Java Programing

Because an operation always produces a known type, by looking at the starting state, it’s possible to determine the types of all items on the stack and in local variables at any point in the future. The collection of all this type information at any given time is called the type state of the stack; this is what Java tries to analyze before it runs an application. Java doesn’t know anything about the actual values of stack and variable items at this time, just what kind of items they are. However, this is enough information to enforce the security rules and to ensure that objects are not manipulated illegally. To make it feasible to analyze the type state of the stack, Java places an additional restriction on how Java byte-code instructions are executed: all paths to the same point in the code have to arrive with exactly the same type state.[4] This restriction makes it possible for the verifier to trace each branch of the code just once and still know the type state at all points. Thus, the verifier can insure that instruction types and stack value types always correspond, without actually following the execution of the code. For a more thorough explanation of all of this, see The Java Virtual Machine, by Jon Meyer and Troy Downing (O’Reilly & Associates). [4] The implications of this rule are of interest mainly to compiler writers. The rule means that Java byte- code can’t perform certain types of iterative actions within a single frame of execution. A common example would be looping and pushing values onto the stack. This is not allowed because the path of execution would return to the top of the loop with a potentially different type state on each pass, and there is no way that a static analysis of the code can determine whether it obeys the security rules. 1.5.2 Class Loaders Java adds a second layer of security with a class loader. A class loader is responsible for bringing the byte-code for one or more Java classes into the interpreter. Every application that loads classes from the network must use a class loader to handle this task. After a class has been loaded and passed through the verifier, it remains associated with its class loader. As a result, classes are effectively partitioned into separate namespaces based on their origin. When a loaded class references another class name, the location of the new class is provided by the original class loader. This means that classes retrieved from a specific source can be restricted to interact only with other classes retrieved from that same location. For example, a Java- enabled web browser can use a class loader to build a separate space for all the classes loaded from a given uniform resource locator (URL). The search for classes always begins with the built-in Java system classes. These classes are loaded from the locations specified by the Java interpreter’s class path (see Chapter 3). Classes in the class path are loaded by the system only once and can’t be replaced. This means that it’s impossible for an applet to replace fundamental system classes with its own versions that change their functionality. 1.5.3 Security Managers Finally, a security manager is responsible for making application-level security decisions. A security manager is an object that can be installed by an application to restrict access to system resources. The security manager is consulted every time the application tries to access items like the filesystem, network ports, external processes, and the windowing environment, so the security manager can allow or deny the request. A security manager is most useful for applications that run untrusted code as part of their normal operation. Since a Java-enabled web browser can run applets that may be retrieved from untrusted sources on the Net, such a browser needs to install a security manager as one of its first actions. This security manager then restricts the kinds of access allowed after that point. This lets the - 17

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