\\\
是文本的换行符。在浏览器中查看源代码，您将看到 \\\
正在为HTML源代码提供换行符。问题是，浏览器不会在呈现的HTML中显示 \\\
作为换行符。这是因为要在HTML中创建一个换行符，您可以使用< br /> 进行换行，或将您的行包装到以 p> ，以< / p> 结尾。如果你要做JSP / Servlet开发，你需要学习HTML的基础知识。

所以：

< / p>）;
out.println（< p> Your UserName is：+ User_name +< / p>）;

或

  out.println（你好POST程序！）; 
 out.println（< br /> Your UserName is：+ User_name）; 
 

它也不建议直接在这样的servlet中打印HTML。您应该设置一个请求属性并转发给作为视图的JSP。这在 Servlets信息页面中有很好的解释。

只需未来的提示：在服务器上更改某些内容（保存到数据库或其他任何内容）的帖子后，您将需要执行服务器端重定向以防止双重发布。

I am new to learning Java Servlet. I am trying to pass parameters through POST query(Apache Tomcat v8.0) using a simple html form that generates two input fields 'UserName' and 'FullName'. The code is running perfectly however; I want 'UserName' and 'FullName' to display on separate new line and I cannot do it by using "/n" inside println() function. Here is my POST query code.

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    response.setContentType("text/html");
    PrintWriter out= response.getWriter();
    String User_name = request.getParameter("UserName");
    String Full_name = request.getParameter("FullName");
    out.println("\nHello from POST method!");
    out.println("\nYour UserName is: " +User_name);
    out.println("\nYour FullName is: " +Full_name);
}

\n is newline for text. View source in browser, and you'll see the \n is giving you a newline in the HTML source. The problem is, browsers don't display that \n as a newline in the rendered HTML. That's because to make a newline in HTML you use either <br /> for linebreak, or wrap your line into a paragraph beginning with <p> and ending with </p>. If you're going to be doing JSP/Servlet development, you need to learn the basics of HTML.

So:

 out.println("<p>Hello from POST method!</p>"); 
 out.println("<p>Your UserName is: " + User_name + "</p>"); 

or

 out.println("Hello from POST method!"); 
 out.println("<br />Your UserName is: " + User_name); 

Its also not recommended to print HTML directly in a servlet like this. You should rather set a request attribute and forward to a JSP that acts as a view. This is explained well in the Servlets info page.

Just a hint for the future: After a post which has changed something on the server (saved to db or whatever) you will want to do a server-side redirect to prevent double posting.