JDK, JRE and JVM

 

Java Development Kit (JDK)

Java Development Kit contains tools needed to develop the Java programs, and JRE to run the programs. The tools include compiler (javac.exe), Java application launcher (java.exe), Applet viewer, etc…

Compiler converts java code into byte code. Java application launcher opens a JRE, loads the class, and invokes its main method.

You need JDK, if at all you want to write your own programs, and to compile them. For running java programs, JRE is sufficient. JRE is targeted for execution of Java. JDK is mainly targeted for java development.

Java Runtime Environment (JRE)

Java Runtime Environment contains JVM, class libraries (like util, math, lang, awt, swing etc), and other supporting files.

Actually JVM runs the program, and it uses the class libraries, and other supporting files provided in JRE. If you want to run any java program, you need to have JRE installed in the system.

Java Virtual Machine (JVM)

Java Virtual Machine executes the program and generates output. To execute any code, JVM utilizes different components.   
JVM is divided into several components like the stack, the garbage-collected heap, the registers and the method area.

The Stack

·    Stack in JVM stores various method arguments as well as the local variables of any method.

Method Area

This is the area where byte codes reside. The program counter points to some byte in the method area. It always keeps tracks of the current instruction which is being executed (interpreted).

Garbage-collected Heap

The Garbage-collected Heap is where the objects in Java programs are stored. Whenever we allocate an object using new operator, the heap comes into picture and memory is allocated from there

Remember that the local object reference resides on Stack but the actual object resides in Heap only. Also, arrays in Java are objects, hence they also resides in Garbage-collected Heap.

Just In Time Compiler (JIT)

JIT is a module inside the JVM which helps in compiling certain parts of byte code into the machine code for higher performance. Note that only certain parts of byte code will be compiled to the machine code, the other parts are usually interpreted and executed.

Questions

What is the use of bin and lib in JDK?

- Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.