data.html

java 基础的 一点东西,,可以看看

  }
}
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="sec"></A>
<H3>Granting Applets Permission</H3>

If you tried to run the applet example, you undoubtedly
saw errors when you clicked the <CODE>Click Me</CODE> button. This is 
because the Java 2 Platform security does not permit an applet 
to write to and read from files without explicit permission.

<P>
An applet has no access to local system resources unless it
is specifically granted the access. So for the <CODE>FileUIAppl</CODE> 
program to read from <CODE>text.txt</CODE> and write to 
<CODE>text.txt</CODE>, the applet has to be given the appropriate read
or write access permission for each file. 

<P>
Access permission is granted with a policy file, and appletviewer
is launched with the policy file to be used for the applet
being viewed.

<H4>Creating a Policy File</H4>

Policy tool is a Java 2 Platform security tool for creating policy files. 
<A HREF="http://java.sun.com/docs/books/tutorial/">The Java Tutorial</A>
trail on 
<A HREF="http://java.sun.com/docs/books/tutorial/security1.2/tour1/step2.html">Controlling Applets</A> explains how to use Policy Tool in good detail. 
Here is the policy file you need to run the applet. You can use
Policy tool to create it or copy the text below into an ASCII file. 

</FONT>

<PRE>
grant {
  permission java.util.PropertyPermission 
	&quot;user.home&quot;, &quot;read&quot;;
  permission java.io.FilePermission 
	&quot;${user.home}/text.txt&quot;, &quot;read,write&quot;;
};
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<H4>Running an Applet with a Policy File</H4>

Assuming the policy file is named <CODE>polfile</CODE> and
is in the same directory with an HTML file named
<CODE>fileIO.html</CODE> that contains the HTML 
to run the <CODE>FileIOAppl</CODE> applet, you would run the
application in appletviewer like this:

</FONT>

<PRE>
appletviewer -J-Djava.security.policy=polfile fileIO.html
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<BLOCKQUOTE>
<HR>
<STRONG>Note:</STRONG>
If your browser is enabled for the Java 2 Platform or if you
have <A HREF="http://java.sun.com/products/plugin/index.html">Java
Plug-in</A> installed, you
can run the applet from the browser if you put the policy file
in your local home directory. 
<HR>
</BLOCKQUOTE>

Here is the <CODE>fileIO.html</CODE> file for running the <CODE>FileIOAppl</CODE>
applet:

</FONT>

<PRE>
&#60;HTML&#62;
&#60;BODY&#62;

&#60;APPLET CODE=FileIOAppl.class WIDTH=200 HEIGHT=100&#62;
&#60;/APPLET&#62;

&#60;/BODY&#62;
&#60;/HTML&#62;
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="res"></A>
<H3>Restricting Applications</H3>

You can use the default security manager and a policy file
to restrict the application's access as follows. 

</FONT>

<PRE>
java -Djava.security.manager 
     -Djava.security.policy=apppolfile  FileIO 
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

Because the application runs within the security manager, which disallows
all access, the policy file needs two additional permissions. One so
the security manager can access the event queue and load the user interface
components, and another so the application does not display
the banner warning that its window was created by another
program (the security manager). 

</FONT>

<PRE>
grant {
  permission java.awt.AWTPermission 
	&quot;accessEventQueue&quot;;
  permission java.awt.AWTPermission 
	&quot;showWindowWithoutWarningBanner&quot;;

  permission java.util.PropertyPermission 
	&quot;user.home&quot;, &quot;read&quot;;
  permission java.io.FilePermission 
	&quot;${user.home}/text.txt&quot;, &quot;read,write&quot;;
};
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="serv"></A>
<H3>File Access by Servlets</H3>

Although servlets are invoked from a browser, they are under the 
security policy in force for the web server under which they run. When 
file input and output code is added to <CODE>ExampServlet.java</CODE> 
from Lesson 5, <A HREF="./Code/FileIOServlet.java">FileIOServlet</A> 
for this lesson executes without restriction under Java WebServer<FONT SIZE="-2"><SUP>TM</SUP></FONT> 1.1.1. 

