Change/Increase Heap Size of the Java Virtual Machine

A Java program can execute in the Java Virtual Machine (JVM) and uses the heap memory to manage the data. If our Java program requires more memory, there is a possibility that the Java Virtual Machine(JVM) will begin to throw OutOfMemoryError instances when attempting to instantiate an object in Java.

How to change/increase the JVM Heap Size?

In Java, it is possible to increase the heap size allocated by the JVM:

• -Xms:
• Set an initial Java heap size
• JVM automatically manages the heap memory but we can tell it how much memory to start with using -Xms

Syntax

java -Xms<size> ClassName

• -Xmx:
• Set the maximum Java heap size.
• When the program uses more memory than the JVM memory also increases, but we can put a limit by using -Xmx.

Syntax:

java -Xmx<size> ClassName

• -Xss
• Set the Java thread stack size

Syntax:

java -Xss<size> ClassName

Methods to Change Heap Size

The following are the different methods to change/increase the JVM Heap size:

• Command Line Method
• Environment Variables
• IDE Configuration

Command Line Method

This sets the initial heap size to 256MB and the maximum heap size to 1024MB:

java -Xms256m -Xmx1024m MyApplication

Set the initial and maximum heap size to 1GB, and the young generation size to 256MB:

java -Xms1024m -Xmx1024m -Xmn256m MyApplication

Common Unit Specifiers:

• k or K for kilobytes (e.g., 256k)
• m or M for megabytes (e.g., 512m)
• g or G for gigabytes (e.g., 2g)

Environment Variables

For Windows, in the environment variables, set the size according to below:

set JAVA_OPTS=-Xms1024m -Xmx2048m

For Linux/Unix, in the environment variables, set the size according to below:

export JAVA_OPTS="-Xms1024m -Xmx2048m"

IDE Configuration

For the Eclipse IDE, follow the following steps:

• Go to Run->Run Configurations
  
• Select your Java application
  
• Arguments tab-> VM arguments: -Xms512m -Xmx2048m

Example

Below is an example for monitoring the heap usage in Java:

public class HeapSizeTest {
   public static void main(String[]args){
      // To get the JVM Heap Size
      long heapSize = Runtime.getRuntime().totalMemory();
      // To print the JVM Heap Size
      System.out.println("Heap Size: " + heapSize);
   }
}

Output

Heap Size: 16252928