How to use the command `jhat` (with examples)

The jhat command is a Java heap analysis tool that allows you to analyze heap dumps generated by the jmap command. It provides a web-based interface to explore and visualize the heap dump, making it easier to identify memory leaks or analyze memory usage patterns.

Use case 1: Analyze a heap dump and view it via HTTP on port 7000

Code:

jhat dump_file.bin

Motivation: By running jhat dump_file.bin, you can analyze a heap dump file (in binary format) and view the analysis results through a web-based interface served on port 7000. This allows you to visually explore the heap dump and understand the memory allocations and references within your Java application.

Explanation:

• jhat: The command itself.
• dump_file.bin: The path to the heap dump file you want to analyze.

Example output:

Listening for transport dt_socket at address: 8000
Listening for transport dt_socket at address: 8001
Reading from dump_file.bin...
Dump file: dump_file.bin
Loading Heap Dump from dump_file.bin ...
Opened Socket Addr,  port = 7000

Use case 2: Analyze a heap dump and specify an alternate port for the HTTP server

Code:

jhat -p port dump_file.bin

Motivation: In some cases, the default port 7000 used by jhat might be already in use by another application. By specifying an alternate port using the -p option, you can make sure that the web-based interface served by jhat is accessible through a different port.

Explanation:

• jhat: The command itself.
• -p port: Specifies the port number to be used by the HTTP server.
• dump_file.bin: The path to the heap dump file you want to analyze.

Example output:

Listening for transport dt_socket at address: 8000
Listening for transport dt_socket at address: 8001
Reading from dump_file.bin...
Dump file: dump_file.bin
Loading Heap Dump from dump_file.bin ...
Opened Socket Addr,  port = port

Use case 3: Analyze a heap dump letting jhat use up to 8 GB RAM

Code:

jhat -J-mx8G dump_file.bin

Motivation: For larger heap dump files, jhat might require more memory to load and analyze the dump efficiently. By providing the -J-mx8G option, you allow jhat to use up to 8 GB of RAM, which can improve the performance and prevent Out-of-Memory errors.

Explanation:

• jhat: The command itself.
• -J-mx8G: Passes the -mx option to the underlying Java Virtual Machine (JVM) running jhat, allowing it to use up to 8 GB of RAM.
• dump_file.bin: The path to the heap dump file you want to analyze.

Example output:

Listening for transport dt_socket at address: 8000
Listening for transport dt_socket at address: 8001
Reading from dump_file.bin...
Dump file: dump_file.bin
Loading Heap Dump from dump_file.bin ...
Opened Socket Addr,  port = 7000

Conclusion:

By using the jhat command with different options, you can effectively analyze heap dump files and gain insights into your Java application’s memory usage. The web-based interface provided by jhat allows you to navigate object instances, view instance details, identify memory leaks, and optimize memory usage. It is a powerful tool in diagnosing and troubleshooting memory-related issues in Java applications.