Cmput 115 - Lecture 8 Department of Computing Science University of Alberta ©Duane Szafron 2000 Revised 1/26/00 The Java Memory Model

Cmput 115 - Lecture 8

Department of Computing Science

University of Alberta

©Duane Szafron 2000

Revised 1/26/00

The Java Memory ModelThe Java Memory Model


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About This LectureAbout This Lecture

In this lecture we will learn how memory is structured and used in Java programs.


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OutlineOutline

Memory References The stack and the heap Parameter passing Objects on the stack Assignment


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Why Study a Memory Model?Why Study a Memory Model?

In this course we are studying data structures and algorithms.

To compare the space and time requirements of these data structures and algorithms we need to know how the data is actually stored in memory and how it is modified.

The way that data is stored and accessed is called a memory model.


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MemoryMemory

When a program is running, the data it needs is stored in memory in a binary format.

In a simple memory model, there are two kinds of memory– a small set of memory that can be manipulated

directly by the CPU is called register memory.– a larger set of memory that cannot be

manipulated directly by the CPU is called main memory.


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Accessing MemoryAccessing Memory

Every byte (8 binary digits) of main memory or word (1, 2, 4 or 8 bytes) is assigned a numerical address.

A fetch instruction is used to copy the contents of one word of main memory at a particular address to register memory.

A store instruction is used to copy the contents of a register to an address in main memory.

00000000

00100110Registers

CPU 01101011101101011111111

000100020003

fetch 0001

store 0003

01101011

0010011000100110

01101011


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ReferencesReferences

Application programmers use symbolic program references to refer to objects and values that are stored in memory, instead of numerical addresses.

The compiler translates each symbolic program reference into an expression that evaluates to an address in main memory or a register.

To understand how memory is used, we must look at how it is organized.


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Direct References in MethodsDirect References in Methods

When a method is executing it can access some objects and some values.

The receiver object can be referenced directly using the pseudo-variable this.

Other objects and values can be referenced directly using method parameters and local variables.

Still other objects and values can only be accessed indirectly by sending messages that return references to them.


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Method ActivationsMethod Activations

A method can only access objects while it is executing or active.

The collection of all direct references in a method is called the frame or stack frame of a method.

The frame is created when the method is invoked, and destroyed when the method finishes.

The stack frames are organized into a region of memory called the run-time stack.


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Example MethodExample Method

Consider a method in the class Person, where each person has two instance variables, name which is bound to a String, and age which is bound to an int:

public void alpha(Person child, int index) {

int anInt;Person newPerson;

anInt = 30;newPerson = new Person(“Fred”, anInt);

}


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Local Variables map to AddressesLocal Variables map to Addresses

The stack frames are stored in main memory where the receiver reference (this), the parameters and the local variables are mapped to consecutive memory locations that are part of the run-time stack.

thischildindexanIntnewPerson

012300012304012308012312012316

stack frame for alpha

{

{


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Assignment of ValuesAssignment of Values

When a value is assigned to a local variable, the value is put into the appropriate memory location in the stack frame.

For example consider the statement:

anInt = 30;

012300012304012308012312012316



30


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Assignment of ObjectsAssignment of Objects

When an object is assigned to a local variable, a reference to that object is put into the appropriate memory location in the stack frame.

For example, consider the statement:

newPerson = new Person(“Fred”, anInt);

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012300012304012308012312012316



object ref


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Object MemoryObject Memory

To understand the nature of the object reference we must consider the memory for the object itself.

When an object is created, memory for that object is allocated in a region of memory called the heap where space is allocated for each instance variable of the object.

For example:

new Person(“Fred”, anInt);

nameage

100234100238

a Person object

}


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Object Memory OrganizationObject Memory Organization

Each instance variable in an object is bound to a value or another object.

If an instance variable is bound to a value, the value is stored directly.

If an instance variable is bound to an object, a reference to that object is stored.

A constructor initializes this memory.

new Person(“Fred”, anInt);

nameage

100234100238

a Person objectobject ref

30


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Object References map to Object References map to AddressesAddresses

An object reference maps to an address in the heap called a pointer.

newPerson = new Person(“Fred”, anInt);

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012300012304012308012312012316 100234


10050030

nameage

100234100238

100500100504100508

valueoffsetcount

10098004


a Person object

a String object

100980101984101988

lengthdata

4‘F’

an Array

‘r’‘e’ ‘d’


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Object References as ArrowsObject References as Arrows

When we draw pictures of memory, we often omit the addresses and represent object references as arrows.

