Class Loading in Java

Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that is already running in the JVM. So how is the very first class loaded? 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. A class loader creates a namespace. All JVMs include at least one class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of

primordial class loader. Let us look at the class loaders created by the JVM.

1. Bootstrap (primordial) - Not reloadable – Loads JDK internal classes, java.* packages. (as defined in the sun.boot.class.path system property, typically loads rt.jar and i18n.jar)
2. Extensions – Not reloadable – Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system property – usually lib/ext directory of the JRE)
3. System – Not reloadable – Loads classes from system classpath (as defined by the java.class.path property, which is set by the CLASSPATH environment variable or –classpath or –cp command line options)

Classes loaded by Bootstrap class loader have no visibility into classes loaded by its descendants (ie Extensions and Systems class loaders). The classes loaded by system class loader have visibility into classes loaded by its parents (ie Extensions and Bootstrap class loaders).
If there were any sibling class loaders they cannot see classes loaded by each other. They can only see the classes loaded by their parent class loader.

Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their parent to load the class first before attempting to load it themselves. When a class loader loads a class, the child class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true.

Two objects loaded by different class loaders are never equal even if they carry the same values, which mean a class is uniquely identified in the context of the associated class loader. This applies to singletons too, where each class loader will have its own singleton.

Static class loading:
Classes are statically loaded with Java’s “new” operator.
class MyClass {
public static void main(String args[])
{
Car c = new Car();
}
}
A NoClassDefFoundException is thrown if a class is referenced with Java’s “new” operator (i.e. static loading) but the runtime system cannot find the referenced class.

Dynamic class loading:
Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time.
Class.forName (String className); //static method which returns a Class
The above static method returns the class object associated with the class name. The string className can be supplied dynamically at run time. Unlike the static loading, the dynamic loading will decide whether to load the class Car or the class Jeep at runtime based on a properties file and/or other runtime conditions. Once the class is dynamically loaded the following method returns an instance of the loaded class. It’s just like creating a class object with no arguments.
class.newInstance (); //A non-static method, which creates an instance of a class (ie creates an object).
Jeep myJeep = null ;
//myClassName should be read from a properties file or Constants interface.
//stay away from hard coding values in your program.
String myClassName = “au.com.Jeep” ;
Class vehicleClass = Class.forName(myClassName) ;
myJeep = (Jeep) vehicleClass.newInstance();
myJeep.setFuelCapacity(50);
A ClassNotFoundException is thrown when an application tries to load in a class through its string name using the following methods but no definition for the class with the specified name could be found:

• The forName(..) method in class – Class.
• The findSystemClass(..) method in class – ClassLoader.
• The loadClass(..) method in class – ClassLoader.

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