Java Heap Dump Analysis using Eclipse Memory Analyzer (MAT)

Posted by Mohaideen Jamil in Core Java, Eclipse, Java, JVM, Software

 
In this article we will learn what a Java heap dump is and how to analyze a heap dumps generated through OutOfMemoryError using Memory Analyzer in Eclipse.
 

What is a heap dump?

 
A heap dump is a snapshot of memory at a given point in time. It contains information on the Java objects and classes in memory at the time the snapshot was taken.
 

Why would we want to read heap dump?

 
If your Java application crashes with an OutOfMemoryError it is possible to automatically get a heap dump to analyze. This view into the memory profile of the application at the time it crashed can help us to figure out what caused the error. This can help decide what to optimize in our code.
 

How to get a heap dump?

 
To generate heap dump we have to execute a JVM with the following parameters in eclipse
 

-XX:+HeapDumpOnOutOfMemoryError writes heap dump on first  OutOfMemoryError

Here the heap dump will be generated in the “current directory” of the JVM by default. It can be explicitly redirected with
 

-XX:HeapDumpPath= for example -XX: HeapDumpPath=/disk/myFolder.

How to read a heap dump?

 
Heap dump will be in binary format so you don’t read the plain file. Instead use a tool like Memory Analyzer Tool.
Download MAT plugin from this location, and install it in your eclipse.
 

OOM Java program

Here the Java program below is used to trigger an OutOfMemoryError. This program is basically creating multiple String instances within a List data structure until the Java Heap depletion.
 

package com.simplecode.heap;

import java.util.ArrayList;
import java.util.List;

public class OOMHeapGenerator
{
public static void main(String[] args)
{
	System.out.println("JVM OutOfMemoryError Simulator");
	List<String> leakingVariable = new ArrayList<String>();
	
	try
	{
		while (1 < 2)
		{
		leakingVariable.add("OutOfMemoryError");
		}
	}
	catch (Throwable exp)
	{
	     if (exp instanceof java.lang.OutOfMemoryError)
	     {
	     System.out.println("OutOfMemoryError triggered! " + "[" + exp + "]");
	     }
	     else
	     {
	     System.out.println("Other Exception! " + "[" + exp + "]");
	     }
	}
	System.out.println("Simulator done!");
}
}

On running this program, when the JVM ran out of memory it created a heap dump file java_ pid1244.hprof.
 

 
Press F5 in your project folder, so now the generated heap dump file will appear in eclipse.

Load Heap Dump

 
Just double click on the heap dump file and select the Leak Suspects Report, and then this file will be loaded into MAT.
 

 

On clicking finish the following Screen is obtained
 

 

Analyze Heap Dump

 
Clicking on the “See stacktrace” link will give you the exact line of code that was building up the huge List causing the application to crash.

Now we know where to go look in at the code to fix this bug. Hence by analysing the Heap Dump using the MAT we could easily identify our primary leaking Java class and data structure.

Solution for “Could not create the Java Virtual Machine” issue

Posted by Mohaideen Jamil in Eclipse, Java | 1 comment

 
If you get the below error message while executing java program
 

Error:Could not create the Java Virtual Machine.
Error:A fatal exception has occurres.Program will exit.

 
To resolve this issue, following steps are needed to be followed (on Windows XP, Windows 7, Windows 8):

• Right Click on My Computer Icon and click on Properties
• Click on Advanced Systems Settings
• Then Environment Variables
• Under System Variables, click New
• In which give Variable Name as: _JAVA_OPTIONS
• Variable Value: -Xmx1024m
• Click OK

That’s it, now your Java program should execute properly.

 
Note:
Heap size allocated by the JVM by default can be Increased/Modified by using Java -Xmx/s command line options.

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

Click here to know about getting JVM heap size, used memory, total memory using Java Runtime programatically.
 

How to increase JVM heap size in Eclipse

Posted by Mohaideen Jamil in Eclipse, Java, Jboss, Tomcat | 1 comment

 
To avoid getting java.lang.OutOfMemoryErrors, while running web application, we should increase heap size allocated by the JVM by using command line options.
 

-Xms<size>        set initial Java heap size
-Xmx<size>        set maximum Java heap size
-Xss<size>        set java thread stack size

To do this, follow these steps:

1. Open the Server Configuration in Eclipse by double-clicking on the Server instance.
 

 

2. Click on “Open launch configuration” link
 

 
3. Click on the Arguments tab and add the following command at the end of VM arguments: -Xms128m –Xmx1024m.
 

 

These values may vary, depending on requirement/available memory

 
If you are not using any server, you can type the following on the command line before you run your program:

java -Xms64m -Xmx1024m MyProgram

Note:

• Do not set -Xmx to too small value
• Set -Xms to a small value

Setting -Xmx to small value mostly leads to OutOfMemoryErrors, because this is the maximum amount of memory you are allocating for Java and it cannot utilize memory beyond the set value.

Also If you set -Xms to higher value you might run out of memory. So try to keep it to a small value like -Xms16m.
 

Eclipse Shortcuts for fast coding

Posted by Mohaideen Jamil in Eclipse | 1 comment

 



List of some Shortcuts for fast coding. Traverse below and improve your coding speed by utilizing Eclipse Shortcuts.

Download It – eclipse.pdf

File navigation  Short cuts
 

Java Editing Short cuts
 

 
Editing Short cuts
 

 
 
Debug and  Run Eclipse short cuts
 

 
Some Extra Short cuts
 

 
String to StringBuffer /StringBuilder
 
In order to change the concatenation from String to StringBuffer /StringBuilder, move the cursor to the String variable, and press Ctrl +1, and select ‘Use StringBuffer//StringBuilder for string concatenation’
For eg, String s = “a”+”b”+”c”+”d”;
will change to StringBuffer sB = new StringBuffer();
sB.append(“a”);
sB.append(“b”);
sB.append(“c”);
sB.append(“d”);
String s = sB.toString();
 

Debugging java application in Eclipse

Posted by Mohaideen Jamil in Eclipse, Java

5 practical Java debugging tips

 
Let’s see some common java debugging tips in eclipse.
 
1) Use conditional breakpoint
 
Eclipse allows you to setup conditional break point for debugging, which is a breakpoint with condition and your thread will only stop at specified line if condition matches instead of just stopping on that line like in case of line breakpoint. To setup a conditional breakpoint just double click on any line where you want to setup a breakpoint and then right click –> properties and then insert the condition. Now program will only stop when that particular condition is true.

 
2) Use Exception breakpoint
 
You can setup Exception breakpoint based on java exception like ArrayIndexOutOfBoundException or NullPointerException. You can setup Exception breakpoint from breakpoint window and your program will stop when exception occurs.

3 ) Inspect and Watch
 
These are two menu options which is used to see the value of expression during debugging java program. To do, just select the statement, right click and inspect and it will show you the value of that statement at debugging time. You can also put watch on that condition, now its value will appear on watch window.

 
4) Step over, Step Into
 
These are simply great debugging options available in any Java IDE, extremely useful if you are debugging multi-threaded application and want to navigate step by step.
 
5) Suspending and resuming thread
 
You can suspend and resume any thread while debugging java program from debug window. Just right click on any thread and select either suspends or resume.