Understanding Java Development Kit

• Java is a compiled and interpreted language.

1. JDK is a combination of development tools and JRE

2. /usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/bin in linux, consist of all the java development tools.

3. JRE stands for Java runtime environment, which consits of API (run time libraries).

4. /usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/rt.jar in linux, consist of all the java package folders and files stored in jar packaging format.

5. To view them using jar tool use the command,

   jar tvf rt.jar

6. How java achieves platform independence i.e philosophy of WORA?

   • When we compile the java source file using java compiler, it generates an intermediate byte code(.class file) which is platform independent. This .class files cannot run on any platform(OS + hardware) directly.

   • As this .class file cannot be run on any real machine, we need a virtual machine to convert this platform independent code to platform specific code. This is where we need a translator and JVM acts as a translator. Hence, JVM is platform specific, and acts as a translator.

7. What does JVM do?

   • JVM load and execute all the required classes.

   • Components of JVM

     1. class loader, which loads all the classes in JVM memory(in to something called method area).

        • Phases involved in class loader

          • Loading

          • Linking

            • Verify : Checks correctness of .class file

            • Prepare : Allocates memory for static fields of class/interface and initialize them with default value.

            • Resolve : Symbolic references with direct references.

          • Initializing

     2. Runtime memory area

        • Method area

        • Heap

        • Stack per thread

        • Program counters per thread

        • Native method stack

     3. Execution Engine

        • Interpreter, which takes intermediate byte code as input and outputs platform specific native code (This code is directly runnable on hardware).

        • JIT(Just in time compiler), basically use native code cache to make code interpretation faster by storing repeatedly occurring code in to native cache after being translated. JIT consists of,

          • Intermdiate code generator : Generates intermediate code.

          • Target code generator : Convert intermediate code to native machine code.

          • Code optimizer : Optimize intermediate code generated

          • Hotspot profiler : Finds the hot spots in the code.

        • Garbage collector

     4. Native method interface (JNI)

     5. Native method library

• More detailed article about it here.
• More about garbage collector is here.
• JDK structure for java 8 is here