CS6270 Virtual Machines - Java Virtual Machine Architecture and APIs

1. Java Virtual MachineJava Virtual Machine
ArchitectureArchitecture
and APIsand APIs
22 Oct 200722 Oct 2007
National University of SingaporeNational University of Singapore
School of ComputingSchool of Computing
OH KWANG SHINOH KWANG SHIN

2. The JVM Architecture
and APIs
22 Oct 2007 2
AgendaAgenda
• Big Picture of Java
• The Java Virtual Machine Architecture
– Data Types & Storage
– Java Instruction Set
– Exceptions and Errors
– Binary Classes
– The Java Native Interface
• Completing the Platform: APIs
– Java Platforms
– Java APIs: Serializability

3. The JVM Architecture
and APIs
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Big Picture of JavaBig Picture of Java
Java Source
( *.java )
Java Compiler
( javac )
Java Class
( *.class )
Java Platform
Java Virtual
Machine
Java APIs
Object.class

4. The JVM Architecture
and APIs
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JVM – Data TypesJVM – Data Types
• Primitive Data Types
– int, char, byte, short, float, double
– Implementation-dependent fashion
• Defined according to the values they can take, not
the number of bits of storage
• Integer in the range -231
to +231
-1
– 32-bit words + two’s complement
– Could use more storage bits
• References
– Can hold reference values
• Reference value points to an object stored in memory

5. The JVM Architecture
and APIs
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JVM – Data TypesJVM – Data Types
• Objects and Arrays
– Object
• Composed of primitive data types and
references that may point to other object
– Array
• Has a fixed number of elements
• Elements of an array
– Must all be of the same primitive type
– Must all be references which point to objects of
the same type

6. The JVM Architecture
and APIs
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JVM – Data StorageJVM – Data Storage
• Global Storage
– Main memory, where globally declared
variables reside
• Local Storage
– Temporary storage for variables that are local
to a method
• Operand Storage
– Holds variables while they are being operated
on by the functional instructions

7. The JVM Architecture
and APIs
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JVM – Data StorageJVM – Data Storage
• The Stack
– Local and operand storage
are allocated on the stack
– Not arrays and objects,
but only references and
individual array elements
on the stack
– As each method is called,
a stack frame is allocated
Locals
Operands
Arguments
Locals
Operands
Arguments
Locals
Operands
Java Stack Structure

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and APIs
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JVM – Data StorageJVM – Data Storage
• Memory Hierarchy

9. The JVM Architecture
and APIs
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JVM – Java Instruction SetJVM – Java Instruction Set
• Instruction Formats
(a) opcode
(b) opcode index
(c) opcode index1 index2
(d) opcode data
(e) opcode data1 data2
Typical Bytecode Instruction Formats

10. The JVM Architecture
and APIs
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JVM – Java Instruction SetJVM – Java Instruction Set
• Data-Movement Instructions
– Push constant values onto the stack
•iconst1: single-byte instruction that pushes
the integer constant 1 onto the stack
•bipush data, sipush data1 data2,
ldc index, ldc_w index1 index2, etc
– Manipulate the stack entries
•pop: pops the top element from the stack
and discards it
•swap: swaps the positions of the top two
stack elements

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and APIs
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JVM – Java Instruction SetJVM – Java Instruction Set
• Data-Movement Instructions
– Moves values : local storage ↔ operand stack
• iload_1: takes the integer from local storage slot 1
and pushes it onto the stack
• istore_1: moves data from the stack to local
storage in the current stack frame
– Deal with global memory data (objects, arrays)
• new: new instance of the object is created on the
heap and initialized. A reference to the object is
pushed onto the stack
• newarray: creates an array containing elements of a
specified primitive type
• getfield, putfield: accesses data held in objects

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and APIs
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JVM – Java Instruction SetJVM – Java Instruction Set
• Type Conversion
– Convert one type of data item on the
stack to another
•i2f: pops an integer from the stack,
converts it to a float, and pushes the float
back onto the stack

