前言

模板模式是指定义一个算法的骨架，并允许子类为一个或者多个步骤提供实现。JDBC操作业务是运用模板模式一个很典型的场景，下面我们就利用模板模式重构JDBC操作业务场景

案例

我们以查询为例，首先我们定义一个RowMapper接口，这个接口定义了mapRow方法，用于实现实体关系映射
RowMapper：

public interface RowMapper<T> {
    T mapRow(ResultSet rs,int rowNum) throws Exception;
}

接下来，我们定义JdbcTemplate这个模板类，在模板类的查询方法中，我们定义实现查询的骨架，实现共同的部分，并且根据传递的参数实现差异化的实现
JdbcTemplate ：

public class JdbcTemplate {
    private DataSource dataSource;
    public JdbcTemplate(DataSource dataSource) {
        this.dataSource = dataSource;
    }
    public List<?> executeQuery(String sql, RowMapper<?> rowMapper, Object[] values) {
        try{
            Connection conn = this.getConnection();
            PreparedStatement pstm = this.createPrepareStatement(conn,sql);
            ResultSet rs = this.executeQuery(pstm,values);
            List<?> result = this.paresResultSet(rs,rowMapper);
            this.closeResultSet(rs);
            this.closeStatement(pstm);
            this.closeConnection(conn);
            return result;
        } catch (Exception e){
            e.printStackTrace();
        }
        return null;
    }

    protected void closeConnection(Connection conn) throws Exception {
        conn.close();
    }
    protected  void closeStatement(PreparedStatement pstm) throws Exception{
        pstm.close();
    }
    protected void closeResultSet(ResultSet rs) throws Exception{
        rs.close();
    }
    protected List<?> paresResultSet(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;
    }
    protected ResultSet executeQuery(PreparedStatement pstm,Object[] values) throws Exception {
        for(int i=0;i<values.length;i++) {
            pstm.setObject(i,values[i]);
        }
        return pstm.executeQuery();
    }
    protected PreparedStatement createPrepareStatement(Connection conn,String sql) throws Exception {
        return conn.prepareStatement(sql);
    }
    public Connection getConnection() throws Exception {
        return this.dataSource.getConnection();
    }
}

定义model
Member ：

public class Member {
    private String username;
    private String password;
    private String nickName;

    private int age;
    private String addr;
    //get set方法
}

然后我们定义DAO继承这个模板类
MemberDao：

public class MemberDao extends JdbcTemplate{

    public MemberDao(DataSource dataSource) {
        super(dataSource);
    }

    public List<?> selectAll() {
        String sql = "select * from t_member";
        return super.executeQuery(sql, new RowMapper<Member>() {
            @Override
            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);
    }
}