GC Interview Questions Answer in Java
Garbage collection
interview questions are very popular in both core Java and advanced Java
Interviews. Apart from Java Collection
and Thread many tricky Java questions stems Garbage
collections which are tough to answer. In this Java Interview article I will
share some questions from GC which is asked in various core Java interviews.These
questions are based upon concept of How Garbage collection works,
Different kinds of Garbage collector and JVM parameters used for
garbage collection monitoring and tuning. As I said GC is an important part of
any Java interview so make sure you have good command in GC. One more thing
which is getting very important is ability to comprehend and understand Garbage collection Output,
more and more interviewer are checking whether candidate can understand GC
output or not. During Java interview they may
provide a snippet of GC output and ask various questions based on that e.g.
Which Garbage collector is used, whether output is from major collection or
minor collection, How much memory is free from GC, What is size of new
generation and old generation after GC etc. I have included few Garbage
collection interview questions and answers from GC output to help with that. It’s recommended to prepare questions from Java collection, multithreading and programming along with Garbage
collection to do well in Java interviews at any stage.
Interview
questions on Java Garbage collection
Here is someGarbage collection Interview questions from my personal collection, which I
have created from my experience and with the help of various friends and colleagues which has shared GC interview
questions with me. Actually there are lot many questions than What I am
sharing here but to keep this post small I thought to only share some
questions, I can think of second part of GC interview question if you guys find
this useful.
Question 1 - What is structure of
Java Heap ? What is Perm Gen space in Heap ?
Answer : In
order to better perform in Garbage collection questions in any Java interview,
It’s important to have basic understanding of
Java Heap space. To learn more about heap, see my post 10 points on Java heap space.
By the way Heap is divided into different generation e.g. new generation, old
generation and PermGen space.PermGen space is used to store
class’s metadata and filling of PermGen space can cause java.lang.OutOfMemory:PermGen space.
Its also worth noting to remember JVM option to configure PermGen
space in Java.
Question 2 - How do you identify
minor and major garbage collection in Java?
Answer: Minor
collection prints “GC” if garbage collection logging is enable using –verbose:gc or -XX:PrintGCDetails, while Major collection prints “Full GC”. This Garbage collection interview
question is based on understanding of Garbage collection output. As more and
more Interviewer are asking question to check candidate’s ability to understand
GC output, this topic become even more important.
Question 3 - What is difference
between ParNew and DefNew Young Generation Garbage collector?
Answer :
This Garbage Collection interview
questions is recently asked to one of my friend. It require more than
average knowledge on GC to answer this question. By the way ParNew and DefNew is two young generation garbage collector. ParNew is a multi-threaded GC used along
with concurrent Mark Sweep while DefNew is single threaded GC used along with Serial Garbage
Collector.
Question 4 - How do you find GC
resulted due to calling System.gc()?
Answer : Another
GC interview question which is based on GC output. Similar to major and minor collection,
there will be a word “System” included in Garbage collection output.
Question 5 - What is difference
between Serial and Throughput Garbage collector?
Answer : Serial
Garbage collector is a stop the world GC which stops application thread from
running during both minor and major collection.
Serial Garbage collector can be enabled using JVM option -XX:UseSerialGC and it's designed for Java
application which doesn't have pause time requirement and have client
configuration. Serial Garbage collector
was also default GC in JDK 1.4 before ergonomics was introduced in JDK 1.5.
Serial GC is most suited for small application with less number of thread while throughput GG is
more suited for large applications. On the other hand Throughput garbage
collector is parallel collector where minor and major collection happens in parallel taking full advantage of all the system resources available like multiple
processor. Though both major and minor collection runs on stop-the-world
fashion and introduced pause in application. Throughput Garbage collector can
be enable using -XX:UseParallelGC or -XX:UseOldParallelGC. It increases overall throughput of
application my minimizing time spent in Garbage collection but still has long
pauses during full GC.This is a kind of Garbage
collection interview questions which gives you an opportunity to show your
knowledge in detail while answering. I always suggest to answer these kind of
questions in detail.
Question 6 – When does an Object
becomes eligible for Garbage collection in Java ?
Answer : An
object becomes eligible for garbage collection
when there is no live reference for that object or it can not be reached by any
live thread. Cyclic reference doesn’t count as live reference and if two
objects are pointing to each other and there is no live reference for any of
them, than both are eligible for GC. Also Garbage collection thread is a daemon thread which will run
by JVM based upon GC algorithm and when runs it collects all objects which are
eligible for GC.
Question 7 - What is finalize method
in Java ? When does Garbage collector calls finalize method in Java ?
Answer : Finalize
method in Java also called finalizer is a method defined in java.lang.Object and called by Garbage collector
before collecting any object which is eligible for GC. Finalize() method provides last chance to
object to do cleanup and free any remaining resource, to learn more about
finalizers, read What is finalize method in Java.
