Servlet database access

This tutorial assumes that you already understand how JDBC applications work. Before you start learning Servlet database access, make sure you have the appropriate JDBC environment setup and database.

Starting from the basic concepts, let us create a simple table and create a few records in the table.

Create database table

To create the Employees table in the test database TEST, please follow the steps below:

Step 1:

Open the Command Prompt, and change to the installation directory, as shown below:

C:\>
C:\>cd Program Files\MySQL\bin
C:\Program Files\MySQL\bin>

Step 2:

C:\Program Files\MySQL\bin>mysql -u root -p
Enter password: ********
mysql>

Step 3:

Create the Employee table in the test database TEST as follows Display:

mysql> use TEST;
mysql> create table Employees
    (
     id int not null,
     age int not null,
     first varchar (255),
     last varchar (255)
    );
Query OK, 0 rows affected (0.08 sec)
mysql>

Create data record

Finally, create several records in the Employee table, as shown below:

mysql> INSERT INTO Employees VALUES (100, 18, 'Zara', 'Ali');
Query OK, 1 row affected (0.05 sec)
 
mysql> INSERT INTO Employees VALUES (101, 25, 'Mahnaz', 'Fatma');
Query OK, 1 row affected (0.00 sec)
 
mysql> INSERT INTO Employees VALUES (102, 30, 'Zaid', 'Khan');
Query OK, 1 row affected (0.00 sec)
 
mysql> INSERT INTO Employees VALUES (103, 28, 'Sumit', 'Mittal');
Query OK, 1 row affected (0.00 sec)
 
mysql>

Access the database

The following The example demonstrates how to use Servlet to access the TEST database.

// 加载必需的库
import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.sql.*;
 
public class DatabaseAccess extends HttpServlet{
    
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // JDBC 驱动器名称和数据库的 URL
      static final String JDBC_DRIVER="com.mysql.jdbc.Driver";  
      static final String DB_URL="jdbc:mysql://localhost/TEST";

      //  数据库的凭据
      static final String USER = "root";
      static final String PASS = "password";

      // 设置响应内容类型
      response.setContentType("text/html");
      PrintWriter out = response.getWriter();
      String title = "数据库结果";
      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");
      try{
         // 注册 JDBC 驱动器
         Class.forName("com.mysql.jdbc.Driver");

         // 打开一个连接
         conn = DriverManager.getConnection(DB_URL,USER,PASS);

         // 执行 SQL 查询
         stmt = conn.createStatement();
         String sql;
         sql = "SELECT id, first, last, age FROM Employees";
         ResultSet rs = stmt.executeQuery(sql);

         // 从结果集中提取数据
         while(rs.next()){
            // 根据列名称检索
            int id  = rs.getInt("id");
            int age = rs.getInt("age");
            String first = rs.getString("first");
            String last = rs.getString("last");

            // 显示值
            out.println("ID: " + id + "<br>");
            out.println(", Age: " + age + "<br>");
            out.println(", First: " + first + "<br>");
            out.println(", Last: " + last + "<br>");
         }
         out.println("</body></html>");

         // 清理环境
         rs.close();
         stmt.close();
         conn.close();
      }catch(SQLException se){
         // 处理 JDBC 错误
         se.printStackTrace();
      }catch(Exception e){
         // 处理 Class.forName 错误
         e.printStackTrace();
      }finally{
         // 最后是用于关闭资源的块
         try{
            if(stmt!=null)
               stmt.close();
         }catch(SQLException se2){
         }// 我们不能做什么
         try{
            if(conn!=null)
            conn.close();
         }catch(SQLException se){
            se.printStackTrace();
         }//end finally try
      } //end try
   }
}

Now let us compile the above Servlet and create the following entry in the web.xml file:

....
 <servlet>
     <servlet-name>DatabaseAccess</servlet-name>
     <servlet-class>DatabaseAccess</servlet-class>
 </servlet>
 
 <servlet-mapping>
     <servlet-name>DatabaseAccess</servlet-name>
     <url-pattern>/DatabaseAccess</url-pattern>
 </servlet-mapping>
....

Now call this Servlet and enter the link: http://localhost:8080/ DatabaseAccess, the following response results will be displayed:

数据库结果
ID: 100, Age: 18, First: Zara, Last: Ali
ID: 101, Age: 25, First: Mahnaz, Last: Fatma
ID: 102, Age: 30, First: Zaid, Last: Khan
ID: 103, Age: 28, First: Sumit, Last: Mittal