English | 简体中文 | 繁體中文 | Русский язык | Français | Español | Português | Deutsch | 日本語 | 한국어 | Italiano | بالعربية
Let's take a look at a flowchart first:
The process of the server handling requests:
(1The server will open a new thread for each request it receives.
(2The server will encapsulate the client's request data into the request object, and the request is the carrier of the request data!
(3The server will also create a response object, which is connected to the client and can be used to send responses to the client.
The flowchart shows that in the request and response of JavaWeb, the two most important parameters are request and response, which are in the Servlet's service() method.
1、Response concept:
The response is a parameter of the Servlet.service method, of type javax.servlet.http.HttpServletResponse. For each request sent by the client, the server will create a response object and pass it to the Servlet.service() method. The response object is used to respond to the client, which means that the response object can be used to complete the response work to the client in the service() method.
The functions of the response object are divided into the following four types:
(1）Set response header information
(2）Send status code
(3）Set response body
(4）Redirect
2、Response body
The response object is used to output the response body (response body) to the client, and the response object provides two response stream objects in total:
(1）PrintWriter out = response.getWriter()：Get character stream；
(2）ServletOutputStream out = response.getOutputStream()：Get byte stream；
Of course, if the response body content is a character, then use response.getWriter(), and if the response content is a byte, such as during a download, then you can use response.getOutputStream().
NoteYou cannot use both streams at the same time in a single request! That is to say, either you use repsonse.getWriter() or response.getOutputStream(), but not both. Otherwise, an illegalStateException exception will be thrown.
3、Set response header information
You can use the setHeader() method of the response object to set response headers! The response headers set by this method will eventually be sent to the client browser!
(1）response.setHeader("content-type”, "text/html;charset=utf-8"): Set content-type response header, which is used to inform the browser that the response content is of html type, encoded in utf-8。And at the same time, it will set the character stream encoding of response to utf-8，i.e., response.setCharaceterEncoding("utf-8");
(2）response.setHeader("Refresh","5; URL=http://www.baidu.com":5Automatically jump to Baidu homepage after 1 second.
4Set status code and other methods
(1) response.setContentType("text}}/html;charset=utf-8"): Equivalent to calling response.setHeader(“content-type”, "text/html;charset=utf-8");
(2) response.setCharacterEncoding("utf-8) Set character encoding of the character response stream to utf-8;
(3) response.setStatus(200): Set status code;
(4) response.sendError(404, "The resource you are looking for does not exist"): When sending an error status code, Tomcat will jump to a fixed error page, but it can display error information.
5, redirection (*****重点*****）
5.1 What is redirection (two requests)?
When you access http://www.sun.com, you will find that the URL in the browser address bar will change to http://www.oracle.com/us/sun/index.htm, which is a redirection. Redirection is when the server notifies the browser to access another address, i.e., to make another request.
5.2 How to complete redirection?
Answer: The status code of redirection is302Firstly, we use the response object to send302status code, and then set a Location, which is to provide a usable URL, and let the browser access the new URL to achieve redirection.
For example:
public class AServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setStatus(302);
response.setHeader("Location", "http://www.baidu.com);
}
}
The purpose of the above code is: after accessing AServlet, it notifies the browser to redirect to Baidu's homepage. The client browser parses the response code as302After that, it knows that the server wants to redirect, so it will immediately retrieve the response header Location and send the second request.
There is also a quick redirection method,even using the response.sendRedirect() method. For example, the two statements in the above example can be replaced with response.sendRedirect("http://www.baidu.com)) instead.
request—wraps all the client request data
1Overview of request
) Request is a parameter of Servlet.service() method, the type is javax.servlet.http.HttpServletRequest. When the client sends each request, the server will create a request object, encapsulate the request data into the request, and then pass it to the service() method when calling Servlet.service() method, which means that the request object can be used to obtain the request data in the service() method.
As shown in the figure:
) The functions of request can be divided into the following types:
(1) Encapsulates the request header data;
(2) Encapsulates the request body data, if it is a GET request, then there is no body;
(3) Request is a domain object, which can be treated as a Map to add and retrieve data;
(4) Request provides request forwarding and request inclusion features.
2Domain methods of request
Request is a domain object! In JavaWeb, there are a total of four domain objects, among which ServletContext is a domain object, which only creates one ServletContext object in the entire application. Request is one of them, and request can share data in a request.
A request creates a request object, if multiple Servlets are experienced in a request, then multiple Servlets can use request to share data. We still do not know how many Servlets are experienced in a request.
The following are the domain methods of request:
(1）void setAttribute(String name, Object value):Used to store an object, which can also be called storing a domain attribute, for example: servletContext.setAttribute(“xxx”, “XXX”);, a domain attribute is stored in the request, the name of the domain attribute is xxx, and the value of the domain attribute is XXX. Please note that if this method is called multiple times and the same name is used, the previous value will be overwritten, which is the same as the characteristic of Map;
(2）Object getAttribute(String name):Used to obtain data from the request, before obtaining it, you need to store it first, for example: String value = (String)request.getAttribute(“xxx”);, to obtain the domain attribute named xxx;
(3）void removeAttribute(String name):Used to remove the domain attribute from the request, if the domain attribute specified by the parameter name does not exist, this method does nothing;
(4）Enumeration getAttributeNames()：获取所有域属性的名称；
3、request传递参数
最为常见的客户端传递参数方式有两种：
(1）浏览器地址栏直接输入：一定是GET请求；
(2）超链接：一定是GET请求；
(3）表单：可以是GET，也可以是POST，这取决与