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30nameage

04


a Person object

a String objectvalueoffsetcount

lengthdata

4‘F’

an Array

‘r’‘e’ ‘d’


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Parameter Passing in JavaParameter Passing in Java

When a method call is made, data from a calling method must be transferred to the method that is called.

How is the argument data passed to the method?

Java uses call by value for each value parameter and call by reference for each object parameter.

In call by value, a copy of the data value is put into the stack frame for the corresponding parameter.

In call by reference, a reference to an object is put into the stack frame.


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Example Parameter PassingExample Parameter Passing

Consider some code that calls the method alpha:aPerson = new Person(“Wilma”, 28);daughter = new Person(“Pebbles”, 5);childIndex = 1;aPerson.alpha(daughter, childIndex);

???100620100844

012004012008012012012016

thisaPersondaughterchildIndex

stack frame for caller100200

28nameage

a Person object100620100621

1003345

nameage

a Person object100844100848

100620100844

1

012300012304012308012312012316


stack frame for alpha1


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Example Parameter Passing - Example Parameter Passing - ArrowsArrows

Consider some code that calls the method alpha:aPerson = new Person(“Wilma”, 28);daughter = new Person(“Pebbles”, 5);childIndex = 1;aPerson.alpha(daughter, childIndex);

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thisaPersondaughterchildIndex

stack frame for caller

28nameage

a Person object

5nameage

a Person object1




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Objects in ObjectsObjects in Objects

Some languages (C++) allow objects to contain other objects rather than just references to other objects.

Some languages (C, Pascal) use structures or records for composite data instead of objects.

In this case the layout of an object is “flattened”:

name

age

100234100238100242100246

a Person object

4‘F’ ‘r’‘e’ ‘d’

28


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Objects on the StackObjects on the Stack

Some languages (C++, C, Pascal) allow objects and structures to be allocated directly in a stack frame instead of the heap.

Person aPerson; int anInt;

aPerson

anInt

100234100238100242100246100250

4‘F’ ‘r’‘e’ ‘d’

2810

stack frame


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The Assignment Operation The Assignment Operation RevisitedRevisited

Assigning a value to a value variable, copies the value.

Assigning an object to an object variable, copies only the object reference, not the entire object.


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Second Java Assignment Second Java Assignment ExampleExample

Person person1;

Person person2;int int1; int int2;person1 = new Person(“Fred”, 30);int1 = 7;person2 = person1;int2 = int1;

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person1person2int1int2

stack frame

30nameage

04

a String objectvalueoffsetcount

lengthdata

4‘F’

an Array

‘r’‘e’ ‘d’


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Stack Object or Structure Stack Object or Structure AssignmentAssignment

If an object or a structure is on the stack (not possible in Java), then assigning it to a variable, copies the entire object or structure.


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Stack Assignment ExampleStack Assignment Example

Person person1;

Person person2;int int1;int int2;person1 = new Person(“Fred”, 30);int1 = 7;person2 = person1;int2 = int1; person1

person2

int1int2

stack frame

4‘F’ ‘r’‘e’ ‘d’

4‘F’ ‘r’‘e’ ‘d’

3030


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The Memory for CollectionsThe Memory for Collections

In Java, an array of Objects or a Vector of objects, actually contains references to objects.

The memory for the collection itself is one four byte object reference per element, regardless of the size of the element object itself.

In other languages all of the memory for the collection may be in the collection itself and this may even be in the stack.


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Java Collection Memory Java Collection Memory ExampleExample

Person people[ ];

people = new Person[4];

for (index = 0; index < 4, index++)

people[index] = new Person(“name” + index, index);

people

stack frame

4lengthdata

an Array

nameage 0

05

a String object

lengthdata

5‘n’

an Array

‘a’‘m’ ‘e’‘0’

1nameage

valueoffsetcount

2nameage

3nameage


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CollectionCollection Stack Memory Stack Memory ExampleExample

stack framepeople 5

‘n’ ‘a’‘m’ ‘e’‘0’

05

‘n’ ‘a’‘m’ ‘e’‘1’

1

5‘n’ ‘a’‘m’ ‘e’‘2’

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‘n’ ‘a’‘m’ ‘e’‘3’

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Documents

Cmput 115 - Lecture 8 Department of Computing Science University of Alberta ©Duane Szafron 2000 Revised 1/26/00 The Java Memory Model