How to Use the Command 'jhsdb' (with Examples)

The jhsdb command is a powerful tool for Java developers and administrators who need to deep dive into the inner workings of Java processes. It offers capabilities such as attaching to live Java processes or analyzing core dumps from crashed Java Virtual Machines (JVMs), making it indispensable for diagnosing and troubleshooting Java applications. By leveraging this command, users can inspect stack traces, monitor lock information, and even set up remote debugging sessions. In this article, we will explore various use cases of the jhsdb command with detailed explanations and example outputs, illustrating its utility in real-world scenarios.

Use Case 1: Print Stack and Locks Information of a Java Process

Code:

jhsdb jstack --pid pid

Motivation:

Understanding the stack traces and lock state of a Java process is crucial for identifying performance bottlenecks, deadlocks, and other issues that might affect the behavior of the application. By using this command, developers can view the current state of all threads within the Java process, making it easier to pinpoint anomalous behavior or lock contention problems.

Explanation:

• jhsdb: The base command used to interface with the Java HotSpot Debugger.
• jstack: A sub-command that prints stack traces of Java threads for the specified process.
• --pid: An argument followed by the process ID of the Java process you want to examine. The process ID (PID) can be obtained using command-line tools like ps on Linux or Task Manager on Windows.

Example Output:

Attaching to process ID 12345, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 11.0.8
Thread 11598: (state = IN_NATIVE)
- java.io.FileInputStream.readBytes(Native Method)
at java.io.FileInputStream.read(FileInputStream.java:233)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
...

Use Case 2: Open a Core Dump in Interactive Debug Mode

Code:

jhsdb clhsdb --core path/to/core_dump --exe path/to/jdk/bin/java

Motivation:

When a JVM crashes, it can generate a core dump containing all the memory and process state at the time of the crash. Analyzing this core dump can provide insights into why the crash occurred. By using the interactive debug mode, users can explore the core dump in great detail, allowing for complex queries and exploration of the JVM’s state.

Explanation:

• jhsdb: The main command to interact with the JVM debugging tools.
• clhsdb: Sub-command that starts an interactive command-line debugger.
• --core: This argument specifies the path to the core dump file generated from the crashed JVM.
• --exe: Points to the Java executable used by the JVM that created the core dump. This helps the tool understand the specific JVM internals that may have been customized or patched.

Example Output:

Opening core dump from /path/to/core_dump
[1] 10178: \path\to\jdk\bin\java -Xmx1024M -classpath /app my.application.Main
(clhsdb) threads
Thread 1001: 0x00007f9c940bb800 - \"Main Thread\" daemon
...

Use Case 3: Start a Remote Debug Server

Code:

jhsdb debugd --pid pid --serverid optional_unique_id

Motivation:

There are situations where developers need to access a running JVM from a different machine or over a network. By starting a remote debug server, the jhsdb tool allows developers to connect to a Java process from afar and perform debugging operations as if they were running commands locally on the host machine.

Explanation:

• jhsdb: The primary command to utilize JVM debugging facilities.
• debugd: A sub-command that launches a debug server.
• --pid: Specifies the process ID of the Java process to which the debug server will attach.
• --serverid: An optional argument to provide a unique identifier for the server, useful in environments where multiple debug servers may be running to avoid conflicts.

Example Output:

Attaching to process ID 12345, please wait...
Debugger server started successfully. Connect with NI Protocol: jni://hostname

Use Case 4: Connect to a Process in Interactive Debug Mode

Code:

jhsdb clhsdb --pid pid

Motivation:

For developers who want an interactive debugging experience similar to using traditional debuggers, attaching to a live Java process in interactive mode can be very beneficial. This setup allows for executing complex queries, inspecting thread states, evaluating expressions, and more without affecting the running state of the application significantly.

Explanation:

• jhsdb: The command used to engage with JVM debugging capabilities.
• clhsdb: Sub-command that enters command-line interactive debugger mode.
• --pid: Specifies the ID of the running Java process to which the tool will attach and provide the debugging interface.

Example Output:

Attaching to process ID 67890, please wait...
(clhsdb) list
[1] 67890: /path/to/jdk/bin/java -Xmx512M -classpath /myapp my.application.Main

Conclusion:

The jhsdb command is an invaluable tool for Java developers and administrators who wish to gain deeper insights into Java processes. Whether it’s checking thread states, exploring core dumps, or setting up remote debugging, jhsdb offers a suite of powerful functionalities that can assist in troubleshooting and optimizing Java applications. With its versatile options and detailed output, it equips users with the necessary information to maintain the stability and performance of their Java applications.