Now that we have understood how the Classloader locates classes, what exactly does loading a class mean? What happens when a class is loaded?
Loading refers to the process of finding the binary form of a class or interface type with a particular name, perhaps by computing it on the fly, but more typically by retrieving a binary representation previously computed from source code by a compiler, and constructing, from that binary form, a Class object to represent the class or interface.
The binary format of a class or interface is normally the class file format described in The Java Virtual Machine Specification, but other formats are possible, provided they meet the specified requirements. The method defineClass of class ClassLoader may be used to construct Class objects from binary representations in the class file format.
The loading process is implemented by the class ClassLoader and its subclasses. Different subclasses of ClassLoader may implement different loading policies. In particular, a class loader may cache binary representations of classes and interfaces, prefetch them based on expected usage, or load a group of related classes together. These activities may not be completely transparent to a running application if, for example, a newly compiled version of a class is not found because an older version is cached by a class loader. It is the responsibility of a class loader, however, to reflect loading errors only at points in the program they could have arisen without prefetching or group loading.
If an error occurs during class loading, then an instance of one of the following subclasses of class LinkageError will be thrown at any point in the program that (directly or indirectly) uses the type:
ClassCircularityError: A class or interface could not be loaded because it would be its own superclass or superinterface.
ClassFormatError: The binary data that purports to specify a requested compiled class or interface is malformed.
NoClassDefFoundError: No definition for a requested class or interface could be found by the relevant class loader.
The above passage is an excerpt from The Java Language Specification.
What it means is, when the JVM needs a class, it is the responsibility of the ClassLoader to locate and loading it. The ClassLoader locates the class file as discussed earlier and creates a 'java.lang.Class' object that represents the class. So for example, when the String class is loaded, a java.lang.Class instance is created to represent the String class. Class instances are created for every class and NOT for every instance of the class. Even if we create 1000 String instances, only one instance of Class will be created to represent the String class. Think of the class Class as a blueprint of the class that was loaded. It contains the name of the class, it's superclasses, implemented interfaces, attributes, and methods.
When the JVM has to instantiate a Student object, it will first delegate the task of loading that object to the ClassLoader. The ClassLoader finds the file Student.class, reads it's contents and creates an object of type java.lang.Class, which represents the Student class. This object is handed over to the JVM, which uses it to create instances of type Student.
Till now we have seen that the ClassLoader locates a class by finding it in the local file system using the CLASSPATH environment variable. This is how classes are loaded most of the time, but it is not the only way. Classes can be loaded from the network over a TCP/IP socket (this is how classes are loaded in an Applet), by using a custom ClassLoader. Custom ClassLoaders can also be used for special checks, like the verification of digital signature of classes that are loaded. You can use, or write your own custom ClassLoader to load classes in an unconventional manner.
The ClassLoader performs several checks on the class that is loaded. We will understand them in greater detail in the next post.
Note: This text was originally posted on my earlier blog at http://www.adaptivelearningonline.net