Java class-loader Interview Questions:-


  1. What is a class loader ? What are the different class loaders used by JVM ?

Part of JVM which is used to load classes and interfaces.

Bootstrap , Extension and System are the class loaders used by JVM.



  2. Difference between loadClass and Class.forName ?

loadClass only loads the class but doesn't initialize the object whereas Class.forName initialize the object after loading it.Part of JVM which is used to load classes and interfaces.



  3. What are various types of Class loaders used by JVM ?

Bootstrap - Loads JDK internal classes, java.* packages.

Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE System - Loads classes from system classpath.



  4. How are classes loaded by JVM ?

Class loaders are hierarchical. The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running.



  5. Difference between static vs. dynamic class loading?

static loading - Classes are statically loaded with Java's "new" operator.

dynamic class loading - Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time.

Class.forName (Test className);



  6. What is dynamic class loading?

Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time. Let us look at how to load classes dynamically by using Class.forName (String className); method, it is a static method.

The above static method returns the class object associated with the class name. The string className can be supplied dynamically at run time. Once the class is dynamically loaded the class.newInstance () method returns an instance of the loaded class. It is just like creating a class object with no arguments.

A ClassNotFoundException is thrown when an application tries to load in a class through its class name, but no definition for the class with the specified name could be found.



  7. How ClassLoader Works in Java

Java class loaders are used to load classes at runtime. ClassLoader in Java works on three principle: delegation, visibility and uniqueness. Delegation principle forward request of class loading to parent class loader and only loads the class, if parent is not able to find or load class.

Visibility principle allows child class loader to see all the classes loaded by parent ClassLoader, but parent class loader can not see classes loaded by child. Uniqueness principle allows to load a class exactly once, which is basically achieved by delegation and ensures that child ClassLoader doesn't reload the class already loaded by parent.

Correct understanding of class loader is must to resolve issues like NoClassDefFoundError in Java and java.lang.ClassNotFoundException, which are related to class loading. ClassLoader is also an important topic in advanced Java Interviews, where good knowledge of working of Java ClassLoader and How classpath works in Java is expected from Java programmer.

I have always seen questions like, Can one class be loaded by two different ClassLoader in Java on various Java Interviews. In this Java programming tutorial, we will learn what is ClassLoader in Java, How ClassLoader works in Java and some specifics about Java ClassLoader.



  8. What is Delegation principles in java class-loader?

As discussed on when a class is loaded and initialized in Java, a class is loaded in Java, when its needed. Suppose you have an application specific class called Abc.class, first request of loading this class will come to Application ClassLoader which will delegate to its parent Extension ClassLoader which further delegates to Primordial or Bootstrap class loader.

Primordial will look for that class in rt.jar and since that class is not there, request comes to Extension class loader which looks on jre/lib/ext directory and tries to locate this class there, if class is found there than Extension class loader will load that class and Application class loader will never load that class but if its not loaded by extension class-loader than Application class loader loads it from Classpath in Java.

Remember Classpath is used to load class files while PATH is used to locate executable like javac or java command.



  9. What is Visibility Principle?

According to visibility principle, Child ClassLoader can see class loaded by Parent ClassLoader but vice-versa is not true. Which mean if class Abc is loaded by Application class loader than trying to load class ABC explicitly using extension ClassLoader will throw either java.lang.ClassNotFoundException.



  10. What is Uniqueness Principle?

According to this principle a class loaded by Parent should not be loaded by Child ClassLoader again. Though its completely possible to write class loader which violates Delegation and Uniqueness principles and loads class by itself, its not something which is beneficial. You should follow all class loader principle while writing your own ClassLoader.



  11. How to load class explicitly in Java?

Java provides API to explicitly load a class by Class.forName(classname) and Class.forName(classname, initialized, classloader), remember JDBC code which is used to load JDBC drives we have seen in Java program to Connect Oracle database.

you can pass name of ClassLoader which should be used to load that particular class along with binary name of class. Class is loaded by calling loadClass() method of java.lang.ClassLoader class which calls findClass() method to locate bytecodes for corresponding class. In this example Extension ClassLoader uses java.net.URLClassLoader which search for class files and resources in JAR and directories.

any search path which is ended using "/" is considered directory. If findClass() does not found the class than it throws java.lang.ClassNotFoundException and if it finds it calls defineClass() to convert bytecodes into a .class instance which is returned to the caller.



  12. Where to use ClassLoader in Java?

ClassLoader in Java is a powerful concept and used at many places. One of the popular example of ClassLoader is AppletClassLoader which is used to load class by Applet, since Applets are mostly loaded from internet rather than local file system, By using separate ClassLoader you can also loads same class from multiple sources and they will be treated as different class in JVM.

J2EE uses multiple class loaders to load class from different location like classes from WAR file will be loaded by Web-app ClassLoader while classes bundled in EJB-JAR is loaded by another class loader. Some web server also supports hot deploy functionality which is implemented using ClassLoader. You can also use ClassLoader to load classes from database or any other persistent store.


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