本文主要是介绍用数字签名的applet解决客户端文件读写，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
最近有个java的B/S项目，要求能在客户端读写文件，查了下资料，解决方案一般是写activex和applet，由于项目是跨浏览器的，所以activex不能满足要求，out了，只能用applet，要解决applet在客户端读写的问题，需要给applet数字签名，最后经过几番尝试，终于调试通过，现发出来给大家共享。首先写applet了
package com.shiningway.applet;
import java.applet.Applet;
import java.io.*;
import java.awt.*;
import java.util.*;
public class ShowMe extends Applet{
public File f=null;
public String text="";//请注意此处的public
Date dd=null;
int n=0;
/**
* Called by the browser or applet viewer to inform
* this applet that it is being reclaimed and that it should destroy
* any resources that it has allocated. The <code>stop</code> method
* will always be called before <code>destroy</code>. <p>
*
* A subclass of <code>Applet</code> should override this method if
* it has any operation that it wants to perform before it is
* destroyed. For example, an applet with threads would use the
* <code>init</code> method to create the threads and the
* <code>destroy</code> method to kill them. <p>
*/
public void destroy() {
// Put your code here
}
public void paint(Graphics g ) {
if(text!=null && !text.equals(""))
{
setTxt(text);
}
//g.drawString(text, 40, 50);
text="";
}
public void setText(String st)
{
text=st;
}
public void init() {
f=new File("D:\\lala.txt");
if (!f.exists())
{
try
{
f.createNewFile();
}
catch(Exception ex)
{
System.out.println(ex.getMessage());
}
}
}
public void setTxt(String tt)
{
try
{
RandomAccessFile raf = new RandomAccessFile(f, "rw");
raf.seek(raf.length());
raf.writeBytes("\n"+tt);
raf.close();
}
catch (Exception ex)
{
System.out.println(ex.getMessage());
}
finally
{
}
}
public void start() {
}
/**
* Called by the browser or applet viewer to inform
* this applet that it should stop its execution. It is called when
* the Web page that contains this applet has been replaced by
* another page, and also just before the applet is to be destroyed. <p>
*
* A subclass of <code>Applet</code> should override this method if
* it has any operation that it wants to perform each time the Web
* page containing it is no longer visible. For example, an applet
* with animation might want to use the <code>start</code> method to
* resume animation, and the <code>stop</code> method to suspend the
* animation. <p>
*/
public void stop() {
// Put your code here
}
}关于applet的数字签名，网上资料很多，自己可以去查，可以参考一下这个帖子http://www.blogjava.net/beansoft/archive/2008/05/11/199801.html
（其实这个帖子里面已经是包含了我这边的大部分内容了）
showMe.html的文件代码
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My applet 'ShowMe' starting page</title>
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="this is my page">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<script>
function setTxt()
{
document.lama.setText(document.getElementById("txtbaby").value);
document.lama.repaint();
}
</script>
<body>
<!--"CONVERTED_APPLET"-->
<!-- HTML CONVERTER -->
<object
classid = "clsid:8AD9C840-044E-11D1-B3E9-00805F499D93"
codebase = "http://java.sun.com/update/1.5.0/jinstall-1_5-windows-i586.cab#Version=1,5,0,0"
WIDTH = "300" HEIGHT = "400" NAME = "lama" >
<PARAM NAME = CODE VALUE = "com.shiningway.applet.ShowMe.class" >
<PARAM NAME = ARCHIVE VALUE = "test.jar" >
<PARAM NAME = NAME VALUE = "lama" >
<PARAM NAME = MAYSCRIPT VALUE = true >
<param name = "type" value = "application/x-java-applet;version=1.5">
<param name = "scriptable" value = "false">
<comment>
<embed
type = "application/x-java-applet;version=1.5" \
CODE = "com.shiningway.applet.ShowMe.class" \
ARCHIVE = "test.jar" \
NAME = "lama" \
WIDTH = "300" \
HEIGHT = "400" \
MAYSCRIPT = true
scriptable = false
pluginspage = "http://java.sun.com/products/plugin/index.html#download">
<noembed>
</noembed>
</embed>
</comment>
</object>
<!--
<APPLET CODE = "com.shiningway.applet.ShowMe.class" ARCHIVE = "test.jar" WIDTH = "300" HEIGHT = "400" NAME = "lama" MAYSCRIPT = true>
</APPLET>
-->
<!--"END_CONVERTED_APPLET"-->
<input type="text" name="txtbaby" id="txtbaby">
<input type="button" οnclick="setTxt()" value="添加内容">
</body>
</html>
这个例子的效果是在showMe.html中的输入框中输入内容，然后点击添加内容按钮，改内容回自动保存在客户端D盘的lala.txt文件中
总结一下，知识点主要是以下几点
[list]
[*]applet编程
[*]applet数字签名
[*]java文本文件的读写操作
[*]javascript如何与applet通信
[/list]
疑问：
网页中javascript不能直接调用applet中文件读写的方法setTxt，查看java控制台信息提示无权限，即使用了签名也是如此，后来只有写在paint方法里面，在javascript中用repaint来调用。有没有遇到这个问题的，你们是如何解决的，欢迎来信讨论,email：rautinee@yahoo.com.cn
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