There are three important part of the JAVA which are given below:
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.
An implementation Its implementation is known as JRE (Java Runtime Environment).It is used to provide runtime environment.It physically exists.It contains set of libraries + other files that JVM uses at runtime.
Runtime Instance Whenever you write java command on the command prompt to run the java class, and instance of JVM is created.
Loads code
Verifies code
Executes code
Provides runtime environment
JVM provides definitions for the:
Memory area
Class file format
Register set
Garbage-collected heap
Fatal error reporting etc.
1) Classloader:
Classloader is a subsystem of JVM that is used to load class files.
2) Class(Method) Area:
Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.
3) Heap:
It is the runtime data area in which objects are allocated.
4) 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.
5) Program Counter Regiser:
PC (program counter) register. It contains the address of the Java virtual machine instruction currently being executed.
6) Native Method Stack:
It contains all the native methods used in the application.
7) Execution Engine:
It contains:
1) A virtual processor
2) Interpreter:Read bytecode stream then execute the instructions.
3) 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.Here the term ?compiler? refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.
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- JDK (Java Development Kit)
- JRE (Java Runtime Environment)
- JVM (Java Virtual Machine)
JDK (Java Development Kit)
It physically exists. It contains JRE + development tools.JRE (Java Runtime Environment)
It used to provide runtime environment. It is the implementation of JVM. It physically exist. It contains set of libraries + other files that JVM use in the runtime. Implementation of JVM are also activity released by other companies besides Sun Micro Systems.JVM (Java Virtual Machine)
JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed. JVMs are available for many hardware and software platforms (i.e.JVM is platform dependent).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.
An implementation Its implementation is known as JRE (Java Runtime Environment).It is used to provide runtime environment.It physically exists.It contains set of libraries + other files that JVM uses at runtime.
Runtime Instance Whenever you write java command on the command prompt to run the java class, and instance of JVM is created.
What it does?
The JVM performs following operation:Loads code
Verifies code
Executes code
Provides runtime environment
JVM provides definitions for the:
Memory area
Class file format
Register set
Garbage-collected heap
Fatal error reporting etc.
Internal Architecture of JVM
Let's understand the internal architecture of JVM. It contains classloader, memory area, execution engine etc.1) Classloader:
Classloader is a subsystem of JVM that is used to load class files.
2) Class(Method) Area: Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.
3) Heap:
It is the runtime data area in which objects are allocated.
4) 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.
5) Program Counter Regiser:
PC (program counter) register. It contains the address of the Java virtual machine instruction currently being executed.
6) Native Method Stack:
It contains all the native methods used in the application.
7) Execution Engine:
It contains:
1) A virtual processor
2) Interpreter:Read bytecode stream then execute the instructions.
3) 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.Here the term ?compiler? refers to a translator from the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.
Frequently Asked Questions
What are the action performed by JVM during compilation of your program?
At compile time, java file is compiled by Java Compiler (It does not interact with OS) and converts the java code into bytecode.
What are the action performed by JVM during runtime of your program?
At runtime, following steps are performed:
Classloader: is the subsystem of JVM that is used to load class files.
Bytecode Verifier: checks the code fragments for illegal code that can violate access right to objects.
Interpreter: read bytecode stream then execute the instructions.
Can you save a java source file by other name than the class name?
Yes, like the figure given below illustrates:
To compile, use:javac Hard.java
To execute, use:java Simple