如何使用JDBC实现数据访问对象层（DAO）

JAVA是面向对象的语言，开发者在操作数据的时候，通常更习惯面对一个特定类型的对象，如一个用户就是一个
User类的对象。DAO层需要做的，就是为上层提供充分的对象支持，让上层再也看不到具体的数据，而是一个个活生生的对象。

增加，删除，查询和修改操作是DAO需要做的最基本的4项操作。查询一般需要提供遍历查询和id查询，对于遍历查询，DAO需要提供
User泛型的list对象，对于id查询则提供已经装配好数据的User对象，至于增加和修改操作，上层一般会提供一个User对象，DAO把
User对象中的数据使用Insert语句插入到表格中。删除操作则只需提供一个id即可

 class User{
	private long id;
	private String name;
	private String gender;
  public User(){
	super();
  }
  public User(long id,String name,String gender){
	super();
	this.id = id;
	this.name = name;
	this.gender = gender;
	}
	//get,set方法
  }

  //DAO类
  public class jdbcDao{
	static{
		try{
			Class.forName("com.mysql.jdbc.Driver");
		}catch(Exception e){
		e.printStackTrace();
			}
		}
	private Connection  getConn(){
		try{
			return DriverManager.getConnection("jdbc:mysql://localhost:3306:xe","root","password");

			}catch(Exception e){
			e.printStackTrace();
			}
		}
		return null;
	}
	private void release(ResultSet rs,Statement ps,Connection conn){
		if(rs!=null){
			try{
				rs.close();
			}catch(Exception e){
				e.printStackTrace();
			}
		}
		if(ps!=null){
			try{
				ps.close();
			}catch(Exception e){
				e.printStackTrace();
			}
		}
		if(conn!=null){
			try{
				conn.close();
			}catch(Exception e){
				e.printStackTrace();
			}
		}
	}
	//用ID获取用户对象
	public User getUserById(long id){
		ResultSet rs = null;
		PreparedStatement ps = null;
		Connection conn = null;
		String sql = "select * from user where id = ?";
		try{
			conn = this.getConnection();
			ps = conn.prepareStatement(sql);
			ps.setLong(1,id);
			rs = ps.executeQuery();
			if(rs.next()){
				//如果存在，则直接构建并返回用户对象
				User user = new User(rs.getLong("id"),rs.getString("name"),rs.getString("gender"));
				return user;
			}
		}catch(Exception e){
			e.printStackTrace();
		}finally{
			this.release(rs,ps,conn);
		}
		return null;
	}
	//查询所有用户
	public List<User> getAllUsers(){
		List<User> list = new ArrayList<User>();
		ResultSet rs = null;
		PreparedStatement ps = null;
		Connection conn = null;
		String sql = "select * from user ";
		try{
			conn = this.getConnection();
			ps = conn.prepareStatement(sql);
			rs = ps.executeQuery();
			//循环添加用户对象
			while(rs.next()){
				User user = new User(rs.getLong("id"),rs.getString("name"),rs.getString("gender"));
				list.add(user);
			}
		}catch(Exception e){
			e.printStackTrace();
		}finally{
			this.release(rs,ps,conn);
		}
		return list;
	}
	//修改用户数据
	public User updateUser(User user){
		PreparedStatement ps = null;
		Connection conn = null;
		String sql = "update user set id =?,name=?,gender=?";
	try{
			conn = this.getConnection();
			conn.setAutoCommit(false);
			ps = conn.prepareStatement(sql);
			ps.setLong(1,user.getId());
			ps.setString(2,user.getName());
			ps.setString(3,user.getGender());
			int rst = ps.executeUpdate();
			if(rst>0){
				return  new User(user.getId(),user.getName(),user.getGender());

			}
			conn.commit();
		}catch(Exception e){
			e.printStackTrace();
			try{
				conn.rollback();
			}catch(Exception e1){
				e1.printStackTrace();
			}
		}finally{
			this.release(null,ps,conn);
		}
		return null;
			}
		}

		//删除用户数据
	public boolean deleteUser(long id){
		PreparedStatement ps = null;
		Connection conn = null;
		String sql = "delete from user where id =?;
	try{
			conn = this.getConnection();
			conn.setAutoCommit(false);
			ps = conn.prepareStatement(sql);
			ps.setLong(1,user.getId());
			ps.setString(2,user.getName());
			ps.setString(3,user.getGender());
			int rst = ps.executeUpdate();
			if(rst>0){
				return user;
			}
			conn.commit();
		}catch(Exception e){
			e.printStackTrace();
			try{
				conn.rollback();
			}catch(Exception e1){
				e1.printStackTrace();
			}
		}finally{
			this.release(null,ps,conn);
		}
		return null;
			}
		}

		//插入用户数据
		public User insertUser(User user){
		PreparedStatement ps = null;
		Connection conn = null;
		String sql = "insert into user values(?,?,?)";
	try{
			conn = this.getConnection();
			conn.setAutoCommit(false);
			ps = conn.prepareStatement(sql);
			ps.setLong(1,user.getId());
			ps.setString(2,user.getName());
			ps.setString(3,user.getGender());
			int rst = ps.executeUpdate();
			if(rst>0){
				return user;
			}
			conn.commit();
		}catch(Exception e){
			e.printStackTrace();
			try{
				conn.rollback();
			}catch(Exception e1){
				e1.printStackTrace();
			}
		}finally{
			this.release(null,ps,conn);
		}
		return null;
			}
		}

	}
  }