[Servlet] 4: Detailed explanation of the request object HttpServletRequest

Table of contents

| Request object HttpServletRequest interface

The request object gets the parameter name

This article belongs to the third part of the back-end full set of notes

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| Request object HttpServletRequest interface

Basic overview of HttpServletRequest

Where is HttpServletRequest used?

Careful observation reveals that the parameter lists of the previous doGet and doPost methods areHttpServletRequest req, HttpServletResponse resp
These two parameters are the HttpServletRequest we are learning now and the HttpServletRespose we will learn later
Let's review the whole process of Tomcat processing requests so that we can use it more clearly

When a request is sent to Tomcat, it first goes through the Service and then it is handed over to the Connector. The Connector is used to receive the request and encapsulate the received request into Request and Response for specific processing. After the Request and Response are encapsulated, they are handed over to the Servlet in the Container. Processing, the Container returns to the Connector after processing the request, and finally the Connector returns the processing result to the client through the Socket, so that the entire request is processed!

What is HttpServletRequest?

HttpServletRequest comes from the Servlet specification and is an interface
Its implementation class is provided by the HTTP server (Tomcat)
The implementation class object of HttpServletRequest is called the request object

effect?

The HttpServletRequest interface is responsible for reading the information in the Http request protocol package when the doGet/doPost method is running
Read the [Request Line] information in the Http request protocol package
Read the request parameter information stored in the [request header] or [request body] in the Http request protocol package
Instead of the browser, apply for a resource file call from the Http server

Request object get URL & Method

grammar

Reference: Difference between URL and URI

request.getRequestURI();  //获取请求行的URI
request.getRequestURL();  //获取请求行的URL
request.getMethod();      //获取请求行的method（请求方式）

example

The following Servlet implementation class-related web.xml configuration has been configured.
The alias of the website deployed in Tomcat is web2. The alias of the Servlet object is web1.、
After the configuration is complete and the code is written, start Tomcat and enter the URL http://localhost:8080/web2/web1 to access the Servlet

public class MyServlet extends HttpServlet {

    @Override
    public void init() throws ServletException {
        super.init();
        System.out.println("MyServlet对象被创建了");
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //super.doGet(req, resp); 如果你是要彻底覆盖父类的doGet方法，不需要父类提供的功能，就可以删除super.doGet(req,resp);这一句
        System.out.println("这是Servlet的doGet方法");

        //request.getRequestURI()
        System.out.println("获取到请求行的URL：" + req.getRequestURL());  //http://localhost:8080/web2/web1
        System.out.println("获取到请求行的URI：" + req.getRequestURI());  ///web2/web1
        //request.getMethod()
        System.out.println("获取到请求行的method（请求方式）：" + req.getMethod());  //?GET
    }
}

The request object gets the parameter name

grammar

Use the getParameterNames method to return an Enumeration enumeration class

Enumeration parameters = request.getParameterNames();
while(parameters.hasMoreElements()){
	String parameter = (String)parameters.nextElement(); //得到的是Object，所以要强转为String
}

example

The following code, web.xml configuration, and Tomcat deployment are exactly the same as the previous section
The URL entered this time is: http://localhost:8080/web2/web1?name=Klee&age=12 to simulate the situation with parameters

public class MyServlet extends HttpServlet {

    @Override
    public void init() throws ServletException {
        super.init();
        System.out.println("MyServlet对象被创建了");
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //...
        
        //request.getParameters
        Enumeration parameters = req.getParameterNames();
        while(parameters.hasMoreElements()){
            System.out.println("参数名：" + (String)parameters.nextElement());//得到的是Object，所以要强转为String
            //参数名：name    参数名：age
        }
    }
}

request object get parameter value

grammar

String value = request.getParameter(参数名);

Contact the code in the previous section to obtain the parameter name, the complete code is as follows

Enumeration parameters = request.getParameterNames();
int i = 1;
while(parameters.hasMoreElements()){
	String parameterName = (String)parameters.nextElement();//得到的是Object，所以要强转为String
	String parameterValue = request.getParameter(parameterName);
	System.out.println("参数" + i + ":" + parameterName + " = " + parameterValue);
    //name = Klee   age = 12
}

example

The following code, web.xml configuration, and Tomcat deployment are exactly the same as the previous section
The URL entered this time is: http://localhost:8080/web2/web1?name=Klee&age=12 to simulate the situation with parameters

public class MyServlet extends HttpServlet {

    @Override
    public void init() throws ServletException {
        super.init();
        System.out.println("MyServlet对象被创建了");
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //...
        
