Download Class Loader : TechnicalStack : http://technicalstack.com

This page was exported from TechnicalStack [ http://technicalstack.com ]
Export date: Wed May 10 16:29:23 2017 / +0000 GMT
Class Loader
Class Loader :
Type of Class Loaders
Every Class Loader Subsystem contains the 3 Class Loaders
1. Bootstrap Class Loader/Primordial class Loader
2. Extension Class Loader
3. Application Class Loader/System Class Loader
Bootstrap Class Loader/Primordial class Loader
This class is responsible for loading Core Java API Classes i.e The classes present in rt.jar
Location:Jdkjre7librt.jar
This location is called Bootstrap Class Path i.e Bootstrap Class Loader is responsible to load classes from
Bootstrap Class Path.
Bootstrap Class Loader is present in JVM by Default and it is implemented in native language C and C++
Extension Class Loader:
It is the child of Bootstrap Class loader and is responsible to load Class from
Location:JDKjre7libext
This Class Loader is implemented in Java and the corresponding .class File Name is
sun.misc.Launcher$ExtClassLoader.class
It also loads various security extension functions.
Application Class Loader/System Class Loader
It is the Child of Extension Class Loader and responsible to load Classes from Application Classpath,It
internally uses Environmental Variable Class Path.It is implemented in Java and the corresponding .class file is
at sun.misc.Launcher$ApplicationClassLoader.class
Example :
class Test {
public static void main (String args[])throws Exception{
System.out.println(String.class.classLoader());
System.out.println(Student.class.classLoader());
System.out.println(Test.class.classLoader());
}
}
Assume that Student.class in present in both extension class loader and application class loader
and Test.class is present in only in Application class path.
For String Class :
From Bootstrap Class path by Bootstrap Class Loader output is null(as Bootstrap Class Loader is not
implemented in Java and it is not Java Object)
For Student Class :
From Extension class path by Extension Class Loader
Output is sun.misc.Launcher$ExtClassLoader@1234
For Test Class : From Application Class Path by Application Class Loader
Output is sun.misc.Launcher$ AppClassLoader@4567
Note:
Bootstrap Class Loader is not Java Object and Hence we are getting null as our first output.
Extension Class Loader and Application Class Loader are Java Object and hence we are getting proper output.
ClassName@HexaDecimal String of HashCode
Class Loader Subsystem will give Highest Priority to Bootstrap Class loader and then Extension Class Loader
followed by Application Class Loader.
How class loader Works ?
1. Class Loader follows delegation Herecary Principle
2. Whenever JVM comes accross a particular class it will first check weather the corresponding class is
already loaded or not.
3. If it is already loaded in Method area then JVM will use that loaded class.
4. If it is not already Loaded then JVM will request Class Loader Subsystem to load that particular
class,then Class Loader Subsystem handovers the request
5. to Application Class Loader.
6. Application Class Loader delegate the request to Extenstion Class Loader and Extension Class loader
inturn delegate to Bootstrap Class Loader.
7. Bootstrap Class Loader searched in Bootstrap class path (jdk/jre/lib) If the specified class is available
then it will load it.Otherwise Bootstrap class Loader delegate it to Extenstion Class Loader.
8. Extension Class Loader searched in the Extensino Class path(jdk/jre/lib/ext).If the specified class is
available then it will load it,Otherwise it will delegate the request to Application Class Loader.
9. Application Class Loader searches in Application class path.If the specified class path is already
available it will load otherwise it will give Runtime Exception : ClassNotFoundException.
What is the need of Custom Class Loader ?
Sometimes we may not satisfy with default class loaders and with default class Loader Mechanism,
Example :
Default class loader will load the ,class file only once even though we are using multiple times that class in our
program.
After Loading .class file if it is modified Outside the clas the Default Class loader won't load updated class
version file on the Fly,because .class file is already available in the method area.
We can resolve this problem by defining our own customized
class loader.
For example whenever we are using a class,first my class loader will check weather updated version is
available or not.If it is available
then load the updated version otherwise use the existing .class file so that always the updated version is
available to our program.
The Main Advantage of Customized class loader is that we can customize class loading mechanism
based on our requirement.
Purpose of java.lang.Classloader Class :
We can use ClassLoader Class to create our own Custom ClassLoaders.
In Java every Custom ClassLoader should extend the ClassLoader class directly or indirectly.
So the ClassLoader Class is the Base Class for all custom ClassLoader
Code:
public Clas CustomizedClasLoader extends ClassLoader{
public Class loadClass(String className) throws ClassNotFoundException
{
//Custom logic
// check for updated clas version if availabe load the updated .class
file and return the updated Class object
//else return class object of already loaded Class
}
}
Important :

Where to use Customize class loader ?
While develoving our own Application Server and WebServers

What is the purpose of ClassLoder class ?
We can use ClassLoader Class to create our own Custom ClassLoaders.

To improve performance while loading class we can go for CustomizedClasLoader.
Post date: 2016-06-07 19:31:20
Post date GMT: 2016-06-07 19:31:20
Post modified date: 2016-07-28 11:06:39
Post modified date GMT: 2016-07-28 11:06:39
Powered by [ Universal Post Manager ] plugin. MS Word saving format developed by gVectors Team www.gVectors.com