I have read many articles on Garbage Collection in Java, some of them are too complex to understand and some of them don’t contain enough information required to understand garbage collection in Java. Then I decided to write my own experience as an article. You can call it a tutorial about garbage collection in simple words, which would be easy to understand and have sufficient information to understand how garbage collection works in Java. Garbage collection works by employing several GC algorithms like Mark and Sweep, G1, etc. There are different kinds of garbage collectors available in Java to collect different areas of heap memory like you have serial, parallel, and concurrent garbage collectors in Java.
A new collector called G1 (Garbage first) is also introduced in JDK 1.7. The first step to learning about GC is to understand when an object becomes eligible for garbage collection?
A new collector called G1 (Garbage first) is also introduced in JDK 1.7. The first step to learning about GC is to understand when an object becomes eligible for garbage collection?
Since JVM provides memory management, Java developers only care about creating an object, they don't care about cleaning up, which is done by the garbage collector, but it can only collect objects which have no live strong reference or it's not reachable from any thread.
If an object, which is supposed to be collected but still lives in memory due to unintentional strong reference then it's known as a memory leak in Java. ThreadLocal variables in Java web applications can easily cause the memory leak.
And, if you are serious about improving your advanced JVM skill and learning things like taking and analyzing heap dumps then highly recommend you to join these advanced Java performance courses on Udemy. It's one of the advanced courses for Java programmers to learn more about Performance and Memory management including troubleshooting memory leaks in Java.
1. Objects are created on the heap in Java irrespective of their scope like local or member variables. while it's worth noting that class variables or static members are created in the method area of Java memory space and both heap and method area is shared between different thread.
2. Garbage collection is a mechanism provided by Java Virtual Machine to reclaim heap space from objects which are eligible for Garbage collection.
3. Garbage collection relieves Java programmers from memory management which is an essential part of C++ programming and gives more time to focus on business logic.
4. Garbage Collection in Java is carried by a daemon thread called Garbage Collector.
5/ Before removing an object from the memory garbage collection thread invokes finalize() method of that object and gives an opportunity to perform any sort of cleanup required.
6. You as a Java programmer can not force garbage collection in Java; it will only trigger if JVM thinks it needs a garbage collection based on Java heap size.
7. There are methods like System.gc() and Runtime.gc() which are used to send requests of Garbage collection to JVM but it’s not guaranteed that garbage collection will happen.
8. If there is no memory space for creating a new object in Heap Java Virtual Machine throws OutOfMemoryError or java. lang.OutOfMemoryError heap space
9. J2SE 5(Java 2 Standard Edition) adds a new feature called Ergonomics goal of ergonomics is to provide good performance from the JVM with a minimum of command line tuning. See Java Performance The Definitive Guide for more details on garbage collection tuning.
Generally, an object becomes eligible for garbage collection in Java in the following cases:
1) All references to that object are explicitly set to null e.g. object = null
2) The object is created inside a block and reference goes out of scope once control exits that block.
3) Parent object set to null if an object holds the reference to another object and when you set the container object's reference null, child or contained object automatically becomes eligible for garbage collection.
4) If an object has only lived weak references via WeakHashMap it will be eligible for garbage collection.
When an object is first created in heap it gets created in a new generation inside Eden space and after subsequent minor garbage collection if an object survives it gets moved to survivor 1 and then survivor 2 before major garbage collection moved that object to old or tenured generation.
Permanent generation of Heap or Perm Area of Heap is somewhat special and it is used to store Metadata related to classes and methods in JVM, it also hosts a String pool provided by JVM as discussed in my string tutorial why String is immutable in Java.
There are many opinions around whether garbage collection in Java happens in the perm area of the Java heap or not, as per my knowledge, this is something that is JVM dependent and happens at least in Sun's implementation of JVM. You can also try this by just creating millions of String and watching for the Garbage collection or OutOfMemoryError.
1) Throughput Garbage Collector: This garbage collector in Java uses a parallel version of the young generation collector. It is used if the -XX:+UseParallelGC option is passed to the runtime via JVM command-line options. The tenured generation collector is the same as the serial collector.
2) Concurrent low pause Collector: This Collector is used if the -Xingc or -XX:+UseConcMarkSweepGC is passed on the command line. This is also referred to as the Concurrent Mark Sweep Garbage collector. The concurrent collector is used to collect the tenured generation and does most of the collection concurrently with the execution of the application. The application is paused for short periods during the collection.
A parallel version of the young generation copying collector is used with the concurrent collector. Concurrent Mark Sweep Garbage collector is the most widely used garbage collector in java and it uses an algorithm to first mark object which needs to collect when garbage collection triggers.
