JSTAT(1)

🦋 jstat(1) — JDK Commands — monitor JVM statistics 🦋

okay bestie, this is the jstat page from the JDK Commands collection. It’s literally here to monitor JVM statistics, and we’re gonna read it together like a group chat. 💖

🌸 NAME 🌸

jstat — monitor JVM statistics

✨ SYNOPSIS ✨

⚠️ Note: This command is experimental and unsupported.

jstat generalOptions
jstat outputOptions [-t] [-h lines] vmid [interval [count]]

• generalOptions — a single general command-line option, periodt. See General Options.
• outputOptions — an option reported by the -options option. It’s one or more output options that consist of a single statOption, plus any of the -t, -h, and -J options. See Output Options for the jstat Command.
• -t — displays a time-stamp column as the first column of output. The time stamp is the time since the start time of the target JVM. ⏱️
• -h n — displays a column header every n samples (output rows), where n is a positive integer. The default value is 0, which displays the column header of the first row of data.
• vmid — a virtual machine identifier: a string that indicates the target JVM. See Virtual Machine Identifier.
• interval — the sampling interval in the specified units, seconds (s) or milliseconds (ms). Default units are milliseconds. This must be a positive integer. When specified, the jstat command produces its output at each interval.
• count — the number of samples to display. The default value is infinity, which causes the jstat command to display statistics until the target JVM terminates or the jstat command is terminated. This value must be a positive integer.

💅 DESCRIPTION 💅

The jstat command displays performance statistics for an instrumented Java HotSpot VM. The target JVM is identified by its virtual machine identifier, or vmid option. No cap.

The jstat command supports two types of options: general options and output options. General options cause the jstat command to display simple usage and version information. Output options determine the content and format of the statistical output.

🫶 All options and their functionality are subject to change or removal in future releases. So don’t get too attached, bestie.

🎀 GENERAL OPTIONS 🎀

If you specify one of the general options, then you can’t specify any other option or parameter. That’s the rule, periodt.

• -help — displays a help message.
• -options — displays a list of static options. See Output Options for the jstat Command.

🌈 OUTPUT OPTIONS FOR THE JSTAT COMMAND 🌈

If you don’t specify a general option, then you can specify output options. Output options determine the content and format of the jstat command’s output, and consist of a single statOption, plus any of the other output options (-h, -t, and -J). The statOption must come first. 👑

Output is formatted as a table, with columns that 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 consistent among the different options. In general, if two options provide a column with the same name, then the data source for the two columns is 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 the target JVM started. The resolution of the time stamp is dependent on various factors and is subject to variation due to delayed thread scheduling on heavily loaded systems. So if it looks a little wiggly, that’s literally why. 😭

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

⚠️ Note: Don’t write scripts to parse the jstat command’s output because the format might change in future releases. If you write scripts that parse the jstat command output, then expect to modify them for future releases of this tool. fr fr, don’t do it.

🌟 -statOption — the stats menu 🌟

• -statOption — determines the statistics information that the jstat command displays. The following lists the available options. Use the -options general option to display the list of options for a particular platform installation. See Stat Options and Output.
  • class — displays statistics about the behavior of the class loader.
  • compiler — displays statistics about the behavior of the Java HotSpot VM Just-in-Time compiler.
  • gc — displays statistics about the behavior of the garbage collected heap.
  • gccapacity — displays statistics about the capacities of the generations and their corresponding spaces.
  • gccause — displays a summary about garbage collection statistics (same as -gcutil), with the cause of the last and current (when applicable) garbage collection events.
  • gcnew — displays statistics about the behavior of the new generation.
  • gcnewcapacity — displays statistics about the sizes of the new generations and their corresponding spaces.
  • gcold — displays statistics about the behavior of the old generation and metaspace statistics.
  • gcoldcapacity — displays statistics about the sizes of the old generation.
  • gcmetacapacity — displays statistics about the sizes of the metaspace.
  • gcutil — displays a summary about garbage collection statistics.
  • printcompilation — displays Java HotSpot VM compilation method statistics.

