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Java's security model is focused on protecting end-users from hostile programs downloaded from untrusted sources across a network. To accomplish this goal, Java provides a customizable "sandbox" in which Java programs run. A Java program can do anything within the boundaries of its sandbox, but it can't take any action outside those boundaries. The sandbox for untrusted Java applets, for example, prohibits many activities, including:
By making it impossible for downloaded code to perform certain actions, Java's security model protects users from the threat of hostile code. For more information on the sandbox concept, see last month's "Under the Hood."
One aspect of the JVM that plays an important role in the security sandbox is the class loader architecture. In the JVM, class loaders are responsible for importing binary data that defines the running program's classes and interfaces. In the block diagram shown in Figure 1, a single mysterious cube identifies itself as "the class loader," but in reality, there may be more than one class loader inside a JVM. Thus, the class loader cube of the block diagram actually represents a subsystem that may involve many class loaders. The JVM has a flexible class loader architecture that allows a Java application to load classes in custom ways.
Figure 1. Java's class loader architecture
A Java application can use two types of class loaders: a "primordial" class loader and class loader objects. The primordial class loader (there is only one of them) is a part of the JVM implementation. For example, if a JVM is implemented as a C program on top of an existing operating system, then the primordial class loader will be part of that C program. The primordial class loader loads trusted classes, including the classes of the Java API, usually from the local disk.
At run time, a Java application can install class loader objects that load classes in custom ways, such as by downloading class files across a network. The JVM considers any class it loads through the primordial class loader to be trusted, regardless of whether or not the class is part of the Java API. It views with suspicion, however, those classes it loads through class loader objects. By default, it considers them to be untrusted. While the primordial class loader is an intrinsic part of the virtual machine implementation, class loader objects are not. Instead, class loader objects are written in Java, compiled into class files, loaded into the virtual machine, and instantiated just like any other object. They really are just another part of the executable code of a running Java application. You can see a graphical depiction of this architecture in Figure 2.