Different Ways to Capture Java Heap Dumps

Introduction

A Java heap dump is a snapshot of the Java Virtual Machine’s (JVM) memory at a specific point in time. It contains information about objects, their references, and their memory usage. Capturing a heap dump is essential for diagnosing memory-related issues, such as memory leaks and excessive memory consumption. In this article, we’ll explore different ways to capture Java heap dumps with various tools and techniques.

Prerequisites

Before we proceed, make sure you have the following:

1. A Java Development Kit (JDK) installed on your machine. You can download the latest JDK from the Oracle website (https://www.oracle.com/java/technologies/javase-downloads.html).
2. A Java application that you want to capture the heap dump for.

Method 1: Using jmap

jmap is a command-line utility provided by the JDK to generate heap dumps. It requires the process ID (PID) of the Java application to capture the heap dump.

Step 1: Find the PID of the Java Process

Open a terminal or command prompt and use the jps command to find the PID of your Java application:

jps

Step 2: Capture the Heap Dump

Once you have the PID, use the jmap command to capture the heap dump:

jmap -dump:file=heap_dump.bin <PID>

The -dump option specifies the output file name (heap_dump.bin in this example). Replace <PID> with the actual process ID of your Java application.

Method 2: Using jcmd

jcmd is another command-line utility that comes with the JDK and provides more options for capturing heap dumps.

Step 1: Find the PID of the Java Process

Use the jcmd command to list all available Java processes along with their PIDs:

jcmd

Step 2: Capture the Heap Dump

To capture the heap dump using jcmd, run the following command:

jcmd <PID> GC.heap_dump <output_file_name>

Replace <PID> with the process ID of your Java application, and <output_file_name> with the desired name for the heap dump file.

Method 3: Using HeapDumpOnOutOfMemoryError JVM Option

The JVM provides an option to automatically generate a heap dump when an OutOfMemoryError occurs.

To enable this feature, add the following JVM option when starting your Java application:

java -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/path/to/dump_folder -jar your_application.jar

With this option enabled, when the JVM encounters an OutOfMemoryError, it will automatically generate a heap dump and save it to the specified folder (/path/to/dump_folder).

Method 4: Using Java Flight Recorder (JFR)

If you have a JDK version 11 or later, you can use Java Flight Recorder (JFR) to capture heap dumps in real-time.

Step 1: Start JFR Recording

To start a JFR recording, use the jcmd command as follows:

jcmd <PID> JFR.start name=heap_dump settings=default

Step 2: Stop JFR Recording and Save Heap Dump

To stop the JFR recording and save the heap dump, run the following command:

jcmd <PID> JFR.dump name=heap_dump filename=heap_dump.jfr

This command will save the heap dump in the JFR format with the filename heap_dump.jfr.

Method 5: Using VisualVM

VisualVM is a powerful graphical tool that comes with the JDK, providing various features for monitoring and profiling Java applications, including the ability to capture heap dumps.

Step 1: Launch VisualVM

Open a terminal or command prompt and run the following command to start VisualVM:

jvisualvm

Step 2: Connect to Your Java Application

In VisualVM, locate your Java application under the “Local” section in the Applications tab. Double-click on your application to connect to it.

Step 3: Capture the Heap Dump

Once connected to your Java application, navigate to the “Memory” tab and click on the “Heap Dump” button. VisualVM will prompt you to save the heap dump to a file.

Choose a filename and location to save the heap dump.

Method 6: Using Your IDE (Eclipse or IntelliJ IDEA)

If you are using an Integrated Development Environment (IDE) like Eclipse or IntelliJ IDEA, you can capture heap dumps directly from the IDE.

Eclipse:

1. Run your Java application in Eclipse.
2. Open the “Debug Perspective.”
3. In the “Debug Perspective,” find your running Java application under the “Debug” tab.
4. Right-click on your application and select “Heap Dump.”

IntelliJ IDEA:

1. Run your Java application in IntelliJ IDEA.
2. Open the “Run” menu.
3. Select “Create Heap Dump” or “Capture Memory Snapshot.”

Both Eclipse and IntelliJ IDEA will prompt you to save the heap dump to a file.

Analyzing Heap Dumps

Once you have captured a heap dump using any of the methods mentioned above, you can analyze it using various tools:

1. Eclipse Memory Analyzer Tool (MAT): MAT is a powerful tool for analyzing heap dumps. You can open the heap dump file with MAT and explore memory usage, identify memory leaks, and analyze object retention paths.
2. VisualVM: Besides capturing heap dumps, VisualVM also offers basic heap analysis features, such as showing the dominator tree, analyzing object references, and monitoring memory usage.
3. Your IDE’s Built-in Heap Dump Analyzer: Some IDEs, like IntelliJ IDEA, have built-in heap dump analysis capabilities that allow you to visualize memory usage and find potential memory issues directly within the IDE.

Conclusion

Capturing and analyzing Java heap dumps are essential steps in diagnosing and resolving memory-related issues in your Java applications. In this article, we explored various methods to capture heap dumps using command-line utilities like jmap and jcmd, JVM options, Java Flight Recorder (JFR), VisualVM, and IDEs like Eclipse and IntelliJ IDEA.

With the ability to capture and analyze heap dumps, you can gain valuable insights into your application’s memory usage, identify memory leaks, and optimize memory allocation, ensuring better performance and stability of your Java applications. Regularly monitoring and analyzing heap dumps can help you maintain a healthy and efficient memory management system in your Java projects.