JVM

Java Virtual Machine

History

Basics

Data Types

JVM Runtimes

History:

•     Java platform was initially developed to address the problem of building s/w for n/w consumer devices.

•     To meet requirements , compiled code has to transport across the networks ,works on any client and assure that the client that it was safe to run.

•     WWW got popularized , but web HTML format was too limited . So the answer is extensibility in the way of HOTJAVA BROWSER.

•     Hot Java Browser implements the java language and platform embedded into HTML webpages.Like HTML pages , compiled programs are n/w and host independent. The programs works the same way regardless of what machine they run on and where they come from.   

Basics-JVM:

•     Cornerstone of the Java platform.

•     is an abstract computing machine . It has its own instruction set and manipulates memory at runtime.

•     Component of technology responsible for its h/w and os independence , a small class file of compiled code and ability to protect users from malicious programs

•     Knows nothing of Java programming language only a binary format ie.., class file format. Class file format consists of

    JVM instructions (Byte code )

    Symbol table (key - value pair)

    Ancillary informations(Support info)

•     Class file format is H/w and Os independent binary format   

Data Types:        

•     VM operates on 2 kinds of Data Types

     1.Primitive types 2. Reference Types (class types , array types & interface types).

•     These are the values actually that can be stored in a variable , passed as an argument and a value returned by methods.

•     Jvm expects all type checking is done prior to the runtime , typically by a compiler .

•     Explicit support for objects. An object is either dynamically allocated class instance or an array.

•      reference to an object is considered to have a JVM reference types.

•     Values of type references can be thought of as a pointer to objects. More than one pointers for an object can exist. Objects are operated via type references.

JVM Runime Areas:

 


PC Register:

•     Each Jvm Thread has its own PC register

•     Contains current executing method    

•     Pc register holds any one of the follwing values

                   if method is not native , PC holds address of JVM instruction

                     if method is native , PC hold either undefined or return addresses(pointer to OPCODES of jvm)

JVM Stacks:

•     Per thread

•     Stores frames(data , partial results , dynamic linking,return method values , dispatch exceptions)

•     Contains local variables , partial results and plays part in method invocation and return  

Heap:

•     Common for all java threads

•     Memory for all class instances and arrays are allocated

•     Created on JVM startup

•     Heap storage for objects is reclaimed by automatic management system(gc) , objects are never explicitly deallocated

•     If your computation requires more memory than heap , then OUTOFMEMORYERROR

Method Area:

•     Common to all Threads

•     Contains per-class structures , runtime constant pool , field , method data , code for constructors & methods , instances and interface initialization

•     Logically is a part of heap

•     VM doesnot mandate location of method area or policies used to manage compiled code  

Run Time Constant Pool:

•     Per Class/per Interface representation in .Class file

•     Contains constants , ranging from numeric literals known at compile time to method/fields resolved at runtime

•     Simillar to symbol table

•     Allocated from the method area

•     Pool for class/interface is constructed when class/interface is created by JVM

Native Method Stacks:

•     Thread based not JVM based

•     Stacks which assists in interpreting the language other than Java

•     JVM cannot load native method stacks