Java Memory Profiling with Eclipse MAT

To quickly locate Java memory issues, analyzing heap dumps using Eclipse MAT is key. 1. Automatically generate heap dumps when using jmap, JVisualVM or OOM; 2. After opening the file, check Histogram, Dominator Tree and Leak Suspects to locate suspicious objects; 3. Analyze the GC Roots reference chain to confirm whether the leak is caused by invalid references; 4. Use Compare Basket to compare snapshots to observe memory trends; 5. Pay attention to large file loading performance and array type display problems. Mastering these operations can efficiently troubleshoot most memory bottlenecks.

Memory problems in Java applications are often headaches, especially when encountering memory leaks or frequent GC. As a powerful memory analysis tool, Eclipse MAT (Memory Analyzer) can help developers quickly locate the root cause of problems. This article does not talk about complex theories, but only talks about a few practical uses and operating methods that you are most concerned about.

How to quickly generate heap dump files available for MAT

Before using MAT, you must first get a heap dump. There are several common ways to generate:

• Use jmap command: jmap -dump:format=b,file=heap.bin <pid></pid>
• Trigger heap dump via JVisualVM
• Automatically export when OOM occurs in the application: Start parameter plus -XX: HeapDumpOnOutOfMemoryError

Note: The heap dump file may be large. Make sure that the disk space is sufficient. It is not recommended to use compression methods such as network disks during transmission to avoid corruption of files.

How to view the main interface of MAT? What indicators are focused on

After opening heap.bin, MAT will automatically generate an overview report (Overview), which has several key areas worth looking at:

• Histogram : Displays the number of instances and memory size of each class, which is suitable for preliminary judgment of whether there is a large amount of duplicate objects piled up.
• Dominator Tree : Display objects according to the "dominate" relationship, and more intuitively see who occupies the most memory.
• Leak Suspects : This is one of the most practical parts of MAT, which automatically analyzes and lists suspicious memory leak points.

For example, if you find that a certain business object has an abnormal number in Histogram, you can right-click to select "Merge Shortest Path to GC Roots" to see why these objects have not been recycled.

How to confirm if it is a memory leak?

The essence of memory leaks is that objects are no longer used but cannot be recycled by GC. In MAT, you can use the following steps to assist in judgment:

1. Find the type of object that is suspected to be leaked
2. View its dominant tree path (Dominator Tree)
3. Analyze the GC Roots reference chain to confirm whether it cannot be released due to unnecessary references

For example: If a cache map continues to grow without a cleanup mechanism, you may see it occupy a high position in the Dominator Tree for a long time. Looking further into its reference chain, it is found that it is held by a static variable, and the cache should have been released with the end of the request - this is a typical leak scenario.

At this time, you can check whether there are singleton errors in the code to hold non-static objects, listeners are not logged out, thread local variables are not cleaned, etc.

Tips and precautions for using MAT

• If the heap dump is too large and loading slowly, you can add startup parameters to adjust the memory settings of the MAT (such as modifying -Xmx in eclipse.ini)
• Comparing heap snapshots at different points in time using the Compare Basket feature helps to discover memory growth trends
• When the class name is displayed in the formats of [B、[C、[I , is actually array types such as byte[], char[], int[], etc., don't panic.

Basically that's it. Mastering these points is enough to deal with most Java memory problems. Although tools are good, understanding business logic is the foundation of positioning problems.

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