Debugging OutOfMemoryError & Memory Leaks in WebLogic Server

OutOfMemoryError (OOM) is a critical issue in Oracle WebLogic Server that can cause application crashes, performance degradation, and system instability. Understanding its causes and applying the correct debugging methodology can help administrators effectively identify and resolve memory leaks.

What is an OutOfMemoryError?

An OutOfMemoryError (OOM) occurs when the Java Virtual Machine (JVM) runs out of available heap memory, preventing applications from allocating new objects.

Two primary causes of OOM:
Genuine Memory Exhaustion → Too many application requests exceed the allocated heap.
Memory Leaks → Unused objects are not released, causing memory accumulation over time.

Example of a Memory Leak:

// Java Memory Leak Example
List<String> leakList = new ArrayList<>();
while (true) {
    leakList.add("Memory Leak " + new Random().nextInt());
}

This code keeps adding data to the list without freeing memory, eventually leading to an OutOfMemoryError.

Identifying Memory Leaks vs. Genuine Exhaustion

A healthy JVM with no memory leaks should see memory usage fluctuate with peak loads

Normal Memory Behavior

Memory Leak Pattern
A leaking application shows a continuous increase in memory usage, never returning to the baseline.

Step-by-Step Debugging Process

Capture JVM Memory Usage Data

Use verbose garbage collection logging to track heap utilization:

java -Xloggc:gc.log -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -jar myApp.jar

Generate a Heap Dump for Analysis

Heap dumps capture all objects in memory, helping identify unused but retained objects.

Generate Heap Dump using JMAP:

jmap -dump:format=b,file=heapdump.hprof <PID>

Analyze Heap Dump using Eclipse MAT (Memory Analyzer Tool):

mat heapdump.hprof

Look for large objects that are never garbage collected—these may indicate memory leaks.

Collect & Analyze Thread Dumps

Thread dumps help in identifying stuck threads that hold memory unnecessarily.

Use JSTACK to capture thread dumps:

jstack -l <PID> > thread_dump.txt

Analyze the output for long-running or stuck threads.

Force Garbage Collection & Monitor Behavior

If memory usage remains high even after forcing garbage collection, there is likely a memory leak.

Trigger Garbage Collection manually:

System.gc();

Observe heap usage before & after GC using JConsole or VisualVM:
If memory usage drops significantly → No leak
If memory remains high → Possible memory leak

Automated Memory Leak Testing Process

Follow this structured testing method to detect memory leaks efficiently:

        +------------------------------------+
        |  Start WebLogic Server            |
        +------------------------------------+
                    |
                    v
        +------------------------------------+
        |  Deploy Application               |
        +------------------------------------+
                    |
                    v
        +------------------------------------+
        |  Take Initial Heap Dump Snapshot  |
        +------------------------------------+
                    |
                    v
        +------------------------------------+
        |  Simulate User Load               |
        +------------------------------------+
                    |
                    v
        +------------------------------------+
        |  Monitor Memory Consumption       |
        +------------------------------------+
                    |
         +-------------------------------------+
         |  Compare Heap Snapshots            |
         |  (Before & After Load Test)        |
         +-------------------------------------+
                    |
          +---------------------------+
          |  Memory Usage Increases?  |
          +---------------------------+
                  |       |
        +--------+       +----------+
        |   NO   |       |   YES    |
        |   No Leak  |   |   Possible Leak  |
        +--------+       +----------+
                    |
        +-------------------------------+
        |  Identify & Fix Retained Objects |
        +-------------------------------+

Best Practices to Prevent Memory Leaks

Avoid Static Collections

static List<String> myList = new ArrayList<>(); // Can cause memory leaks

Close Database & Network Connections

Connection conn = DriverManager.getConnection(url, user, pass);
conn.close(); // Always close connections

Use Weak References to allow GC to reclaim objects

WeakReference<String> weakStr = new WeakReference<>(new String("Test"));

Limit Session & Cache Size

<session-config>
    <session-timeout>30</session-timeout>
</session-config>

Enable Heap Dump on OOM Errors

java -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=./heapdump.hprof -jar myApp.jar

Final Thoughts

Memory leaks in WebLogic Server can severely impact application performance, but by following a structured debugging approach, you can:
Detect & Fix Memory Leaks Early
Optimize Application Performance
Ensure WebLogic Server Stability

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