13. The JVM Architecture
and APIs
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JVM – Java Instruction SetJVM – Java Instruction Set
• Functional Instructions
– Take input operands, perform operations on
them, and produce a result
– Single byte: operands are always taken from the
stack and results are placed onto the stack
– Example
• iadd: pops two integers from the stack  adds them
 pushes the sum onto the stack
• iand: pops two integers from the stack  performs a
logical AND on them  pushes the result onto the
stack
• ishfl: pops two integers from the stack  shifts the
top element left by an amount specified by the second
element  pushes the result onto the stack

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and APIs
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JVM – Java Instruction SetJVM – Java Instruction Set
• Control Flow Instructions
– ifeq data1 data2
• pops an integer from the stack  compares it with zero
– True: PC relative branch to an offset found by
concatenating the two data bytes
– if_icmpeq data1 data2
• pops two integer values from the stack  compares the
first with the second
– True: PC relative branch to an offset found by
concatenating the two data bytes
– ifnull data1 data2
• pops a reference from the stack  check it for null
– True: PC relative branch to an offset found by
concatenating the two data bytes

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and APIs
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JVM – Java Instruction SetJVM – Java Instruction Set
• Example Program
– javap: The Java Class File Disassembler
class Rectangle {
protected int sides [];
………………
………………
public int perimeter () {
return 2*(sides[0] + sides[1]);
}
public int area () {
return (sides[0] * sides[1]);
}
}
Java Source Rectangle.java
public int perimeter();
Code:
0: iconst_2
1: aload_0
2: getfield #2; //Field sides:[I
5: iconst_0
6: iaload
7: aload_0
8: getfield #2; //Field sides:[I
11: iconst_1
12: iaload
13: iadd
14: imul
15: ireturn
Disassembled Code

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and APIs
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Rectangle Objectsides[0]
JVM – Java Instruction SetJVM – Java Instruction Set
• Example Program Scenario
public int perimeter () {
return 2*(sides[0] + sides[1]);
}
public int perimeter();
Code:
0: iconst_2
1: aload_0
2: getfield #2;
5: iconst_0
6: iaload
7: aload_0
8: getfield #2;
11: iconst_1
12: iaload
13: iadd
14: imul
15: ireturn
Constant 2
Operand Stack
Pushes a constant 2 onto the operand stack
Pushes local variable 0 onto the stack
(argument  reference to the rectangle object)
Pushes the reference to the sides array
sides Array
Pushes index number 0 onto the stack
Index Number 0
Load element 0 from the array sides
Pushes local variable 0 onto the stack
(argument  reference to the rectangle object)
Pushes the reference to the sides array
Pushes index number 1 onto the stack
Load element 1 from the array sides
Rectangle Object
sides Array
Index Number 1
sides[1]
Pushes addition of the top two stack elements
Pushes multiplication of top two stack elements
Returns with the integer result on top of stack
sides[0]+sides[1]
2*(
sides[0]+sides[1])

17. The JVM Architecture
and APIs
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JVM – Exceptions & ErrorsJVM – Exceptions & Errors
• All exceptions must be handled
somewhere
– No global way to turn them off
– If there is no handler, then calling method
takes over
– Overall program robustness consideration
• Errors
– Caused by limitation of the VM implementation
or VM bugs
• Exceptions
– Caused by program behavior that occurs
dynamically – as the program executes

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and APIs
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JVM – Exceptions & ErrorsJVM – Exceptions & Errors
• Exception table with each method
– Makes it possible to specify an exception
handler, depending on where an exception
occurs
From To Target Type
8 12 96 Arithmetic Exception
Exception Table

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and APIs
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JVM – Exceptions & ErrorsJVM – Exceptions & Errors
• Example of exception table
class Rectangle {
protected int sides [];
………………
public int perimeter () {
try {
return 2*(sides[0] + sides[1]);
} catch(ArithmeticException e) {
return -1;
}
}
public int area () {
return (sides[0] * sides[1]);
}
}
public int perimeter();
Code:
0: iconst_2
1: aload_0
2: getfield #2; //Field sides:[I
5: iconst_0
6: iaload
7: aload_0
8: getfield #2; //Field sides:[I
11: iconst_1
12: iaload
13: iadd
14: imul
15: ireturn
16: astore_1
17: iconst_m1
18: ireturn
Exception table:
from to target type
0 15 16 Class java/lang/ArithmeticException

