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Java Fundamental Classes Reference

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9. Security

Contents:
SecurityManager
ClassLoader

Java uses a "sandbox" security model to ensure that applets cannot cause security problems. The idea is that an applet can do whatever it wants within the constraints of its sandbox, but that nothing done inside the sandbox has any consequences outside of the sandbox.

9.1 SecurityManager

Java implements the sandbox model using the java.lang.SecurityManager class. An instance of SecurityManager is passed to the method System.setSecurityManager() to establish the security policy for an application. Before setSecurityManager() is called, a Java program can access any resources available on the system. After setSecurityManager() is called, however, the SecurityManager object is responsible for providing a security policy. Once a security policy has been set by calling setSecurityManager, the method cannot be called again. Subsequent calls simply throw a SecurityException.

All methods in the Java API that can access resources outside of the Java environment call a SecurityManager method to ask permission before doing anything. If the SecurityManager method throws a SecurityException, the exception is thrown out of the calling method, and access to the resource is denied. The SecurityManager class defines a number of methods for asking for permission to access specific resources. Each of these methods has a name that begins with the word "check." Table 9.1 shows the names of the check methods provided by the SecurityManager class.

Table 9.1: The Check Methods of SecurityManager

Method Name

Permission

checkAccept()

To accept a network connection

checkAccess()

To modify a Thread or ThreadGroup

checkAwtEventQueueAccess()

To access the AWT event queue

checkConnect()

To establish a network connection or send a datagram

checkCreateClassLoader()

To create a ClassLoader object

checkDelete()

To delete a file

checkExec()

To call an external program

checkExit()

To stop the Java virtual machine and exit the Java environment

checkLink()

To dynamically link an external library into the Java environment

checkListen()

To listen for a network connection

checkMemberAccess()

To access the members of a class

checkMulticast()

To use a multicast connection

checkPackageAccess()

To access the classes in a package

checkPackageDefinition()

To define classes in a package

checkPrintJobAccess()

To initiate a print job request

checkPropertiesAccess()

To get or set the Properties object that defines all of the system properties

checkPropertyAccess()

To get or set a system property

checkRead()

To read from a file or input stream

checkSecurityAccess()

To perform a security action

checkSetFactory()

To set a factory class that determines classes to be used for managing network connections and their content

checkSystemClipboardAccess()

To access the system clipboard

checkTopLevelWindow()

To create a top-level window on the screen

checkWrite()

To write to a file or output stream

The SecurityManager class provides implementations of these methods that always refuse the requested permission. To implement a more permissive security policy, you need to create a subclass of SecurityManager that implements that policy.

In Java 1.0, most browsers consider an applet to be trusted or untrusted. An untrusted applet is one that does not come from the local filesystem. An untrusted applet is treated as follows by most popular browsers:

  • It can establish network connections to the network address from which it came.

  • It can create new windows on the screen. However, a notice is displayed on the bottom of the window that the window was created by an untrusted applet.

  • It cannot access any other external resources. In particular, untrusted applets cannot access local files.

As of Java 1.1, an applet can have a digital signature attached to it. When an applet has been signed by a trusted entity, a browser may consider the applet to be trusted and relax its security policy.


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