        //request.getParameters
        Enumeration parameters = req.getParameterNames();
        int i = 1;
        while(parameters.hasMoreElements()){
            String parameterName = (String)parameters.nextElement();//得到的是Object，所以要强转为String
            String parameterValue = req.getParameter(parameterName);
            System.out.println("参数" + i++ + ":" + parameterName + " = " + parameterValue);
            //参数1:name = Klee  参数2:age = 12
        }
    }
}

Parameter value garbled problem

Why are the parameter values obtained from the POST request garbled?

If the browser sends a request in GET mode, the request parameters are stored in [Request Header ]. After the Http request protocol packet arrives at the Http server, the first thing to do is to decode the binary content of the request header, which is decoded by Tomcat. Tomcat9.0 uses [by default] utf-8] Character set decoding request header , which can explain all national characters
If the browser sends a request in POST mode, the request parameters are stored in the [request body] . After the Http request protocol packet arrives at the Http server, the first thing to do is to decode the binary content of the request body and the current request object (request) is responsible for decoding it. request uses the [IS0-8859-1] character set to decode the request body by default . If an Eastern European character set is used, if the content of the request body parameter is Chinese, it will not be able to decode and only get garbled characters.
It should be noted that any version of Tomcat has the problem of garbled POST requests, and only Tomcat before version 8.0 has garbled GET requests (so this section does not introduce garbled GET requests, you need Baidu yourself)

In addition, in addition to the above-mentioned garbled characters caused by the mismatch between the encoding between the Tomcat server and the browser , we also need to consider the garbled characters caused by the encoding settings output by the console , and also consider the garbled characters caused by the local code file. Garbled characters caused by character settings .

If the browser sends a request in POST mode, the request parameters are stored in the [request body] . After the Http request protocol packet arrives at the Http server, the first thing to do is to decode the binary content of the request body and the current request object (request) is responsible for decoding it. request uses the [IS0-8859-1] character set to decode the request body by default . If an Eastern European character set is used, if the content of the request body parameter is Chinese, it will not be able to decode and only get garbled characters.

The basic idea of solving POST garbled characters

In the Post request mode, before reading the content of the request body, the request object should be notified to use the utf-8 character set to re-decode the content of the request body

Solution to POST garbled characters, console output garbled characters, and local code Chinese garbled characters

Add code in the first line of the doPost method req.setCharacterEncoding("UTF-8");(this method is only valid for POST garbled characters, that is, it can only be used for doPost)

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        System.out.println("编码方式" + req.getCharacterEncoding());

        String parameterValue = req.getParameter("userName");
        System.out.println(parameterValue);
    }

If the POST is still garbled after adding the first line of code, even GET is also garbled, it can be considered that the console output is garbled

Solution: [Click Edit Configuration, enter in VM Options]-Dfile.encoding=utf-8

(The solution of this method is effective for both POST and GET: the function is to solve the garbled output of the console) Reference 1 Reference 2

If after using the above two methods, the Chinese from the browser is not garbled, but the Chinese in the code is garbled again, you can consider entering [Settings - Editor - File Encdings] to set it

example

    <form action="/web2/postWeb" method="GET">
      请求参数：<input type="text" name="userName"/>
      <br/>
      <input type="submit" value="POST方式访问Servlet">
    </form>

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        System.out.println("编码方式" + req.getCharacterEncoding());

        String parameterValue = req.getParameter("userName");
        System.out.println(parameterValue);

    <!--Servlet实现类PostServlet的配置-->
    <servlet>
        <servlet-name>MyPostServlet</servlet-name>
        <servlet-class>controller.PostServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>MyPostServlet</servlet-name>
        <url-pattern>/postWeb</url-pattern>
    </servlet-mapping>