3) The Incremental (Sometimes called train) low pause collector: This collector is used only if -XX:+UseTrainGC is passed on the command line. This garbage collector has not changed since java 1.4.2 and is currently not under active development. It will not be supported in future releases so avoid using this and please see the 1.4.2 GC Tuning document for information on this collector.
An important point to note is that -XX:+UseParallelGC should not be used with -XX:+UseConcMarkSweepGC. The argument parsing in the J2SE Platform starting with version 1.4.2 should only allow the legal combination of command-line options for garbage collectors but earlier releases may not find or detect all illegal combinations and the results for illegal combinations are unpredictable. It’s not recommended to use this garbage collector in java.
See these Java Performance courses and books to learn more about different JVM options and garbage collection tuning and troubleshooting.
Garbage collection tuning is a long exercise and requires a lot of profiling of application and patience to get it right. While working with High volume low latency Electronic trading system I have worked with some of the projects where we need to increase the performance of Java applications by profiling and finding what causing full GC and I found that Garbage collection tuning largely depends on an application profile, what kind of object application has and what is their average lifetime etc.
For example, if an application has too many short-lived objects then making Eden space wide enough or larger will reduce the number of minor collections. you can also control the size of both young and Tenured generations using JVM parameters for example setting -XX:NewRatio=3 means that the ratio of the young and tenured generation is 1:3, you got to be careful on sizing this generation.
Making the young generation larger will reduce the size of the tenured generation which will force the Major collection to occur more frequently which pauses application thread during that duration results in degraded or reduced throughput. The parameters NewSize and MaxNewSize are used to specify the young generation size from below and above. Setting these equal to one another fixes the young generation.
In my opinion, before doing garbage collection tuning a detailed understanding of how garbage collection works in Java is a must and I would recommend reading the Garbage collection document provided by Sun Microsystems for detailed knowledge of garbage collection in Java.
Also to get a full list of JVM parameters for a particular Java Virtual machine please refer to official documents on garbage collection in Java. I found this link quite helpful though http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html
As a fallback, if the concurrent garbage collector is unable to finish before the tenured generation fills up, the application is paused and the collection is completed with all the application threads stopped.
Such Collections with the application stopped are referred to as full garbage collections or full GC and are a sign that some adjustments need to be made to the concurrent collection parameters.
Always try to avoid or minimize full garbage collection or Full GC because it affects the performance of Java applications.
When you work in the finance domain for an electronic trading platform and on a high volume low latency systems performance of Java application becomes extremely critical you definitely like to avoid full GC during the trading hours.
2) New objects are created by the young generation and subsequently moved to the old generation.
3) String pool is created in PermGen area of Heap, garbage collection can occur in perm space but depends upon JVM to JVM. By the way from JDK 1.7 update, the String pool is moved to the heap area where objects are created.
4) Minor garbage collection is used to move an object from Eden space to survivor 1 and survivor 2 space and major collection is used to move an object from young to tenured generation.
5) Whenever Major garbage collection occurs application threads stop during that period which will reduce the application’s performance and throughput.
6) There are few performance improvements that have been applied in garbage collection in java 6 and we usually use JRE 1.6.20 for running our application.
7) JVM command-line options –Xmx and -Xms are used to set up starting and max size for Java Heap. The ideal ratio of this parameter is either 1:1 or 1:1.5 based on my experience, for example, you can have either both –Xmx and –Xms as 1GB or –Xms 1.2 GB and 1.8 GB.
8) There is no manual way of doing garbage collection in Java, but you can use various reference classes like WeakReference or SoftReference to assist garbage collectors.
And, if you prefer to watch than read then you can also check out this tutorial on our Youtube channel where we have talked about the internal working of HashMap and How HashMap works in general. If you want to receive notifications about new tutorials, you can subscribe to our Youtube channel here.
If an object, which is supposed to be collected but still lives in memory due to unintentional strong reference then it's known as a memory leak in Java. ThreadLocal variables in Java web applications can easily cause the memory leak.
And, if you are serious about improving your advanced JVM skill and learning things like taking and analyzing heap dumps then highly recommend you to join these advanced Java performance courses on Udemy. It's one of the advanced courses for Java programmers to learn more about Performance and Memory management including troubleshooting memory leaks in Java.
1. Important points about Garbage Collection in Java
This article is in continuation of my previous articles How Classpath works in Java and How to write the Equals method in Java and before moving ahead let's recall a few important points about garbage collection in Java.1. Objects are created on the heap in Java irrespective of their scope like local or member variables. while it's worth noting that class variables or static members are created in the method area of Java memory space and both heap and method area is shared between different thread.
2. Garbage collection is a mechanism provided by Java Virtual Machine to reclaim heap space from objects which are eligible for Garbage collection.
