Java Fundamentals - JVM Architecture

ClassLoader

1.Loading:

When a Java program is converted into .class file by Java compiler which is collection of byte code. ClassLoader is responsible to load that class file from file system, network or any other location.

Our Java class is depends up on any other class let’s say JdbcDriver.class, it will search by following ClassLoaders.

·         Bootstrap ClassLoader - JRE/lib/rt.jar

First bootstrap class loader tries to find the class. It scans the rt.jar file in JRE lib folder

·         Extension ClassLoader - JRE/lib/ext or any directory denoted by java.ext.dirs

If class is not found then extension class loader searches the class file in inside jre\lib\ext folder

·         Application ClassLoader - CLASSPATH environment variable, -classpath or -cp option

Again if class is not found then application classloader searches all the Jar files and classes in CLASSPATH environment variable of system.

2.Linking:

·         Bytecode verifier will verify whether the generated bytecode is proper or not.

·         Prepare(memory allocation): allocates memory to static variables & methods.

3.Initialization: In prepare only memory is allocated, here all static variable will be assigned with the original values and the static blocks will be executed.

Runtime area

Fields(Data members) and methods are also known as class members

·         Method Area : all Class level Data members, Method definitions stored here

·         Heap All Objects & instance variable Data stored Here.

·         Stacks : All Methods executions & Thread Executions done here. store local variables, and intermediate results. Each thread has its own JVM stack, created simultaneously as the thread is created. So all such local variable are called thread-local variables.

·         PC register store the physical memory address of the statements which is currently executing. In Java, each thread has its separate PC register.

·         Java supports and uses native code as well. Many low level code is written in languages like C and C++. Native method stacks hold the instruction of native code.

Execution Engine

All code assigned to JVM is executed by an execution engine. The execution engine reads the byte code and executes one by one. It uses two inbuilt interpreter and JIT compiler to convert the bytecode to machine code and execute it.

1.Interpreter A JVM interpreter pretty much converts each byte-code instruction to corresponding native instruction. It directly executes the bytecode only one instruction at a time and does not perform any optimization.

2.JIT Compiler JIT compiler takes a block of code (not one statement at a time as interpreter), optimize the code and then translate it to optimized machine code. To improve performance, it will Optimizes the bytecode

Differences between JDK, JRE and JVM

JVM = Just a Specification. HotSpot VM is implementation of IT

JRE = JVM + libraries to run Java application.

JDK = JRE + tools to develop Java Application.

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