Namespaces in Java

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In general, a running Java environment can have many Class Loaders active, each defining its own namespace. Namespaces allow Java classes to see different views of the world depending on where they originate (see Figure 2.6). Simply put, a namespace is a set of unique names of classes loaded by a particular Class Loader and a binding of each name to a specific class object. Though some people say that namespaces are disjoint and do not overlap, this is not true in general. There is nothing to stop namespaces from overlapping.
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Figure 2.6 Class Loaders have two distinct jobs (which we believe would have been better off separated): (1) fetching and instantiating byte code as classes, and (2) managing name spaces. This figure shows how Class Loaders typically divide classes into distinct name spaces according to origin. It is especially important to keep local classes distinct from external classes. This figure implies that name spaces do not overlap, which is not entirely accurate.
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Most VM implementations have used different class loaders to load code from different origins. This allowed these implementations to assign a single security policy to all code loaded by the same class loader, and to make security decisions based on which class loader loaded the class that is asking to perform a dangerous operation. With the addition of code signing in JDK 1.1, there are now two characteristics for categorization of code: origin (usually represented as a URL) and signer (the identity associated with the private key used to sign the file). Only the Class Loader that loaded a piece of code knows for sure where the code was loaded from. Applet Class Loaders, which are typically supplied by the browser vendor, load all applets and the classes they reference, usually getting the classes from HTTP servers. When an applet loads across the network, its Applet Class Loader receives the binary data and instantiates it as a new class. Under normal operation, applets are forbidden to install a new Class Loader, so Applet Class Loaders are the only game in town. A trusted Java application (such as the Java interpreter built in to Netscape Navigator or Internet Explorer)
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can, however, define its own class loaders. Sun Microsystems provides three template class loader modules as part of the JDK (discussed earlier). If an untrusted applet could somehow install a Class Loader, the applet would be free to define its own namespace. Prior to Java 2, this would allow an attack applet to breach security (see 5). If you are writing an application or built-in extension that defines its own Class Loader, you should be very careful to follow the rules; otherwise, your Class Loader will almost certainly introduce a security hole. It is unfortunate that in order to get the ability to use your own code-finding mechanism, you must also take on responsibility for managing namespaces. One criticism often raised against the Java security model is that because of the presence of objects like application-definable class loaders, the security model is too distributed and lacks central control. Applet Class Loaders install each applet in a separate namespace. This means that each applet sees its own classes and all of the classes in the standard Java library API, but it doesn't see classes belonging to other applets. Hiding applets from each other in this way has two advantages: It allows multiple applets to define classes with the same name without ill effect, so applet writers don't have to worry about name collisions. It also makes it harder, though not impossible, for applets to team up. As an example, consider a class called laptop with no explicit package name (that is, laptop belongs to the default package). Imagine that the laptop class is loaded by an Applet Class Loader from as you surf the Java Security Web Site. Then you surf over to and load a different class named laptop (also in the default package). What we have here is two different classes with the same name. How can the VM distinguish between them The tagging of classes according to which Class Loader loaded them provides the answer. Applets from different CodeBases are loaded by different instances of the browser's Applet Class Loader class. (By the way, distinct namespaces will be created even if the two sites use explicit package names that happen to be the same.) Although the same class is involved in loading the two different classes (i.e., the Applet Class Loader), two different instances of the Applet Class Loader class are involved-one for each CodeBase. Recall that the default object protection and encapsulation scheme covered earlier in this chapter allows classes that are members of a package to access all other classes in the same package. That means it is important for the VM to keep package membership straight. As a result, Class Loaders have to keep track of packages as well as classes. When a class is imported from the network, the Applet Class Loader places it into a namespace labeled with information about its origin. Whenever one class tries to reference another, the Applet Class Loader follows a particular order of search. The first place it looks for a class is in the set of classes loaded by the Primordial Class Loader. If the Primordial Class Loader doesn't have a class with the indicated name, the Applet Class Loader widens the search to include the namespace of the class making the reference. Because the Applet Class Loader searches for built-in classes first, it prevents imported classes from pretending to be built-in classes (something known as "class name spoofing"). This policy prevents such things as applets redefining file I/O classes to gain unrestricted access to the file system. Clearly, the point is to protect fundamental primitives from outside corruption.
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Since all applets from a particular source are put in the same namespace, they can reference each other's methods. A source is defined as a particular directory on a particular Web server. According to the Java specification, every Class Loader must keep an inventory of all the classes it has previously loaded. When a class that has already been loaded is requested again, the class loader must return the already loaded class.
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