Jvm, Jre And Javac Are The Main Part For The Java Program

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JVM :

JVM is known as Java Virtual machine.It is an interpreter .Java Virtual Machine is an abstract machine. It provides the runtime environment in which java bytecode can be executed.
JVM is platform dependent so it is available for many hardware and software.
What is JVM ?
[1] JVM is a specification where working of Java Virtual Machine is specified. But implementation provider is independent to choose the algorithm. Its implementation has been provided by Sun and other companies.
[2] JVM is an implementation Its implementation is called as JRE (Java Runtime Environment).
[3] JVM is a Runtime InstanceWhenever we write java command on the command prompt to run the java class, an instance of JVM is created.
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Working of JVM:
The JVM performs following works :
1)Loads the java code
2)Verifies the code
3)Executes the code
4)Provides runtime environment
Architecture of JVM:
[1] Classloader:
It is a subsystem of JVM which is used to load the class files.
[2] Class Area(Method Area):
Class Area stores ...
... per-class structures such as the runtime constant pool, field and method data, the code for methods.
[3] Stack:
Java Stack stores frames.It holds local variables and partial results, and plays a part in method invocation and return.
Each thread has a private JVM stack, created at the same time as thread.

A new frame is created each time a method is invoked. A frame is destroyed when its method invocation completes.java training center bangalore
[4] Native Method Stack:
Native Method Stack contains all the native methods used in the application.
[5] Heap:
It is the runtime data area in which objects are allocated and stored.
[6] Program Counter Register:
PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
[7] Execution Engine :
Execution Engine contains :
i) A virtual processor
ii) Interpreter: Read bytecode stream then execute the instructions.
ii) Just-In-Time(JIT) compiler: It is used to improve the performance.JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.
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JRE:
JRE is known as Java Runtime Environment.It is used to provide runtime environment to java program.It is the implementation of JVM. It physically exists. It contains set of libraries and other files that JVM uses at runtime.
Javac :
Javac is the primary Java compiler included in the JDK(Java Development Key) .The compiler accepts source code conforming to the Java language specification JLS) and generates bytecode conforming to the Java Virtual Machine Specification (JVMS).
Javac is itself written in Java. The compiler can also be invoked programmatically.
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Working of Javac :

- The javac command compiles Java source code (.java file)into Java bytecodes (.class file). we then use the Java interpreter to interprete the Java bytecodes.

- Java source code must be contained in files whose filenames end with the .java extension. The file name must be having the class name, as classname.java, if the class is public or is referenced from another source file.
- For every class defined in each source file compiled by javac compiler, the compiler stores the resulting bytecodes in a class file with a name of the form classname.class.
- When the compiler must refer to your own classes you need to specify their location. java/j2ee classes bangalore Use the classpath option or CLASSPATH environment variable to do this.
- Class path is a sequence of directories (or zip files) which javac searches for classes not already defined in any of the files specified directly as command arguments.
- The compiler looks in the class path for both a source file and a class file, recompiling the source (and regenerating the class file) if it is newer.
This is the short description about the JVM,JRE and Javac . To know more about the java for developing purpose or getting a job as java developer join Infocampus Software Training Institute for the java course.

Author :
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