Download Activation Records in Java (Updated!)

Programming Languages and
Paradigms
Activation Records in Java
Activation Records in Java
Activation record: A chunk of computer
memory which contains the data
needed for the activation of a routine
Java Virtual Machine’s Role



Loads class files needed and executes
bytecodes they contain in a running
program
Organizes memory into structured areas
Java Virtual Machine
Java Virtual Machine

Runtime data areas in JVM





Method area – contains class information
Heap – memory for class instances and
arrays
PC registers – program counters for each
thread
Java stack – stores stack frames
Native method stacks – Native methods:
written in other languages
Runtime Data Areas

Method Area





Contains class information
One for each JVM instance
Shared by all threads in
JVM
One thread access at a time
Heap




Contains class instance or
array (objects)
One for each JVM instance
Facilitates garbage
collection
Expands and contracts as
program progresses
Objects Representation in Heap
Runtime Data Areas: Java Stack

Java Stack



Each thread creates separate Java stack
Contains frames: current thread’s state
Pushing and popping of frames
Stack Frame

Local Variables



Operand Stack





Organized in an array
Accessed via array indices
…
Incoming parameter 2
Incoming parameter 1
Organized in an array
Accessed via pushing and popping
Always on top of the stack frame
Work space for operations
Frame Data


Constant Pool Resolution:
Dynamic Binding
Normal Method Return



No exception thrown
Returns a value to the previous
frame
Exception Dispatch
Local variables
Current frame
Frame data
Operand stack
Outgoing
parameters
Next frame
Example: Separate Stack Frames
class StackFrameExample {
public static void addAndPrint() {
double result = addTwoTypes(1,
System.out.println(result);
}
88.88);
public static double addTwoTypes(int i, double d) {
return i + d;
}
}
Example: Contiguous Stack
Bank Account Example
public class BankAccount {
private double balance;
public static int totalAccounts = 0;
public BankAccount() {
balance = 0;
totalAccounts++;
}
public void deposit( double amount ) {
balance += amount;
}
}
public class Driver {
public static void main( String[] args ) {
BankAccount a = new BankAccount();
BankAccount b = new BankAccount();
b.deposit( 100 );
}
}
// In command prompt
java Driver
// In Java Virtual Machine
Driver.main( args )
*
a
main()
A stack frame for
main() is pushed
into the Java stack
Driver class
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
Method Area
b
Parameters
Local variables
Frame data
100
ClassLoader loads
Driver to the method
area
…
args[0]
Heap
Operand stack
// Driver.java
public static void main( String[] args ) {
BankAccount a = new BankAccount();
BankAccount b = new BankAccount();
b.deposit( 100 );
}
…
args[0]
*
a
main()
100
*
ClassLoader
loads
BankAccount()
BankAccount to the
method area
b
Parameters
Local variables
A pointer to the
Frame data
BankAccount
pointer in the
heap is
Operand stack
created (Constant
Pool Resolution)
Parameters
Local variables
Frame data
Operand stack
Parameters
Driver class
BankAccount class
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
A stack frame for
the BankAccount
constructor is
pushed into the Java
stack
Methods
BankAccount()
deposit( double )
Constant Pool
0
Method Area
pointer
A pointer to the
BankAccount class
data is created
Heap
// BankAccount.java
private double balance;
private static int totalAccounts = 0;
…
args[0]
*
a
main()
b
Parameters
Local variables
Frame data
public BankAccount() {
balance = 0;
totalAccounts++;
}
100
Operand stack
Parameters
*
public void deposit( double amount ) { BankAccount()
balance += amount;
}
Local variables
Frame data
Operand stack
Parameters
Driver class
BankAccount class
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
Methods
BankAccount()
deposit( double )
Constant Pool
0
Static Variables
totalAccounts
10
Method Area
The
totalAccounts
totalAccounts
is
incremented
static variable
byof1
BankAccount is
initialized with a
default value and
then assigned with 0
pointer
balance
0.0
Heap
The balance variable
of this instance is
initialized
assigned with
with0a
default value
// Driver.java
public static void main( String[] args ) {
BankAccount a = new BankAccount();
BankAccount b = new BankAccount();
b.deposit( 100 );
}
*
a
