What is an OutOfMemoryError and steps to find the root cause of OOM in Java?

The OutOfMemoryError is thrown by JVM, when JVM does not have enough available memory, to allocate. OutOfMemoryError falls into the Error category in Exception class hierarchy.

To generate OutOfMemoryError

• We will allocate a large chunk of memory, which will exhaust heap memory storage.
• We will keep on allocating the memory and point will reach, when JVM will not have enough memory to allocate, then OutOfMemoryError will be thrown.
• Once we will catch the OutOfMemory error, we can log the error.

Example

public class OutOfMemoryErrorDemo {
   public static void main(String[] args) throws Exception {
      int dummyArraySize = 15;
      System.out.println("Max JVM memory: " + Runtime.getRuntime().maxMemory());
      long memoryConsumed = 0;
      try {
         long[] memoryAllocated = null;
         for(int loop = 0; loop < Integer.MAX_VALUE; loop++) {
            memoryAllocated = new long[dummyArraySize];
            memoryAllocated[0] = 0;
            memoryConsumed += dummyArraySize * Long.SIZE;
            System.out.println("Memory Consumed till now: " + memoryConsumed);
            dummyArraySize *= dummyArraySize * 2;
            Thread.sleep(500);
         }
      } catch (OutOfMemoryError outofMemory) {
         System.out.println("Catching out of memory error");
         //Log the information, so that we can generate the statistics
         throw outofMemory;
      }
   }
}

Output

Max JVM memory: 119537664
Memory Consumed till now: 960
Memory Consumed till now: 29760
Memory Consumed till now: 25949760
Catching out of memory error
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at OutOfMemoryErrorDemo.main(OutOfMemoryErrorDemo.java:9)

Steps to find the root cause of the OOM

Step 1: Generate a heap dump on OutOfMemoryError

Start the application with the VM argument -XX:+HeapDumpOnOutOfMemoryError. This will tell the JVM to produce a heap dump when an OOM occurs

$ java -XX:+HeapDumpOnOutOfMemoryError ...

Step 2: Reproduce the problem

If we cannot reproduce the problem in the development environment, we may have to use the production environment. When we reproduce the problem and the application throws an OOM, it will generate a heap dump file.

Step3: Investigate the issue using the heap dump file

Use VisualVM to read the heap dump file and diagnose the issue. VisualVM is a program located in JDK_HOME/bin/jvisualvm. The heap dump file has all information about the memory usage of the application.