how to increase java heap memory permanently?

I have one problem with java heap memory. I developed one client server application in java which is run as a windows service it requires more than 512MB of memory. I have 2GB of RAM but when I run my application it throws an exception

Out of memory error:java heap space

but I have already set heap size (maximum 512MB) in the java control panel and I still get the same error. I can't set heap size through the command line because my application runs as a windows service so how can I increase the default heap size?

Answer : - 

The Java Virtual Machine takes two command line arguments which set the initial and maximum heap sizes: -Xms and -Xmx. You can add a system environment variable named _JAVA_OPTIONS, and set the heap size values there.
For example if you want a 512Mb initial and 1024Mb maximum heap size you could use:
under Windows:

SET _JAVA_OPTIONS = -Xms512m -Xmx1024m

under Linux:

export _JAVA_OPTIONS="-Xms512m -Xmx1024m"

It is possible to read the default JVM heap size programmatically by using totalMemory() method of Runtime class. Use following code to read JVM heap size.

public class GetHeapSize {
    public static void main(String[]args){

        //Get the jvm heap size.
        long heapSize = Runtime.getRuntime().totalMemory();

        //Print the jvm heap size.
        System.out.println("Heap Size = " + heapSize);
    }
}