jstat: Monitor Java heap, threads, classes

Jstat is a command line virtual machine statistics monitoring tool available as part of Java installation.

If you install java, open command line or terminal and execute the command ‘jstat’.

$jstat
invalid argument count
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.

I am going to use below application to experiment with jstat command.

App.java

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.lang.management.RuntimeMXBean;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class App {

 private static class BigObject {
  int i;
  int ii;
  int iii;
  int iiii;
  int iiiii;
  int iiiiii;
  int iiiiiii;
  int iiiiiiii;
  int iiiiiiiii;
  int iiiiiiiiii;

  public BigObject() {
   i = 1;
   ii = 11;
   iii = 111;
   iiii = 1111;
   iiiii = 11111;
   iiiiii = 111111;
   iiiiiii = 1111111;
   iiiiiiii = 11111111;
   iiiiiiiii = 111111111;
   iiiiiiiiii = 1111111111;
  }

 }

 private static void printGarbageCollectorBeansInfo() {
  System.out.println("---------------------------------------------");
  List<GarbageCollectorMXBean> beans = ManagementFactory.getGarbageCollectorMXBeans();

  for (GarbageCollectorMXBean bean : beans) {
   System.out.println("Name : " + bean.getName());
   System.out.println("Collection Count : " + bean.getCollectionCount());
   System.out.println("Collection Time : " + bean.getCollectionTime());

   String[] memoryPoolNames = bean.getMemoryPoolNames();
   for (String poolName : memoryPoolNames) {
    System.out.println(poolName);
   }

  }
  System.out.println("---------------------------------------------");
 }

 public static void main(String args[]) throws InterruptedException {

  RuntimeMXBean bean = ManagementFactory.getRuntimeMXBean();
   String jvmName = bean.getName();
   long pid = Long.valueOf(jvmName.split("@")[0]);
         System.out.println("PID  = " + pid);
         
  int counter = 0;
  
  List<BigObject> bigObjs = new ArrayList<> ();
  

  while (true) {
   BigObject obj = new BigObject();
   
   counter++;
   
   bigObjs.add(obj);
   
   //System.out.println(counter);

   if (counter % 100000 == 0) {
    bigObjs = new ArrayList<> ();
    counter = 0;
    TimeUnit.SECONDS.sleep(30);
    printGarbageCollectorBeansInfo();
   }
  }

 }
}

Run App.java

It prints the Process Id of the application

$javac App.java

$java App

PID  = 86867

You can even get the process id of java application by executing the command  ‘jps’.

Open another terminal and experiment with jstat command.

a. class : Get Statistics on the behavior of the class loader

Syntax

jstat -class <PID>

$ jstat -class 86867

Loaded  Bytes  Unloaded  Bytes     Time  

  1483  2750.5        0     0.0       0.18

Below table summarizes the columns of output.

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.

b. -compiler : Just-In-Time compiler statistics

Syntax

jstat -compiler <PID>

$ jstat -compiler 86867

Compiled Failed Invalid   Time   FailedType FailedMethod

     517      0       0     0.25          0     

Below table summarizes the columns of output.

Column	Description
Compiled	Number 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.

c. -gc : Garbage Collected heap statistics

Syntax

Jstat -gc <PID>

$ jstat -gc 86867

 S0C    S1C    S0U    S1U      EC       EU        OC         OU       MC     MU    CCSC   CCSU   YGC     YGCT    FGC    FGCT     GCT  

10752.0 10752.0 6080.9  0.0   65536.0  37294.3   175104.0     16.0    8576.0 8143.4 1152.0 974.6       2    0.009   0      0.000    0.009

Below table summarizes the columns of output.

Column	Description
S0C	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.

d. -gccapacity: Memory Pool Generation and Space Capacities

Syntax

jstat -gccapacity <PID>

$ jstat -gccapacity 86867

 NGCMN    NGCMX     NGC     S0C   S1C       EC      OGCMN      OGCMX       OGC         OC       MCMN     MCMX      MC     CCSMN    CCSMX     CCSC    YGC    FGC

 87040.0 1397760.0  87040.0 10752.0 10752.0  65536.0   175104.0  2796544.0   175104.0   175104.0      0.0 1056768.0   8576.0      0.0 1048576.0   1152.0      3     0

Below table summarizes the columns of output.