3. Garbage collection relieves Java programmers from memory management which is an essential part of C++ programming and gives more time to focus on business logic.
4. Garbage Collection in Java is carried by a daemon thread called Garbage Collector.
5/ Before removing an object from the memory garbage collection thread invokes finalize() method of that object and gives an opportunity to perform any sort of cleanup required.
6. You as a Java programmer can not force garbage collection in Java; it will only trigger if JVM thinks it needs a garbage collection based on Java heap size.
7. There are methods like System.gc() and Runtime.gc() which are used to send requests of Garbage collection to JVM but it’s not guaranteed that garbage collection will happen.
8. If there is no memory space for creating a new object in Heap Java Virtual Machine throws OutOfMemoryError or java. lang.OutOfMemoryError heap space
9. J2SE 5(Java 2 Standard Edition) adds a new feature called Ergonomics goal of ergonomics is to provide good performance from the JVM with a minimum of command line tuning. See Java Performance The Definitive Guide for more details on garbage collection tuning.
2. When an Object becomes Eligible for Garbage Collection?
An object becomes eligible for Garbage collection or GC if it's not reachable from any live threads or by any static references. In other words, you can say that an object becomes eligible for garbage collection if all references are null. Cyclic dependencies are not counted as the reference so if object A has a reference to object B and object B has a reference to Object A and they don't have any other live reference then both Objects A and B will be eligible for Garbage collection.Generally, an object becomes eligible for garbage collection in Java in the following cases:
1) All references to that object are explicitly set to null e.g. object = null
2) The object is created inside a block and reference goes out of scope once control exits that block.
3) Parent object set to null if an object holds the reference to another object and when you set the container object's reference null, child or contained object automatically becomes eligible for garbage collection.
4) If an object has only lived weak references via WeakHashMap it will be eligible for garbage collection.
3. Heap Generations for Garbage Collection in Java
Java objects are created in Heap and Heap is divided into three parts or generations for the sake of garbage collection in Java, these are called the Young generation, Tenured or Old Generation, and Perm Area of the heap. New Generation is further divided into three parts known as Eden space, Survivor 1, and Survivor 2 space.When an object is first created in heap it gets created in a new generation inside Eden space and after subsequent minor garbage collection if an object survives it gets moved to survivor 1 and then survivor 2 before major garbage collection moved that object to old or tenured generation.
Permanent generation of Heap or Perm Area of Heap is somewhat special and it is used to store Metadata related to classes and methods in JVM, it also hosts a String pool provided by JVM as discussed in my string tutorial why String is immutable in Java.
There are many opinions around whether garbage collection in Java happens in the perm area of the Java heap or not, as per my knowledge, this is something that is JVM dependent and happens at least in Sun's implementation of JVM. You can also try this by just creating millions of String and watching for the Garbage collection or OutOfMemoryError.
4. Types of Garbage Collectors in Java? Concurrent vs Serial Garbage Collectors
Java Runtime (J2SE 5) provides various types of Garbage collection in Java which you can choose based on your application's performance requirement. Java 5 adds three additional garbage collectors except for the serial garbage collectors. Each is a generational garbage collector that has been implemented to increase the throughput of the application or to reduce garbage collection pause times.1) Throughput Garbage Collector: This garbage collector in Java uses a parallel version of the young generation collector. It is used if the -XX:+UseParallelGC option is passed to the runtime via JVM command-line options. The tenured generation collector is the same as the serial collector.
2) Concurrent low pause Collector: This Collector is used if the -Xingc or -XX:+UseConcMarkSweepGC is passed on the command line. This is also referred to as the Concurrent Mark Sweep Garbage collector. The concurrent collector is used to collect the tenured generation and does most of the collection concurrently with the execution of the application. The application is paused for short periods during the collection.
A parallel version of the young generation copying collector is used with the concurrent collector. Concurrent Mark Sweep Garbage collector is the most widely used garbage collector in java and it uses an algorithm to first mark object which needs to collect when garbage collection triggers.
3) The Incremental (Sometimes called train) low pause collector: This collector is used only if -XX:+UseTrainGC is passed on the command line. This garbage collector has not changed since java 1.4.2 and is currently not under active development. It will not be supported in future releases so avoid using this and please see the 1.4.2 GC Tuning document for information on this collector.
An important point to note is that -XX:+UseParallelGC should not be used with -XX:+UseConcMarkSweepGC. The argument parsing in the J2SE Platform starting with version 1.4.2 should only allow the legal combination of command-line options for garbage collectors but earlier releases may not find or detect all illegal combinations and the results for illegal combinations are unpredictable. It’s not recommended to use this garbage collector in java.
