“jhat” is a command-line tool provided as part of the Java Development Kit (JDK), used for analyzing Java heap dumps. Heap dumps are snapshots of the Java Virtual Machine’s (JVM) memory at a specific point in time, capturing information about objects and their relationships in memory. “jhat” allows developers to inspect and analyze these heap dumps to diagnose memory-related issues such as memory leaks, excessive memory usage, and inefficient object allocation.
Here’s a more detailed explanation of its functionalities:
- Heap Analysis: The primary purpose of “jhat” is to perform analysis on Java heap dumps. Heap dumps contain detailed information about objects allocated in the JVM’s heap memory, including their class names, sizes, references, and other metadata. “jhat” reads and processes this information to provide insights into memory usage patterns and object lifecycles.
- Memory Leak Detection: “jhat” helps developers identify memory leaks by analyzing the relationships between objects in memory. It can detect situations where objects are unintentionally retained in memory, preventing them from being garbage-collected and leading to memory exhaustion over time. By examining object references and retention paths, “jhat” can pinpoint the source of memory leaks in Java applications.
- Dominator Tree: One of the key features of “jhat” is the ability to generate a dominator tree from the heap dump. The dominator tree represents the hierarchy of objects in memory, with each node representing an object and its immediate dominator. This visualization helps developers understand which objects are dominating the memory usage and identify potential memory hotspots.
- Object Querying: “jhat” provides a query interface that allows developers to search for specific objects or classes within the heap dump. Developers can perform queries based on various criteria, such as class name, object size, or reference chains. This allows for targeted analysis and debugging of memory-related issues.
- Web-Based Interface: “jhat” includes a built-in web server that serves as the interface for interacting with the analyzed heap dump. After processing the heap dump, “jhat” starts a web server that allows developers to navigate through the analyzed data using a web browser. This provides a user-friendly interface for exploring memory usage and conducting detailed analysis.
- Integration with Development Tools: While “jhat” is primarily a command-line tool, it can be integrated into development environments and build tools for automated heap analysis. Developers can incorporate “jhat” into their build scripts or debugging workflows to automatically analyze heap dumps generated during testing or debugging sessions.
- Documentation and Resources: “jhat” is documented as part of the JDK documentation, providing detailed information on its usage, command-line options, and output format. Additionally, there are numerous resources and tutorials available online that cover heap analysis techniques using “jhat” and other related tools.
jhat Command Example
1. Analyze a heap dump (from jmap), view via HTTP on port 7000:
# jhat [dump_file.bin]
2. Analyze a heap dump, specifying an alternate port for the http server:
# jhat -p [port] [dump_file.bin]
3. Analyze a dump letting jhat use up to 8 GB RAM (2-4x dump size recommended):
# jhat -J-mx8G [dump_file.bin]
Summary
In summary, “jhat” is a valuable tool for Java developers, providing insights into memory usage and helping diagnose memory-related issues in Java applications. Its capabilities for heap analysis, memory leak detection, dominator tree visualization, and object querying make it an essential tool for optimizing and troubleshooting Java applications’ memory usage.