前言

使用的是比较原始的方法写一个web后端Demo.
简单介绍一下：
后端操作数据库使用的是BaseDao，后端接口用的是Servlet。
基本功能：增删改查
工具：IDEA、MySQL、Jdk1.8、Postman

项目结构截图

项目结构.png

数据库

DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT 'ID',
  `name` varchar(10) DEFAULT NULL COMMENT '姓名',
  `age` int(11) DEFAULT NULL COMMENT '年龄',
  `sex` varchar(10) DEFAULT NULL COMMENT '性别',
  `school` varchar(20) DEFAULT NULL COMMENT '学校',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=19 DEFAULT CHARSET=utf8 COMMENT='学生信息表--测试';

BaseDao

BaseDao代码稍微有点长....

public abstract class BaseDao<T> {
  public final String dirver = "com.mysql.jdbc.Driver";
  public final String name = "root";
  public final String pass = "123456";
  public final String url = "jdbc:mysql://localhost:3306/student";
  Class<T> clazz;
  //反射获得clazz
  @SuppressWarnings("unchecked")
  public BaseDao() {
    clazz = (Class<T>) ((ParameterizedType) this.getClass()
        .getGenericSuperclass()).getActualTypeArguments()[0];
  }
  // DQL 带参数的查询
  public List<T> executeQuery(String sql, Object[] obj) {
    Connection conn = null;
    PreparedStatement stat = null;
    ResultSet rs = null;
    List<T> list = new ArrayList<T>();
    try {
      conn = createConn();
      // 3、创建传输对象statmemnt
      stat = conn.prepareStatement(sql);// ？不确定:类型、数量
      // 3+、绑定替换数据
      for (int i = 0; i < obj.length; i++) {
        stat.setObject(i + 1, obj[i]);
      }
      // 4、发送sql语句，并且接收返回结果 : DML -> executeUpdate ; DQL -> executeQuery
      rs = stat.executeQuery();
      // 5、如果返回rs类型的数据，需要将数据转换成list
      ResultSetMetaData rsmd = rs.getMetaData();//列名信息
      int columuCount = rsmd.getColumnCount();//列的数量
      while (rs.next()) {
        T t = (T) clazz.newInstance();//创建对象
        for (int i = 0; i < columuCount; i++) {//封装数据
          Field f = clazz.getDeclaredField(rsmd.getColumnName(i + 1));//列名->属性名->属性对象
          f.setAccessible(true);
          f.set(t, rs.getObject(i + 1));//将rs列中的值赋给属性
        }
        list.add(t);
      }
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      closeAll(rs, stat, conn);
    }
    return list;
  }
  // 关闭一切
  public void closeAll(ResultSet rs, Statement stat, Connection conn) {
    // 6、关闭一切
    try {
      if (rs != null)
        rs.close();
      if (stat != null)
        stat.close();
      if (conn != null)
        conn.close();
    } catch (SQLException e) {
      e.printStackTrace();
    }
  }
  // DML  增删改
  public int executeUpdate(String sql, Object[] obj) {
    Connection conn = null;
    PreparedStatement stat = null;
    int ret = 0;
    try {
      conn = createConn();
      // 3、创建传输对象statmemnt
      stat = conn.prepareStatement(sql);// ？不确定:类型、数量
      // 3+、绑定替换数据
      for (int i = 0; i < obj.length; i++) {
        stat.setObject(i + 1, obj[i]);
      }
      // 4、发送sql语句，并且接收返回结果 : DML -> executeUpdate ; DQL -> executeQuery
      ret = stat.executeUpdate();
      // 5、如果返回rs类型的数据，需要将数据转换成list
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      closeAll(stat, conn);
    }
    return ret;
  }
  // 创建连接
  public Connection createConn() {
    Connection conn = null;
    try {
      // 1、加载驱动
      Class.forName(dirver);
      // 2、创建连接connection
      conn = DriverManager.getConnection(url, name, pass);
      System.out.println("数据库连接成功！");
    } catch (Exception e) {
      e.printStackTrace();
      System.out.println("数据库连接失败！");
    }
    return conn;
  }
  // 关闭一切
  public void closeAll(Statement stat, Connection conn) {
    // 6、关闭一切
    try {
      if (stat != null)
        stat.close();
      if (conn != null)
        conn.close();
    } catch (SQLException e) {
      e.printStackTrace();
    }
  }
  // DQL 不带参数查询
  protected List<T> executeQuery(String sql) {
    Connection conn = null;
    PreparedStatement stat = null;
    ResultSet rs = null;
    List<T> list = new ArrayList<T>();
    try {
      conn = createConn();
      // 3、创建传输对象statmemnt
      stat = conn.prepareStatement(sql);// ？不确定:类型、数量
      // 3+、绑定替换数据
      // 4、发送sql语句，并且接收返回结果 : DML -> executeUpdate ; DQL -> executeQuery
      rs = stat.executeQuery();
      // 5、如果返回rs类型的数据，需要将数据转换成list
      ResultSetMetaData rsmd = rs.getMetaData();//列名信息
      int columuCount = rsmd.getColumnCount();//列的数量
      while (rs.next()) {
        T t = (T) clazz.newInstance();//创建对象
        for (int i = 0; i < columuCount; i++) {//封装数据
          try {
            Field f = clazz.getDeclaredField(rsmd.getColumnName(i + 1));//列名->属性名->属性对象
            f.setAccessible(true);
            f.set(t, rs.getObject(i + 1));//将rs列中的值赋给属性
          } catch (Exception e) {
            System.out.println("对象中没有该属性：" + rsmd.getColumnName(i + 1));
          }
        }
        list.add(t);
      }
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      closeAll(rs, stat, conn);
    }
    return list;
  }
  // DQL
  protected int getRecordCount(String sql) {//select count(*) from msg;
    Connection conn = null;
    PreparedStatement stat = null;
    ResultSet rs = null;
    int count = 0;
    try {
      conn = createConn();
      // 3、创建传输对象statmemnt
      stat = conn.prepareStatement(sql);// ？不确定:类型、数量
      // 3+、绑定替换数据
      // 4、发送sql语句，并且接收返回结果 : DML -> executeUpdate ; DQL -> executeQuery
      rs = stat.executeQuery();
      // 5、如果返回rs类型的数据，需要将数据转换成list
      if (rs.next()) {
        count = rs.getInt(1);
      }
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      closeAll(rs, stat, conn);
    }
    return count;
  }
}