🧚 -J — send options to Java 🧚

• -JjavaOption — passes javaOption to the Java application launcher. For example, -J-Xms48m sets the startup memory to 48 MB. For a complete list of options, see java.

✨ STAT OPTIONS AND OUTPUT ✨

okay bestie, here's the tea on what columns jstat actually spits out for each statOption. every field, every unit, no skips 💅

-class option

Class loader statistics — the whole "who's loading what" vibe.

• Loaded: Number of classes loaded.
• Bytes: Number of KB loaded.
• Unloaded: Number of classes unloaded.
• Bytes: Number of KB unloaded.
• Time: Time spent performing class loading and unloading operations.

(yes, Bytes shows up twice, that's literally how it is — one for loaded, one for unloaded 🫶)

-compiler option

Java HotSpot VM Just-in-Time compiler stats, aka the JIT doing its thing ✨

• Compiled: Number of compilation tasks performed.
• Failed: Number of compilations tasks 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 of the last failed compilation.

-gc option

Garbage collected heap statistics — the main character of the whole page honestly 👑

• 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).
• MC: Metaspace Committed Size (KB).
• MU: Metaspace utilization (KB).
• CCSC: Compressed class committed size (KB).
• CCSU: Compressed class space used (KB).
• YGC: Number of young generation garbage collection (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 option

Memory pool generation and space capacities — all the ceilings and floors 🏠

• 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).
• MCMN: Minimum metaspace capacity (KB).
• MCMX: Maximum metaspace capacity (KB).
• MC: Metaspace Committed Size (KB).
• CCSMN: Compressed class space minimum capacity (KB).
• CCSMX: Compressed class space maximum capacity (KB).
• CCSC: Compressed class committed size (KB).
• YGC: Number of young generation GC events.
• FGC: Number of full GC events.

-gccause option

This one is lowkey -gcutil's sibling — it shows the exact same summary of garbage collection statistics as the -gcutil option, but it also throws in the causes of the last garbage collection event and, when applicable, the current garbage collection event. So on top of every column listed for -gcutil, you get these two extra beauties:

• LGCC: Cause of last garbage collection
• GCC: Cause of current garbage collection

-gcnew option

New generation statistics — the nursery era 🌱

• 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 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.

-gcnewcapacity option

New generation space size statistics:

• 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 option

Old generation size statistics — the retirement home arc 🧓

• MC: Metaspace Committed Size (KB).
• MU: Metaspace utilization (KB).
• CCSC: Compressed class committed size (KB).
• CCSU: Compressed class space used (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 option

Old generation statistics:

• 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.

-gcmetacapacity option

Metaspace size statistics:

• MCMN: Minimum metaspace capacity (KB).
• MCMX: Maximum metaspace capacity (KB).
• MC: Metaspace Committed Size (KB).
• CCSMN: Compressed class space minimum capacity (KB).
• CCSMX: Compressed class space maximum 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 option

The summary of garbage collection statistics — all percentages, super readable 💯

• 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.
• M: Metaspace utilization as a percentage of the space's current capacity.
• CCS: Compressed class space utilization as a percentage.
• 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 option

Java HotSpot VM compiler method statistics:

• Compiled: Number of compilation tasks performed by the most recently compiled method.
• Size: Number of bytes of byte code 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 a slash (/) instead of a dot (.) as a name space separator. The method name is the method within the specified class. The format for these two fields is consistent with the HotSpot -XX:+PrintCompilation option.

---

🦋 VIRTUAL MACHINE IDENTIFIER 🦋

So the vmid string? Its syntax matches the syntax of a URI, fr fr:

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

The syntax of the vmid string corresponds to the syntax of a URI. The vmid string can vary from a simple integer that represents a local JVM to a more complex construction that specifies a communications protocol, port number, and other implementation-specific values. It's giving "simple but also could be a whole situation" ✨

• protocol — The communications protocol. If the protocol value is omitted and a host name isn't specified, then the default protocol is a platform-specific optimized local protocol. If the protocol value is omitted and a host name 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 provided the JVM processes is not running in a separate docker instance. You can also determine the lvmid on Linux and macOS platforms with the ps command, and on Windows with the Windows Task Manager.
• hostname — A host name or IP address that indicates the target host. If the hostname value is omitted, then the target host is the local host.
• port — The default port for communicating with the remote server. If the hostname value is omitted or the protocol value specifies an optimized, local protocol, then the port value is ignored. Otherwise, treatment of the port parameter is implementation-specific. For the default rmi protocol, the port value indicates the port number for the rmiregistry on the remote host. If the port value is omitted and the protocol value indicates rmi, then the default rmiregistry port (1099) is used.
• servername — The treatment of the servername parameter depends on implementation. 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.

