Essentials for JSP development: Detailed explanation of the built-in objects and their functions in JSP

Introduction:
JSP (JavaServer Pages) is a type of application in Java applications Technology embedded in HTML designed to simplify the development of dynamic web pages. In JSP, built-in objects are a set of objects that developers can use directly when writing JSP pages. They provide many useful functions that can simplify the development process and improve efficiency. This article will analyze the built-in objects and their functions in JSP in detail, and give specific code examples.

1. Request object
The request object is an instance of the javax.servlet.http.HttpServletRequest class, which provides methods related to the client's HTTP request. Developers can use the request object to obtain request parameters, obtain request header information, obtain session status, send redirections, etc. The following are some examples of common methods:

1. Get request parameters:

   String username = request.getParameter("username");

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2. Get request header information:

   String userAgent = request.getHeader("User-Agent");

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3. Get session status:

   HttpSession session = request.getSession(); session.setAttribute("userId", userId);

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4. Send redirection:

   response.sendRedirect("index.jsp");

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2. Response object
The response object is javax. An instance of the servlet.http.HttpServletResponse class, which provides methods related to the client's HTTP response. Developers can use the response object to set response header information, set response content, send redirections, etc. The following are some examples of common methods:

1. Set response header information:

   response.setHeader("Content-Type", "text/html;charset=UTF-8");

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2. Set response content:

   PrintWriter out = response.getWriter(); out.println("
   Welcome to my website!
   ");

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3. Send redirection:

   response.sendRedirect("index.jsp");

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3. out object
The out object is an instance of the javax.servlet.jsp.JspWriter class, which provides output text and HTML label method. Developers can use out objects to send text and HTML content to clients. Here are some examples of common methods:

1. Output text:

   out.print("Hello, World!");

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2. Output HTML tag:

   out.println("
   Welcome to my website!
   ");

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4. Application object
The application object is an instance of the javax.servlet.ServletContext class, which represents the Web application on the current server. Developers can use the application object to share global data, obtain the initialization parameters of the Web application, obtain the real path of the Web application, etc. Here are some examples of common methods:

1. Share global data:

   application.setAttribute("visitCount", visitCount);

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2. Get the initialization parameters of the web application:

   String dbUrl = application.getInitParameter("dbUrl");

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3. Get the real path of the Web application:

   String realPath = application.getRealPath("/");

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5. Session object
The session object is an instance of the javax.servlet.http.HttpSession class. It represents the session between client and server. Developers can use session objects to store and retrieve session state information. Here are some examples of common methods:

1. Store session state information:

   session.setAttribute("username", username);

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2. Get session state information:

   String username = (String) session.getAttribute("username");

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3. Set session expiration time:

   session.setMaxInactiveInterval(60 * 30); // 设置会话过期时间为30分钟

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6. PageContext object
The pageContext object is an instance of the javax.servlet.jsp.PageContext class, which represents the current JSP Contextual information for the page. Developers can use the pageContext object to access other built-in objects and obtain the config object, request object, response object, etc. of the JSP page. The following are some examples of common methods:

1. Access other built-in objects:

   HttpServletRequest request = (HttpServletRequest) pageContext.getRequest();

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2. Get the config object of the JSP page:

   ServletConfig config = pageContext.getServletConfig();

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3. Get the request object of the JSP page:

   HttpServletRequest request = (HttpServletRequest) pageContext.getRequest();

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Conclusion:
This article introduces the built-in objects and their functions in JSP in detail, and gives specific code example. These built-in objects can greatly simplify the JSP development process and improve development efficiency. I hope this article will be helpful to developers who are learning or using JSP.

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