Question 8 - If Object A has
reference to Object B and Object B refer to Object A, apart from that there is
no live reference to either object A or B, Does they are eligible to Garbage
collection ?
This Garbage
collection interview questions is related question 5 “When object become
eligible for Garbage collection”. An object becomes eligible for Garbage
collection if there is no live reference for it. It can not be accessible from
any Thread and cyclic dependency doesn’t prevent Object from being Garbage
collected. Which means in this case both Object A and Object B are eligible of
Garbage collection. See How Garbage collection works in Java
for more details.
Question 9 -Can we force Garbage
collector to run at any time ?
Answer : No,
you can not force Garbage collection in Java. Though you can request it by
calling Sytem.gc() or its cousin Runtime.getRuntime().gc(). It’s not guaranteed that GC will
run immediately as result of calling these method.
Question 10 - Does Garbage collection
occur in permanent generation space in JVM?
Answer : This is a tricky Garbage collection interview
question as many programmers are not sure whether PermGen space is part of Java heap space or not and
since it maintains class Meta data and String
pool, whether its eligible for garbage collection or not. By the way Garbage Collection
does occur in PermGen space and if PermGen space is full
or cross a threshold, it can trigger Full GC. If you look at output of GC you
will find that PermGen space is also garbage collected. This is why correct
sizing of PermGen space is important to avoid frequent full GC. You can control
size of PermGen space by JVM options -XX:PermGenSize and -XX:MaxPermGenSize.
Question 11 : How to you monitor
garbage collection activities?
Answer : One
of my favorite interview questions on Garbage collection, just to
check whether candidate has ever monitored GC activities or not. You can monitor
garbage collection activities either offline or real-time. You can use tools
like JConsole and VisualVM VM with its Visual GC plug-in
to monitor real time garbage collection activities and memory status of JVM or
you can redirect Garbage collection output to a log file for offline analysis
by using -XlogGC=<PATH> JVM parameter. Anyway you should
always enable GC options like -XX:PrintGCDetails -X:verboseGC and -XX:PrintGCTimeStamps as it doesn't impact application performance much
but provide useful states for performance monitoring.
Question 12: Look at below Garbage
collection output and answer following question :
[GC
[ParNew: 1512K->64K(1512K), 0.0635032
secs]
15604K->13569K(600345K), 0.0636056
secs]
[Times: user=0.03 sys=0.00, real=0.06
secs]
1. Is this output of Major Collection or Minor
Collection ?
2. Which young Generation Garbage collector is
used ?
3. What is size of Young Generation, Old
Generation and total Heap Size?
4. How much memory is freed from Garbage
collection ?
5. How much time is taken for Garbage collection ?
6. What is current Occupancy of Young Generation
?
This Garbage
collection Interview questions is completely based on GC output. Following are
answers of above GC questions which will not only help you to answer these
question but also help you to understand and interpret GC output.
Answer 1:
It's Minor collection because of "GC" word, In case of Major collection, you would see "Full GC".
Answer 2: This output is of multi-threaded
Young Generation Garbage collector "ParNew", which is used along with CMS concurrent Garbage
collector.
Answer 3: [1512K] which is written in bracket
is total size of Young Generation, which include Eden and two survivor space.
1512K on left of arrow is occupancy of Yong Generation before GC and 64K is
occupancy after GC. On the next line value if bracket is total heap size which
is (600345K). If we subtract size of young
generation to total heap size we can calculate size of Old Generation. This
line also shows occupancy of heap before and after Garbage collection.
Answer 4: As answered in previous garbage
collection interview question, second line shows heap occupancy before and
after Garbage collection. If we subtract value of right side 13569K, to value
on left side 15604K, we can get total memory freed by GC.
Answer 5: 0.0636056 secs on second line denotes total time it took to collect dead
objects during Garbage collection. It also include time taken to GC young
generation which is shown in first line (0635032 secs).
Answer 6: 64K
Here are few
more interesting Garbage collection
Interview question for your practice, I haven’t provided answers of all
garbage collection interview questions. If you know the answer than you can post via comments.
Question - What is difference between -XX:ParallelGC and -XX:ParallelOldGC?
Question -
When do you ConcurrentMarkSweep Garbage collector and Throughput GC?
Question - What is difference between
ConcurrentMarkSweep and G1 garbage collector?
Question - Have you done any garbage collection tuning?
What was your approach?
These were
some Garbage collection interview questions and answers, may help on your Java
Interview preparation. If you have got any interesting interview questions
related to GC than don’t forget to share with us.
Other Java Interview resources from
Javarevisited
Source:http://javarevisited.blogspot.com/2012/10/10-garbage-collection-interview-question-answer.html
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