帮我用java代码写一个serverlet可以对数据库进行增删改查

好的，下面是一个简单的Java Servlet示例代码，可以进行数据库的增删改查操作。假设数据库使用MySQL，并且已经连接成功。

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.*;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/DBServlet")
public class DBServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    private Connection conn;
    private Statement stmt;

    public void init() throws ServletException {
        // 连接数据库
        String url = "jdbc:mysql://localhost:3306/test";
        String username = "root";
        String password = "123456";
        try {
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection(url, username, password);
            stmt = conn.createStatement();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();

        // 查询数据库
        String sql = "select * from user";
        ResultSet rs = null;
        try {
            rs = stmt.executeQuery(sql);
            while (rs.next()) {
                out.println("id=" + rs.getInt("id") + ", name=" + rs.getString("name") + ", age="
                        + rs.getInt("age") + "<br>");
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null)
                    rs.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();

        // 插入数据
        String name = request.getParameter("name");
        int age = Integer.parseInt(request.getParameter("age"));
        String sql = "insert into user(name,age) values('" + name + "'," + age + ")";
        try {
            stmt.executeUpdate(sql);
            out.println("插入成功");
        } catch (SQLException e) {
            e.printStackTrace();
        }

        // 更新数据
        int id = Integer.parseInt(request.getParameter("id"));
        sql = "update user set name='" + name + "', age=" + age + " where id=" + id;
        try {
            stmt.executeUpdate(sql);
            out.println("更新成功");
        } catch (SQLException e) {
            e.printStackTrace();
        }

        // 删除数据
        id = Integer.parseInt(request.getParameter("id"));
        sql = "delete from user where id=" + id;
        try {
            stmt.executeUpdate(sql);
            out.println("删除成功");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public void destroy() {
        // 关闭数据库连接
        try {
            if (stmt != null)
                stmt.close();
            if (conn != null)
                conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

在上面的代码中，我们使用了`init()`方法来连接数据库，使用`doGet()`方法来查询数据，使用`doPost()`方法来插入、更新和删除数据，使用`destroy()`方法来关闭数据库连接。其中，`request.getParameter()`方法用于获取前端页面传来的参数，`stmt.executeUpdate()`方法用于执行增删改操作，`stmt.executeQuery()`方法用于执行查询操作。

需要注意的是，上面的代码仅仅是一个简单的示例，实际项目中需要考虑更多的异常处理、安全性等问题。