heap and stack space in java

MEMORY MANAGEMENT in JAVA

STACK SPACE, HEAP SPACE AND STRING POOLS

JAVATHLON.COM BY TALHA OCAKÇI

MOTIVATION

javathlon.com by Talha Ocakçı

What happens when you run this code:A) Operating system fails. Because all memory is exhausted by our program.B) All running programs lose data since all memory is exhausted by our program.C) Operating system does not allow program execution by detecting the memory consumption.D) Program halts and throw an error.

JVM MEMORY

OPERATING

SYSTEM

MEMORY

JVM MEMORY

(CONSTANT)

When main method is executed, JVM asks for some memory fromthe OS and size of this space can not be changed during theprogram execution.

JVM MEMORY

Every object is stored in JVM memory and can be accessed by its address value.

A Java application can not interfere with a memory block outside the JVM.

Memory Management

HEAP AREA STACK AREA NON-HEAP AREA

Class instances and arrays Stacks for executing methods perthread

Permanent Generation1. Per-class data 2. Interned Strings

HEAP AREA

▪ Objects (excluding interned strings) created in anythread are stored in this area. So this is a commonplace for all threads.

▪ Interned strings stored in non-heap areaseparately.

HEAP AREA

▪ If you create an object with new() keyword, it willbe stored in heap area.

▪If no reference left for a given object, it will be a candidate for garbage collection.

STACK AREA

MOTIVATION

We will try toswap thecustomers andsee if they werereally swapped.

WHAT IS STACK?

Element 2

Element 1Element 1

Element 2

Element 1

HOW IS METHOD ORDER PRESERVED?

When a methodis called, method-specificdata is collected, tied together andput into stackspace. It is removed whenthe execution is done.

secondMethod

firstMethod

Element 1

firstMethod

secondMethod

firstMethod

thirdMethod

secondMethod

firstMethod

thirdMethod

Stack frame

STACK SPACE

JVM uses this space for controlling the execution of methods.

Stores local values of ongoing methods.

Stores signatures of ongoing methods.

STACK AREA

Stack for

Thread 1

Stack for

method A

Stack for

Thread 2

Stack frame per method

Stack areaFor each thread, a stack is stored.

Each stack stores a stack frame permethod call. Stack frame stores returntype, arguments and address values of local objects and values of localprimitive values.

STACK FRAME

Storedinformation

Arguments String

Local object(customer)

0x65899

Local primitive(age)

Return value True

STACK FRAMES

Located in a stack allocated to a thread in stack area. Stores

1. Address values of local objects

2. Values of local primitives

3. Return value of the method

4. Current class’s definition’s address value

5. Operands stack

MOTIVATION

What happens when you runthis code?A) Prints 5 infinitelyB) Exhausts all the memory of JVMC) Throws an exception

MOTIVATION

Stack space is limited. Here, without popping a stack frame fromthe stack, a new one is pushed. Soallocated memory for methodinformation grows rapidly.

JVM throws a StackOverflowException and haltsthe execution.

PASSING OBJECTS in JAVA

In Java, objects are passed betweenmethods by their values.

1. Objects are passed by values of theiraddress on heap space

2. Primitives are passed by values

3. Strings are also passed by values of their value. This value is checked againstthe interned strings in non-heap area.

EXAMPLE

We will try toswap thecustomers andSee if they werereally swapped.

HEAP SPACE

1Talha

Turkey

2JohnUS

customer1 1000

customer2 2000

Stack frame for main method

public void swapCustomers(Customer cust1, Customer cust2) {Customer temp = cust2;cust2 = cust1;cust1 = temp;

HEAP SPACE

1Talha

Turkey

2JohnUS

customer1 1000

customer2 2000

cust1 1000

cust2 2000

Stack frame for swapCustomers method

HEAP SPACE

1Talha

Turkey

2JohnUS

customer1 1000

customer2 2000

temp 2000

cust1 1000

cust2 2000

HEAP SPACE

1Talha

Turkey

2JohnUS

customer1 1000

customer2 2000

temp 2000

cust1 1000

cust2 1000

HEAP SPACE

1Talha

Turkey

2JohnUS

customer1 1000

customer2 2000

temp 2000

cust1 2000

cust2 1000

HEAP SPACE

1Talha

Turkey

2JohnUS

customer1 1000

customer2 2000

temp 2000

cust1 2000

cust2 1000

HEAP SPACE

1Talha

Turkey

2JohnUS

customer1 1000

customer2 2000

So nothing has been changed onCustomer1 and customer2 references.

SECOND EXAMPLE

We will pass the object to a method, change some attributes of it and check whether the original object has been changed.

HEAP SPACE

1Talha

Turkey

customer1 1000

public void changeCountry(Customer cust) {cust.address = "new zealand";

HEAP SPACE

1Talha

Turkey

customer1 1000

cust 1000

HEAP SPACE

1Talha

new zealand

customer1 1000

cust 1000

Stack frame for changeCountry method

HEAP SPACE

1Talha

new zealand

customer1 1000

So, the original object is really modified on heap space

EXAMPLE 3

We will send a primitive to a method,multiply the argument value and will check Whether te original value has been changed.

HEAP SPACE

public void doubleTheValue(int value){

value = value *2;}

Since there is no object defined, no object exists for this program

value 10

Stack frame for doubleTheValue method

HEAP SPACE

value = value *2;}

value 20

Stack frame for doubleTheValue method

HEAP SPACE

value = value *2;}

Example 4

Now we will try the same thing with a String. We will demonstrate the specialcase of the String class and string interningidea.

Example 4

This code will output «current».Because updateValue method could not change the value of String string variable

The reason is string interning also known as «string pooling».

Lets start with an example

What is the output of this code snippet?

Lets start with an example

If you answered this as «true», you probably know what is going on.

Second example

What is the output of this code snippet?

Second example

Answer is «false»

Third example

Now, let’s try it with string interning.

Output is true.

Now lets inspect what is going on.

String interningOne of the most used method in an ordinary program is string comparison. And in an average program, there are so many strings which has an average length between 10-20.

So, to compare two strings, JVM must compare that much character everytime causing a real performance problem. That’s why, JVM puts a used String to a String pool and uses the internal HashMap for comparing the string variables.

STRING POOL

s1 6000

s2 6000

String s1 = "talha_ocakci";

talha_ocakci

STRING POOL

s1 6000

s2 6000

talha_ocakci

s1 == s26000 == 6000true

STRING POOL

s3 9500

s4 9600

String s3 = new String("talha_ocakci");

String s4 = new String("talha_ocakci");

talha_ocakci

STRING POOL

s3 9500

s4 9600

String s3 = new String("talha_ocakci");s3 = s3.intern();String s4 = new String("talha_ocakci");s4= s4.intern();

talha_ocakci

STRING POOL

s3 6000

s4 6000

String s3 = new String("talha_ocakci");s3 = s3.intern();String s4 = new String("talha_ocakci");s4= s4.intern();

talha_ocakci

www.javathlon.com

By Talha OCAKÇI

heap and stack space in java

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