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Now let's look at the specifics of runtime Byerly us and let's start with the method area under a heap

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.

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We will look at what goes into that area and also how heap and method area are interconnected.

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Let's see how things get populated on heap and area we know the objects are created on heap on these

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objects which include regular class instances that are created via a new operator Arrius are not class

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objects when we say objects are stored on the heap.

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What does that mean.

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It means that we are mainly talking about the instance data present within those objects that is instance

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literally sites on the heap.

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So we have literally a look at space to an object based on how much of the instance it contains which

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is based on the number of incidents variables and also Data Types of those instance variables.

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No objects also can be in behavior which is nothing but methods.

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The method however is something that is specific to the class or not to the object.

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So that would be the last data and any class data will be stored in the metal area.

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So that area is very strong.

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Class data which is not limited to just method code but would also include Mad-Eye information like

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the feeling team freetime on other stuff that people will see something that is all about classes while

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heap is all about objects.

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Non-X if we want to create an instance of a class in this because there are few things that can happen

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.

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First classloader checks of the class object for that particular class is already there on the heap

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.

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If it is not there it would then create a class object on the heap and the corresponding class data

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itself in the method area and then JVM would also create a new instance of that class and store it on

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the heap.

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Now if a new object of the same class is to be created then only the object will be created and stored

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on the heap.

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Classloader knows that the class object and class data have already been created and so it does not

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create them once again.

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And we already know this.

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Similarly as a program continues to exist even how most of getting creator on board heap on Methody

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idea we can also see pointers going back and forth between heap and Methody.

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On one of them is a byte original point or two.

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Will see how this point will be useful not it's focus only on metadata and let's see what gets stored

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in it that it's what gets stored as part of the class data.

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What we will see here are general things that would be part of class data.

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The exact implementation details are GBM specific classes they don't include Medine permission of the

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type like fully qualified name of their type fully qualified name off its direct superclass fully qualified

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name of any direct super interfaces whether that type is a class or interface.

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And information about pipes modifiers like abstract final under access modifier public.

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We know that such information is accessible through class object.

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So what it means is that class object in dentally uses the class data which is stored in the metro area

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.

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So in the previous slide that was the reason why there was a connection from the class object to the

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class data but the connection was by that action.

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And that's because Glassner includes a reference back to the class object and this reference is needed

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whenever access to class object is needed.

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For example during class throating process classloader Fosdick's a class object for that pipe is already

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on heap.

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For that it may actually Serj the method area Boosey of the class data for the pipe exists and if it

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doesn't exist it would then simply return the reference to the class object.

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We know that classes in group feet like static and instance variables so Klausner don't include field

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information like free name free type and any associated modifiers like static Feiner any access modifiers

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under modifiers.

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Volatile and transparent.

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Whether it is related to multi-threading while consumed is related to serialization which isn't either

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related topic well then intransient will be discussed later.

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Note that here we are talking about free Mad-Eye information not the file happens to be an instance

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variable with the parameter data type then the value itself will be stored on the heap.

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But if the field happens to be a static variable with a primitive build up then the value will be stored

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in the metro area itself.

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Now if a field regardless of whether it is instance or static reference is an object then the object

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will be on heap as data placed off objects.

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But the reference itself which is the memory address of the object will be stored in the metro area

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in case of static variables.

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And I would strongly assume that the reference would be stored on a heap for instance variables as any

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instance where you would go on the heap closeted I would also include runtime constant put me under

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to discuss the dot classifiers would include something called constant pull on runtime constant it is

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simply at runtime version of that constant pool.

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It would basically include constants likely gross that is string literals are literally assigned to

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primitive variables.

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Constants can also include symbolic references which were discussed earlier in the lifetime of a type

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while discussing the resolution strip classes also include methods subclass data.

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I would also include method information like the method name methods Rikan type Nemeroff method parameters

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and also type off those parameters with the arder maintain.

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We know that methods can have modifiers too like static final access modifiers abstract synchronized

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or native Glassner can include all this method modifier ARFs synchronize is related to multithreading

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while native as the name implies would imply a native method.

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Which would mean that the method itself will be implemented in some other programming language method

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information will also include the actual code corresponding to the method body.

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Next Klausner.

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I will also include metor table which is simply an audio for references to instance methods.

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Note that we are talking about instance methods here are not static methods so each reference is basically

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a pointer to actual method bytecode itself which is also part of Gloucester to this table would be used

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during method in location process.

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That is when an instance method is invoked on an object then the method table would be used to find

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the method that needs to be executed.

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This is one reason for having a pointer from object on the heap with the class data.

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So in invoking a method on an object JVM would simply follow the pointer fremd the object to the class

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data and would then use the method table to find an exit to the target method and not admit that it

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would also include references to any methods inherited from superclasses in the next lesson.

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We will look at a very interesting example.

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Very we'll see how their table will be useful.

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So those are some of the things stored in the method area at the class level.

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No one thing to note about method area is that prior to Djala 8 method area was located in the so called

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permanent generation space are simply Bohmian.

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Angela ate.

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This area has been completely removed and the method area has been moved to near a heap and is not referred

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to as model space.

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So what was imagined then.

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Now is Matt auspice the problem.

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Namely does matter space because Metromedia and groats Mad-Eye information.

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One reason for this change is because for bombon there was a default maximum size set of the max size

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was reached and out of memory or would be generated with medals space.

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There is not before max size that is system memory is the limit.

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So you may not see order of memory error or even if you see it it may be delayed.

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Another reason is related to improving the both foments of garbage collection process.

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Earlier when discussing method tables we saw how JVM use method be able to invoke an instance method

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that is at runtime.

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The class due information associated with the type of the object has been used such type information

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used at runtime is referred to as runtime type information or RTTY in shock.

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RTD is also used to check the correctness of cost.

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For example here an object of type subclass is assigned to a variable whose reference type is superclass

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in the next statement.

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A typecast is inserted to assign the same object to a variable whose reference type is of type subgraphs

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.

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Whether this is a valid cost or not is performed only at runtime.

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On For that the class data of the object is consulted and run by so runtime type information is being

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used here.

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Arkadi is also used well-performing instance of operation.

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For example here at runtime it needs to be checked whether or not the object type of the reference variable

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is the class subclass so class state of the object is used here.

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So these are a few places where type information is utilized on time.

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So we have seen what constitutes method area and we have also seen how metatheory are a heap are interconnected

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.

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BLUEJACKET the resources section for some interesting pointers.

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One of the pointer is a chapter from the book Inside the Java Virtual Machine by build winners.

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Lots of material for the JVM stuff you're covering here comes from this book and all of it is available

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online and the link posted in the resources section is a reference to the online version of the book

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so you can take a look at that.

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And that's about it for now.