See these Java Performance courses and books to learn more about different JVM options and garbage collection tuning and troubleshooting.
5. JVM Parameters for Garbage Collection in Java
For example, if an application has too many short-lived objects then making Eden space wide enough or larger will reduce the number of minor collections. you can also control the size of both young and Tenured generations using JVM parameters for example setting -XX:NewRatio=3 means that the ratio of the young and tenured generation is 1:3, you got to be careful on sizing this generation.
Making the young generation larger will reduce the size of the tenured generation which will force the Major collection to occur more frequently which pauses application thread during that duration results in degraded or reduced throughput. The parameters NewSize and MaxNewSize are used to specify the young generation size from below and above. Setting these equal to one another fixes the young generation.
In my opinion, before doing garbage collection tuning a detailed understanding of how garbage collection works in Java is a must and I would recommend reading the Garbage collection document provided by Sun Microsystems for detailed knowledge of garbage collection in Java.
Also to get a full list of JVM parameters for a particular Java Virtual machine please refer to official documents on garbage collection in Java. I found this link quite helpful though http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html
6. Full GC and Concurrent Garbage Collection in Java
The concurrent garbage collector in java uses a single garbage collector thread that runs concurrently with the application threads with the goal of completing the collection of the tenured generation before it becomes full. In normal operation, the concurrent garbage collector is able to do most of its work with the application threads still running, so only brief pauses are seen by the application threads.As a fallback, if the concurrent garbage collector is unable to finish before the tenured generation fills up, the application is paused and the collection is completed with all the application threads stopped.
Always try to avoid or minimize full garbage collection or Full GC because it affects the performance of Java applications.
When you work in the finance domain for an electronic trading platform and on a high volume low latency systems performance of Java application becomes extremely critical you definitely like to avoid full GC during the trading hours.
7. Summary of Garbage collection in Java
1) Java Heap is divided into three generations for the sake of garbage collection. These are a young generation, tenured or old generation, and Perm area.2) New objects are created by the young generation and subsequently moved to the old generation.
3) String pool is created in PermGen area of Heap, garbage collection can occur in perm space but depends upon JVM to JVM. By the way from JDK 1.7 update, the String pool is moved to the heap area where objects are created.
4) Minor garbage collection is used to move an object from Eden space to survivor 1 and survivor 2 space and major collection is used to move an object from young to tenured generation.
5) Whenever Major garbage collection occurs application threads stop during that period which will reduce the application’s performance and throughput.
6) There are few performance improvements that have been applied in garbage collection in java 6 and we usually use JRE 1.6.20 for running our application.
7) JVM command-line options –Xmx and -Xms are used to set up starting and max size for Java Heap. The ideal ratio of this parameter is either 1:1 or 1:1.5 based on my experience, for example, you can have either both –Xmx and –Xms as 1GB or –Xms 1.2 GB and 1.8 GB.
8) There is no manual way of doing garbage collection in Java, but you can use various reference classes like WeakReference or SoftReference to assist garbage collectors.
That's all about garbage collection in Java. In this tutorial, we learn how the heap is divided into different regions like Eden, survivor spaces, and perm gen space. An object becomes eligible for garbage collection when there is no strong reference pointing to it or it is not reachable from any thread.
When garbage collector realizes the need for garbage collection it triggers minor collection and sometimes stops-the-world major collection. It's all automatic as you cannot force garbage collection in Java.
When garbage collector realizes the need for garbage collection it triggers minor collection and sometimes stops-the-world major collection. It's all automatic as you cannot force garbage collection in Java.
Other Java Garbage Collections tutorials you may find useful
- How ClassLoader works in Java
- 10 Garbage Collection Interview question Answer
- How to remote debug Java program in Eclipse
- How to increase heap size in Maven and ANT script
- How to fix invalid heap errors in Java
- Java ArrayList and HashMap Performance improvement
- How to find CPU and memory usage of Java application
- Difference between JRE, JVM, and JDK in Java
- 10 ClassLoader interview Questions for experienced
- 5 Books to learn Java Performance in depth
- Difference between Stack and Heap in Java?
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88 comments :
Nice Explanation :)
Anand
Garbage Collection Explained from the SCJP Certification Perspective
Garbage collection is quite important if you are working in finance domain. since for any kind of trading e.g. Electornic, DMA, Forex, Fixed Income or Derivaties performance is most important given high volume and low latency nature of such application. no mater which are of Finance domain you work you always need to tune garbage collection parameter to get maximum performance and provide ultra low latency to your Direct to Market Access (DMA) Clients. even equity exchanges are now days upgrading there systems like Tokyo Stock exchange moved to Arrowhead.
