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 from
the OS and size of this space can not be changed during the
program 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 per
thread
Permanent Generation
1. Per-class data
2. Interned Strings

HEAP AREA
▪ Objects (excluding interned strings) created in any
thread are stored in this area. So this is a common
place for all threads.
▪ Interned strings stored in non-heap area
separately.

HEAP AREA
▪ If you create an object with new() keyword, it will
be 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 to
swap the
customers and
see if they were
really swapped.

WHAT IS STACK?
Element 2
Element 1
Element 1
Element 2
Element 1
Element 1

HOW IS METHOD ORDER
PRESERVED?
When a method
is called,
method-specific
data is collected,
tied together and
put into stack
space. It is
removed when
the execution is
done.

secondMethod
firstMethod
Element 1
firstMethod
firstMethod
secondMethod
secondMethod
firstMethod
thirdMethod
secondMethod
firstMethod
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 area
For each thread, a stack is stored.
Each stack stores a stack frame per
method call. Stack frame stores return
type, arguments and address values of
local objects and values of local
primitive values.

STACK FRAME
Stored
information
Value
Arguments String
Local object
(customer)
0x65899
Local primitive
(age)
30
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 run
this code?
A) Prints 5 infinitely
B) Exhausts all the memory of
JVM
C) Throws an exception

MOTIVATION
Stack space is limited. Here,
without popping a stack frame from
the stack, a new one is pushed. So
allocated memory for method
information grows rapidly.
JVM throws a
StackOverflowException and halts
the execution.

PASSING OBJECTS in JAVA
In Java, objects are passed between
methods by their values.
1. Objects are passed by values of their
address on heap space
2. Primitives are passed by values
3. Strings are also passed by values of
their value. This value is checked against
the interned strings in non-heap area.

EXAMPLE
We will try to
swap the
customers and
See if they were
really swapped.

...
HEAP SPACE
1
Talha
Turkey
2
John
US
customer1 1000
customer2 2000
1000
2000
Stack frame for main method
public void swapCustomers(Customer cust1, Customer cust2) {
Customer temp = cust2;
cust2 = cust1;
cust1 = temp;
}

...
HEAP SPACE
1
Talha
Turkey
2
John
US
customer1 1000
customer2 2000
1000
2000
cust2 = cust1;
cust1 = temp;
}
cust1 1000
cust2 2000
Stack frame for swapCustomers method

...
HEAP SPACE
1
Talha
Turkey
2
John
US
customer1 1000
customer2 2000
1000
2000
cust2 = cust1;
cust1 = temp;
}
temp 2000
cust1 1000
cust2 2000

...
HEAP SPACE
1
Talha
Turkey
2
John
US
customer1 1000
customer2 2000
1000
2000
cust2 = cust1;
cust1 = temp;
}
temp 2000
cust1 1000
cust2 1000

...
HEAP
SPACE
1
Talha
Turkey
2
John
US
customer1 1000
customer2 2000
1000
2000
cust2 = cust1;
cust1 = temp;
}
temp 2000
cust1 2000
cust2 1000

...
HEAP SPACE
1
Talha
Turkey
2
John
US
customer1 1000
customer2 2000
1000
2000
cust2 = cust1;
cust1 = temp;
}
So nothing has been changed on
Customer1 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
1
Talha
Turkey
customer1 1000
1000
public void changeCountry(Customer cust) {
cust.address = "new zealand";
}

...
HEAP
SPACE
1
Talha
Turkey
customer1 1000
1000
}
cust 1000

...
HEAP
SPACE
1
Talha
new zealand
customer1 1000
1000
}
cust 1000
Stack frame for changeCountry method

...
HEAP
SPACE
1
Talha
new zealand
customer1 1000
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
x 10
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
x 10
{
value = value *2;
}
value 20
Stack frame for doubleTheValue method

...
HEAP
SPACE
x 10
{
value = value *2;
}

Example 4
Now we will try the same thing with
a String. We will demonstrate the special
case of the String class and string interning
idea.

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 interning
One 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
6000

...
STRING POOL
s1 6000
s2 6000
talha_ocakci
6000
s1 == s2
6000 == 6000
true

STRING POOL
s3 9500
s4 9600
String s3 = new String("talha_ocakci");
...
talha_ocakci
6000
HEAP
...
talha_ocakci
9500
talha_ocakci
9600

STRING POOL
s3 9500
s4 9600
s3 = s3.intern();
s4= s4.intern();
...
talha_ocakci
6000
HEAP
...
talha_ocakci
9500
talha_ocakci
9600

STRING POOL
s3 6000
s4 6000
s3 = s3.intern();
s4= s4.intern();
...
talha_ocakci
6000
HEAP
...
talha_ocakci
9500
talha_ocakci
9600

www.javathlon.com
By Talha OCAKÇI

Heap and stack space in java

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