In our experience, every type safety violation has created an opportunity for an untrusted applet to break out of Java's security restrictions. This example shows what can go wrong if type safety is violated.

The applet would then be allowed (incorrectly) to delete files. Since the object was really in the Applet class, setting the first field to true allows the applet to access the hard disk. If the operation were allowed to go ahead, it would do what turnOn was supposed to do, and set the first field of the object to true.

Suppose a program tried to apply the turnOn operation to an Applet object. Type safety checks ensure that any object a method may try to manipulate is of the proper type.

Setting the first field in these classes to 'true' is not equivalent. In this figure, two classes, Alarm and Applet, each include a number of fields. Note that the first field of Applet is fileAccessAllowed, which determines whether the applet is allowed access to files on the hard disk.įigure 2.10 Type safety provides an important foundation for the Java security model. The Java runtime library defines another class called Applet, whose memory layout is also shown in Figure 2.10. Alarm defines an operation turnOn, which sets the first field to true. This class is represented in memory as shown in Figure 2.10. A calendar-management applet defines a class called Alarm. To understand why, consider the following slightly contrived example. Type safety is the most essential element of Java's security. Type safety means that a program cannot perform an operation on an object unless that operation is valid for that object. In our calendar management example, the Alarm class might define a turnOn operation, but the Date class would neither need nor allow turnOn. Each class defines both a set of objects and operations to be performed on the objects of that class. For example, a calendar-management applet might use classes like Date, Appointment, Alarm, and GroupCalendar. More specifically, every piece of memory is part of some Java object. This means that programs are prevented from accessing memory in inappropriate ways. The Java language is designed to enforce type safety. The Base Java Security Model: The Original Applet SandboxĬHAPTER SECTIONS: 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13

Is java safe to