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<TITLE>Writing Advanced Applications, Chapter 10: Writing a Security Manager</TITLE>
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<H2>Writing Advanced Applications<BR>
<FONT SIZE="3">Chapter 10 Continued: Writing a Security Manager</FONT></H2>
<FONT SIZE="-1">[<A HREF="signed.html"><<BACK</A>] [<A HREF="index.html#contents">CONTENTS</A>] [<A HREF="appA.html">NEXT>></A>]</FONT></DIV>
<P>
A security manager is a Java<FONT SIZE="-2"><SUP>TM</SUP></FONT> virtual machine (VM) object that implements
a security policy. By default, the Java 2® platform software provides a
security manager that disallows all access to local system
resources apart from read access to the directory and its subcirectories
where the program is invoked.
<P>
You can extend the default security manager to implement customized
verifications and approvals for applets and applications, but the
implementation must include the appropriate access verification code
for every <CODE>checkXXX</CODE> method you override.
If you do not include this code, no access verfication check
happens, and your code breaches the system security policy.
<P>
This section uses an example application to explain how to write a
custom security manager that prompts the end user for password
identification before reading from and writing to specific
files. The implementation includes access verification code
so once the end user makes it through the password check, he or she
still needs the file read and write permissions in their policy file.
<P>
The example consists of the <CODE>FileIO</CODE> application,
and the <CODE>PasswordSecurityManager</CODE> program that provides
the custom security manager implementation.
<UL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#file">The FileIO Program</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#pwd">The PasswordSecurityManager Program</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#run">Run The FileIO Program</A></FONT>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A HREF="#ref">Reference Information</A></FONT>
</UL>
<HR>
<A NAME="file"></A>
<H3>The FileIO Program</H3>
The <A HREF="./Code/FileIO.java">FileIO</A> program displays
a simple user interface and asks the end user to enter some text.
When the end user clicks the <CODE>Click Me</CODE> button,
the text is saved to a file in the end user's home directory,
and a second file is opened and read. The text read from
the second file is displayed to the end user.
<P>
<TABLE>
<TR><TD>
<IMG SRC="./Art/FileIO.gif"><P>
<DIV ALIGN="CENTER"><FONT FACE="Verdana, Arial, Helvetica, sans-serif">Before Button Click</FONT></DIV></TD>
<TD>
<IMG SRC="./Art/FileIO2.gif"><P>
<DIV ALIGN="CENTER"><FONT FACE="Verdana, Arial, Helvetica, sans-serif">After Button Click</FONT></DIV></TD></TR>
</TABLE>
<P>
The custom security manager for this program prompts the
end user to enter a password before it allows <CODE>FileIO</CODE>
to write text to or read text from a file. The <CODE>main</CODE>
method of <CODE>FileIO</CODE> creates a custom security manager
called <CODE>PasswordSecurityManager</CODE>.
</FONT>
<PRE>
public static void main(String[] args){
BufferedReader buffy = new BufferedReader(
new InputStreamReader(System.in));
try {
System.setSecurityManager(
new PasswordSecurityManager("pwd", buffy));
} catch (SecurityException se) {
System.err.println("SecurityManager already set!");
}
</PRE>
<FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<A NAME="pwd"></A>
<H3>The PasswordSecurityManager Class</H3>
The
<A HREF="./Code/PasswordSecurityManager.java">PasswordSecurityManager</A>
class declares two private instance variables, which are initialized by
the constructor when the custom security manager is installed.
The <CODE>password</CODE> instance variable contains the
actual password, and the <CODE>buffy</CODE> instance variable
is an input buffer that stores the end user's password input.
</FONT>
<PRE>
public class PasswordSecurityManager
extends SecurityManager{
private String password;
private BufferedReader buffy;
public PasswordSecurityManager(String p,
BufferedReader b){
super();
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