SpringJdbc之queryForXXX大全解读

Posted 2020-09-14 鼠标

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1.查询单个字段

Object queryForObject(String sql, Object[] args, Class requiredType)

其中Class requiredType 参数为返回值的类型（可以为常见的类库中的类类型）比如String.class

参数“requiredType”不可以是自定义的类

2.查询多个字段

Object queryForObject(String sql, Object[] args, RowMapper rowMapper)

其中Rowmapper 需要new一下给该接口一个类类型，它负责给你返回一个对象（需要什么值要自己set，建议全部set）

如果要把查询结果封装为自定义的类，需要采用第2个方法

//只查询一列：name
String sql = "SELECT NAME FROM CUSTOMER WHERE CUST_ID = ?";

String name = (String)getJdbcTemplate().queryForObject(
sql, new Object[] { custId }, String.class);

return name;

//查询返回自定义的类
String sql = "SELECT * FROM CUSTOMER WHERE CUST_ID = ?";

Customer customer = (Customer)getJdbcTemplate().queryForObject(
sql, new Object[] { custId },
new BeanPropertyRowMapper(Customer.class));

return customer;

我们接着分析下BeanPropertyRowMapper

其实不用看源码都知道思路了：
new BeanPropertyRowMapper(User.class)
mapRow(ResultSet rs, int rowNumber)

遍历ResultSet，每一行对应一个User 对象

遍历每一行时，顺序遍历各列，取得该列的值通过反射调用对应的setter方法，赋值给到User 对象

注意到无论是取列名还是取列的值，都是通过index（1~columnCount）来取的

public T mapRow(ResultSet rs, int rowNumber) throws SQLException {
                T mappedObject = BeanUtils.instantiate(this.mappedClass);
                BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(mappedObject);

                ResultSetMetaData rsmd = rs.getMetaData();
                int columnCount = rsmd.getColumnCount();
                Set<String> populatedProperties = (isCheckFullyPopulated() ? new HashSet<String>() : null);

                for (int index = 1; index <= columnCount; index++) {
                        String column = JdbcUtils.lookupColumnName(rsmd, index);
                        PropertyDescriptor pd = this.mappedFields.get(column.replaceAll(" ", "").toLowerCase());
                        if (pd != null) {
                                try {
                                        Object value = getColumnValue(rs, index, pd);
                                    bw.setPropertyValue(pd.getName(), value);
                                        if (populatedProperties != null) {
                                                populatedProperties.add(pd.getName());
                                        }
                                }
                        }
                }

                if (populatedProperties != null && !populatedProperties.equals(this.mappedProperties)) {
                        throw new InvalidDataAccessApiUsageException("Given ResultSet does not contain all fields " +
                                        "necessary to populate object of class [" + this.mappedClass + "]: " + this.mappedProperties);
                }

                return mappedObject;
        }

注意到mapRow方法里面，populatedProperties变量是用来检查是否所有的property都正确地实现了赋值
再看看BeanPropertyRowMapper的构造函数：

public BeanPropertyRowMapper(Class<T> mappedClass) {
initialize(mappedClass);
}

protected void initialize(Class<T> mappedClass) {
this.mappedClass = mappedClass;

/*保存field
注意到对于驼峰式命名的field：
例如，对于gameId, mappedFields 会同时保存"gameId"和"game_id"
因此在sql语句中，
select id as game_id, name from game和
select id as gameId, name from game的效果是一样的
*/
this.mappedFields = new HashMap<String, PropertyDescriptor>();

//保存property
this.mappedProperties = new HashSet<String>();
PropertyDescriptor[] pds = BeanUtils.getPropertyDescriptors(mappedClass);
for (PropertyDescriptor pd : pds) {
if (pd.getWriteMethod() != null) {
this.mappedFields.put(pd.getName().toLowerCase(), pd);
String underscoredName = underscoreName(pd.getName());
if (!pd.getName().toLowerCase().equals(underscoredName)) {
this.mappedFields.put(underscoredName, pd);
}
this.mappedProperties.add(pd.getName());
}
}
}

//1. "game", return "game", unchanged
//2. "gameId", return "game_id"
private String underscoreName(String name) {
//......

}

二、查询返回对象或者list

思路：首先在Bean里面实现RowMapper和序列化接口（序列化可有可无）

接着在maprow方法中将rs取出来的值set到当前Bean对象里面

当然命名也要响应的变化下比如：UserPo,然后返回该对象最

后在new 的RowMapper接口中直接new 该Po就ok

【此方法适用于查询单个对象，或者多个对象组成的List或者Map】






/**
 * 实现数据表与字段的映射
 * 
 * @author andy
 * 
 */
public class UserInfo implements RowMapper<UserInfo>, Serializable {

    /**
     * 
     */
    private static final long serialVersionUID = -8823504831198719837L;

    private Integer id;

    private String uname;

    private Integer unumber;

    private Date uRegisterTime;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getUname() {
        return uname;
    }

    public void setUname(String uname) {
        this.uname = uname == null ? null : uname.trim();
    }

    public Integer getUnumber() {
        return unumber;
    }

    public void setUnumber(Integer unumber) {
        this.unumber = unumber;
    }

    public Date getuRegisterTime() {
        return uRegisterTime;
    }

    public void setuRegisterTime(Date uRegisterTime) {
        this.uRegisterTime = uRegisterTime;
    }

    @Override
    public UserInfo mapRow(ResultSet rs, int rowNum) throws SQLException {
        UserInfo userInfo = new UserInfo();
        userInfo.setId(rs.getInt("id"));
        userInfo.setUname(rs.getString("uname"));
        userInfo.setUnumber(rs.getInt("unumber"));
        userInfo.setuRegisterTime(rs.getDate("uregister_time"));
        return userInfo;
    }

}

@Autowired
    private JdbcTemplate jdbcTemplate;

    @Autowired
    private NamedParameterJdbcTemplate namedParameterJdbcTemplate;

    @Override
    public UserInfo getById(Integer id) {

        String sql = "SELECT * FROM user_info WHERE id = ?";

        UserInfo userInfo = jdbcTemplate.queryForObject(sql, new UserInfo(),
                new Object[] { id });

        return userInfo;
    }

    @Override
    public List<UserInfo> findAll() {
        String sql = "SELECT * FROM user_info";
        List<UserInfo> userInfos = jdbcTemplate.query(sql, new UserInfo());
        return userInfos;
    }