>> "An Object becomes eligible for Garbage collection or GC if its not reachable from any live threads"
or additionally if It isn't reachable via static references, it's important because they are out of live threads scope. The roots of GC are all initialized static variables and all variables in live thread stacks. Cyclic references aren't problems because GC in Java doesn't count references: it goes thru the live objects graph starting from roots and mark it, all other objects that aren't accessible via roots marked as garbage and are eligible for collection.
@Anonymous, Indeed Garbage collection affects performance of high volume low latency electronic trading platform severely if not tuned properly and if major collection occurs multiple time during normal trading hours then latency increases multiple times which is not at all acceptable.for such sophisticated system you need to properly tune your garbage collection parameter and -Xms and -Xmx parameters.
Hi , we have one stock trading system on exchange connectivity side which connects to exchange and sends order to exchange for equity trading. I am working on to optimize that application , can you suggest some tool or method to optimize garbage collection usage for that java application. Since its Equity stock trading system we have latency as big concern.
If you're using quickfix/j for FIX, you might want to consider using the C++ API.
This is an wonderful article on garbage collection, i would request to update it with the G1 garbage collector as well
Hi,
I recently applied for etrading company but I dont have much experience in Garbage collection and Multi threading cos I was involved in application development before which was related to creating web pages.Now how do I prepare for trading software?
Hi Ramya,
Excellent knowledge of Java and FIX Protocol is required for getting into electronic trading firm. in Java they focus mostly on multithreading because all electronic trading applications are multi-threaded and driven by high volume which could be challenge if you are not familiar with concurrent programming. try to prepare these topic well by reading tutorials, articles, blog post etc and you will surely get into a decent job in big IBs.
Thanks a lot for your reply Javin...But some of the garbage collection topics is too theoretical to say.They focus even on memory model rt?I dint know where to start from as I just know basics of all these...Your help will be appreciated for sure:)
Hi Ramya,
If you are going for an interview next week then based on my experience at least prepare following topic:
1) different generations of heap space
2) how concurrent mark sweep garbage collection works ?
3) what is major and minor collection ? why you need to avoid major collections ?
All the best for your interview :)
Javin
oh thanks a lot Javin..this will surely help..I need not have trading experience to clear the interview rt?I will update myself with multithreading,synchronization,GC and Memory Model..Lets see how it goes..
I was looking for answer of "What is garbage collection in Java" in simple plain english and thanks to you now I know exactly what is garbage collection in java and how garbage collection works in Java.
Thanks Anonymous.just to answer your question "What is garbage collection in Java" "in case if you still have doubt, Garbage collection is the process inside Java Virtual Machine which recollect memory from java heap allocated to object which are eligible for garbage collection.
more important question is how an object becomes eligible for garbage collection , I think here are the situations
1) object will be eligible for garbage collection in java if there is no live reference from any thread.
2) if there is no static variable referencing object.
3) There is no object graph which is referencing the object
Hi Anonymous thanks for your contribution on garbage collection eligibility , indeed its important to know when a particular object eligible to garbage collection .
Thanks Anonymous and Gautam. Garbage collection in java is a vast topic and this tutorial aims to provide at least an overview of different areas of garbage collection in java.
Hi Javin, Roger again I have dropped you one message on one of your post, it looks like it doesn't appear here it is again " I am working on electronic trading system which we are going to design for foreign exchange and currency trading. I have some question related to FIX Protocol and how we can use FIX Protocol for currency trading. Can you please help me. since I don't have any prior experience on writing any electronic trading system" any suggestion will be appreciated
Hi Roger, I would be glad to help you. I have dropped you an email about FIX Protocol and electronic trading system. by the way this post is about garbage collection java :)
Does choice of garbage collector in Java affect performance of your application. if yes than how to choose correct garbage collector ?
How to minimize the use of Garbage collection?
@ab, head first java is my favorite java book for beginners. you may like that.
@Mukesh , to minimise garbage collection in java you need to do some garbage collection tuning and optimization. by properly setting size of eden space and survival space you can minimize garbage collection in java.
@Anonymous , yes choice of garbage collector in java affects performance, mostly people use ConcurrentMarkSweep garbage collection for better performance because it does both minor collection concurrently with application thread and minimize major collection.
I found answered to my basic questions like What is garbage collection and why do garbage collection needed . thanks a lot its indeed very easy to understand and grasp. In my opinion one of the best garbage collection tutorial in java
very nice article javin. can you elaborate on mark sweep algorithm. like how it starts tha application graph from root objects and all. it will be very help full for me.
thanks...........wonderful explanation
@Anonymous and imran , thanks for you comment , good to know that you find this Java Garbage Collection tutorial useful.
