The basics of Java class loaders

The class loader concept, one of the cornerstones of the Java virtual machine, describes the behavior of converting a named class into the bits responsible for implementing that class. Because class loaders exist, the Java run time does not need to know anything about files and file systems when running Java programs.

What class loaders do

Classes are introduced into the Java environment when they are referenced by name in a class that is already running. There is a bit of magic that goes on to get the first class running (which is why you have to declare themain()method as static, taking a string array as an argument), but once that class is running, future attempts at loading classes are done by the class loader.

At its simplest, a class loader creates a flat name space of class bodies that are referenced by a string name. The method definition is:

Class r = loadClass(String className, boolean resolveIt);

The variableclassNamecontains a string that is understood by the class loader and is used to uniquely identify a class implementation. The variableresolveItis a flag to tell the class loader that classes referenced by this class name should be resolved (that is, any referenced class should be loaded as well).

All Java virtual machines include one class loader that is embedded in the virtual machine. This embedded loader is called the primordial class loader. It is somewhat special because the virtual machine assumes that it has access to a repository oftrusted classeswhich can be run by the VM without verification.

The primordial class loader implements the default implementation ofloadClass(). Thus, this code understands that the class namejava.lang.Objectis stored in a file with the prefixjava/lang/Object.classsomewhere in the class path. This code also implements both class path searching and looking into zip files for classes. The really cool thing about the way this is designed is that Java can change its class storage model simply by changing the set of functions that implements the class loader.

Digging around in the guts of the Java virtual machine, you will discover that the primordial class loader is implemented primarily in the functionsFindClassFromClassandResolveClass.

So when are classes loaded? There are exactly two cases: when the new bytecode is executed (for example,FooClassf= newFooClass();) and when the bytecodes make a static reference to a class (for example,System.out).