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Java's class loader architecture controls interaction between code loaded from different sources by using different class loaders to load code from different sources.  This prevents malicious code from gaining access to and subverting trusted code.  A class loader that loads untrusted code should not interact with trusted code that invokes any of the methods from the following table:

Certain standard APIs in the core libraries of the Java runtime enforce SecurityManager checks but allow those checks to be bypassed depending on the immediate caller's class loader. When the java.lang.Class.newInstance method is invoked on a Class object, for example, the immediate caller's class loader is compared to the Class object's class loader. If the caller's class loader is an ancestor of (or the same as) the Class object's class loader, the newInstance method bypasses a SecurityManager check. (See Section 4.3.2 in [1] for information on class loader relationships). Otherwise, the relevant SecurityManager check is enforced.

The difference between this class loader comparison and a SecurityManager check is noteworthy. A SecurityManager check investigates all callers in the current execution chain to ensure each has been granted the requisite security permission. (If AccessController.doPrivileged was invoked in the chain, all callers leading back to the caller of doPrivileged are checked.) In contrast, the class loader comparison only investigates the immediate caller's context (its class loader). This means any caller who invokes Class.newInstance and who has the capability to pass the class loader check--thereby bypassing the SecurityManager--effectively performs the invocation inside an implicit AccessController.doPrivileged action. Because of this subtlety, callers should ensure that they do not inadvertently invoke Class.newInstance on behalf of untrusted code.

        package yy.app;

        class AppClass {
           OtherClass appMethod() throws Exception {
               return OtherClass.class.newInstance();
           }
        }

        +--------------------------------+
        | xx.lib.LibClass                |
        |   .LibClass                    |
        +--------------------------------+
        | java.lang.Class                |
        |   .newInstance                 |
        +--------------------------------+
        | yy.app.AppClass |<-- AppClass.class.getClassLoader
        | .appMethod |       determines check
        +--------------------------------+

        |                                |

Code has full access to its own class loader and any class loader that is a descendent. In the case of Class.newInstance access to a class loader implies access to classes in restricted packages (e.g., sun.* in the Sun JDK).

In the diagram below, classes loaded by B have access to B and its descendents C, E, and F. Other class loaders, shown in grey strikeout font, are subject to security checks.

        +-------------------------+

        | bootstrap loader | <--- null
        +-------------------------+
         ^ ^
        +------------------+ +---+

        | extension loader | | A |
        +------------------+ +---+
         ^
        +------------------+

        | system loader | <--- Class.getSystemClassLoader()
        +------------------+
         ^ ^
        +----------+   +---+

        | B |   | F |
        +----------+   +---+
          ^      ^     ^

        +---+  +---+   +---+
        | C |  | E |   | G | 
        +---+  +---+   +---+
          ^
        +---+
        | D |
        +---+

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