Introduction

It is a virtual machine that runs java byte code by creating run-time areas to execute class members. The byte code is generated by a java compiler in a JVM understandable format. Using the “java” tool we can start the JVM process.  “java” initiates the java virtual machine instance.

To understand the above definition we need to know the virtual machine.

 Virtual machine

The virtual machine is a program that creates an environment between the computer platform and the end-user in which the end-user can operate programs. The creation of a number of different identical execution environments on a single computer to execute programs is called a Virtual Machine as shown in the below figure multiple VM’s on a single host machine.

 

 

 

JVM Types

The Java hotspot client VM is the default virtual machine of the Java SDK and JRE. It is tuned for best performance when running applications in a client environment by reducing application start-up time and memory footprint.

The Java Hotspot Server version loads more slowly, putting more effort into producing highly optimized JIT compilations that yield higher performance.

ClassLoader

ClassLoader is a class that is responsible to load classes into JVM’s method area.

Basic class loaders  are listed below:

ApplicationClassLoader: ApplicationClassLoader to load the classes from the application classpath,(current working directory). It basically uses a classpath environment variable to locate the class i.e. “.class” file.

ExtensionClassLoader :ExtensionClassLoader to load the classes from extension classpath,(%JAVA_HOME%\jre\lib\ext” folder).

BootStrapClassLoader:BootStrapClassLoader to load the classes from BootStrap classpath, (%JAVA_HOME%\jre\lib\rt.jar” folder). These classes are predefined classes.

 

How ClassLoader works ?

Whenever JVM comes across a type, it checks for that class byte code in the method area. If it’s not loaded, it requests the class loader subsystem to load the corresponding class byte code in the method area from that class respective Classpath. Alternatively, if it is already loaded it makes use of that type.

ClassLoader then handovers the request to ApplicationClassLoader, which searches for that class in the folders configured in the Classpath environment variable. If a class is not found, it forwards the request to ExtensionClassLoader and then to BootStrapClassLoader.

If the search fails in all the ClassLoaders then then jvm throws an exception “java.lanf.NoClassDefFoundError” / “java.lang.ClassNotFoundException”.

If class found in any one of the classpaths, the respective ClassLoader loads that class into jvm’s method area.

Please find the below figure for more clarity.

 

Heap vs Stack

Whenever a class is executed by specifying its corresponding class name by using the command “java {classname}” the java launcher JAVA immediately initiates the java run-time environment for the class execution as a layer on top of the operating system.

Heap:

• Main memory of JVM.
• Objects of classes.
• Non-static variables memory is created.
• Method area and heap area both are memory sharable areas.

Java Stack:

• All java methods are executed.
• JVM by defaults creates Main Thread and Garbage collector thread
• Each method execution JVM creates a separate block in the main thread. This is called the Stack Frame.
• Java operations are called Stack Frame operations because every method is executed only in the stack.

Stack Frame is a sub-block inside a thread created to execute a method or block and is destroyed automatically after the completion of the method.If the method has any local variable they all are created inside the method’s stack frame and are destroyed automatically when the stackframe is destroyed. Stack Frame is divided into 3 blocks:

• Variables storage area:for method’s local variable
• Instruction storage area: To store instruction information
• Method stack:To execute method.