IBM User Guide for Java V7 on Windows > IBM SDK for Java > Class loading

Namespaces and the runtime package

Loaded classes are identified by both the class name and the class loader that loaded it. This separates loaded classes into namespaces that the class loader identifies.

A namespace is a set of class names that are loaded by a specific class loader. When an entry for a class has been added into a namespace, it is impossible to load another class of the same name into that namespace. Multiple copies of any given class can be loaded because a namespace is created for each class loader.

Namespaces cause classes to be segregated by class loader, thereby preventing less-trusted code loaded from the application or custom class loaders from interacting directly with more trusted classes. For example, the core API is loaded by the bootstrap class loader, unless a mechanism is specifically provided to allow them to interact. This prevents possibly malicious code from having guaranteed access to all the other classes.

You can grant special access privileges between classes that are in the same package by the use of package or protected access. This gives access rights between classes of the same package, but only if they were loaded by the same class loader. This stops code from an untrusted source trying to insert a class into a trusted package. As discussed earlier, the delegation model prevents the possibility of replacing a trusted class with a class of the same name from an untrusted source. The use of namespaces prevents the possibility of using the special access privileges that are given to classes of the same package to insert code into a trusted package.

Parent: Class loading

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