JVM

By,
PRITY KUMARI
205111037
NIT-Trichy(MCA)

 JVM is an interpreter for bytecode.
 JVM needs to be implemented on each
platform.
 Enhance portability.
 Assure security.
 Encourage Fast execution of the program.
 Def : JVM is a component of the Java system
that interprets and executes the instructions in
our class files.

 Compiled code to be executed by the Java
Virtual Machine is represented using a
hardware- and operating system-independent
binary format, typically (but not necessarily)
stored in a file, known as the class file format.
 The class file format precisely defines the
representation of a class or interface, including
details such as byte ordering that might be
taken for granted in a platform-specific object
file format.

 Like the Java programming language, the Java
Virtual Machine operates on two kinds of types:
primitive types and reference types.
 There are, correspondingly, two kinds of values
that can be stored in variables, passed as
arguments, returned by methods, and operated
upon: primitive values and reference values.
 The instruction set of the Java Virtual Machine
distinguishes its operand types using instructions
intended to operate on values of specific types.
 For instance, iadd, ladd, fadd, and dadd

 numeric types,
 the boolean type
 returnAddress type
1)Numeric types
The integral types are:
 Byte(8-bit signed two's-complement integers)
 Short (16-bit signed two's-complement integers)
 Int(32-bit signed two's-complement integers)
 Long(64-bit signed two's-complement integers)
 Char(16-bit unsigned )and default value is the null
code point ('u0000')

2) Floating-point:
 float, whose values are elements of the float value set
and whose default value is positive zero
 double, whose values are elements of the double value
and whose default value is positive zero
3)Boolean-type:
 encode the truth values true and false, and the default
value is false.
4)returnAddress Type:
 The values of the returnAddress type are pointers to
the opcodes of Java Virtual Machine instructions.
 Of the primitive types, only the returnAddress type is
not directly associated with a Java programming
language type.

 There are three kinds of reference types:
class types
array types
interface types.
 Their values are references to dynamically created
class instances, arrays, or class instances or arrays
that implement interfaces, respectively.
 A reference value may also be the special null
reference, a reference to no object, which will be
denoted here by null
 The Java Virtual Machine specification does not
mandate a concrete value encoding null.

FIG: Memory Configuration of JVM

The JVM defines various run-time data areas that
are used during execution of a program. Some
of these data areas are created on JVM start-up
and are destroyed only when the Java Virtual
Machine exits.
Other data areas are per thread. Per-thread data
areas are created when a thread is created and
destroyed when the thread exits.

1)The pc Register:
 The JVM can support many threads of execution at once
 Each Java Virtual Machine thread has its own pc (program
counter) register.
2) JVM Stacks:
 Each JVM thread has a private JVM stack, created at the same
time as the thread.
 It holds local variables and partial results, and plays a part in
method invocation and return.
 Because the JVM stack is never manipulated directly except to
push and pop frames, frames may be heap allocated.
 The memory for a JVM stack does not need to be contiguous.
 Throws two exceptions
- OutOfMemoryError.
- StackOverflowError.

3) Heap
 The JVM has a heap that is shared among all Java Virtual
Machine threads.
 The heap is the run-time data area from which memory for
all class instances and arrays is allocated.
 The heap is created on virtual machine start-up.
 Throws one exception
- OutOfMemoryError
4) Method Area
 The method area is analogous to the storage area for
compiled code of a conventional language or analogous to
the "text" segment in an operating system process.
 It stores per-class structures such as the run-time constant
pool, field and method data, and the code for methods and
constructors, including the special methods used in class
and instance initialization and interface initialization.
 Throws one exception
- OutOfMemoryError

5) Run-Time Constant Pool
It contains several kinds of constants, ranging from numeric
literals known at compile-time to method and field references
that must be resolved at run-time.
 The run-time constant pool serves a function similar to that of a
symbol.
6) Native Method Stacks
 An implementation of the Java Virtual Machine may use
conventional stacks, called "C stacks," to support native
methods
 Java Virtual Machine implementations that cannot load native
methods and that do not themselves rely on conventional
stacks need not supply native method stacks.
 If supplied, native method stacks are typically allocated per
thread when each thread is created.

Figure 2: Content of Memory Blocks at runtime.

 Loading means reading the class file for a type, parsing it to get its
information, and storing the information in the method area.
 For each type it loads, the JVM must store the following information in the
method area:
 The fully qualified name of the type
 Whether the type is a class or an interface
 The type's modifiers ( public, abstract, final, etc)
 Method info: name, return type, number & types of parameters, modifiers,
bytecodes, size of stack frame and exception table.

The end of the loading process is the creation of an instance of
java.lang.Class for the loaded type.
The purpose is to give access to some of the information
captured in the method area for the type, to the programmer.
Some of the methods of the class java.lang.Class are:
Note that for any loaded type T, only one instance of java.lang.Class is
created even if T is used several times in an application.
public String getName()
public Class getSupClass()
public boolean isInterface()
public Class[] getInterfaces()
public Method[] getMethods()
public Fields[] getFields()
public Constructor[] getConstructors()

 The next process handled by the class loader is Linking. This
involves three sub-processes: Verification, Preparation and
Resolution
 Example of some of the things that are checked at verification are:
 Every method is provided with a structurally correct signature
 Every instruction obeys the type discipline of the Java language
 Every branch instruction branches to the start not middle of
another instruction

 In this phase, the JVM allocates memory for the class (i.e static)
variables and sets them to default initial values.
 Note that class variables are not initialized to their proper
initial values until the initialization phase - no java code is
executed until initialization.
 The default values for the various types are shown below:

 Resolution is the process of replacing symbolic names
for types, fields and methods used by a loaded type
with their actual references.
 Symbolic references are resolved into a direct references
by searching through the method area to locate the
referenced entity.
 For the class below, at the loading phase, the class
loader would have loaded the classes: TestClassClass,
String, System and Object.
 The names of these classes would have been stored in
public class TestClassClass{
public static void main(String[] args){
String name = new String(“Ahmed”);
Class nameClassInfo = name.getClass();
System.out.println("Parent is: “ + nameClassInfo.getSuperclass());
}
}

 After a class is loaded, linked, and initialized, it is
ready for use. Its static fields and static methods can be
used and it can be instantiated.
 When a new class instance is created, memory is
allocated for all its instance variables in the heap.
 Memory is also allocated recursively for all the instance
variables declared in its super class and all classes up is
inheritance hierarchy.
 All instance variables in the new object and those of its
superclasses are then initialized to their default values.
 Finally, the reference to the newly created object is
returned as the result.

Rules for processing a constructor:
 Assign the arguments for the constructor to its parameter
variables.
 If this constructor begins with an explicit invocation of another
constructor in the same class (using this), then evaluate the
arguments and process that constructor invocation recursively.
 If this constructor is for a class other than Object, then it will begin
with an explicit or implicit invocation of a superclass constructor
(using super). Evaluate the arguments and process that
superclass constructor invocation recursively.
 Initialize the instance variables for this class with their proper
values.
 Execute the rest of the body of this constructor.

JVM

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