servlet.html

java 基础的 一点东西,,可以看看

class in this package to indicate a servlet problem.</FONT>
<P> 
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
<CODE>javax.servlet.http</CODE>, which contains HTTP
servlet classes. The <CODE>HttpServlet</CODE> class is in this
package. </FONT>
</UL>

</FONT>

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

public class ExampServlet 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;&#60;title&#62;Example&#60;/title&#62;&quot; +
       &quot;&#60;body bgcolor=FFFFFF&#62;&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(DATA);
    } else {
      out.println(&quot;No text entered.&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">

<H3>Class and Method Declarations</H3>

All servlet classes extend the <CODE>HttpServlet</CODE> abstract class.
<CODE>HttpServlet</CODE> simplifies writing HTTP servlets by providing a
framework for handling the HTTP protocol. Because <CODE>HttpServlet</CODE>
is <CODE>abstract</CODE>, your servlet class must extend it and override at
least one of its methods. An <CODE>abstract</CODE> class is a class
that contains unimplemented methods and cannot be instantiated itself.

</FONT>

<PRE>
public class ExampServlet extends HttpServlet {
  public void doPost(HttpServletRequest request,
         HttpServletResponse response)
         throws ServletException, IOException
  {
</PRE>

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

The <CODE>ExampServlet</CODE> class is declared <CODE>public</CODE> 
so the web server that runs the servlet, which is not
local to the servlet, can access it. 

<P>
The <CODE>ExampServlet</CODE> class defines a <CODE>doPost</CODE>
method with the same name, return type, and parameter list as the
<CODE>doPost</CODE> method in the <CODE>HttpServlet</CODE> class.
By doing this, the <CODE>ExampServlet</CODE> class overrides and
implements the <CODE>doPost</CODE> method in the 
<CODE>HttpServlet</CODE> class.

<P>
The <CODE>doPost</CODE> method performs the <CODE>HTTP POST</CODE>
operation, which is the type of operation specified in 
the HTML form used for this example. The other possibility is the 
<CODE>HTTP GET</CODE> operation, in which case you would implement 
the <CODE>doGet</CODE> method instead. 

<P>
In short, <CODE>POST</CODE> requests are for sending any amount of 
data directly over the connection without changing the URL, and 
<CODE>GET</CODE> requests are for getting limited amounts of 
information appended to the URL.  <CODE>POST</CODE> requests 
cannot be bookmarked or emailed and do not change the Uniform 
Resource Locators (URL) of the response.  <CODE>GET</CODE> 
requests can be bookmarked and emailed and add information to the URL
of the response. 

<P>
The parameter list for the <CODE>doPost</CODE> method takes a
<CODE>request</CODE> and a <CODE>response</CODE> object. The browser
sends a request to the servlet and the servlet sends a response back
to the browser.

<P>
The <CODE>doPost</CODE> method implementation accesses information
in the <CODE>request</CODE> object to find out who made the
request, what form the request data is in, and which HTTP headers
were sent, and uses the <CODE>response</CODE> object to create
an HTML page in response to the browser's request. 
 
The <CODE>doPost</CODE> method throws an <CODE>IOException</CODE>
if there is an input or output problem when it handles the request, 
and a <CODE>ServletException</CODE> if the request could not be 
handled. These exceptions are handled in the <CODE>HttpServlet</CODE> 
class.

<H3>Method Implementation</H3>

The first part of the <CODE>doPost</CODE> method uses
the <CODE>response</CODE> object to create an HTML page.
It first sets the response content type to be 
<CODE>text/html</CODE>,
then gets a <CODE>PrintWriter</CODE> object for formatted text output.

</FONT>

<PRE>
response.setContentType(&quot;text/html&quot;);
    PrintWriter out = response.getWriter();

    out.println(&quot;&#60;title&#62;Example&#60;/title&#62;&quot; +
       &quot;&#60;body bgcolor=#FFFFFF&#62;&quot;);

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

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

The next line uses the <CODE>request</CODE> object to
get the data from the text field on the form and store it 
in the <CODE>DATA</CODE> variable. The <CODE>getparameter</CODE> 
method gets the named parameter, returns <CODE>null</CODE> if the
parameter was not set, and an empty string if the parameter
was sent without a value. 

</FONT>

<PRE>
    String DATA = request.getParameter(&quot;DATA&quot;);
</PRE>

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

The next part of the <CODE>doPost</CODE> method gets the data out of the 
<CODE>DATA</CODE> 
parameter and passes it to the <CODE>response</CODE> object to add to the
HTML response page.
 
</FONT>

<PRE>
    if(DATA != null){
      out.println(DATA);
    } else {
      out.println(&quot;No text entered.&quot;);
    }
</PRE>

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

The last part of the <CODE>doPost</CODE> method creates a link to take the 
end user from the HTML response page back to the original form, and 
closes the response.

</FONT>

<PRE>
    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">

<BLOCKQUOTE>
<HR>
<STRONG>Note:</STRONG> 
To learn how to use the other methods available in the 
<CODE>HttpServlet</CODE>, <CODE>HttpServletRequest</CODE>,
and <CODE>HttpServletResponse</CODE> classes, see 
<A HREF="http://java.sun.com/docs/books/tutorial/index.html">The
Java Tutorial</A> trail on 
<A HREF="http://java.sun.com/docs/books/tutorial/servlets/index.html">Servlets</A>. 
<HR>
</BLOCKQUOTE>

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

You can find more information on servlets in the
<A HREF="http://java.sun.com/docs/books/tutorial/servlets/index.html">Servlets</A>
trail in
<A HREF="http://java.sun.com/docs/books/tutorial">The Java Tutorial</A>. 

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