设计模式之模板模式

1. 模板模式使用场景

模板模式是大家常见的设计模式之一，相信你早已使用过JdbcTemplate、RestTemplate等，很显然这就是模板模式的体现。

 

2. 代码实现

我们以封装数据库操作的JdbcTemplate为例。

public class JdbcTemplate {

	private DataSource dataSource;
	
	public JdbcTemplate(DataSource dataSource) {
		this.dataSource = dataSource;
	}
	
	private Connection getConnection() throws SQLException {
		return this.dataSource.getConnection();
	}
	
	private PreparedStatement createPreparedStatement(Connection conn, String sql) throws SQLException {
		return  conn.prepareStatement(sql);
	}
	
	private ResultSet executeQuery(PreparedStatement ps, RowMapper<?> rowMapper, Object[] values) throws SQLException {
		for (int i = 0; i < values.length; i++) {
			ps.setObject(i,values[i]);
		}
		return ps.executeQuery();
	}

    private void closeStatement(Statement st) throws  Exception{
        st.close();
    }

    private void closeResultSet(ResultSet rs) throws  Exception{
        rs.close();
    }

    private void closeConnection(Connection conn) throws  Exception{
    	conn.close();
    }
	
    private List<Object> parseResultSet(ResultSet rs,RowMapper rowMapper) throws  Exception{
        List<Object> result = new ArrayList<Object>();
        int rowNum = 1;
        while (rs.next()){
            result.add(rowMapper.mapRow(rs,rowNum ++));
        }
        return result;
    }
    
	public List<Object> executeQuery(String sql,RowMapper<?> row,Object [] values){
		
		try {
			// 1.获取连接
			Connection conn = this.getConnection();
			// 2.创建语句集
			PreparedStatement ps = this.createPreparedStatement(conn,sql);
			// 3.执行语句集，获取结果集
			ResultSet res = ps.executeQuery();
			
			// 4.解析语句集
			List<Object> result = this.parseResultSet(res, null);
			
			// 5.关闭结果集
			this.closeResultSet(res);
			// 6.关闭语句集
			this.closeStatement(ps);
			
			// 7.关闭连接
			this.closeConnection(conn);
			
			
			return result;
		} catch (Exception e) {
			e.printStackTrace();
		}
		return null;
	}



	public DataSource getDataSource() {
		return dataSource;
	}

	public void setDataSource(DataSource dataSource) {
		this.dataSource = dataSource;
	}
	
	
}

public interface RowMapper<T> {

	public T mapRow(ResultSet rs, int rowNum) throws Exception;
	
	
}

public class Member {

    private String username;
    private String password;
    private String nickName;

    private int age;
    private String addr;

    //get&set略
}

//这里我们不进行继承JdbcTemplate类，原因是继承会重写很多方法
public class MemberDao{

	private JdbcTemplate jdbc = new JdbcTemplate(null);
	
	
	public List<Object> query(){
		String sql = "SELECT * FORM member ";
		return jdbc.executeQuery(sql,new RowMapper<Member>() {
			public Member mapRow(ResultSet rs, int rowNum) throws Exception{
				Member member = new Member();
				member.setUsername(rs.getString("username"));
				member.setPassword(rs.getString("password"));
				member.setAge(rs.getInt("age"));
				member.setAddr(rs.getString("addr"));
				return member;
			}
		},null);
	}

    public Object processResult(ResultSet rs, int rowNum)throws Exception {
        Member member = new Member();
        member.setUsername(rs.getString("username"));
        member.setPassword(rs.getString("password"));
        member.setAge(rs.getInt("age"));
        member.setAddr(rs.getString("addr"));
        return member;
    }
	
}

public class MemberDaoTest {

	public static void main(String[] args) {
		MemberDao memberDao = new MemberDao();
		memberDao.query();
	}
	
}

3. 总结

模板模式优点很明显，定义统一的执行标准，对于调用者不需要关系具体实现逻辑。它可以把公共的代码写在父类抽象类中，提高了代码的复用性，减少重复代码。父类进行行为管控，在子类中可以对父类方法进行扩展，具有扩展性，不同子类方法处理的灵活性。

模板模式的缺点是：由于继承关系父类的方法，子类都要重写一遍。