Mybatis存取数据格式转换

使用举例：

@TableField(typeHandler = StringListTypeHandler.class)
private List<String> topList;

1. 对需要转换类型的字段添加TableField注解指定对应执行当class（这个class一定要是对应此字段class类型的构造函数）

2. 别忘了在配置文件中配置转换文件所在包的位置

mybatis-plus.type-handlers-package: com.zpf.demo.global.handler

3. TypeHanlder源码：

@MappedTypes({Object.class})
@MappedJdbcTypes(JdbcType.VARCHAR)
public class JsonTypeHandler<T> extends BaseTypeHandler<T> {
    protected static final ObjectMapper objectMapper = new ObjectMapper();
    protected Class<T> type;
    public JsonTypeHandler(Class<T> type) {
        if (type == null) {
            throw new NullPointerException("Type argument cannot be null");
        }
        this.type = type;
    }
    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, Object parameter, JdbcType jdbcType) throws SQLException {
        ps.setString(i, toJsonString(parameter));
    }
    @Override
    public T getNullableResult(ResultSet rs, String columnName) throws SQLException {
        return parse(rs.getString(columnName));
    }
    @Override
    public T getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        return parse(rs.getString(columnIndex));
    }
    @Override
    public T getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        return parse(cs.getString(columnIndex));
    }
    protected String toJsonString(Object parameter) {
        try {
            return objectMapper.writeValueAsString(parameter);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        }
    }
    protected T parse(String json) {
        try {
            return objectMapper.readValue(json, type);
        } catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        }
    }
}

@MappedJdbcTypes(JdbcType.VARCHAR)
@MappedTypes({List.class})
public class StringListTypeHandler extends JsonTypeHandler<List<String>> {
    public StringListTypeHandler(Class<List<String>> type) {
        super(type);
    }
    @Override
    protected List<String> parse(String json) {
        try {
            return objectMapper.readValue(json, type);
        } catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        }
    }
}

其中MappedJdbcTypes注解描述对应数据库字段类型，MappedTypes注解描述转换字段类型了；

2020-10-08