Loading a Class in Java

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Loading a Class
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Class loading proceeds according to the following general algorithm:
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Determine whether the class has been loaded before. If so, return the previously loaded class. Consult the Primordial Class Loader to attempt to load the class from the CLASSPATH. This prevents external classes from spoofing trusted Java classes. See whether the Class Loader is allowed to create the class being loaded. The Security Manager makes this decision. If not, throw a security exception. Read the class file into an array of bytes. The way this happens differs according to particular class loaders. Some class loaders may load classes from a local database. Others may load classes across the network. Construct a Class object and its methods from the class file. Resolve classes immediately referenced by the class before it is used. These classes include classes used by static initializers of the class and any classes that the class extends. Check the class file with the Verifier.
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Summary
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Each Java class begins as source code. This is then compiled into byte code and distributed to machines anywhere on the Net. A Java-enabled browser automatically downloads a class when it encounters the <APPLET> tag in an HTML document. The Verifier examines the byte code of a class file to ensure that it follows Java's strict safety rules. The Java VM interprets byte code declared safe by the Verifier. The Java specification allows classes to be unloaded when they are no longer needed, but few current Java implementations unload classes. Java's ability to dynamically load classes into a running Java environment is fraught with security risks. The class-loading mechanisms mitigate these risks by providing separate namespaces set up according to where mobile code originates. This capability ensures that essential Java classes cannot be spoofed (replaced) by external, untrusted code. The Applet Class Loader in particular is a key piece of the Java
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security model.
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... Preface -- 1 -- 2 -- 3 -- 4 -- 5 -- 6 -- 7 -- 8 -- 9 -- A -- B -- C -- Refs Front -- Contents -- Help
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Copyright 1999 Gary McGraw and Edward Felten. All rights reserved. Published by John Wiley & Sons, Inc.
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The Base Java Security Model: The Original Applet Sandbox
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CHAPTER SECTIONS: 1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13
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Section 8 -- The Security Manager
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The third part of the base Java security model is the Security Manager. This part of the security model restricts the ways an applet uses visible interfaces (Java API calls). The Security Manager implements a good portion of the entire security model and is the part of the security model most often encountered (in terms of a SecurityException) by Java applet developers. The job of the Security Manager is to keep track of who is allowed to do which dangerous operations. A standard Security Manager will disallow most operations when they are requested by untrusted code, and will allow trusted code to do whatever it wants. The old distinction between applets and applications in JDK 1.0.2 used to directly affect which code the Security Manager managed. Applets, being completely untrusted, were subject to the strict rules of the Security Manager, while applications, being completely trusted, were not. This often led to confusion in the past, especially among managers who believed that the way the Security Manager was set up somehow made trusted Java applications more secure. The fact is, running trusted Java applications is just as risky as running any other executable code written in any language. If the trusted code is malicious or buggy, you could be in big trouble. Although the browser vendors have designed very similar Security Managers for the most popular Javaenabled browsers, there is no strict edict forcing them to do this. The fact that Security Managers applied to untrusted applets generally enforce the same policies makes things easier to understand and leads to a more uniform experience for both Java users and Java applet developers. But Security Managers don't have to follow an all-or-nothing approach to controlling dangerous resources. They could, for example, be written to give specialized access to particular classes. The fact is, Security Managers can be as simple or as complicated as their authors decide to make them. To the extent that applet security is a major concern to you (and it must be, or you would not be reading this book), your choice of a browser should also be of great concern. This discussion focuses on two central characteristics of Security Managers: how they work, and how they are set up to restrict the activities of untrusted applets in most browsers. Before the introduction of code
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signing with JDK 1.1, the security policy for untrusted applets was easy to understand (although its enforcement was complicated), and the goal of a Security Manager was thus straightforward. Now that applets come in myriad trust levels, talk of a single Security Manager makes less sense. To be sure, the Security Manager's default rules do serve as the default under there somewhere, but like the rest of the sandbox, they are only a default.
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