU    CCSC   CCSU   YGC     YGCT    FGC    FGCT     GCT
17472.0 17472.0  0.0   1248.6 139776.0  7359.2   349568.0   233485.7  149632.0 144590.5 16768.0 15863.7   1119   11.856  11      2.871   14.727

Field meanings:

  • S0C: current capacity of Survivor 0, in KB.
  • S1C: current capacity of Survivor 1, in KB.
  • S0U: used space in Survivor 0, in KB.
  • S1U: used space in Survivor 1, in KB.
  • EC: current Eden capacity, in KB.
  • EU: used Eden space, in KB.
  • OC: current old generation capacity, in KB.
  • OU: used old generation space, in KB.
  • MC: metaspace capacity, in KB.
  • MU: used metaspace, in KB.
  • CCSC: compressed class space capacity, in KB.
  • CCSU: used compressed class space, in KB.
  • YGC: number of young generation GC events.
  • YGCT: total time spent in young generation GC.
  • FGC: number of Full GC events.
  • FGCT: total time spent in Full GC.
  • GCT: total GC time.

jstat -gccapacity: memory pool capacities

This option focuses on the size of each generation and space rather than current utilization.

[root@root ~]# jstat -gccapacity 1
 NGCMN    NGCMX     NGC     S0C   S1C       EC      OGCMN      OGCMX       OGC         OC       MCMN     MCMX      MC     CCSMN    CCSMX     CCSC    YGC    FGC
 10880.0 174720.0 174720.0 17472.0 17472.0 139776.0    21888.0   349568.0   349568.0   349568.0      0.0 1181696.0 149632.0      0.0 1048576.0  16768.0   1119    11

Field meanings:

  • NGCMN: minimum new generation capacity (KB).
  • NGCMX: maximum new generation capacity (KB).
  • NGC: current new generation capacity (KB).
  • S0C: current Survivor 0 capacity (KB).
  • S1C: current Survivor 1 capacity (KB).
  • EC: current Eden 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).
  • MCMN: minimum metaspace capacity (KB).
  • MCMX: maximum metaspace capacity (KB).
  • MC: current metaspace capacity (KB).
  • CCSMN: minimum compressed class space capacity (KB).
  • CCSMX: maximum compressed class space capacity (KB).
  • CCSC: current compressed class space capacity (KB).
  • YGC: number of young generation GC events.
  • FGC: number of Full GC events.

jstat -gccause: GC summary with causes

This output is similar to -gcutil, but it also tells you why the last GC happened and whether a GC is currently in progress.

[root@root ~]# jstat -gccause 1
  S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT    LGCC                 GCC
  6.67   0.00  90.76  66.79  96.66  94.62   1130   11.949    11    2.871   14.820 Allocation Failure   No GC

Field meanings:

  • S0: Survivor 0 utilization as a percentage.
  • S1: Survivor 1 utilization as a percentage.
  • E: Eden utilization as a percentage.
  • O: old generation utilization as a percentage.
  • M: metaspace utilization as a percentage.
  • CCS: compressed class space utilization as a percentage.
  • YGC: number of young generation GC events.
  • FGC: number of Full GC events.
  • LGCC: cause of the most recent GC.
  • GCC: cause of the current GC.

jstat -gcnew: new generation statistics

[root@root ~]# jstat -gcnew 1
 S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT
17472.0 17472.0    0.0 1350.4 15  15 8736.0 139776.0  73826.1   1131   11.957

Field meanings:

  • S0C: Survivor 0 capacity (KB).
  • S1C: Survivor 1 capacity (KB).
  • S0U: used Survivor 0 space (KB).
  • S1U: used Survivor 1 space (KB).
  • TT: tenuring threshold.
  • MTT: maximum tenuring threshold.
  • DSS: desired survivor size (KB).
  • EC: Eden capacity (KB).
  • EU: used Eden space (KB).
  • YGC: number of young generation GC events.
  • YGCT: total young generation GC time.

jstat -gcnewcapacity: new generation size details

[root@root ~]# jstat -gcnewcapacity 1
  NGCMN      NGCMX       NGC      S0CMX     S0C     S1CMX     S1C       ECMX        EC      YGC   FGC
   10880.0   174720.0   174720.0  17472.0  17472.0  17472.0  17472.0   139776.0   139776.0  1131    11

Field meanings:

  • NGCMN: minimum new generation capacity (KB).
  • NGCMX: maximum new generation capacity (KB).
  • NGC: current new generation capacity (KB).
  • S0CMX: maximum Survivor 0 capacity (KB).
  • S0C: current Survivor 0 capacity (KB).
  • S1CMX: maximum Survivor 1 capacity (KB).
  • S1C: current Survivor 1 capacity (KB).
  • ECMX: maximum Eden capacity (KB).
  • EC: current Eden capacity (KB).
  • YGC: number of young generation GC events.
  • FGC: number of Full GC events.

jstat -gcold: old generation and metaspace usage

[root@root ~]# jstat -gcold 1
   MC       MU      CCSC     CCSU       OC          OU       YGC    FGC    FGCT     GCT
149632.0 144638.0  16768.0  15866.4    349568.0    233488.9   1132    11    2.871   14.841