StudentDao

public interface StudentDao {
  /**
   * 获取所有的学生
   * @return
   */
  List<Student> getAllStudent();
  /**
   * 添加学生
   * @param student
   * @return
   */
  int insertStudent(Student student);
  /**
   * 根据id删除学生
   * @param id
   * @return
   */
  int deleteStudentById(Integer id);
  /**
   * 根据id修改学生信息
   * @param student
   * @return
   */
  int updateStudentById(Student student);
  /**
   * 获取学生总数
   * @return
   */
  int getStudentCount();
}

StudentDaoImpl

/**
 * @author wangxl
 * @ClassName StudentDaoImpl
 * @Description TODO
 * @date 2019/10/23 10:50
 */
public class StudentDaoImpl extends BaseDao<Student> implements StudentDao {
  @Override
  public List<Student> getAllStudent() {
     String sql = "select * from student";
    return executeQuery(sql);
  }
  @Override
  public int insertStudent(Student student) {
    String sql = "insert into student(name,age,sex,school) values(?,?,?,?)";
    return executeUpdate(sql,new Object[]{student.getName(),student.getAge(),student.getSex(),student.getSchool()});
  }
  @Override
  public int deleteStudentById(Integer id) {
    String  sql = "delete from student where id = ?";
    return executeUpdate(sql,new Object[]{id});
  }
  @Override
  public int updateStudentById(Student student) {
    String sql = "update student set name = ? , age = ? ,sex = ? ,school = ? where id = ?";
    return executeUpdate(sql,new Object[]{student.getName(),student.getAge(),student.getSex(),student.getSchool(),student.getId()});
  }
  @Override
  public int getStudentCount() {
    String sql = "select count(*) from student";
    return getRecordCount(sql);
  }
}

