java memory architecture

7/29/2019 Java Memory Architecture

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Joydeep Dey


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CODE

STATIC Area

STACK

HEAP


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Typeinformation

constant pool Fieldinformation

Method

information

Class

variables

Method Table

Reference toclass loader

and classClass


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TypeInformation

Constantpool

FieldInforma

tion

MethodInforma-

tion

Classvariables

MethodTable

Ref. toclass loaderand class

Class

Fully qualified types

name.

Fully qualified direct super

class name.

Whether class or an

interface

type's modifiers

list of the fully qualified

names of any direct super

interfaces


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Constant PoolType

Information

FieldInforma

tion

MethodInforma-

tion

Classvariables

MethodTable


Class

Ordered set of constants- string- integer

- floating point- final variables

symbolic referencesto

- types

- fields- Methods


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FieldInformation

TypeInforma

tion

Constantpool

MethodInforma-

tion

Classvariables

MethodTable


Class

fields name fields type fields modifiers (subset )

- public- private- protected- static- final- volatile

- transient


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MethodInformation

TypeInforma

tion

Constantpool

FieldInforma

tion

Classvariables

MethodTable


Class

Methods name Methods return type Number and type of

parameters

Modifiers (subset)- public- private- protected- static- final

- synchronized- native- abstract


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Classvariables

TypeInforma

tion

Constantpool

FieldInforma

tion

MethodInforma

tion

MethodTable


Class

ordered set of classvariables

- static variables


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Ref. toClass loader

And class

Class

TypeInforma

tion

Constantpool

FieldInforma

tion

MethodInforma

tion

MethodTable

Classvariables

Reference to class loaderis used for dynamic linking. instance java.lang.Class

is created every type forthe following info.

- getName();- getSuperClass();- isInterface();- getInterfaces();

- getClassLoader();


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Method

Table

TypeInforma

tion

Constantpool

FieldInforma

tion

MethodInforma

tion

Classvariables

Used for quick ref. to

method.

Contains name and indexin symbol ref. array


Class


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Objects and arrays are allocated in this area.

Two different threads of the same application,however, could trample on each other's heap

data.


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object is associated with a lock (or mutex) tocoordinate multi-threaded access to the object.

Only one thread at a time can "own" an object's

lock. Once a thread owns a lock, it can request the

same lock again multiple times, but then has torelease the lock the same number of timesbefore it is made available to other threads.


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The name of an array's class has one opensquare bracket for each dimension plus a letteror string representing the array's type.

The class name for an array of ints is "[I.

The class name for three-dimensional array ofbytes is "[[[B".

The class name for a two-dimensional array of

Objects is "[[Ljava.lang.Object".


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Java stack stores a thread's state in discreteframes.

Each frame contains

- local variables Area.- operand stack

- frame data


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organized as a zero-based array of cells. Variables are accessed through their indices.

Values of type int, float, reference, and returnAddress occupy one cell.

Values of type byte, short, and char alsooccupy one cell.

Values of type long and double occupy two

consecutive cells in the array.


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class Example3a {public static int runClassMethod(int i, long l, float f, double d, Object o, byte b) {

return 0; }public int runInstanceMethod(char c, double d, short s, boolean b) {

return 0; }

}


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operand stack is also organized as an array ofcells.

local variables are accessed via array indices, the

operand stack is accessed by pushing andpopping values.

instructions take their operands from

- operand stack

- immediately following the opcode

- constant pool


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iload_0 // push the int in local variable 0 iload_1 // push the int in local variable 1 iadd // pop two ints, add them, push result

istore_2 // pop int, store into local variable 2


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Frame data is needed to support

- constant pool resolution

- normal method return

- exception dispatch

- debugging.


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class Example3c {

public static voidaddAndPrint() {

double result =

addTwoTypes(1, 88.88);System.out.println(result);

}

public static doubleaddTwoTypes(int i, double d)

{return i + d;

}

}


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class abc {

public int a;

String str;

abc() {

a=10; atr=string1;}

public void print{ System.out.print(a+ +str); }

}

interface def {

void add();

}

class pqr extends abc implements def {

static int b;

final int c=50;String s;

pqr(int m) { super(); b=m; s= new String(string2); }

void add() { a=a+c; add1(); }

static void add1() { c=b+50; }

}

Example


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class Main {public static void main(String[] s) {

pqr p=new pqr(20);

p.add();p.print();

}

}


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class abc {public int a;

String str;

abc() {a=10;str=string1;

}public void print{

System.out.print(a+ +str);}

}

abc

java.lang.Object

Isclass=true

modifier=4

Type info

a str print

Symbol ref. array

10 string1

Constant pool

name Type Modifiera int 5 0

str String 4 1

index

Field info

name ret.type npar modifier parlist

void 0 1

print void 0 5

codeptr

Method info

name index

2

print 3

Method Table

null

Class variables

Class A ea of abc in Method a ea


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abc

java.lang.ObjectIsclass=true

modifier=4

ptr. to interface list

ptr. to symbolic ref. array

name Type Modifier index

a int 5 0

str int 4 1

ptr to field infoname ret.type npar modifier parlist codeptr

void 0 5

print void 0 5

ptr to method info

ptr to class variable list

ref. to class loader

ref. to Class

ptr to method table

Method name index in sym ref.

