java heap memory check

[Jacob Sonia]

Hi,

I am trying this code for checking java heap memory, but somehow the stats doesn't look right

package com.ebay.ewa2.util;

import java.text.NumberFormat;

public class Main {
	
	public static void main(String[] args){
		
		Runtime runtime = Runtime.getRuntime();

	    NumberFormat format = NumberFormat.getInstance();

	    StringBuilder sb = new StringBuilder();
	    long maxMemory = runtime.maxMemory();
	    long allocatedMemory = runtime.totalMemory();
	    long freeMemory = runtime.freeMemory();

	    sb.append("free memory: " + format.format(freeMemory / 1024) + "<br/>");
	    sb.append("allocated memory: " + format.format(allocatedMemory / 1024) + "<br/>");
	    sb.append("max memory: " + format.format(maxMemory / 1024) + "<br/>");
	    sb.append("total free memory: " + format.format((freeMemory + (maxMemory - allocatedMemory)) / 1024) + "<br/>");
	    
	    System.out.println(sb);
	    
	    long total = Runtime.getRuntime().totalMemory();
	    long used  = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
	   
	    System.out.println("total " + total + "used " + used);
	    
	    long a = 1232;

		//MappingInitializationsForGeo.init();
		//MappingsInitializationForChnl.init();
		
		StringBuilder sb1 = new StringBuilder();
		long maxMemory1 = runtime.maxMemory();
	    long allocatedMemory1 = runtime.totalMemory();
	    long freeMemory1 = runtime.freeMemory();
	    
	    sb1.append("free memory: " + format.format(freeMemory1 / 1024) + "<br/>");
	    sb1.append("allocated memory: " + format.format(allocatedMemory1 / 1024) + "<br/>");
	    sb1.append("max memory: " + format.format(maxMemory1 / 1024) + "<br/>");
	    sb1.append("total free memory: " + format.format((freeMemory1 + (maxMemory1 - allocatedMemory1)) / 1024) + "<br/>");
	    
	    System.out.println(sb1);
	    
	    long total1 = Runtime.getRuntime().totalMemory();
	    long used1 = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
	    
	    System.out.println("total1 " + total1 + "used1 " + used1);
	    

	}

}
    

free memory: 3,117
allocated memory: 4,096
max memory: 1,048,576
total free memory: 1,047,597

total 4194304used 1004064
free memory: 3,115
allocated memory: 4,096
max memory: 1,048,576
total free memory: 1,047,595

total1 4194304used1 1004192


Looking at the first stats, it looks like a long takes 2 kb of memory. And the second stats, which is in bytes say that it is taking around 128 bytes of memory. Not sure if it is correct.

Thanks,


[HENRY: Fixed code tags]

[Paul Clapham]

There are so many things wrong with that, I don't even know where to start.

Okay. Let's try one thing: you assign values to three long variables, then you do some String concatenations, then you create a StringBuilder object. And you assume the difference between the free memory before this and the free memory after that is... the amount of memory used by a long variable?

Let's try another thing: the space for local variables is allocated at the start of the method. So even if you didn't create all those other objects, you still wouldn't be measuring the space required for a long variable.

And rounding your numbers down to the next lower multiple of 1024 isn't going to help you tell if a long variable requires 8 or 16 bytes.

[Jacob Sonia]

Hi Paul,



Thanks for the answer. I tried this..is this the same...

package com.ebay.ewa2.util;

import java.text.NumberFormat;

public class Main {
	
	public static void main(String[] args){
		
		Runtime runtime = Runtime.getRuntime();

	    NumberFormat format = NumberFormat.getInstance();

	    StringBuilder sb = new StringBuilder();
	    long maxMemory = runtime.maxMemory();
	    long allocatedMemory = runtime.totalMemory();
	    long freeMemory = runtime.freeMemory();

	    sb.append("free memory: " + format.format(freeMemory / 1024) + "<br/>");
	    sb.append("total free memory: " + format.format((freeMemory + (maxMemory - allocatedMemory)) / 1024) + "<br/>");
	    
	    System.out.println(sb);
	    	    
	    new Main().init();
	    
	    
	    StringBuilder sb1 = new StringBuilder();
		long maxMemory1 = runtime.maxMemory();
	    long allocatedMemory1 = runtime.totalMemory();
	    long freeMemory1 = runtime.freeMemory();
	    
	    sb1.append("free memory: " + format.format(freeMemory1 / 1024) + "<br/>");
	    sb1.append("total free memory: " + format.format((freeMemory1 + (maxMemory1 - allocatedMemory1)) / 1024) + "<br/>");
	    
	    System.out.println(sb1);
	    
	    	    

	}
	
	public void init(){
		long a = 1232;
	}

}

[HENRY: Fixed code tags]

[Henry Wong]

Jacob,

Please fix your code tags, it's not very readable the way you have it.

Henry

[Paul Clapham]

It looks to me like that code is trying to calculate how much heap memory is used. Am I right? But local variables use stack memory, not heap memory.