20. The JVM Architecture
and APIs
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JVM – Binary ClassesJVM – Binary Classes
• Binary class
– Typically included in a class file
– Code + Metadata
• Metadata is a detailed specification of the
data structures and their relationships
– Can be loaded on demand
• At the time they are needed by the program
• Saves bandwidth for loading binary classes
that are never used
• Allows a Java program to start up quickly

21. The JVM Architecture
and APIs
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JVM – Binary Class FormatJVM – Binary Class Format
Magic Number
Version Information
Constant Pool Size
Constant Pool
Access Flags
This Class
Super Class
Interface Count
Interfaces
Field Count
Field Information
Method Count
Methods
Attribute Count
Attributes
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
• • • • • •
Identifier of java binary class
Minor and major version numbers of this class file
Number of entries in the constant pool + 1
All the constant values and references
Provide access information
Name of this class (Valid index of Constant Pool)
Name of super class (Valid index of Constant Pool or 0)
Number of direct superinterfaces
Number of references to the superinterfaces
(Valid index into the Constant Pool table)
Number of field_info structures in the Field Information
field_info structures giving a complete description of
a field in this class or interface
Number of method_info structures in the Methods
method_info structure giving a complete description of
a method in this class or interface
Number of attributes in the Attributes
Detailed information regarding
the other components listed earlier

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and APIs
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JVM – Java Native InterfaceJVM – Java Native Interface
• JNI (Java Native Interface)
– Allows java code and native compiled
code to interoperate

23. The JVM Architecture
and APIs
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Java PlatformJava Platform
• Java Platform = JVM + Java APIs
• Java APIs
– A set of standard libraries
– Provide most of the features that are
visible to users and software developers
• Support for secure network computing,
component-based software, graphical user
interfaces (GUIs), etc.

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and APIs
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Java PlatformJava Platform
• J2EE, J2SE and J2ME

25. The JVM Architecture
and APIs
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Java APIs - SerializationJava APIs - Serialization
• Process of converting an object into
an implementation-independent form
Object
Platform A-
Dependent
Representation
Serialization Deserialization
Object
Platform B-
Dependent
Representation
Platform-
Independent
Representation
Persistent Storage
Network

26. The JVM Architecture
and APIs
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Java APIs - ThreadJava APIs - Thread
• Multithreading support is provided by Java
libraries that are part of java.lang
• Monitors
– Support the synchronization among threads
– Lock and two Java bytecode instructions
• Lock
– Associated with each object and each class
– Operated as a counter, rather than flag
• Two Java bytecode instructions
– Monitorenter
– monitorexit

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and APIs
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210
Java APIs - ThreadJava APIs - Thread
public int perimeter () {
synchronized (sides) {
synchronized (sides) {
return 2*(sides[0] + sides[1]);
}
}
}
public int perimeter();
Code:
………
6: monitorenter
………
13: monitorenter
14: iconst_2
………
35: aload_2
36: monitorexit
………
41: aload_1
42: monitorexit
………
Lock of sides
Acquires the lock for the object, the lock is incremented
Acquiring thread may already hold the lock
, the lock is incremented
Decrements the lock for the object
Decrements the lock for the object
If the lock becomes zero,
then it is released and can
be acquired by a waiting
thread (if there is one)

28. The JVM Architecture
and APIs
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ReferencesReferences
• James E. Smith and Ravi Nair, Virtual Machines: Versatile
Platform for Systems and Processes. Morgan Kaufmann
Publishers, 2005.
• Java Technology – The Source for Java Developers
Available: http://java.sun.com
• Bill Venners, Inside the Java 2 Virtual Machine, McGraw-
Hill, 1999.
• Tim Lindholm and Frank Yellin, The JavaTM
Virtual
Machine Specification – Second Edition, Addison-Wesley
Longman Publishing Co., Inc., Boston, MA, 1999.