Friday, April 29, 2011

Classloader types

Bootstrap class loader

There is only one Primordial Class Loader, which is an essential part of each Java VM. It cannot be overridden. The Primordial Class Loader is involved in bootstrapping the Java environment. Since most VMs are written in C, it follows that the Primordial Class Loader is typically written in C. This special class loader loads trusted classes, usually from the local disk.

ClassLoaders other than Bootstrap by jdk
There are three distinct types of Class Loader objects defined by the JDK itself:
  • Applet Class Loaders, 
  • RMI Class Loaders, and 
  • Secure Class Loaders.
From the standpoint of a Java user or a system administrator, Applet Class Loaders are the most important variety. Java developers who are interested in rolling their own Class Loaders will likely subclass or otherwise use the RMI Class Loader and Secure Class Loader classes.

Applet Class Loaders are responsible for loading classes into a browser and are defined by the vendor of each Java-enabled browser. Vendors generally implement similar Applet Class Loaders, but they do not have to. Sometimes seemingly subtle differences can have important security ramifications. For example, Netscape now tracks a class not by its name, but by a pointer to actual code, making attacks that leverage Class Loading complications harder to carry out.

Applet Class Loaders help to prevent external code from spoofing important pieces of the Java API. They do this by attempting to load a class using the Primordial Class Loader before fetching a class across the network. If the class is not found by the Primordial Class Loader, the Applet Class Loader typically loads it via HTTP using methods of the URL class. Code is fetched from the CODEBASE specified in the <APPLET> tag. If a fetch across the Web fails, a ClassNotFound exception is thrown.
It should be clear why external code must be prevented from spoofing the trusted classes of the Java API. Consider that the essential parts of the Java security model (including the Applet Class Loader class itself) are simply Java classes. If an untrusted class from afar were able to set up shop as a replacement for a trusted class, the entire security model would be toast!

The RMI Class Loader and Secure Class Loader classes were introduced with JDK 1.1 and Java 2, respectively. RMI Class Loaders are very similar to Applet Class Loaders in that they load classes from a remote machine. They also give the Primordial Class Loader a chance to load a class before fetching it across the Net. The main difference is that RMI Class Loaders can only load classes from the URL specified by Java's rmi.server.codebase property. Similar in nature to RMI Class Loaders, Secure Class Loaders allow classes to be loaded only from those directories specified in Java's property. Secure Class Loaders can only be used by classes found in the package and are extensively used by the Java 2 access control mechanisms.

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