What is jcmd?
jcmd is a Java diagnostic tool that can be used to send diagnostic commands to a Java Virtual Machine (JVM). It can be used to troubleshoot and diagnose JVM and Java applications, as well as to control Java Flight Recordings (JFRs).
How to use jcmd
The jcmd utility can be used to send commands to a JVM by specifying the process ID (PID) of the JVM or the main class of the application. The following is an example of how to use jcmd to send the `help` command to a JVM with a PID of 1234:
jcmd 1234 help
The `help` command will list all of the available jcmd commands.
Available jcmd commands
The following are some of the available jcmd commands:
* `JFR.start`: Starts a Java Flight Recording.
* `JFR.dump`: Dumps the contents of a Java Flight Recording.
* `check`: Checks the status of the JVM.
* `native_memory`: Prints information about the native memory usage of the JVM.
* `check_commercial_features`: Checks if the JVM has any commercial features enabled.
* `unlock_commercial_features`: Unlocks commercial features in the JVM.
* `ManagementAgent`: Controls the Java Management Agent.
* `Thread`: Prints information about the threads in the JVM.
* `GC`: Prints information about the garbage collector in the JVM.
* `heap_dump`: Dumps the heap of the JVM.
For more information on jcmd commands, please refer to the jcmd documentation.
Here are some common use cases and examples of how to use `jcmd`:
1. List Java processes:
To list all Java processes running on a system along with their process IDs (PIDs), you can use the following command:
jcmd
2. List available diagnostic commands:
To list the diagnostic commands available for a specific Java process, you can use the following command, replacing `<PID>` with the PID of the target process:
jcmd <PID> help
3. Execute a diagnostic command:
You can execute a specific diagnostic command on a Java process using the following syntax:
jcmd <PID> <command-name> [options]
For example, to generate a heap dump of a Java process with PID 12345, you can use:
jcmd 12345 GC.heap_dump /path/to/heapdump.hprof
4. Print system properties:
To view the system properties of a running Java process, you can use:
jcmd <PID> VM.system_properties
5. Print thread stack traces:
To print the stack traces of all threads in a Java process, you can use:
jcmd <PID> Thread.print
6. Trigger a garbage collection:
To manually trigger a garbage collection in a Java process, you can use:
jcmd <PID> GC.run
7. Monitor performance counters:
You can use `jcmd` to monitor various performance counters, such as CPU usage, memory usage, and more, depending on the available diagnostic commands.
Keep in mind that the available diagnostic commands and their usage may vary depending on the version of the JDK and the specific JVM implementation you are using (e.g., Oracle HotSpot, OpenJDK, etc.). You can use `jcmd` to explore the available options and diagnostic capabilities of a specific Java process by running `jcmd <PID> help`.
Advantages of using jcmd
jcmd is a powerful tool that can be used to troubleshoot and diagnose JVM and Java applications. It is also a valuable tool for controlling Java Flight Recordings.
Disadvantages of using jcmd
jcmd can be a complex tool to use. It is important to understand the available commands and options before using jcmd.
Conclusion
jcmd is a valuable tool for JVM and Java application troubleshooting and diagnosis. It is also a valuable tool for controlling Java Flight Recordings. If you are a Java developer, it is important to be familiar with jcmd.