Debugging

Use jpsand jstacto learn more about Java processes.jstatcan also be useful for Java statistics monitoring.

jvmtop.sh --profile <PID>

cd /opt/www/java/jdk/bin/

# jmap -heap <JAVA_PID>

# java -XX:+PrintFlagsFinal -version | grep -iE 'HeapSize|PermSize|ThreadStackSize'

https://blogs.sap.com/2014/04/02/using-jvmtop-and-jvmmon-to-diagnose-tomcat-cpu-bottlenecks/

Thread StackTrace

In *nix, withtopby pressingHyou can see the threads.

Then withjpsyou can see thepidbear in mind that if the process was started with privileges then you must execute it withsudofor instance.

If you take the thread id and converted it to hexadecimal then you can cross that data with thejstack pidoutput.

Both tools are in$JAVA_HOME/bin.

Explain memory management in java.

Ans. In java, memory is managed via garbage collector. Few techniques for memory management are:

1. Reference Counting:

A count of references to each object is maintained. When garbage collector runs, it deletes objects with zero reference count.

Drawback:

Circular references are maintained in memory.

2. Tracing collectors/Copy Collector/Stop and copy collector:

Start from a root object and keep a track of all references which have direct/indirect reference to the root object. Then all the live objects are moved to another heap, taking care of references properly.

Drawback:

At each point of time, you will have 2 heaps thus consuming twice the memory.

3. Mark sweep collectors/Stop and work collector:

Similar to tracing collector except that instead of copying the references to the new heap, they are swept out of memory, after a list of live and dead objects is known.

Mark and sweep is a stop-the-world garbage collection technique; that is all application threads stop until garbage collection completes or until a higher-priority thread interrupts the garbage collector. If the garbage collector is interrupted it must restart which can lead to application thrashing with little apparent result.