How to Increase Heap Space in Java

1. Increase Java Heap Size Using Command Line
2. Increase Java Heap Size in an Integrated Development Environment (IDE)

In Java, Heap Space is mainly used for garbage collection and allocating memory to objects.

A default Heap space is allocated when installing the JVM on our machine, but it can vary. The following points show how we can increase the heap size using two ways in Java.

Increase Java Heap Size Using Command Line

The first method to increase the heap size in Java is to use the command line to run the commands that override the default Java Heap Space. We follow the below two steps.

Step 1: Check the Default Maximum Heap Size in Java

Before we change the size of the heap in Java, we must know the default maximum heap size allocated to the JVM. To do that we use the code Runtime.getRuntime().maxMemory() that returns the maximum heap size in bytes.

In the program, we get the maximum heap space in bytes and convert it to a more readable format like megabytes or gigabytes. The output shows that our current default size is around 1.48 GB.

We perform this step so that we do not allocate a smaller size than the heap’s default size.

public class JavaExample {
  public static void main(String[] args) {
    double maxHeapSize = Runtime.getRuntime().maxMemory();
    String sizeInReadableForm;

    double kbSize = maxHeapSize / 1024;
    double mbSize = kbSize / 1024;
    double gbSize = mbSize / 1024;

    if (gbSize > 0) {
      sizeInReadableForm = gbSize + " GB";
    } else if (mbSize > 0) {
      sizeInReadableForm = mbSize + " MB";
    } else {
      sizeInReadableForm = kbSize + " KB";
    }

    System.out.println("Maximum Heap Size: " + sizeInReadableForm);
  }
}

Output:

Maximum Heap Size: 1.48046875 GB

Step 2: Use the Command Line to Set the Maximum Heap Size

Now that we know the maximum Java heap space, we increase it using the command line.

To get the commands that we can use to modify the heap size, we open the command line and use the command java -X that returns a whole list of commands, but we are interested in only the following two commands.

The command -Xms sets the initial and minimum heap size while the -Xms sets the maximum size; we need to use the second command.

 -Xms<size>        set initial Java heap size
 -Xmx<size>        set maximum Java heap size

In the command line, we write the following command that runs the program that we see in the first step.

We use the -Xmx2g to increase the heap size by 2GB. Notice that we write the storage unit as a single character; for example, if we want to set a 2GB heap size, we write 2g.

The output shows that the default size of around 1.4GB is now modified to 2.0GB.

java -Xmx2g JavaExample

Output:

Maximum Heap Size: 2.0 GB

Increase Java Heap Size in an Integrated Development Environment (IDE)

We can also increase Java heap space in an IDE if we run the program. Although some IDEs might have different steps to perform the task, if we follow these steps, we can modify the heap size in the majority of the IDEs.