This
document takes one through an example code deliberately executed in order to
obtain the “out Of Memory” Exception. The below is the sequence
- Execute the program using the
JVM option HeapDumpOnOutOfMemoryError.
This ensures the JVM to dump data in file
something like java_pidpid.hprof in Java working
directory
- Execute jhat <data file
name>
- The above step will start a
local server, accessible via web browsers. This step will read the data
file obtained from step #1 and then analyze the information, resolves the
references and be ready for the further steps
- Open the IE (or Mozilla) with
the below default URL
http://localhost:7000
- The
IE will list out all the references. This has the capability of drilling
down from one wrapper object (or container object) to the contained
objects. Example: On the landing page:
- Click on class WrapperClass”. This will show all
the reference to the objects held by WrapperClass instance. Similarly,
the step can be repeated to drill down on the references held to reveal
the references held by the clicked reference. This is a top-down drilling.
- Click on “Show
instance counts for all classes (including platform)” This will show
ALL the instances that were held at the moment the system crashed
i. Our example
deliberately pushes Integer objects onto an arraylist contained as a member
variable inside the WrapperClass.
ii. We should see a
large number of instances of Integers (because, we have deliberately did that).
Click on the “instances of Integer”.
iii. This should reveal ALL individual Integer Object instances
iv. Click on any of the
object instance shown
v. That should show the
“references to this object”
vi. Repeat the step
until the container object, WrapperClass, in our case is revealed
This is the
bottom-up drilling
Hence,
jhat comes extremely handy while debugging above scenarios.
Main Program
import java.io.File;
import
java.io.FileInputStream;
import
java.io.FileNotFoundException;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
public class MemorCrashTest {
public static void main(String args[])
{
for(;;)
{
WrapperClass w=new
WrapperClass();
File f=new
File("D:\\Workflow.zip");
try {
FileInputStream
fis=new FileInputStream(f);
while(fis.read()!=-1)
{
w.a.add(fis.read());
}
System.out.println(w.a);
} catch
(FileNotFoundException e) {
//
TODO Auto-generated catch block
e.printStackTrace();
}catch (IOException
e) {
//
TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
WrapperClass.java -> This contains
the ArrayList
import java.util.ArrayList;
public class WrapperClass {
public ArrayList a =new ArrayList();
}
Compile and execute using the JVM parameters HeapDumpOnOutOfMemoryError
The
program has crashed with “Out of Memory” Error. And, has produced the data file
in c:\tmp\java_pid4072.hprof
Run
jhat and pass the generated profile data for analysis
Jhat has now analyzed the data
and is ready for reports. Open the browser for further steps
Click on the WrapperClass
Instance
Bottom
Up Drilldown – From Landing Page
Now you can dig into any level of classes to find the memory leakage.
