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Designing with Dynamic Extension
How Dynamic Extension Works in Java and How to Use it in Your Designs
by Bill Venners
First Published in JavaWorld, December 1998

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Name spaces
When you start a Java application, you give the name of a class that has a main() method to a JVM. The JVM starts executing Java at the indicated main() method, and the application is off and running.

Each application lives inside its very own "instance" of the JVM, with memory areas that are private just to that application. Although different threads of the same application can all access a common heap, for example, threads in one Java application can't directly access the heap in another Java application. Each Java application gets its own Java virtual machine.

Inside a JVM, each class loader (be it primordial or object) gets its own name space. A name space is simply the set of fully qualified names of all the classes and interfaces that a class loader has loaded so far. A fully qualified name of a class is the name of its package, plus a dot, plus its simple name. For example, the fully qualified name of the Hashtable class from the java.util package is java.util.Hashtable.

Within a single name space, all fully qualified names are unique. Two different name spaces, however, can contain identical fully qualified names. This architecture enables a single Java application to load and use two (or more) different classes that have the same fully qualified name. So long as those like-named classes are loaded by different class loaders, which will cause the classes to be placed into different name spaces, the Java application will be able to load and use them.

As an example, Figure 1 shows a snapshot of two name spaces in a single Java application. Two fully qualified names are common to both name spaces: java.lang.Object and Mouse. The upper name space contains two other fully qualified names (Animal and Cat) that don't appear in the lower name space. Likewise, the lower name space contains two fully qualified names (Device and Keyboard) that don't appear in the upper name space. Within each name space, however, all names are unique.

Figure 1. Two name spaces

All of this is fine and good, but begs the question: How are the names in a name space related to the actual types being named? When a JVM loads a type (class or interface), it parses and extracts information about the type from a class file. It places this information (in some data structure devised by the people who implement the JVM) into a logical area of the JVM's memory called the method area. So when the JVM loads a class, a chunk of type data is added to the method area and a fully qualified name is inserted into a name space. Every fully qualified name in every name space is associated with a chunk of data in the method area that defines that named type.

For example, Figure 2 shows the relationship between names (Mouse and java.lang.Object) in the name spaces of the previous example and type data in the method area. One fully qualified name that both name spaces contain is java.lang.Object. In the case of this name, both entries (one in each name space) are associated with the same chunk of type data in the method area.

By contrast, although both name spaces also contain the fully qualified name Mouse, each entry (one in each name space) is associated with a different chunk of type data in the method area. In the upper name space, the name Mouse is associated with a chunk of type data that describes a small, furry animal with a pink nose. In the lower name space, the name Mouse is associated with a chunk of type data that describes a device you attach to your computer.

Figure 2. Names and definitions

Dynamic linking and name spaces
When you compile a Java program, you get one class file for each type (class or interface) you define in source code. The class file is an intermediate compiled binary format for the type. Class files, which haven't been linked, contain symbolic references to other types in a list called the constant pool. At runtime, the JVM dynamically links the Java application by resolving the symbolic references contained in the constant pools of class files. This process is called constant pool resolution.

Name spaces in the JVM arise from a simple rule that all JVMs must follow when they resolve the symbolic references contained inside class files. Sometimes, an entry in a constant pool may refer symbolically to a type that hasn't yet been loaded. When such entries are resolved, the JVM must load the referred-to type. Because all types must be loaded by a class loader, the JVM must at that point decide which class loader to ask to load the type.

To choose a class loader, the JVM uses this simple rule:

The Resolution Rule - The JVM loads referenced types via the same class loader that loaded the referencing type.

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