How to open/read/write a local file from an applet:
This page shows a method for accessing/reading/writing local files from a JAVA applet in your browser.
Bascially you just need to sign the applet with a certificate and you're ready to go.
The code: displays a filedialog, opens, reads and displays the file (in the JAVA console):
See below for a full applet
int arrlen = 10000;
byte[] infile = new byte[arrlen];
Frame parent = new Frame();
FileDialog fd = new FileDialog(parent, "Please choose a file:",
FileDialog.LOAD);
fd.show();
String selectedItem = fd.getFile();
if (selectedItem == null) {
// no file selected
} else {
File ffile = new File( fd.getDirectory() + File.separator +
fd.getFile());
// read the file
System.out.println("reading file " + fd.getDirectory() +
File.separator + fd.getFile() );
try {
FileInputStream fis = new FileInputStream(ffile);
BufferedInputStream bis = new BufferedInputStream(fis);
DataInputStream dis = new DataInputStream(bis);
try {
int filelength = dis.read(infile);
String filestring = new String(infile, 0, filelength);
System.out.println("FILE CONTENT=" + filestring);
} catch(IOException iox) {
System.out.println("File read error...");
iox.printStackTrace();
}
} catch (FileNotFoundException fnf) {
System.out.println("File not found...");
fnf.printStackTrace();
}
}
Now we need to make
an (inofficial) certificate. It IS a certificate, but the browser-plugin
will display a certificate-requester, which says that it is unofficial.
We do not need to care about this - the user has to decide if he trusts
you or not. Maybe you want to state on your page, that the plugin says
it is unofficial and therefore not trustworthy, but the user has to accept
it in order to run the applet. The permissions are the same - official
or unofficial - that doesn't count.
OK, here we go:
'keytool' is delivered with Sun's development kit.
keytool -genkey -keyalg rsa -alias yourkey
Follow the instructions and type in all needed information.
Now we make the certificate:
keytool -export -alias yourkey -file yourcert.crt
Now we have to sign the applet:
Just make a *.bat file including this:
javac yourapplet.java
jar cvf yourapplet.jar yourapplet.class
jarsigner yourapplet.jar yourkey
The batch-file compiles
the applet, makes a jar-archive and signs the jar-file.
The HTML-code to display the applet:
<applet code="yourapplet.class" archive="yourapplet.jar" width="600" height="500">
</applet>
Now we are done! The applet is signed and if the user accepts the certificate,
the applet is allowed to access local files. If the user doesn't agree,
this appears in the console:
java.security.AccessControlException: access denied
(java.io.FilePermission yourfilename read) at
java.security.AccessControlContext.checkPermission(AccessControlContext.java:270)
at
java.security.AccessController.checkPermission(AccessController.java:401)
at java.lang.SecurityManager.checkPermission(SecurityManager.java:542)
at java.lang.SecurityManager.checkRead(SecurityManager.java:887)
Doesn't look good. It isn't! The accesscontroller checks if it is allowed
to read the file. The certificate was not accepted, therefore the applet
isn't allowed.
The full applet source code:
The applet displays a small button "open". Click on it, the file requester shows up and
select a small text file i.e. on your harddisk. The content of the text file is displayed in the JAVA console.
localfile.java
import java.applet.*;
import java.awt.*;
import java.util.*;
import java.lang.*;
import java.text.*;
import java.awt.event.*;
import java.io.*;
public class localfile extends Applet {
public localfile() {
Panel p = new Panel();
Font f;
String osname = System.getProperty("os.name","");
if (!osname.startsWith("Windows")) {
f = new Font("Arial",Font.BOLD,10);
} else {
f = new Font("Verdana",Font.BOLD,12);
}
p.setFont(f);
p.add(new Button("Open"));
p.setBackground(new Color(255, 255, 255));
add("North",p);
}
public boolean action(Event evt, Object arg) {
if (arg.equals("Open")) {
System.out.println("OPEN CLICKED");
int arrlen = 10000;
byte[] infile = new byte[arrlen];
Frame parent = new Frame();
FileDialog fd = new FileDialog(parent, "Please choose a file:",
FileDialog.LOAD);
fd.show();
String selectedItem = fd.getFile();
if (selectedItem == null) {
// no file selected
} else {
File ffile = new File( fd.getDirectory() + File.separator +
fd.getFile());
// read the file
System.out.println("reading file " + fd.getDirectory() +
File.separator + fd.getFile() );
try {
FileInputStream fis = new FileInputStream(ffile);
BufferedInputStream bis = new BufferedInputStream(fis);
DataInputStream dis = new DataInputStream(bis);
try {
int filelength = dis.read(infile);
String filestring = new String(infile, 0,
filelength);
System.out.println("FILE CONTENT=" + filestring);
} catch(IOException iox) {
System.out.println("File read error...");
iox.printStackTrace();
}
} catch (FileNotFoundException fnf) {
System.out.println("File not found...");
fnf.printStackTrace();
}
}
} else return false;
return true;
}
}
index.html
<html>
<body bgcolor="#dddddd">
<applet code="localfile.class" archive="localfile.jar" width="100" height="100">
</applet>
</body>
</html>
Compile, pack and sign the applet with:
javac localfile.java
jar cvf localfile.jar localfile.class
jarsigner localfile.jar yourkeyname
last update: 29-JUL-2005
