模板模式讲解一

一、模板

模板，很经典、也很常用的一种模式。无处不在，如，spring jdbcTemplate。模板，个人理解， 讲大同小异的东西抽出它们的共性，提供标准的套路。

二、为什么要用模板？

假设场景，我们需要查询个人资料信息，现提供两个api，getPersonById(int id)、 getPersonListByName(String name)

1、我们先来看下不使用模板的情况

①、注册jdbc、建立连接

static final String DB_URL = "jdbc:mysql://localhost/testdb";
    static final String USER = "root";
    static final String PWD = "root";

    private static Connection connection = null;

    static {
        try {
            // 注册JDBC驱动程序：以打开与数据库的通信信道
            Class.forName("com.mysql.jdbc.Driver");
            // 获取连接
            connection = DriverManager.getConnection(DB_URL, USER, PWD);
        } catch (ClassNotFoundException e) {
            throw new RuntimeException("ClassNotFoundException");
        } catch (SQLException e) {
            throw new RuntimeException("SQLException");
        }
    }

②、根据Id查询

public static List<Person> getPersonById(int id) {
        List<Person> res = new ArrayList<Person>();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            String sql = "select * from person where id = ?";
            ps = (PreparedStatement) connection.prepareStatement(sql);
            ps.setInt(1, id);
            rs = ps.executeQuery();
            while (rs.next()) {
                res.add(new Person(rs.getInt("id"), rs.getString("name")));
            }
            return res;
        } catch (SQLException e) {

        } finally {
            if (connection != null) {
                try {
                    connection.close();
                } catch (SQLException e) {

                }
            }
            if (ps != null) {
                try {
                    ps.close();
                } catch (SQLException e) {

                }
            }
            if (rs != null) {
                try {
                    rs.close();
                } catch (SQLException e) {

                }
            }
        }
        return null;
    }

③、根据名字查询

public static List<Person> getPersonListByName(String name) {
        List<Person> res = new ArrayList<Person>();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            String sql = "select * from person where name = ?";
            ps = (PreparedStatement) connection.prepareStatement(sql);
            ps.setString(1, name);
            rs = ps.executeQuery();
            while (rs.next()) {
                res.add(new Person(rs.getInt("id"), rs.getString("name")));
            }
            return res;
        } catch (SQLException e) {

        } finally {
            if (connection != null) {
                try {
                    connection.close();
                } catch (SQLException e) {

                }
            }
            if (ps != null) {
                try {
                    ps.close();
                } catch (SQLException e) {

                }
            }
            if (rs != null) {
                try {
                    rs.close();
                } catch (SQLException e) {

                }
            }
        }
        return null;
    }

从②、③我们可以看出来，都一段冗余的判断关闭连接等代码，而且这些非业务性代码不应该浸入系统。代码看起来也不是很清晰。那有没有办法改进呢？答案当然是有的，模板可以很好解决这种诟病。

2、使用模板+回调结果（业务按照自身的需求封装，灵活性高）的场景

①、定义一个回调接口

public interface AssemblyResService<T> {

    public T assemblyRes(ResultSet rs) throws SQLException;
}

②、采用匿名类的方式

根据id查询

public static List<Person> getPersonById(int id) {
        List<Person> res = new ArrayList<Person>();
        try {
            String sql = "select * from person where id = ?";
            res = queryTemp(sql, new Object[]{id}, new AssemblyResService<Person>() {
                
                @Override
                public Person assemblyRes(ResultSet rs) throws SQLException {
                    return new Person(rs.getInt("id"), rs.getString("name"));
                }
            });
            return res;
        } catch (Exception e) {
            
        }
        return null;
    }

根据名字查询

public static List<Person> getPersonListByName(String name) {
        List<Person> res = new ArrayList<Person>();
        try {
            String sql = "select * from person where name = ?";
            res = queryTemp(sql, new Object[]{name}, new AssemblyResService<Person>() {
                
                @Override
                public Person assemblyRes(ResultSet rs) throws SQLException {
                    return new Person(rs.getInt("id"), rs.getString("name"));
                }
            });
            return res;
        } catch (Exception e) {
            
        }
        return null;
    }

③、模板（类似spring jdbcTemplate）

public static List<Person> queryTemp(String sql, Object[] params, AssemblyResService<Person> resService) {
        List<Person> res = new ArrayList<Person>();
        PreparedStatement ps = null;
        ResultSet rs = null;
        try {
            ps = (PreparedStatement) connection.prepareStatement(sql);
            for(int i=0; i<params.length; i++) {
                ps.setObject(i+1, params[i]);
            }
            rs = ps.executeQuery();
            while (rs.next()) {
                res.add(resService.assemblyRes(rs));
            }
            return res;
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                try {
                    connection.close();
                } catch (SQLException e) {

                }
            }
            if (ps != null) {
                try {
                    ps.close();
                } catch (SQLException e) {

                }
            }
            if (rs != null) {
                try {
                    rs.close();
                } catch (SQLException e) {

                }
            }
        }
        return null;
    }