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
100
Static Variables
totalAccounts
*
Operand stack
Local variables
Frame data
Operand stack
The stack frame for
the BankAccount
constructor is
popped from the
Java stack
Parameters
pointer
balance
0.0
1
Method Area
*
Parameters
Methods
BankAccount()
deposit( double )
Constant Pool
0
Parameters
The pointer
is
returned to the
callingLocal
framevariables
Frame data
BankAccount()
BankAccount class
b
main()
The pointer is popped
from the operand stack
and assigned to a
Driver class
…
args[0]
Heap
// Driver.java
public static void main( String[] args ) {
BankAccount a = new BankAccount();
BankAccount b = new BankAccount();
b.deposit( 100 );
}
Since the
BankAccount class
was already loaded
in the method area,
no other loading
happens
…
args[0]
*
a
main()
100
*
BankAccount()
b
Parameters
Local variables
A pointer to the
Frame data
BankAccount
pointer in the
heap is
Operand stack
created (Constant
Pool Resolution)
Parameters
Local variables
Frame data
Operand stack
Parameters
Driver class
BankAccount class
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
A stack frame for
the BankAccount
Constructor is
pointer
pushed into the Java
stack
Methods
BankAccount()
deposit( double )
Constant Pool
0
Static Variables
totalAccounts
pointer
balance
0.0
1
Method Area
Heap
A pointer to the
BankAccount class
data is created
// BankAccount.java
private double balance;
private static int totalAccounts = 0;
…
args[0]
*
a
Parameters
b
Local variables
main()
Frame data
public BankAccount() {
balance = 0;
totalAccounts++;
}
100
Nothing happens
since the
totalAccounts was
public void deposit( double amountalready
) { initialized
BankAccount()
when the
balance += amount;
BankAccount class
}
was first loaded
Operand stack
Parameters
*
Local variables
Frame data
Operand stack
Parameters
Driver class
BankAccount class
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
Methods
BankAccount()
deposit( double )
Constant Pool
0
Static Variables
totalAccounts
The balance variable
of this instance is
initialized
assigned with
with0a
default value
totalAccounts is
incremented by 1
pointer
balance
0.0
21
Method Area
Heap
pointer
balance
0.0
// Driver.java
public static void main( String[] args ) {
BankAccount a = new BankAccount();
BankAccount b = new BankAccount();
b.deposit( 100 );
}
…
args[0]
*
a
Parameters
b
Local variables
main()
Frame data
100
The pointer is popped
from the operand stack
and assigned to b
*
Operand stack
Parameters
*
BankAccount()
The pointer is
Local variables
returned
to the
calling
frame
Frame data
Operand stack
Driver class
BankAccount class
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
Methods
BankAccount()
deposit( double )
Constant Pool
0
Static Variables
totalAccounts
The stack frame for
the BankAccount
Constructor is
popped from the
Java stack
pointer
balance
pointer
balance
0.0
2
Method Area
Parameters
Heap
0.0
// Driver.java
public static void main( String[] args ) {
BankAccount a = new BankAccount();
BankAccount b = new BankAccount();
b.deposit( 100 );
}
*
a
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
Static Variables
totalAccounts
Operand stack
amount=100
Parameters
The object reference
to the instance is
always put as the
first local variable of
a stack frame
Local variables
100 is popped from
the operand
Framestack
data
and put into the next
frame’s Operand
parameters
stack
Parameters
pointer
balance
0.0
2
Method Area
Local variables
100
Methods
BankAccount()
deposit( double )
Constant Pool
0
Parameters
Frame data
deposit( double )
BankAccount class
b
main()
b
Driver class
…
args[0]
Heap
A stack frame for
the deposit method
of instance ‘b’ is
pointer
pushed into the Java
balance
stack
0.0
// BankAccount.java
private double balance;
private static int totalAccounts = 0;
…
args[0]
*
a
Parameters
b
Local variables
main()
Frame data
public BankAccount() {
balance = 0;
totalAccounts++;
}
Operand stack
amount=100
Parameters
b
Local variables
public void deposit( double amount ) { BankAccount()
balance += amount;
}
Frame data
Operand stack
Parameters
Driver class
BankAccount class
Methods
main()
Constant Pool
“BankAccount” a
“BankAccount” b
100
Methods
BankAccount()
deposit( double )
Constant Pool
0
Static Variables
totalAccounts
pointer
balance
pointer
balance
0.0
2
Method Area
100.0
0.0
Heap
The frame knows
which balance to
modify because of
the object reference