查询 GetAllStudentServlet

@WebServlet("/getAllStudent")
public class GetAllStudentServlet extends HttpServlet {
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
      super.doGet(request,response);
  }
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");
    //返回结果
    Map<String,Object> map = new HashMap<>();
    StudentDao studentDao = new StudentDaoImpl();
    List<Student>  list = studentDao.getAllStudent();
    if(list.size()>=0){
      map.put("code",200);
      map.put("msg","返回结果成功");
      map.put("data",list);
      map.put("count",studentDao.getStudentCount());
    }else{
      map.put("code",500);
      map.put("msg","返回结果成失败");
      map.put("data",null);
    }
    JSONObject json = JSONObject.fromObject(map);
    PrintWriter out = response.getWriter();
    out.print(json.toString());
    out.flush();
    out.close();
  }
}

第一次查询结果.png

出现了乱码？？？
别慌！
我们在GetAllStudentServlet.java里面加上这一句，就OK了。或者写一个字符过滤器就好了。

    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");

再查询一次！

第二次查询结果.png

删除 DeleteStudentByIdServlet

@WebServlet("/deleteStudentById")
public class DeleteStudentByIdServlet extends HttpServlet {

  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    super.doGet(request,response);
  }
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    //字符过滤
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");

    //返回结果
    Map<String,Object> map = new HashMap<>();
    //获取传入的参数
    String id = request.getParameter("id");
    StudentDao studentDao = new StudentDaoImpl();
    int result = studentDao.deleteStudentById(Integer.parseInt(id));
    if(result==1){
      map.put("code",200);
      map.put("msg","删除成功");
      map.put("data",result);
    }else{
      map.put("code",500);
      map.put("msg","删除失败");
      map.put("data",result);
    }

    JSONObject json = JSONObject.fromObject(map);
    PrintWriter out = response.getWriter();
    out.print(json);
    out.flush();
    out.close();
  }
}

删除.png

修改 UpdateStudentServlet

@WebServlet("/updateStudent")
public class UpdateStudentServlet extends HttpServlet {
  @Override
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    //字符过滤
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");
    //返回结果
    Map<String,Object> map = new HashMap<>();
    Student student = new Student();
    //获取传入的参数
    String id =request.getParameter("id");
    String name =request.getParameter("name");
    String age =request.getParameter("age");
    String sex =request.getParameter("sex");
    String school =request.getParameter("school");
    //给对象赋值
    student.setAge(Integer.parseInt(age));
    student.setName(name);
    student.setSchool(school);
    student.setSex(sex);
    student.setId(Integer.parseInt(id));
    StudentDao studentDao = new StudentDaoImpl();
    int result = studentDao.updateStudentById(student);
    if(result==1){
      map.put("code",200);
      map.put("msg","修改成功");
      map.put("data",result);
    }else{
      map.put("code",500);
      map.put("msg","修改失败");
      map.put("data",result);
    }
    //封装成json
    JSONObject json = JSONObject.fromObject(map);
    PrintWriter out = response.getWriter();
    out.print(json);
    out.flush();
    out.close();
  }
  @Override
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    super.doPost(request,response);
  }
}

修改.png
修改完后，去数据库瞄了一眼，结果：哦吼，完蛋！！
修改后的数据库.png

解决办法：
就是在你的tomcat的conf下面的server.xml修改即可。

useBodyEncodingForURI="true" URIEncoding="UTF-8"

修改位置.png

添加 insertStudenServlet

@WebServlet("/insertStudent")
public class insertStudenServlet extends HttpServlet {
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    //字符过滤
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");

    //返回结果
    Map<String,Object> map = new HashMap<>();
    Student student = new Student();
    //获取传入的参数
    String name =request.getParameter("name");
    String age =request.getParameter("age");
    String sex =request.getParameter("sex");
    String school =request.getParameter("school");

    //给对象赋值
    student.setAge(Integer.parseInt(age));
    student.setName(name);
    student.setSchool(school);
    student.setSex(sex);

    StudentDao studentDao = new StudentDaoImpl();
    int result = studentDao.insertStudent(student);

    if(result==1){
      map.put("code",200);
      map.put("msg","添加成功");
      map.put("data",result);
    }else{
      map.put("code",500);
      map.put("msg","添加失败");
      map.put("data",result);
    }
    //封装成json
    JSONObject json = JSONObject.fromObject(map);
    PrintWriter out = response.getWriter();
    out.print(json);
    out.flush();
    out.close();

  }
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    super.doPost(request,response);
  }
}

添加.png