Java Servlet：PrintWriter带有变量的奇怪行为
[英]Java Servlet: PrintWriter strange behaviour with variable
问题描述
I'm learning how to program servlet by following these 1 2 official tutorials. 
我通过遵循这些1 2官方教程来学习如何编程servlet。 I cannot find a solution to a simple question about the strange behaviour of PrintWriter class with variables. 我找不到有关带变量的PrintWriter类的奇怪行为的简单问题的解决方案。 The code is the following: 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();
}
}
The output of the servlet on servlet container is displayed up to servlet容器上servlet的输出显示到
out.println("<h2>System information</h2>");
I cannot figure out why, but I discovered that by splitting the following line into these lines works: 我不知道为什么，但是我发现将以下行拆分为这些行是可行的：
out.println("Host name: ");
out.println(request.getRemoteHost())
out.println("<br>");
I searched a lot on the Web, but I cannot find and answer. 我在网上搜索了很多内容，但找不到并回答。
I'm under ubuntu mate 16.04 LTS with apache tomcat 8 installed from repository. 我在ubuntu mate 16.04 LTS下，从存储库安装了apache tomcat 8。
Edit I further investigate about such behaviour by trying different tomcat 7 under ubuntu mate 16.04 LTS and different OS windows 10 with xampp and tomcat 7. The result is the same. 编辑我通过在ubuntu mate 16.04 LTS下尝试使用不同的tomcat 7以及使用xampp和tomcat 7的不同OS Windows 10来进一步研究此类行为。结果是相同的。
So I took the Request Info official example present into examples folder installed together with tomcat. 因此，我将出现的“请求信息”官方示例放到与tomcat一起安装的examples文件夹中。 The code of the example is this: 该示例的代码是这样的：
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);
}
}
As expected it does not work and its behaviour it is the same of the previous code. 如预期的那样，它不起作用，并且其行为与先前的代码相同。 However, if I open prebuilt version inside the example folder it works. 但是，如果我在example文件夹中打开预构建版本，它将起作用。
Finally, by splitting each line into several lines it works as described before. 最后，通过将每行分成几行，其工作方式如前所述。 It is really strange behaviour. 这确实是奇怪的行为。
2 个解决方案
解决方案1
0 2018-04-08 19:39:23
I would recommend that you use a StringBuilder object to generate the result, then. 我建议您使用StringBuilder对象生成结果。 You can, if desired, calculate the content length, which is useful in some situations. 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);
解决方案2
0 2018-05-26 16:06:37
I found the solution at my strange problem. 我找到了解决我的奇怪问题的方法。 I tried also with latest tomcat v9.0.8 that uses jvm v1.8.0_171-b11 while I compiled my code with javac 1.9. 我在使用javac 1.9编译代码时也尝试了使用jvm v1.8.0_171-b11的最新tomcat v9.0.8。 By using javac v1.8 it works. 通过使用javac v1.8，它可以工作。
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