Monday, 15 August 2011


Many times we get OutOfMemoryError in our glassfish logs. When no more memory is remaining in the Java process, it throws the OutOfMemoryError exception. This problem may indicate memory leak in the application. But not every OutOfMemoryError occurs due to memory leak, it may be due to load also. Some application needs more memory to run and this may also cause the OutOfMemoryError error.
To profile I will first take a heap dump using jmap, heap dump is a list of objects in the memory of JVM as well as the content of the memory occupied by those objects.  Then I will use Heap Analysis Tool (HAT) to deal with the heap dump. The Heap Analysis Tool (HAT) helps to debug unnecessary object retention by providing a simple way to browse the object topology in a heap snapshot.
jmap and jhat is available with Java SE 6.

Find the process id of the java process we are going to take heap dump. We can do that using Linux ps command or from Windows task manager or we can use the jps (Java Virtual Machine Process Status) tool.

Use jmap to generate the heap dump.

Now start jhat to read the heap dump.

Once jhat is ready it starts a web server, (the default port for the web server is 7000, and we can change it by command line option of jhat). We can use a web browser to inspect the heap dump.
Our jhat web server is running in the computer on port 7000

Some examples of using jhat:

1) All Classes (excluding platform):
The default page shows all classes present in the heap, excluding platform classes. By clicking on the name of a class, we can query the Class
Link to run for this query:

2) All Classes (including platform):
This query will also show the platform classes.
Link to run for this query:

3) Class Query:
We can use this query to view information about a class.
Suppose we want the information for the class org.mkcl.ef.AO.dao.AORegistrationDAO, use the link:

4) Instances Query
It shows all instances of a class.
To see all the instances of the class com.mysql.jdbc.Buffer, use the link

5) Object Query:
Object query shows information about an object that was in the heap.
To display information about the object 0×6a8b880, use the link:×6a8b880

6) Rootset query:
This query gives reference chains from the rootset to a given object.
For the object 0×6a8b880, use the link:×6a8b880

7) Objects reachable from a given object:
It shows the transitive closure of all objects reachable from a given object.
Objects reachable from 0×03b60518×03b60518

8) Instance Counts for All Classes (excluding platform):
It shows the count of instances for every class in the system, excluding platform classes.

9) Instance Counts for All Classes (including platform):
It shows the count of instances for every class in the system, including platform classes.

10) All Roots Query:
This query shows all members of the rootset.

11) Heap Histogram:
It shows the number of allocated objects at the time the jmap was executed.
To view the heap histogram use the link:

12) Object Query Language (OQL) query
We can use Object Query Language (OQL is SQL-like query language to query Java heap, OQL allows to filter/select information wanted from Java heap.) using the link:

Help for using OQL can be found in the link
Using the above method we can analyze the heap and may find out the probable reason of the OutOfMemoryError.

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