OutOfMemoryError in Java applications: The main reason for Java heap space is insufficient heap memory, which is usually caused by low heap size settings, memory leaks, frequent creation of large objects, or unreasonable GC configuration. 1. When the heap is insufficient, the -Xmx parameter can be adjusted to increase the maximum heap; 2. Memory leaks can be used to analyze the heap dump to find unfree references; 3. Avoid creating large objects in the loop and use streaming or cache pools instead; 4. Select the appropriate GC type according to the load and optimize parameter configuration. The solution order should be: increase the heap, troubleshoot leakage, optimize object usage, and adjust GC strategy.

OutOfMemoryError: Java heap space occurs during running a Java application, usually because the heap memory is insufficient to accommodate the objects required by the program. This problem is common, especially in scenarios where big data, high concurrency, or memory leaks exist.

The most direct reason for insufficient heap memory

When a Java program is running, all objects are allocated on the heap. A maximum heap memory limit is set when the JVM is started (via the -Xmx parameter). This error will be thrown when the program applies for a new object and the heap space is full and the garbage collector (GC) cannot free up enough space.

To give a simple example: if your application wants to load a file of several hundred MB into memory for processing, and the maximum heap set by the JVM is only 512MB, it is likely to trigger OOM.

One solution is to increase the heap size appropriately, such as:

• -Xms512m -Xmx2g means that the initial heap is 512MB, which can be expanded to 2GB at most

But note that blindly increasing the pile is not a universal solution, and may also cover up deeper problems.

Memory leak causes useless objects to not be recycled

Even if a larger heap is set, if there is a memory leak in the application, the stack will eventually fill up. Memory leaks are when objects are no longer used, but because some references are not released, GC cannot recycle them.

Common leak scenarios include:

• Static collection classes (such as static List ) continue to add objects without cleaning
• The cache has no expiration mechanism or capacity limit
• Listeners and callbacks are not logged out in time (such as event listening, anonymous internal classes hold external class references)

To troubleshoot such problems, you can use tools such as:

• Use VisualVM, MAT, or JProfiler to analyze heap dumps (heap dump)
• Check the GC log to see if the memory in the elderly has dropped significantly after Full GC

The key to fixing memory leaks is to find "who holds a reference that shouldn't be held."

Frequent creation of large objects increases memory pressure

Some application logic creates large objects frequently, such as:

• Read the entire file to the byte array at once
• Build super large string stitching results
• Operate high-definition pictures or video data

If these operations do not have a reasonable frequency control or optimized structure, it is easy to cause memory jitter or even OOM.

Suggested practices include:

• Avoid creating large objects in loops
• Use streaming to replace one-time loading
• Reduce duplicate allocation using cache pools or multiplexing mechanisms

For example, when processing files, use BufferedReader to read them line by line, rather than reading the entire content at once.

Unreasonable GC configuration affects memory management efficiency

Different garbage collectors have different memory management strategies. If the GC is configured unreasonably, it may lead to untimely memory recovery or inefficient efficiency.

for example:

• Using Serial GC may have poor performance under large data volumes
• G1 GC If the pause time target is not set correctly, it may also affect the recovery rhythm.

It can be optimized by:

• Select the appropriate GC type (such as CMS, G1, ZGC) according to the application characteristics
• Turn on GC log analysis to view the changes in the heap before and after each recycling
• Adjust the ratio of the new generation to the old generation (via -Xmn or -XX:NewRatio )

GC performance tuning requires testing and observation based on actual load, and cannot be generalized.

Basically that's it. When encountering OutOfMemoryError: Java heap space , first confirm whether the heap is too small, then check whether there is any memory leak, then check whether there is any abuse of large objects, and finally consider whether the GC configuration is reasonable. Any of these problems are not handled properly, which may lead to OOM.

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