2

print 3

Symbolic ref. array

Class Area of abc in Method area


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interface def{

void add();

}

def

java.lang.Object

Isclass=false

modifier=4

Type info

add

Symbol ref. array

Constant pool

Field info

null

Method info


add void 0 4

codeptr

Classvariables

null

add 0

Method Table

name index

Class Area of def in Method area


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def

java.lang.ObjectIsclass=false

modifier=4


ptr. to symbolic ref. arrayptr to field info

name ret.type npar modifier parlist codeptr

add void 0 4ptr to method info



ref. to Class

ptr to method table


add 0

Symbolic ref. array

Class Area of def in Method area

l d b l d f {


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class pqr extends abc implements def {static int b;final int c=50;String s;pqr(int m) { super(); b=m;

s= new String(string2); }void add() { a=a+c; add1(); }static void add1() { c=b+50; }}

pqr

abc

Isclass=true

modifier=4

Type info

Symbolic ref. array

b c s super add add1

50

name Type Modifier

b int 4,6 0

c int 4,7 1

S String 4 2

index

Field infoConstant pool


void 1 4

add void 0 4

add1 void 0 4

super void 0 4

Method info

codeptr

b

Class variables

3

add 5

Method Tablename index

add1 6

super 4

Class Area of pqr in Method area


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pqr

abcIsclass=true

modifier=4

ptr. to interface list ( to def)

ptr. to symbolic ref. array

name Type Modifier index

b int 4,6 0

c int 4,7 1

ptr to field infoname ret.type npar modifier parlist codeptr

void 1 4

add void 0 4

ptr to method info

ptr to class variable list (b)


ref. to Class

ptr to method tableMethod name index in sym ref.

3

add 4

Symbolic ref. array

Class Area of pqr in Method area

S String 4 2

add1 void 0 4super void 0 4

l M i {


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class Main {public static void main(String[] s) {

pqr p=new pqr(20);p.add();p.print();

}}

Main

java.lang.Object

Isclass=true

modifier=4

Type info

main

Symbol ref. array

Constant pool

Field info

null

Method info


main void 0 4

codeptr

Classvariables

null

main 0

Method Table

name index

20

Class Area of Main in Method area


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Main

java.lang.ObjectIsclass=true

modifier=4


ptr. to symbolic ref. arrayptr to field info

name ret.type npar modifier parlist codeptr

main void 0 5,6ptr to method info



ref. to Class

ptr to method table


main 0

Symbolic ref. array

Class Area of Main in Method area


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Main

stack

Heap

Main

main

Main pqr

p

Main pqr

p

main

main

Pqr.

Main pqr

p

Pqr.

abc.

Main pqr

p

Main pqr

p

a,b,c

str

s

string2

abc


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Main pqr

p

a,b,c

str

s

string2

add

abc Main pqr

p

a,b,c

str

s

string2

add

abc

add1

Main pqr

p

a,b,c

s

string2

abc

Main pqr

p

a,b,c

s

string2

print

abc Main pqr

p

a,b,c

s

string2

abc

str

str str

strs


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CODE

STATIC Area

Metadata

MethodArea

Constantpool

Classvariables

TypeInforma

tion

FieldInforma

tion

MethodInforma

tion

MethodTable


Class


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C++ Heap

Garbage collection

JVM Heap


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C++ stack

OperandStack

Frame data

ThreadedJVMStack

JVM stackThreads


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Chop the chunk.

Allocate Requested number of bytes.

Provide necessary information to garbage

collector. Collect the bytes returned by garbage

collector.

Defragment the chunks.


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How to allocate bytes- Contiguous memory allocation.

- Non contiguous memory allocation.

How many bytes to be allocated

- exact requested number of bytes.

- Allocate bytes in terms of 2^n (Buddy System).

How defragmentation to be done

- Compaction.

- Buddy system.


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Algorithms Followed

Noncontiguous Memory Allocation.

First fit to choose approperiate chunk.

Buddy system to chop a chunk.

Buddy system for defragmentation.


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Need of 4 threads

- Memory allocator.

- Memory Defragmenter.

- Chunk cleaner.

- Data manager.


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Allocator

1. Checks for first chunk with size

>=m+4.

2. If available chops it and returns to VM

else requests OS for new chunk. Data Manager inserts an entry in Memory

Allocation Table (for garbage collector ).

Starting address Size requested Size Allocated


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For each entry in MA Table GC moves tostarting address+ size requested address,

checks the ref. count. If zero returns the index oftable entry to Data Manager.

Data Manager deletes the entry from MATand adds in Defragment Table.

For each entry in Defragment Table,

Defragmenter combines the consecutive

fragments.

Starting Address Size


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VM requests an array of bytes

- to store data in Method Area.- for stack frame.

- for object on Heap.

MM doesnt know the purpose of bytes.How does MM allocates memory in three

different address spaces for different type of

requests.Use Three MMs. Each MM has its own addr.Space. Its responsibility of VM to direct therequests to appropriate MM.


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For Method Area MM 10K

For Stack 16K

For Heap 1K, 10K, 32K

Initial State of Chunk pool Trade off between performance and

efficient memory usage. first and top pointers of each pool is set

to null


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similar to Garbage collector.

For every 5s cleaner returns all chunks morethan 5.


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Method Area as Type Area

Stack as Roots

Heap as Heap, but two heaps- Managed heap

- Unmanaged heap

In JVM entire heap is managed.


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Java

M/C 1

JVMM/C 2

CLR

M/C 1

M/C 2

C#

VB

Some Languages allow pointers. So to supportpointers Unmanaged heap is supported


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JVM MM- Allocation is complex.

- Defragmentation is simple.

ClR MM

- Allocation is simple.

- Defragmentation is complex.


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Algorithms Followed

Contiguous Memory Allocation.

Compaction for defragmentation.

Lazy Defragmenter


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Type Memory manager.

Roots Memory manager.

Managed heap manager.

Unmanaged heap manager.

GC runs.

No GC.

GC runs.

No GC.

To deallocate Unmanaged Memory Finalizersshould be used.


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Obj 2

Obj 1

Obj 3

Next Ptr

MemoryAllocation

Before GC After GC


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java memory architecture

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