Field meanings:

  • MC: metaspace capacity (KB).
  • MU: used metaspace (KB).
  • CCSC: compressed class space capacity (KB).
  • CCSU: used compressed class space (KB).
  • OC: old generation capacity (KB).
  • OU: used old generation space (KB).
  • YGC: number of young generation GC events.
  • FGC: number of Full GC events.
  • FGCT: Full GC time.
  • GCT: total GC time.

jstat -gcoldcapacity: old generation capacity

[root@root ~]# jstat -gcoldcapacity 1
   OGCMN       OGCMX        OGC         OC       YGC   FGC    FGCT     GCT
    21888.0    349568.0    349568.0    349568.0  1132    11    2.871   14.841

Field meanings:

  • OGCMN: minimum old generation capacity (KB).
  • OGCMX: 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 GC time.
  • GCT: total GC time.

jstat -gcmetacapacity: metaspace capacity

[root@root ~]# jstat -gcmetacapacity 1
   MCMN       MCMX        MC       CCSMN      CCSMX       CCSC     YGC   FGC    FGCT     GCT
       0.0  1181696.0   149632.0        0.0  1048576.0    16768.0  1131    11    2.871   14.828

Field meanings:

  • MCMN: minimum metaspace capacity (KB).
  • MCMX: maximum metaspace capacity (KB).
  • MC: current metaspace capacity (KB).
  • CCSMN: minimum compressed class space capacity (KB).
  • CCSMX: maximum compressed class space capacity (KB).
  • CCSC: current compressed class space capacity (KB).
  • YGC: number of young generation GC events.
  • FGC: number of Full GC events.
  • FGCT: Full GC time.
  • GCT: total GC time.

jstat -gcutil: utilization summary

If you want a compact view focused on percentages and GC counts, this is often the easiest option to read.

[root@root ~]# jstat -gcutil 1
  S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT
  6.68   0.00  39.54  66.79  96.66  94.62   1132   11.970    11    2.871   14.841

Field meanings:

  • S0: Survivor 0 usage percentage.
  • S1: Survivor 1 usage percentage.
  • E: Eden usage percentage.
  • O: old generation usage percentage.
  • M: metaspace usage percentage.
  • CCS: compressed class space usage percentage.
  • YGC: young generation GC count.
  • YGCT: young generation GC time.
  • FGC: Full GC count.
  • FGCT: Full GC time.
  • GCT: total GC time.

jstat -printcompilation: recently compiled method statistics

[root@root ~]# jstat -printcompilation 1
Compiled  Size  Type Method
   31174   1055    1 sun/reflect/GeneratedMethodAccessor214 invoke

Field meanings:

  • Compiled: number of compilation tasks performed by the most recently compiled method.
  • Size: bytecode size of the most recently compiled method.
  • Type: compilation type of the most recently compiled method.
  • Method: class name and method name of the most recently compiled method. HotSpot uses / instead of . in the class name.

Sampling output repeatedly

jstat can also print statistics at intervals instead of only once. For example, -h3 tells it to reprint the header every three lines, and 1s sets the sampling interval to one second:

[root@root ~]# jstat -gcnew -h3 1 1s
 S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT
17472.0 17472.0    0.0 1359.4 15  15 8736.0 139776.0  51283.7   1133   11.980
17472.0 17472.0    0.0 1359.4 15  15 8736.0 139776.0  51283.8   1133   11.980
17472.0 17472.0    0.0 1359.4 15  15 8736.0 139776.0  51674.1   1133   11.980
 S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT
17472.0 17472.0    0.0 1359.4 15  15 8736.0 139776.0  51816.1   1133   11.980
17472.0 17472.0    0.0 1359.4 15  15 8736.0 139776.0  51824.8   1133   11.980
17472.0 17472.0    0.0 1359.4 15  15 8736.0 139776.0  51828.9   1133   11.980
 S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT
17472.0 17472.0    0.0 1359.4 15  15 8736.0 139776.0  52158.8   1133   11.980
17472.0 17472.0    0.0 1359.4 15  15 8736.0 139776.0  52312.8   1133   11.980

This kind of output is handy when you want to watch Eden growth, survivor movement, or GC counters changing over time.

Notes on reading the metrics

A few patterns are especially useful when looking at jstat output on JDK 8:

  • If E or EU rises quickly and YGC keeps increasing, the application is generating many short-lived objects.
  • If O or OU stays high and FGC increases, pressure may be moving into the old generation.
  • If M or MU is close to its limit, metaspace usage deserves attention.
  • -gccause is helpful when you want to know not just that GC happened, but what triggered it.
  • -gcutil is usually the fastest way to get an overall impression, while -gc, -gcnew, and -gcold are better when you need more detail.

For day-to-day troubleshooting, a common flow is: use jps to find the target JVM, use jstat -gcutil or jstat -gc for a quick health check, and then switch to more specific options depending on whether you are investigating young generation pressure, old generation growth, metaspace, or compilation behavior.

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