Servlets 简明教程

Servlets - Database Access

本教程假定您了解 JDBC 应用程序的工作原理。在使用 servlet 访问数据库之前,确保您已经设置好了适当的 JDBC 环境以及数据库。

要详细了解如何使用 JDBC 及其环境设置访问数据库,可以参阅我们的 JDBC Tutorial

首先了解基本概念,我们要创建一个简单表格并在该表格中创建一些记录,如下所示 −

Create Table

要创建 TEST 数据库中的 Employees 表格,请使用以下步骤 −

Step 1

打开 Command Prompt 并更改到安装目录,如下所示 −

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

Step 2

按照以下方法登录数据库:

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

Step 3

TEST 数据库中按照如下方式创建表 Employee

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 Records

最后,在 Employee 表中创建一些记录,如下所示 −

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>

Accessing a Database

以下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
   }
}

现在我们编译上述 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 [role="bare"] [role="bare"]http://localhost:8080/DatabaseAccess 调用此 servlet,它将显示以下响应:

Database Result
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