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  Servlet - 数据库访问

  • 简述

    本教程假设您了解 JDBC 应用程序的工作原理。在开始通过 servlet 访问数据库之前，请确保您有正确的 JDBC 环境设置以及数据库。
    从基本概念开始，让我们创建一个简单的表并在该表中创建几条记录，如下所示 -

  • 创建表

    要创建 Employees TEST 数据库中的表，使用以下步骤 -

    第1步

    打开一个 Command Prompt 并更改到安装目录如下 - 
    
C:\>
C:\>cd Program Files\MySQL\bin
C:\Program Files\MySQL\bin>

    第2步

    登录数据库如下 
    
C:\Program Files\MySQL\bin>mysql -u root -p
Enter password: ********
mysql>

    第3步

    创建表 Employee 在 TEST 数据库如下 - 
    
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>

  • 创建数据记录

    最后，您在 Employee 表中创建了几条记录，如下所示 - 
    
mysql> INSERT INTO Employees VALUES (100, 18, 'Alex', 'Moo');
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>

  • 访问数据库

    这是一个示例，显示了如何使用 Servlet 访问 TEST 数据库。 
    
// Loading required libraries
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 driver name and database URL
      static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
      static final String DB_URL="jdbc:mysql://localhost/TEST";
      //  Database credentials
      static final String USER = "root";
      static final String PASS = "password";
      // Set response content type
      response.setContentType("text/html");
      PrintWriter out = response.getWriter();
      String title = "Database Result";
      
      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 {
         // Register JDBC driver
         Class.forName("com.mysql.jdbc.Driver");
         // Open a connection
         Connection conn = DriverManager.getConnection(DB_URL, USER, PASS);
         // Execute SQL query
         Statement stmt = conn.createStatement();
         String sql;
         sql = "SELECT id, first, last, age FROM Employees";
         ResultSet rs = stmt.executeQuery(sql);
         // Extract data from result set
         while(rs.next()){
            //Retrieve by column name
            int id  = rs.getInt("id");
            int age = rs.getInt("age");
            String first = rs.getString("first");
            String last = rs.getString("last");
            //Display values
            out.println("ID: " + id + "<br>");
            out.println(", Age: " + age + "<br>");
            out.println(", First: " + first + "<br>");
            out.println(", Last: " + last + "<br>");
         }
         out.println("</body></html>");
         // Clean-up environment
         rs.close();
         stmt.close();
         conn.close();
      } catch(SQLException se) {
         //Handle errors for JDBC
         se.printStackTrace();
      } catch(Exception e) {
         //Handle errors for Class.forName
         e.printStackTrace();
      } finally {
         //finally block used to close resources
         try {
            if(stmt!=null)
               stmt.close();
         } catch(SQLException se2) {
         } // nothing we can do
         try {
            if(conn!=null)
            conn.close();
         } catch(SQLException se) {
            se.printStackTrace();
         } //end finally try
      } //end try
   }
}
    现在让我们编译上面的 servlet 并在 web.xml 中创建以下条目 
    
....
<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>
....
    现在使用 URL http://localhost:8080/DatabaseAccess 调用这个 servlet，它将显示以下响应 - 
    
ID: 100, Age: 18, First: Alex, Last: Moo
ID: 101, Age: 25, First: Mahnaz, Last: Fatma
ID: 102, Age: 30, First: Zaid, Last: Khan
ID: 103, Age: 28, First: Sumit, Last: Mittal
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