模板模式讲解

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

 1 static final String DB_URL = "jdbc:mysql://localhost/testdb";
 2     static final String USER = "root";
 3     static final String PWD = "root";
 4
 5     private static Connection connection = null;
 6
 7     static {
 8         try {
 9             // 注册JDBC驱动程序：以打开与数据库的通信信道
10             Class.forName("com.mysql.jdbc.Driver");
11             // 获取连接
12             connection = DriverManager.getConnection(DB_URL, USER, PWD);
13         } catch (ClassNotFoundException e) {
14             throw new RuntimeException("ClassNotFoundException");
15         } catch (SQLException e) {
16             throw new RuntimeException("SQLException");
17         }
18     }

②、根据Id查询

 1 public static List<Person> getPersonById(int id) {
 2         List<Person> res = new ArrayList<Person>();
 3         PreparedStatement ps = null;
 4         ResultSet rs = null;
 5         try {
 6             String sql = "select * from person where id = ?";
 7             ps = (PreparedStatement) connection.prepareStatement(sql);
 8             ps.setInt(1, id);
 9             rs = ps.executeQuery();
10             while (rs.next()) {
11                 res.add(new Person(rs.getInt("id"), rs.getString("name")));
12             }
13             return res;
14         } catch (SQLException e) {
15
16         } finally {
17             if (connection != null) {
18                 try {
19                     connection.close();
20                 } catch (SQLException e) {
21
22                 }
23             }
24             if (ps != null) {
25                 try {
26                     ps.close();
27                 } catch (SQLException e) {
28
29                 }
30             }
31             if (rs != null) {
32                 try {
33                     rs.close();
34                 } catch (SQLException e) {
35
36                 }
37             }
38         }
39         return null;
40     }

③、根据名字查询

 1 public static List<Person> getPersonListByName(String name) {
 2         List<Person> res = new ArrayList<Person>();
 3         PreparedStatement ps = null;
 4         ResultSet rs = null;
 5         try {
 6             String sql = "select * from person where name = ?";
 7             ps = (PreparedStatement) connection.prepareStatement(sql);
 8             ps.setString(1, name);
 9             rs = ps.executeQuery();
10             while (rs.next()) {
11                 res.add(new Person(rs.getInt("id"), rs.getString("name")));
12             }
13             return res;
14         } catch (SQLException e) {
15
16         } finally {
17             if (connection != null) {
18                 try {
19                     connection.close();
20                 } catch (SQLException e) {
21
22                 }
23             }
24             if (ps != null) {
25                 try {
26                     ps.close();
27                 } catch (SQLException e) {
28
29                 }
30             }
31             if (rs != null) {
32                 try {
33                     rs.close();
34                 } catch (SQLException e) {
35
36                 }
37             }
38         }
39         return null;
40     }

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;
    }