<P>
<IMG SRC="./Art/servIO1.gif" WIDTH="297" HEIGHT="205" ALT="">
<TABLE BORDER="2" CELLSPACING="2" CELLPADDING="2">
<TR><TD>
<IMG SRC="./Art/servIO2.gif" WIDTH="313" HEIGHT="115" ALT="">
</TD></TR>
</TABLE>

</FONT>

<PRE>
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class FileIOServlet extends HttpServlet {

  public void doPost(HttpServletRequest request,
	HttpServletResponse response)
	throws ServletException, IOException
  {
    response.setContentType(&quot;text/html&quot;);
    PrintWriter out = response.getWriter();
    out.println(&quot;&lt;title&gt;Example&lt;title&gt;&quot; +
                     &quot;&lt;body bgcolor=FFFFFF&gt;&quot;);

    out.println(&quot;&#60;h2&#62;Button Clicked&#60;/h2&#62;&quot;);

    String DATA = request.getParameter(&quot;DATA&quot;);

    if(DATA != null){
      out.println(&quot;&#60;STRONG&#62;Text from 
			form:&#60;/STRONG&#62;&quot;);
      out.println(DATA);
    } else {
      out.println(&quot;No text entered.&quot;);
    }

    try{
//Code to write to file
      String outputFileName=
        System.getProperty(&quot;user.home&quot;,
          File.separatorChar + &quot;home&quot; +
          File.separatorChar + &quot;monicap&quot;) +
          File.separatorChar + &quot;text.txt&quot;;
      File outputFile = new File(outputFileName);
      FileWriter fout = new FileWriter(outputFile);
      fout.write(DATA);
      fout.close();

//Code to read from file
      String inputFileName =
	System.getProperty(&quot;user.home&quot;,
          File.separatorChar + &quot;home&quot; +
          File.separatorChar + &quot;monicap&quot;) +
          File.separatorChar + &quot;text.txt&quot;;
      File inputFile = new File(inputFileName);
      FileReader fin = new 
	FileReader(inputFile);
      char c[] = new 
	char[(char)inputFile.length()];
      int i;
      i = fin.read(c);
      String s = new String(c);
      out.println(&quot;&#60;P&#62;
	&#60;STRONG&#62;Text from file:&#60;/STRONG&#62;&quot;);
      out.println(s);
      fin.close();
    }catch(java.io.IOException e){
     System.out.println(&quot;Cannot access text.txt&quot;);
    }

    out.println(&quot;&#60;P&#62;Return to 
	&#60;A HREF=&quot;../simpleHTML.html&quot;&#62;Form&#60;/A&#62;&quot;);
    out.close();
   }
}
</PRE>

<FONT FACE="Verdana, Arial, Helvetica, sans-serif">

<A NAME="append"></A>
<H3>Appending</H3>

So far the examples have shown you how to read in and
write out streams of data in their entirety.  But often,
you want to append data to an existing file or read in
only certain amounts.  Using the 
<A HREF="http://java.sun.com/products/jdk/1.2/docs/api/java/io/RandomAccessFile.html">RandomAccessFile</A> 
class, alter the <A HREF="./Code/FileIO.java">FileIO.java</A>
class to append to the file.

<P>
Give it a try before taking a peek at the 
<A HREF="./Code/AppendIO.java">Solution</A>.

<A NAME="more"></A>
<H3>More Information</H3>

For more infomation on file input and output, see the 
<A HREF="http://java.sun.com/docs/books/tutorial/essential/io/index.html">Reading 
and Writing</A> trail in 
<A HREF="http://java.sun.com/docs/books/tutorial">The Java Tutorial</A>.

<P>
You can learn more about component sizing in
<A HREF="http://java.sun.com/docs/books/tutorial">The Java Tutorial</A>
sections on
<A HREF="http://java.sun.com/docs/books/tutorial/ui/swingLayout/problems.html">Solving 
Common Layout Problems</A>
and
<A HREF="http://java.sun.com/docs/books/tutorial/ui/swingOverview/layout.html">Layout 
Management</A>.


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