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).
YGC	Number of Young generation GC Events.
FGC	Number of Full GC Events.

e. – gcutil : Summary of garbage collection statistics

Syntax

Jstat -gcutil <PID>

$ jstat -gcutil 86867

  S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT  

 66.82   0.00  64.62   0.02  95.21  84.65      4    0.016     0    0.000    0.016

Below table summarizes the columns of output.

Column	Description
S0	Survivor space 0 utilization as a percentage of the space's current capacity.
S1	Survivor space 1 utilization as a percentage 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 percentage 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.

e. -gccause

Displays same summary as gcutil, apart from this it also includes the causes of the last garbage collection event and (if applicable) the current garbage collection event.

Syntax

Jstat -gccause <PID>

$ jstat -gccause 86867

  S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT    LGCC                 GCC                

  0.00  92.43  37.99   0.02  95.24  84.65      5    0.019     0    0.000    0.019 Allocation Failure   No GC

Below table summarizes the columns of output.

Column	Description
S0	Survivor space 0 utilization as a percentage of the space's current capacity.
S1	Survivor space 1 utilization as a percentage 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 percentage 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.
LGCC	Cause of last Garbage Collection.
GCC	Cause of current Garbage Collection.

f. -gcnew : New Generation statistics

Syntax

jstat -gcnew <PID>

$ jstat -gcnew 86867

 S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT 

7680.0 5632.0    0.0 5205.9  6  15 7680.0  64000.0  58188.8      5    0.019

Below table summarizes the columns of output.

Column	Description
S0C	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 (Old Generation) threshold.
MTT	Maximum tenuring threshold.
DSS	Desired survivor size (KB).
EC	Current eden space capacity (KB).
EU	Eden space utilization (KB).
YGC	Number of young generation GC events.
YGCT	Young generation garbage collection time.

g. -gcnewcapacity: New Generation Space Size Statistics

Synatx

jstat –gcnewcapacity <PID>

$ jstat -gcnewcapacity 86867

  NGCMN      NGCMX       NGC      S0CMX     S0C     S1CMX     S1C       ECMX        EC      YGC   FGC

   87040.0  1397760.0    78848.0 465920.0   8704.0 465920.0   1536.0  1396736.0    61440.0     7     0

Below table summarizes the columns of output.

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.

h. -gclod: Old and Permanent Generation Statistics

Syntax

jstat -gcold <PID>

$ jstat -gcold 86867

   MC       MU      CCSC     CCSU       OC          OU       YGC    FGC    FGCT     GCT  

  8576.0   8179.3   1152.0    975.2    175104.0      1668.1      8     0    0.000    0.026

Below table summarizes the columns of output.

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.

i. –gcoldcapacity: Old Generation Statistics

Syntax

jstat -gcoldcapacity <PID>

$ jstat -gcoldcapacity 86867

   OGCMN       OGCMX        OGC         OC       YGC   FGC    FGCT     GCT  

   175104.0   2796544.0    175104.0    175104.0     9     0    0.000    0.027

Below table summarizes the columns of output.

Column	Description
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 garbage collection time.
GCT	Total garbage collection time.

j. -printcompilation: HotSpot Compiler Method Statistics

Syntax

jstat -printcompilation <PID>

$ jstat -printcompilation 86867

Compiled  Size  Type Method

     642     34    1 java/net/AbstractPlainSocketImpl isClosedOrPending

Below table summarizes the columns of output.

Column	Description
Compiled	Number of compilation tasks performed by the most recently compiled method.
Size	Number of bytes of bytecode of the most recently compiled method.
Type	Compilation type of the most recently compiled method.
Method	Class name and method name identifying the most recently compiled method. Class name uses "/" instead of "." as namespace separator.

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