Question

我正在学习如何通过遵循这些1 2官方教程来编写servlet。我找不到一个关于PrintWriter类与变量的奇怪行为的简单问题的解决方案。代码如下：

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class ServletExample extends HttpServlet {

   public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws IOException, ServletException
   {
    response.setContentType("text/html");       
    PrintWriter out = response.getWriter();
    out.println("<html>");
    out.println("<head>");  
    out.println("<title>First example</title>");
    out.println("</head>");
    out.println("<h2>System information</h2>");
    out.println("Host name: " + request.getRemoteHost() + "<br>");
    out.println("Remote address: " + request.getRemoteAddr() + "<br>");
    out.println("Port: " + request.getServerPort() + "<br>");
    out.println("Encoding: " + request.getCharacterEncoding() + "<br>");
    out.println("Method: " + request.getMethod() + "<br>");
    out.println("Protocol: " + request.getProtocol() + "<br>");
    out.println("Address: " + request.getRequestURI() + "<br>");
    out.println("Path: " + request.getPathInfo() + "<br>");
    out.println("</body>");
    out.println("</html>");
    out.close();
    }
}

servlet容器上servlet的输出显示为

out.println("<h2>System information</h2>");

我无法弄清楚为什么，但我发现通过将以下行分成这些行可以起作用：

out.println("Host name: ");
out.println(request.getRemoteHost())
out.println("<br>");

我在网上搜索了很多，但我找不到并回答。

我在ubuntu mate 16.04 LTS下，从存储库安装了apache tomcat 8。

编辑我通过在ubuntu配对16.04 LTS和不同操作系统窗口10下使用xampp和tomcat 7尝试不同的tomcat 7来进一步研究这种行为。结果是一样的。

所以我将Request Info官方示例呈现在与tomcat一起安装的examples文件夹中。该示例的代码是：

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class RequestInfo extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException
{
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println("<html>");
    out.println("<head>");
    out.println("<title>Request Information Example</title>");
    out.println("</head>");
    out.println("<body>");
    out.println("<h3>Request Information Example</h3>");
    out.println("Method: " + request.getMethod());
    out.println("Request URI: " + request.getRequestURI());
    out.println("Protocol: " + request.getProtocol());
    out.println("PathInfo: " + request.getPathInfo());
    out.println("Remote Address: " + request.getRemoteAddr());
    out.println("</body>");
    out.println("</html>");
}

/**
 * We are going to perform the same operations for POST requests
 * as for GET methods, so this method just sends the request to
 * the doGet method.
 */

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException
{
    doGet(request, response);
}
}

正如所料，它不起作用，它的行为与之前的代码相同。但是，如果我在示例文件夹中打开预构建版本，则可以正常工作。

最后，通过将每一行分成几行，它如前所述工作。这真是奇怪的行为。

Answer 1

我建议你使用StringBuilder对象来生成结果。如果需要，您可以计算内容长度，这在某些情况下很有用。

StringBuilder sbText = new StringBuilder();
sbText.append("<html>")
    .append("<head>")
    .append(etc...);

String text = sbText.toString();
// Option A:
response.getWriter().print(text);

// Option B:
byte[] bytes = text.getBytes();
response.setContentLength(bytes.length);
response.getOutputStream().write(bytes);

Answer 2

我找到了解决我奇怪问题的方法。我还尝试使用最新的tomcat v9.0.8，它使用jvm v1.8.0_171-b11，而我使用javac 1.9编译代码。通过使用javac v1.8，它可以工作。

Java Servlet：PrintWriter带变量的奇怪行为

2 个答案: