Servlet form data

May 14, 2021 Servlet

1. Servlet form data

2. GET method

3. POST method

4. Use the servlet to read form data

5. An instance of the GET method that uses the URL

6. Use an example of the form's GET method

7. Use an instance of the form's POST method

8. Pass check box data to the servlet program

9. Read all form parameters

Servlet form data

In many cases, you need to pass some information, from the browser to the Web server, and eventually to the background program. Browsers use two methods to pass this information to the Web server, the GET method and the POST method, respectively.

GET method

The GET method requests the page to send encoded user information. B etween the page and encoded information? Character separation, as follows:

http://www.test.com/hello?key1=value1&key2=value2

The GET method is the default way to pass information from the browser to the Web server, and it produces a long string that appears in the browser's address bar. D o not use the GET method if you are passing passwords or other sensitive information to the server. The GET method has a size limit: the request string can only have a maximum of 1024 characters.

This information is QUERY_STRING and can be accessed through QUERY_STRING environment variables, and Servlet uses the doGet() method to handle this type of request.

POST method

Another more reliable way to pass information to background programs is the POST method. T he POST method packages information in essentially the same way as the GET method, but the POST method does not treat the information as a URL? T he text string after the character is sent, but the information is treated as a separate message. M essages are sent to the background program as standard output, which you can parse and use. Servlet uses the doPost() method to handle this type of request.

Use the servlet to read form data

Servlet processes form data, which is automatically resolved in different ways depending on the situation:

getParameter(): You can call the request.getParameter() method to get the value of the form parameters.
getParameterValues(): If the argument appears more than once, the method is called and multiple values are returned, such as check boxes.
getParameterNames(): If you want a complete list of all the parameters in the current request, call the method.

An instance of the GET method that uses the URL

Here's a simple URL that will use the GET method to pass two values to the HelloForm program.

http://localhost:8080/HelloForm?first_name=ZARA&last_name=ALI

Here's the HelloForm and Servlet .java web browser input. We'll use the getParameter() method to easily access the information passed:

// 导入必需的 java 库
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

// 扩展 HttpServlet 类
public class HelloForm extends HttpServlet {
 
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // 设置响应内容类型
      response.setContentType("text/html");

      PrintWriter out = response.getWriter();
      String title = "使用 GET 方法读取表单数据";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +       "transitional//en\">\n";
      out.println(docType +
                "<html>\n" +
                "<head><title>" + title + "</title></head>\n" +
                "<body bgcolor=\"#f0f0f0\">\n" +
                "<h1 align=\"center\">" + title + "</h1>\n" +
                "<ul>\n" +
                "  <li><b>名字</b>："
                + request.getParameter("first_name") + "\n" +
                "  <li><b>姓氏</b>："
                + request.getParameter("last_name") + "\n" +
                "</ul>\n" +
                "</body></html>");
  }
}

Assuming that your environment is set up correctly, compile HelloForm .java, as follows:

$ javac HelloForm.java

If all goes well, the compilation above will result in a HelloForm .class file. Next, you must copy the file to the webapps/ROOT/WEB-INF/classes and create the following entries in the webapps/ROOT-INF/classes file located in the .lt;Tomcat-design-directory/webapps/ROOT/WEB-INF/.xml file:

    <servlet>
        <servlet-name>HelloForm</servlet-name>
        <servlet-class>HelloForm</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloForm</servlet-name>
        <url-pattern>/HelloForm</url-pattern>
    </servlet-mapping>

Now enter the address bar in your browser to http://localhost:8080/HelloForm?first_name=ZARA&last_name=ALI and make sure that the Tomcat server is started before triggering the above command. If all goes well, you'll get the following results:

Use the GET method to read form data

Name: ZARA
Last name: ALI

Use an example of the form's GET method

Here's a simple example of using an HTML form and a submit button to pass two values. We'll use the same Servlet HelloForm to process the input.

<html>
<body>
<form action="HelloForm" method="GET">
名字：<input type="text" name="first_name">
<br />
姓氏：<input type="text" name="last_name" />
<input type="submit" value="提交" />
</form>
</body>
</html>

Save this HTML to hello .htm file and place it in the directory of the .lt; Tomcat-design-directory/webapps/ROOT directory. When you access http://localhost:8080/Hello.htm, here is the actual output of the form above.

Servlet form data

Try entering your first and last name, then click the Submit button to see the output on your machine. Based on the input provided, it produces a similar result to the last instance.

Use an instance of the form's POST method

Let's make a small change to the servlet above so that it can handle get and POST methods. The following HelloForm .java Servlet program uses the GET and POST methods to process input given by a web browser.

// 导入必需的 java 库
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

// 扩展 HttpServlet 类
public class HelloForm extends HttpServlet {
 
  // 处理 GET 方法请求的方法
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // 设置响应内容类型
      response.setContentType("text/html");

      PrintWriter out = response.getWriter();
      String title = "Using GET Method to Read Form Data";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +       "transitional//en\">\n";
      out.println(docType +
                "<html>\n" +
                "<head><title>" + title + "</title></head>\n" +
                "<body bgcolor=\"#f0f0f0\">\n" +
                "<h1 align=\"center\">" + title + "</h1>\n" +
                "<ul>\n" +
                "  <li><b>名字</b>："
                + request.getParameter("first_name") + "\n" +
                "  <li><b>姓氏</b>："
                + request.getParameter("last_name") + "\n" +
                "</ul>\n" +
                "</body></html>");
  }
  // 处理 POST 方法请求的方法
  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
     doGet(request, response);
  }
}

Now compile and deploy the servlet above and test it with hello .htm post method, as follows:

<html>
<body>
<form action="HelloForm" method="POST">
名字：<input type="text" name="first_name">
<br />
姓氏：<input type="text" name="last_name" />
<input type="submit" value="提交" />
</form>
</body>
</html>

Here's the actual output of the form above, try entering your first and last name, and then clicking the Submit button to see the output on your machine.

