java startup option "-Xmx512M -XX:MaxPerSize=1024M" doesn't work for my jvm-CodePudding

I'm using oracle jdk1.8 on win10, trying to set max heap size of JVM and see what happens when OOM. Using this code snippet:

package mygroup;
import java.util.ArrayList;
import java.util.List;

public class App 
{
    public static class AUser{
        private String name;
        private String gender;
        private int age;
        public AUser(String n,String g,int a){
            name=n;
            gender=g;
            age=a;
        }
    }

    public static void main( String[] args ) throws Exception
    {
        System.out.println( "App!" );
        List<AUser> users=new ArrayList<AUser>();
        int max=40960000;
        int c=0;
        while(c < max){
              c;
            users.add(new AUser("Me","male",35));
        }
        System.in.read();
    }
}

I compile and run it with

java mygroup/App -Xmx512M -XX:MaxPerSize=1024M

I expect that before it hits "System.in.read()", it should crash and report OOM error for JVM. But nothing happened. Acturally it runs normally, and wait for my key strike.

Then I started java visualVM, connect to "mygroup.App", and executed "heap dump". I could see that this JVM is using around 3.8G memory and totally 40960000 instances of "mygroup.App@AUser" is using 1.47G of memory.

Seems that "-Xmx512M -XX:MaxPerSize=1024M" is not working.

And if I change my program to

        while(true){
            users.add(new AUser("Me","male",35));
        }

to create objects, actually this program will run until there's no more memory of my machine(I have 16G memory!)

So how to make "max heap size" work? Thansk!

CodePudding user response：

The JVM arguments need to go before the class you're running, as Slaw pointed out in a comment above.

Here's a really simple program, all it does it print out whatever string data was sent to the program as input parameters:

import java.util.Arrays;

public class example {
    public static void main(String[] args) {
        System.out.println("arguments: "   Arrays.toString(args));
    }
}

Here are a few runs showing that everything after example.java is simply passed into the program as input arguments:

$ java example.java
arguments: []

$ java example.java one two three
arguments: [one, two, three]

Here's a variation identical to what you posted, with your intended JVM arguments isntead being treated as regular input parameters to the program:

$ java example.java -Xmx512M -XX:MaxPerSize=1024M
arguments: [-Xmx512M, -XX:MaxPerSize=1024M]

So, you'll want to re-order the command input to set JVM parameters instead of sending input parameters to your program.