在Servlet中有两种机制用于打印响应：PrintWriter和ServletOutputStream。Servlet文档告诉我们应该使用ServletOutputStream打印二进制数据，用PrintWriter打印字符文本，如果在Servlet中同时需要打印二进制数据和字符文本，比较明智的做法是使用ServletOutputStream,而不要用if-else分开。ServletOutputStream继承基于字节的java.io.OutputStream,但增加了println()和print()方法，用于打印基本数据类型和字符串（请参看Servlet API),象下面的代码并不值得推荐,甚至随着代码的增加可能会越来越难于管理代码,甚至可能会引入一些bug,因为你必须时刻记住你正在使用的是哪个流。
PrintWriter pw
ServletOutputStream out;
try{
out=response.getOutputStream();
//写二进制数据
}catch(Exception e){
pw=response.getWriter();
pw.println("There was an error: "+e.getMessage())
}
下面是一个正确的实例：这是一个简单投票程序。
import java.io.*;
import java.text.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class GoodVoterServlet extends HttpServlet
{
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
doPost(request,response);
}
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
String vote = request.getParameter("vote");
ServletOutputStream out = response.getOutputStream();
VoterApp voter = VoterApp.getInstance();
if ( vote == null )//创建投票页面
{
//Let's print out the Poll of the Day!
response.setContentType("text/html");
out.println("Poll of the Day!");
out.println(" request.getRequestURI() + "'>");
out.println(voter.getPollOfTheDay());
out.println("");
out.println("");
}
else//统计结果
{
voter.addToPollResults(vote);
//Get the generated poll results graph
byte[] generatedGraph = voter.generateImageBytes();
if ( generatedGraph == null )//没有输出图像时输出文本字符
{
response.setContentType("text/html");
out.println("Technical difficulties.. Please see " +
"your administrator for details.");
return;
}
else//输出统计图像
{
response.setContentType("image/gif");
out.write(generatedGraph, 0, generatedGraph.length);
out.flush();
}
}
}
}
帮助类的代码：
public class VoterApp {
private static VoterApp instance = null;
private VoterApp() {}
public static VoterApp getInstance()
{
if ( instance == null )
{
instance = new VoterApp();
}
return(instance);
}
public String getPollOfTheDay()//投票的表单
{
String poll =
"What is the best thing about Utah?" +
"The Mormon Tabernacle Choir
" +
"The Utah Jazz
" +
"Winter Olympics
" +
"Donnie Osmond
";
return(poll);
}
public void addToPollResults(String foo)//点票计数
{
}
public byte[] generateImageBytes()//获取图像的二进制数据。
{
byte[] b = new byte[1];
b[0] = 0;
return b;
}
}
(责任编辑：包春林)
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