---

🌈 EXAMPLES 🌈

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

💖 THE GCUTIL OPTION 💖

Okay bestie, so this example attaches to lvmid 21891 and takes 7 samples at 250 millisecond intervals, and it displays the output exactly as the -gcutil option says to. Slay.

The output shows that a young generation collection happened between the third and fourth sample. That collection took 0.078 seconds and promoted objects from the eden space (E) to the old space (O). That made old space utilization go up from 66.80% to 68.19%. Before the collection, the survivor space was 97.02% utilized, but after this collection it’s 91.03% utilized. The math is mathing.

jstat -gcutil 21891 250 7
  S0     S1     E      O      M     CCS    YGC     YGCT    FGC    FGCT     GCT
  0.00  97.02  70.31  66.80  95.52  89.14      7    0.300     0    0.000    0.300
  0.00  97.02  86.23  66.80  95.52  89.14      7    0.300     0    0.000    0.300
  0.00  97.02  96.53  66.80  95.52  89.14      7    0.300     0    0.000    0.300
 91.03   0.00   1.98  68.19  95.89  91.24      8    0.378     0    0.000    0.378
 91.03   0.00  15.82  68.19  95.89  91.24      8    0.378     0    0.000    0.378
 91.03   0.00  17.80  68.19  95.89  91.24      8    0.378     0    0.000    0.378
 91.03   0.00  17.80  68.19  95.89  91.24      8    0.378     0    0.000    0.378

---

🎀 REPEAT THE COLUMN HEADER STRING 🎀

This example attaches to lvmid 21891 and takes samples at 250 millisecond intervals, and it displays the output as specified by the -gcnew option. It also uses the -h3 option to output the column header after every 3 lines of data. Repeating headers? We love a consistent queen.

Besides showing the repeating header string, this example shows that between the second and third samples, a young GC occurred. Its duration was 0.001 seconds. The collection found enough active data that the survivor space 0 utilization (S0U) would have exceeded the desired survivor size (DSS). As a result, objects were promoted to the old generation (not visible in this output), and the tenuring threshold (TT) was lowered from 31 to 2.

Another collection occurs between the fifth and sixth samples. This collection found very few survivors and returned the tenuring threshold to 31. It’s giving “we’re so back.”

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

---

🌸 INCLUDE A TIME STAMP FOR EACH SAMPLE 🌸

This example attaches to lvmid 21891 and takes 3 samples at 250 millisecond intervals. The -t option is used to generate a time stamp for each sample in the first column. Timestamps? Cute and useful.

The Timestamp column reports the elapsed time in seconds since the start of the target JVM. Also, the -gcoldcapacity output shows the old generation capacity (OGC) and the old space capacity (OC) increasing as the heap expands to meet allocation or promotion demands. The old generation capacity (OGC) has grown from 11,696 KB to 13,820 KB after the eighty-first full garbage collection (FGC). The maximum capacity of the generation (and space) is 60,544 KB (OGCMX), so it still has room to expand. Growth mindset, literally.

              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. Remote monitoring era.

The lvmid is combined with the name of the remote host to construct a vmid of 40496@remote.domain. 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 command on remote.domain that’s bound to the default port of the rmiregistry command (port 1099). Networking but make it brat.

jstat -gcutil 40496@remote.domain 1000
... output omitted

---

And that’s the whole vibe check, bestie. This page is from JDK 21, dated 2023, and it’s JSTAT(1) — go forth and read your JVM’s fortune like the main character you are. 💅✨