This week I am starting a series on the Java classloader. Before we start talking about the classloader here's a little foundation.
If you have programmed with C++, you will remember there are two ways of compiling and linking programs.
Any large software consists of many classes and dependencies on external libraries.
Statically compiling a program means creating one large executable file containing all the classes and dependencies. Since all that it needs is bundled in one file which is loaded when the program starts, such a program does not need to locate any files when running. However, because statically linked programs have a very large footprint, this approach is not favored, and an approach known as dynamic linking is used.
The output of a dynamically compiled program is an executable file containing the core program, and many libraries files. When the program starts running only the main executable is loaded in memory. Dependencies are loaded as and when required. Some of these dependencies may be distributed with the software while others may already exist in the underlying operating system, and the program simply expects them to be there. When the program needs to invoke a class that is in an external library, the runtime locates the library and loads the class.
Java uses an approach closer to the latter. In fact Java programs can NEVER be statically linked (unless you compile them into statically linked native code). Software written in Java is distributed as a bundle of class or jar files. When the program is executing, the JVM has to find, load, and invoke methods of various classes. All the classes are not loaded at once, instead they are located and loaded as they are required. The task of loading these class files is the responsiility of the Java classloader.
We will discuss the Java classloader in greater detail in the next post.
Note: This text was originally posted on my earlier blog at http://www.adaptivelearningonline.net