Javin, very simple and clear explanation of garbage collection. thanks a lot for the article!!
I want to know the destruction time of any object. Can it be done with the help of garbage collection or some other way? Please suggest some ways.
nice article ....
But I have a question regarding it..
Is there any way to find out the no of live objects of a class(or if we can figure out how many objects were garbage collected within a specific time interval) ?
@Lets Share, There is no precise way of knowing which objects gets garbage collected at which time, at-least in my knowledge. though you can estimate it by overriding finalize() method of Object class. Since Garbage Collector in Java calls finalize() method before garbage collecting object you can log the time in finalize method. though GC only calls finalize() one time per object and there might be delay between call to finalize and actual garbage collection.
@Anuj, I think number of Objects in heap for a class can be obtained by either profiling the application or by using JConsole. most of pro-filer shows this data. you can also take heap dump and analyze it for object based on counts and type.
Hi Javin, I have a doubt.
You have mentioned "Objects are created on heap in Java irrespective of there scope e.g. local or member variable. while its worth noting that class variables or static members are created in method area of Java memory space"
My doubt is suppose we have a class like below. I think arrayList, i3 and i4 instances will be created on heap.
Could you please tell me where
arraylist refernce variable is kept?
where m,k is created?
where set instance in created?
where set reference variable is kept?
where i3 and i4 reference variables are created? method stack?
where string test1 and test2 are ceated?
where s and newString are created?
class A {
int m = 0;
ArrayList arrayList = new ArrayList();
static int k = 0;
static HashSet set = new HashSet ();
String s = new String("newString");
void method(){
Integer i3 = new Integer(3);
Integer i4 = new Integer(4);
arrayList.add(i3);
arrayList.add("Test1");
set.add(i4);
set.add("Test2");
}
}
One more doubt Javin. You said Perm area of heap stores meta data of classes. That means java.lang.Class Object corresponding to each type created is kept in Perm space? then what is stored in the method area (which is not part of heap)
Javin, also please go through http://www.artima.com/insidejvm/ed2/jvm5.html
Its about method area.
After reading the link, i am in a confusion about what is stored in PERM area of heap.
Liked your comment about GC articles at the beginning. Most GC articles are indeed extreme ends of the pendulum.
In any case, I just have a minor comment about the part which I think is not accurate.
"....and then Survivor 2 before Major Garbage collection moved that object to Old or tenured generation."
It is still the Minor Garbage Collector which moves objects from S1, S2 to Tenured-Generation, not the Major Garbage Collector.
Thanks for your correction Suhas, indeed its minor garbage collector which moves object from S2 to tenured generation and major garbage collector only cleans tenured generation.
hi Javin, Excellent article
Could you please eloberate JVM command line options –Xmx and -Xms
Hi Priya, glad to know that you like this Java Garbage collection article. I have discussed about -Xmx and -Xms in couple of my post like
How to increase heap size in ANT and Maven
let me know if you still have any question.
What would happen if I instantiate an object and then force it to be set to null?
eg. String s = String.new();
s = null;
What happens in this case?
there is nothing like String.new() , it should be new String() but I got your point. in this case string pointed by s is eligible for garbage collection and memory used by this instance is reclaimed when Garbage collector will run.
I know that there is an old Garbage Collector in JAVA, but I cant find information about it. Can you help me?
Excellent, but I want to one more thing that's: in-case Exception occurred at the time when GarbageCollection is running, then what will happen
Hi Atul, Exception can occur while execution finalize() method which called before Garbage collection in java in that case GC just continue, nothing special.
Hi Javin, we generally keep -Xms512m -Xmx1024m whenever we get Heap Space problem. It means we are following a ratio of 1:2. But you said ideal ratio is 1:1 or 1:1.5. Will my ratio of 1:2 will impact anything badly on the heap space?
@Seenu, As such not a problem but experience shows that having heap ratio 1:1 or 1:5 JVM expands itself better from -Xms to -Xmx but as I said its GC tuning exercise.
Thanks Sreedhar, Glad to hear that you like this tutorial on Garbage Collection. Please consider sharing with your circle.
Hi..
I Read some books Garbage collection is internally fallowing “Mark and Sweep ” algorithm.
When the object is eligible to for the GC , that particular object is going to be marked .before exiting that method or class level. Marked object is sweep out.
can you please elaborate on the below comment.
Important point to not is that -XX:+UseParallelGC should not be used with -XX:+UseConcMarkSweepGC.
PermGen area also get GCed. When PermGen are reach to its certain size, it will also GCed and reclaim space from String Pool too.
you can check-out by below example which will lead to GC in PermGen area too.
for (int i = 0; i < 99999; i++) {
String s = String.valueOf(i).intern();
}
I THINK......
