Analyzing Heap Dump with Jhat ( Java Heap Analysis Tool )

The jhat command parses a java heap dump file and launches a web server. jhat enables you to browse heap dumps using your favorite web browser 

This tool helps you to debug and anaiyze the objets running in the java program. This is usefull in debugging memory leaks and high object creations. The tool when started allows one to  view the object allocation in a browser. It also provides a Query based syntax for extracting various level of information from the Heap dump.

Noramlly a Heap File generated by the jmap command are not readable. jhat command helps to read the file. Once the jhat command is started , a server is started at 7000 .Once the command says,server is ready.Open a browser and type , localhost:7000 to see all about object information.

Dev:localhost:root-~ $ jhat -J-Xmx2G hprof.bin

Reading from myA2.hprof...

Dump file created Tue Sep 02 03:08:19 CDT 2014

Snapshot read, resolving...

Resolving 2208439 objects...

Chasing references, expect 441 dots................................................................................

...................................................................................................................

................

Eliminating duplicate references...................................................................................

...................................................................................................................

...................................................................................................................

.............

Snapshot resolved.

Started HTTP server on port 7000

Server is ready.

Some times when processing bigger dumps it is necessary to provide heap size for the jhat command itself. This can be done by passing the heap size as jhat -J-Xmx2G.

Once we open the browser with URL "http://localhost:7000" we can see the object allocation obtained from the heap dump.

The main page shows you all the classes and objects present leaving platform classes. The list is sorted by Full-Qualified class name and broken out package.

http://localhost:7000/histo/

This url provides the information about the heap 

http://localhost:7000/finalizerSummary/

Provides the details about the finalizer object information

http://localhost:7000/class/myNameClass

This URL provides us the information about the Class name myNameClass

http://localhost:7000/instances/MyNameClass

http://localhost:7000/allInstances/MyNameClass

The instances query shows you all instances of a given class. The "allInstances" variant includes instances of subclasses of the given class as well

http://localhost:7000/object/myObject

The object query gives you information about an object that was on the heap.

http://localhost:7000/showInstanceCounts/

This query shows the count of instances for every class in the system

Normally during OOMR we will look for "our" classes. We will need to exclude platform classes to easily detect abnormalities in our classes quantity (or size) use,

http://localhost:7000/showInstanceCounts/excludePlatform/

Query Language

As we said earlier there is a Query syntax facility available which will allow us to Extract various levels of information. Use the below url to see the Text box where we can enter the Queries

http://localhost:7000/oql/

Here are some of the sample Queries that we can use to extract information

select filter(heap.classes(),"/com.pack/(it.name)") : Show all classes that are loaded in heap that have com.pack pattern

select heap.findClass("com.pack.app.pqs.PQSLookupColumn").statics

lists the static variables in the class

select i from [Ljava.lang.Object; i where i.length == 10

select count(heap.objects('java.lang.String')) : Total Number of String objects in a Heap

select file.path.toString() from java.io.File file  : show path value of all File objects     

show names of all ClassLoader classes : select classof(cl).name from instanceof java.lang.ClassLoader cl

select o from instanceof 0xd404b198 o : show instances of the Class identified by given id string

         

Note that 0xd404b198 is id of a Class (in a session). This is found by looking at the id shown in that class's page.

select heap.findClass("java.lang.String").fields.length : get number of fields of java.lang.String class

select heap.findObject("0xf3800b58") : find the object whose object id is given

select filter(heap.classes(), "/java.net./(it.name)") :  select all classes that have name pattern java.net.*

         

System properties

select map(filter(heap.findClass('java.lang.System').props.table, 'it != null && it.key != null && it.value != null'),

            function (it) {

                var res = it.key.toString() + ' = ' + it.value.toString();

                return res;

            });

More to Come ,Happy learning :-)