The following documents contain important information you will need to know to get the most out of the JDK tools.
jps
This utility lists the instrumented Java HotSpot VMs on the target system. The utility is very useful in environments where the VM is embedded, that is, it is started using the JNI Invocation API rather than the java launcher.
jps [ options ] [ hostid ]
The output of the jps command follows the following pattern:
lvmid [ [ classname | JARfilename | "Unknown"] [ arg* ] [ jvmarg* ] ]
For example,
$ jps
32515 LogPuller
17461 SparkServerKt
17770 SparkServerKt
31163 UploadToHDFS
9308 Jps
893 MainKt
26991 Logmgr
26303 LogAdmin
6703 jar
The utility lists the virtual machines for which the user has access rights. This is determined by access-control mechanisms specific to the operating system. On Oracle Solaris operating system, for example, if a non-root user executes the jps utility, then the output is a list of the virtual machines that were started with that user’s uid.
If the jps command is run without specifying a hostid, then it searches for instrumented JVMs on the local host. If started with a hostid, then it searches for JVMs on the indicated host, using the specified protocol and port. A jstatd process is assumed to be running on the target host.
jmap
This utility can obtain memory map information, including a heap histogram, from a Java process, a core file, or a remote debug server.
jmap [ options ] pid
jmap [ options ] executable core
jmap [ options ] [ pid ] server-id@ ] remote-hostname-or-IP
$ jmap -heap 32515
Attaching to process ID 32515, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.112-b15
using thread-local object allocation.
Parallel GC with 23 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 17179869184 (16384.0MB)
NewSize = 2863136768 (2730.5MB)
MaxNewSize = 5726273536 (5461.0MB)
OldSize = 5726797824 (5461.5MB)
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 = 2733113344 (2606.5MB)
used = 479095272 (456.9008560180664MB)
free = 2254018072 (2149.5991439819336MB)
17.529286630273024% used
From Space:
capacity = 25690112 (24.5MB)
used = 25681312 (24.491607666015625MB)
free = 8800 (0.008392333984375MB)
99.96574557557398% used
To Space:
capacity = 65011712 (62.0MB)
used = 0 (0.0MB)
free = 65011712 (62.0MB)
0.0% used
PS Old Generation
capacity = 5726797824 (5461.5MB)
used = 4829785576 (4606.042457580566MB)
free = 897012248 (855.4575424194336MB)
84.33658257952149% used
15987 interned Strings occupying 1405880 bytes.
The -heap option is used to obtain the following Java heap information:
- Information specific to the garbage collection (GC) algorithm, including the name of the GC algorithm (for example, parallel GC) and algorithm-specific details (such as number of threads for parallel GC).
- Heap configuration that might have been specified as command-line options or selected by the VM based on the machine configuration.
- Heap usage summary: For each generation (area of the heap), the tool prints the total heap capacity, in-use memory, and available free memory. If a generation is organized as a collection of spaces (for example, the new generation), then a space specific memory size summary is included.
jstack
The jstack command-line utility attaches to the specified process or core file and prints the stack traces of all threads that are attached to the virtual machine, including Java threads and VM internal threads, and optionally native stack frames. The utility also performs deadlock detection.
The utility can also use the jsadebugd daemon to query a process or core file on a remote machine.
$ jstack -F 8321
Attaching to process ID 8321, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 1.6.0-rc-b100
Deadlock Detection:
Found one Java-level deadlock:
=============================
"Thread2":
waiting to lock Monitor@0x000af398 (Object@0xf819aa10, a java/lang/String),
which is held by "Thread1"
"Thread1":
waiting to lock Monitor@0x000af400 (Object@0xf819aa48, a java/lang/String),
which is held by "Thread2"
Found a total of 1 deadlock.
Thread t@2: (state = BLOCKED)
Thread t@11: (state = BLOCKED)
- Deadlock$DeadlockMakerThread.run() @bci=108, line=32 (Interpreted frame)
Thread t@10: (state = BLOCKED)
- Deadlock$DeadlockMakerThread.run() @bci=108, line=32 (Interpreted frame)
Thread t@6: (state = BLOCKED)
Thread t@5: (state = BLOCKED)
- java.lang.Object.wait(long) @bci=-1107318896 (Interpreted frame)
- java.lang.Object.wait(long) @bci=0 (Interpreted frame)
- java.lang.ref.ReferenceQueue.remove(long) @bci=44, line=116 (Interpreted frame)
- java.lang.ref.ReferenceQueue.remove() @bci=2, line=132 (Interpreted frame)
- java.lang.ref.Finalizer$FinalizerThread.run() @bci=3, line=159 (Interpreted frame)
Thread t@4: (state = BLOCKED)
- java.lang.Object.wait(long) @bci=0 (Interpreted frame)
- java.lang.Object.wait(long) @bci=0 (Interpreted frame)
- java.lang.Object.wait() @bci=2, line=485 (Interpreted frame)
- java.lang.ref.Reference$ReferenceHandler.run() @bci=46, line=116 (Interpreted frame)
jstat
This utility uses the built-in instrumentation in Java to provide information about performance and resource consumption of running applications. The tool can be used when diagnosing performance issues, especially those related to heap sizing and garbage collection.
The jstat utility does not require the VM to be started with any special options. The built-in instrumentation in the Java HotSpot VM is enabled by default.
$ jstat -gcutil 2834 250 7
S0 S1 E O M YGC YGCT FGC FGCT GCT
0.00 99.74 13.49 7.86 95.82 3 0.124 0 0.000 0.124
0.00 99.74 13.49 7.86 95.82 3 0.124 0 0.000 0.124
0.00 99.74 13.49 7.86 95.82 3 0.124 0 0.000 0.124
0.00 99.74 13.49 7.86 95.82 3 0.124 0 0.000 0.124
0.00 99.74 13.49 7.86 95.82 3 0.124 0 0.000 0.124
0.00 99.74 13.49 7.86 95.82 3 0.124 0 0.000 0.124
0.00 99.74 13.49 7.86 95.82 3 0.124 0 0.000 0.124
jcmd
The jcmd utility is used to send diagnostic command requests to the JVM, where these requests are useful for controlling Java Flight Recordings. The JFRs are used to troubleshoot and diagnose JVM and Java Applications with flight recording events.
A special command jcmd <process id/main class> PerfCounter.print prints all performance counters in the process.
See more: The jcmd Utility
Reference