How to Handle the OutOfMemoryError in Java

A java.lang.OutOfMemoryError is a runtime error in Java which occurs when the Java Virtual Machine (JVM) is unable to allocate an object due to insufficient space in the Java heap. The Java Garbage Collector (GC) cannot free up the space required for a new object, which causes a java.lang.OutOfMemoryError. This error can also be thrown when the native memory is insufficient to support the loading of a Java class.

What Causes java.lang.OutOfMemoryError

The JVM's memory management scheme sets aside a portion of the heap memory to store newly allocated objects. Any referenced objects remain active in the heap throughout their lifespan (until their reference is closed) and occupy memory. When objects are no longer referenced, they become eligible for the GC to remove them and free up the occupied heap memory.

The Java heap size is determined by two JVM attributes, which can be set when launching Java:

• -Xms to set the initial heap size
• -Xmx to set the maximum heap size

The amount of heap memory used by a Java application impacts the number of objects that can be allocated and their size. If an object requires more memory than is available in the heap, the application can encounter a java.lang.OutOfMemoryError.

A java.lang.OutOfMemoryErrorusually means that something is wrong in the application - for example, the application code is referencing large objects for too long or trying to process large amounts of data at a time. The problems could also exist in third-party libraries used within an application.

java.lang.OutOfMemoryError Example

Here is an example of a java.lang.OutOfMemoryError thrown due to insufficient Java heap space:

public class OutOfMemoryErrorExample {
    public static void main(String[] args) {
        Integer[] myArray = new Integer[1000 * 1000 * 1000];
    }
}

In this example, an Integer array with a very large size is attempted to be initialized. Because the Java heap is insufficient to allocate this array, it throws a java.lang.OutOfMemoryError: Java heap space

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at OutOfMemoryErrorExample.main(OutOfMemoryErrorExample.java:8)

A java.lang.OutOfMemoryError: Java heap space can also occur in applications that use finalizers excessively. If a class has a finalize() method, the GC does not clean up any objects of that class and they are instead queued for finalization, which occurs at a later stage. If the finalizer thread cannot keep up with the finalization queue (due to excessive usage of finalizers), the Java heap space can fill up and a java.lang.OutOfMemoryError can occur.

How to Catch java.lang.OutOfMemoryError

Since the java.lang.OutOfMemoryError descends from the Throwable class, it can be caught and handled in application code. In some cases, especially when the lines of code that may be causing the OutOfMemoryError are known, it can be a good idea to handle the error. Where it’s possible to do so, it is best practice to clean up the resources, log the reason for the failure and exit the program gracefully. As an example:

public class OutOfMemoryErrorExample {
    public void createArray(int size) {
        try {
            Integer[] myArray = new Integer[size];
        } catch (OutOfMemoryError oome) {
            //Log the info
            System.err.println("Array size too large");
            System.err.println("Max JVM memory: " + Runtime.getRuntime().maxMemory());
        }
    }

    public static void main(String[] args) {
        OutOfMemoryErrorExample oomee = new OutOfMemoryErrorExample();
        oomee.createArray(1000 * 1000 * 1000);
    }
}

In this case, because the line of code that may cause an OutOfMemoryError is known, it is handled in a try-catch block and the reason for the error is logged (the large size of the array) along with the maximum size of the JVM, which helps the caller of the method to take corrective action. The program exits with the following message:

Array size too large
Max JVM memory: 4294967296

It is also a good idea to handle an OutOfMemoryError when the application needs to be left in a consistent state in case the error occurs. This enables the program to continue running normally if new objects are not attempted to be allocated.

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