OBJECT ARE DYNEMICALLY ALLOCATED BY USING NEW OPERATOR IT IS CALLED GARBAGE COLLECTION
very gud.......article...
i think object are dynemically allocated by new operator this is called garbage collection...
beautiful article...Please keep writing such articles for the sake of knowledge seekers like me :)
It would have been really gr8 if there were some example programs..It could be understandable ,if it was explained by a simple diagrams for the example progs ...
Both Serial Garbage collector and Parallel Garbage collector or Throughput GC are Stop the world GC, it means application thread stopped when Garbage collection happens. Only difference between Serial Garbage Collection and Parallel (Throughput GC) is that in Serial both Young Generation (Eden Space + survivor) or minor collection and full GC (major collection) happens serially which introduced larger pause time while in case of Parallel GC minor collection and major collection happens in parallel which results in lower or reduced pause time. Another worth noting difference between parallel and serial GC is that former is more suited for Servers written in Java while later is best suited for Client application with lesser heap size.
On the other hand Java now has two concurrent Garbage collector as well 1) CMS (Concurrent mark sweep) and Garbage first (G1) Garbage collector which is introduced in Java 7. Both CMS and G1 are concurrent collector as oppose to stop the world Parallel GC and most of there operation performed in parallel of Application thread to further reduce application pause time but that require more overhead in terms of larger heap space because actual freeing of space require more time. By the way G1 Garbage collector is introduced as replacement of CMS GC and it employes different algorithm for garbage collection like it divide whole space in different region and target region which has most Garbage on it. that's why known as Garbage first.
It's important to choose right kind of Garbage collector based upon your application need. Concurrent Mark Sweep (CMS) or G1 Garbage collector are more suited for high response time java application like Web Servers.
There is always couple of Garbage collection interview questions on Java interviews, following are some interview question on garbage collection which I had collected recently from my colleagues, I am looking for answers now:
1) Difference between Serial Garbage collector and Parallel Garbage Collector?
2) What is ConcurrentMarkSweep Garbage Collector, Can you explain how Concurrent Mark Sweep GC works?
3) What is Garbage collection tuning ?
4) What is difference between major collection and minor collection in GC ?
5) Can we run Garbage collector explicitly? How do you recognize full garbage collection because of Runtime.gc() or System.gc()
6) What is difference between CMS and G1 garbage collector?
7) What is Eden space in Heap?
8) Have you ever done Garbage collection tuning? What are the heap sizes you have used?
9) have you used -XX:+UseCompressedOops in 64 bit JVM? why you should use it?
10) Can you extend Garbage Collection mechanism to provide your own Garbage collector ?
if anybody has answers for these Garbage Collection interview questions than please let me know.
Hello, What is difference between -XX:+UseParallelOldGC or -XX:+UseParallelGC command line options ? isn't it both used to enable Throughput Garbage Collector ?
Awesome Explanation!! I never knew about these so much stuffs about garbage collection
One the best tutorials on GC that I have seen as yet on the Internet. Simple, clear and informative....
Keep up the great work and looking forward to more of your articles.
Hi @Javin : This is an excellent document. Thanks a Lot.
I have a few questions.
1. Is there any specific reason when only Major Collection will take place? I need a little more elaboration on the difference between Major n Minor Collection.
2. When are the times that the JVM does not go for Garbage Collection when it actually should have, and ultimately ends up in causing Memory Leaks? [Leaving out the options of not closing File,DB Handlers]
Anyone is welcome with an Answer
Thanks in Advance ....
Really very nice article.
=> Ideal ratio of this parameter is either 1:1 or 1:1.5.
I have set –Xmx and –Xms as 512GB or –Xms 1 GB this is good or not?
One doubt. You have mentioned about method area where static variables are stored. I hope method area is different from the perm space of heap.
You have mentioned class metadat is stored perm space of heap.
In another site, http://www.artima.com/insidejvm/ed2/jvm2.html, it is said that class metadata is stored in the method area.
So Could you please clarify
1. whether method area or perm space actually stores class metadata?
2. Whether gc occurs in method area?
Hi. In my project recently getting problem with Connection hits to DB, As we are closing properly bt still some where its giving the problem while asking db dba he told that lot of connections are open. pls let us know the thing. this is happening with WAPT tool while testing the app for performance. could you please tell me to test my code for connection failuer scenarios and gc possible code.
@Javin
...When an object first created in heap its gets created in new generation inside Eden space and after subsequent Minor Garbage collection if object survives its gets moved to survivor 1 and then Survivor 2 before Major Garbage collection moved that object to Old or tenured generation... needs a little bit of correction.