Servlet form data

Based on the input provided, it produces a similar result to the last instance.

Pass check box data to the servlet program

Use the check box when you need to select more than one option.

Below is an HTML code instance, CheckBox.htm, and a form with two check boxes.

<html>
<body>
<form action="CheckBox" method="POST" target="_blank">
<input type="checkbox" name="maths" checked="checked" /> 数学
<input type="checkbox" name="physics"  /> 物理
<input type="checkbox" name="chemistry" checked="checked" /> 
                                                化学
<input type="submit" value="选择学科" />
</form>
</body>
</html>

The result of this code is the following form:

Servlet form data

Here's .java CheckBox or Servlet program that handles check box input given by your web browser.

// 导入必需的 java 库
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

// 扩展 HttpServlet 类
public class CheckBox extends HttpServlet {
 
  // 处理 GET 方法请求的方法
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // 设置响应内容类型
      response.setContentType("text/html");

      PrintWriter out = response.getWriter();
     String title = "读取复选框数据";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +       "transitional//en\">\n";
      out.println(docType +
                "<html>\n" +
                "<head><title>" + title + "</title></head>\n" +
                "<body bgcolor=\"#f0f0f0\">\n" +
                "<h1 align=\"center\">" + title + "</h1>\n" +
                "<ul>\n" +
                "  <li><b>数学标识：</b>: "
                + request.getParameter("maths") + "\n" +
                "  <li><b>物理标识：</b>: "
                + request.getParameter("physics") + "\n" +
                "  <li><b>化学标识：</b>: "
                + request.getParameter("chemistry") + "\n" +
                "</ul>\n" +
                "</body></html>");
  }
  // 处理 POST 方法请求的方法
  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
     doGet(request, response);
  }
}

The above example shows the following results:

Read the check box data

Mathematical identification: on
Physical identity: null
Chemical identification: on

Read all form parameters

The following is a generic example of reading all available form parameters using httpServletRequest's getParameterNames() method. The method returns an enumeration that contains parameter names that are not specified in order.

Once we have an enumeration, we can loop the enumeration in a standard way, using the hasMoreElements() method to determine when to stop, and the nextElement() method to get the name of each parameter.

// 导入必需的 java 库
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

// 扩展 HttpServlet 类
public class ReadParams extends HttpServlet {
 
  // 处理 GET 方法请求的方法
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // 设置响应内容类型
      response.setContentType("text/html");

      PrintWriter out = response.getWriter();
     String title = "读取所有的表单数据";
      String docType =
      "<!doctype html public \"-//w3c//dtd html 4.0 " +       "transitional//en\">\n";
      out.println(docType +
        "<html>\n" +
        "<head><title>" + title + "</title></head>\n" +
        "<body bgcolor=\"#f0f0f0\">\n" +
        "<h1 align=\"center\">" + title + "</h1>\n" +
        "<table width=\"100%\" border=\"1\" align=\"center\">\n" +
        "<tr bgcolor=\"#949494\">\n" +
        "<th>参数名称</th><th>参数值</th>\n"+
        "</tr>\n");

      Enumeration paramNames = request.getParameterNames();
      
      while(paramNames.hasMoreElements()) {
         String paramName = (String)paramNames.nextElement();
         out.print("<tr><td>" + paramName + "</td>\n<td>");
         String[] paramValues =
                request.getParameterValues(paramName);
         // 读取单个值的数据
         if (paramValues.length == 1) {
           String paramValue = paramValues[0];
           if (paramValue.length() == 0)
             out.println("<i>No Value</i>");
           else
             out.println(paramValue);
         } else {
             // 读取多个值的数据
             out.println("<ul>");
             for(int i=0; i < paramValues.length; i++) {                 out.println("<li>" + paramValues[i]);
             }
             out.println("</ul>");
         }
      }
      out.println("</tr>\n</table>\n</body></html>");
  }
  // 处理 POST 方法请求的方法
  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
     doGet(request, response);
  }
}

Now try the servlet above from the form below:

<html>
<body>
<form action="ReadParams" method="POST" target="_blank">
<input type="checkbox" name="maths" checked="checked" /> 数学
<input type="checkbox" name="physics"  /> 物理
<input type="checkbox" name="chemistry" checked="checked" /> 化学
<input type="submit" value="选择学科" />
</form>
</body>
</html>

Calling the servlet now using the form above will produce the following results:

Read all form data

The name of the argument	The value of the argument
maths	on
chemistry	on

You can try using the servlet above to read other form data, such as text boxes, turn buttons, or pull-down boxes.

Servlet form data

Table of contents

Servlet form data

GET method

POST method

Use the servlet to read form data

An instance of the GET method that uses the URL

Use the GET method to read form data

Use an example of the form's GET method

Use an instance of the form's POST method

Pass check box data to the servlet program

Read the check box data

Read all form parameters

Read all form data

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