Young generation divided into three parts eden, From Space(S1), To Space(S2). Initially the objects are allocated to eden and from space. when the minor collection occurs live objects are moved from eden and From Space(S1) to To Space(S2). Then the role of S1 and S2 is changed. The objects surviving one or more minor collection are then moved to the tenured generation.
I want to know how the objects would be allocated to heap and what would be the structure of heap if there is no garbage collector. Further tell me whether G1 is implemented fully or it is still in experiment and the default arrangement of generation in G1.
Thanks
Read more: http://javarevisited.blogspot.com/2011/04/garbage-collection-in-java.html#ixzz2NZw9TldD
Hi i have some questions related to GC
1. GC give guarantee that object will be removed ?
2. What happens when GC call finalize() methhod , and why it calls ?
3. And the last one , difference regions of heap space , i cant understand that regions ?
Thanks !
How many object are elligible for garbage collection for below program:
class A{
}
class B extends A{
public static void main(String a[]){
B b =new B();
B b1= new B();
b=b1;
}
}
As this is falls under inheritance, how many object are eligible for garbage collection?
Thanks!
From Java 7. String Pool is moved to the Heap.
http://stackoverflow.com/a/14194112
1) stop abusing the HEAP
2) stop using lesser platform like Java for critical production systems
3) stop making garbage to start with. learn the damn C/C++
gc() runs the finalization methods of any objects pending finalization.
Calling this method suggests that the Java virtual machine expend
effort toward running the finalize methods of objects
that have been found to be discarded but whose finalize
methods have not yet been run. When control returns from the
method call, the virtual machine has made a best effort to
complete all outstanding finalizations.
went through some other links...but this is only one provides the necessary info which i was looking for. Thank you for very clear, straight nd simple explanation.
oncurrent-Mark-Sweep collector is most popular garbage collector of Java. CMS collector is popular for its better throughput and less pause time. Because for many applications, end-to-end throughput is not as important as fast response time. For example gaming applications need fast response time to make their gaming experience better, if any game hang for a second only, it lost its charm. As you know young generation collections do not typically cause long pauses, because of its small size and less amount of live objects survived. However, old generation collections is uncertain, can impose long pauses, especially when large heaps are involved. To address this issue, the Java HotSpot JVM includes a collector called the concurrent-mark-sweep (CMS) collector, also known as the low-latency collector
To read more click here http://www.somanyword.com/2014/01/concurrent-mark-sweep-cms-garbage-collector-in-java/
What is the condition for garbage collector to start? When does it start in your application? There are some conditions that trigger the start of garbage collector-what are they?
How long Garbage Collection takes to complete? We are seeing pause time upto 4 to 5 seconds in our Java application? is this normal? Our heap size is not very large and its somewhere around 2 to 4 GB.
Hi,
For example if 10 objects are eligible for GC, then which object will get first GC? in what basis it will assign.
Thanks
Kamal
Hi,
I have one doubt. I am beginner.I am not understood if Object A has reference of object B and object B has reference of Object A then how both Objects A and B will be eligible for Garbage collection?
Ex:If we take Some class like
S a=new S();
S b=new S();
S t=b;
b=a;
a=t;
t=null;
Still a pointing b and b pointing a. Then how a and b both eligible for garbage collection? I am not understood please help me.
What is the use of gc() method if there is no guarantee of garbage collection ?
in which condition we should call it ?
@Manish Sahu, its not guaranteed but it does cleanup every time you call it. What Java tells you that you cannot build your solution based upon that method, garbage collection should be left to Garbage collector. If you use Eclipse, you can see that GC collector icon at bottom right, that use this method to run GC and free memory.
Hi, I would really appreciate if someone will explain how the concepts above changed since Java 8 introduced Metaspace instead of the PermGen.
How does Java Garbage Collection work? Short answer: it doesn't. EVER.
I was looking for a tool that can help me to understand the GC log in a graphical way and this tool http://gceasy.io has given the exact functionalities that I was looking for.
Excellent explanation.....
@Javin : If possible please update this post reflecting Java 8 memory model, especially substitution of PermGen with MetaSpace.
Obviously Javin's blog entries are getting so numerous he sometimes misses an update - but here is the update, just somewhere else: http://javarevisited.blogspot.co.uk/2016/07/difference-in-string-pool-between-java6-java7.html
@Sudip and @Adam, yes, I need to update this article, with respect to removal of PermGen space in Java 8 and introduction of MetaSpace. Also, I need to add some more information on G1 garbage collector, that's on my TODO list. Thanks for your input and attention guys. Thanks
your blog is owesome brother keep it up.
i never saw such a clean explanation. about any topic.
Thankyou again
Well written, clearly explain. Thank you :)
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