【Java】MyBatis注解SelectProvider等的使用

@SelectProvider(method=”xxx”,type=xxx.class)

MyBatis3提供的这个注解可以让程序员用自定义的类构造SQL语句（method属性为类中要执行获取sql语句的方法，type属性为获取sql语句的指定类）

---

实现

可以写一个通用的简单CRUD的接口，让mapper类去继承，这样就不用每个mapper都去写这些简单的CRUD的SQL了（类似MyBatis-Plus）

接口：
基本覆盖了所有简单的CRUD

public interface BaseMapper<T> {
     

    @SelectProvider(method = "selectById", type = BaseSelectMapperProvider.class)
    T selectById(Long id);

    @SelectProvider(method = "selectByIds", type = BaseSelectMapperProvider.class)
    List<T> selectByIds(@Param("ids") List<Long> ids);

    @SelectProvider(method = "count", type = BaseSelectMapperProvider.class)
    Long count(T entity);

    @SelectProvider(method = "select", type = BaseSelectMapperProvider.class)
    List<T> select(T entity);

    @SelectProvider(method = "selectPage", type = BaseSelectMapperProvider.class)
    List<T> selectPage(T entity, Long pageStart, Long pageSize);

    @InsertProvider(method = "save", type = BaseInsertMapperProvider.class)
    @Options(useGeneratedKeys = true)
    int save(T entity);

    @InsertProvider(method = "batchSave", type = BaseInsertMapperProvider.class)
    int batchSave(@Param("entityList") List<T> entityList);

    @UpdateProvider(method = "updateById", type = BaseUpdateMapperProvider.class)
    int updateById(T entity);

    @DeleteProvider(method = "deleteById", type = BaseDeleteMapperProvider.class)
    int deleteById(Long id);

    @DeleteProvider(method = "delete", type = BaseDeleteMapperProvider.class)
    int delete(T entity);

}

Provider实现类：

public class BaseSelectMapperProvider<T> {
     

	// 封装了通用的获取表名、泛型类型等的helper类
    private final BaseMapperHandler handler = new BaseMapperHandler();

    public String selectById(ProviderContext context){
     
        Class<?> clazz = handler.getEntityClass(context);
        String tableName = handler.getTableName(clazz);
        SQL sql = new SQL();
        sql.SELECT(handler.generateSelectColumnString(clazz));
        sql.FROM(tableName);
        sql.WHERE("id = #{id}");
        return sql.toString();
    }

    @SuppressWarnings({
     "unused", "unchecked"})
    public String selectByIds(Map<String, Object> map, ProviderContext context) {
     
        Class<?> clazz = handler.getEntityClass(context);
        List<String> list = new ArrayList<>();
        List<Long> ids = (List<Long>) map.get("ids");
        ids.forEach(id -> list.add(String.valueOf(id)));
        String tableName = handler.getTableName(clazz);
        SQL sql = new SQL();
        sql.SELECT(handler.generateSelectColumnString(clazz));
        sql.FROM(tableName);
        sql.WHERE("id in (" + String.join(",", list) + ")");
        return sql.toString();
    }

    public String select(T entity, ProviderContext context){
     
        Class<?> clazz = handler.getEntityClass(context);
        String tableName = handler.getTableName(clazz);
        SQL sql = new SQL();
        sql.SELECT(handler.generateSelectColumnString(clazz));
        sql.FROM(tableName);
        String whereSql = generateWhereSql(entity, clazz);
        if (null != whereSql) {
     
            sql.WHERE(whereSql);
        }
        return sql.toString();
    }

    public String count(T entity, ProviderContext context) {
     
        Class<?> clazz = handler.getEntityClass(context);
        String tableName = handler.getTableName(clazz);
        SQL sql = new SQL();
        sql.SELECT("count(id)");
        sql.FROM(tableName);
        String whereSql = generateWhereSql(entity, clazz);
        if (null != whereSql) {
     
            sql.WHERE(whereSql);
        }
        return sql.toString();
    }

    public String selectPage(T entity, Long pageStart, Long pageSize,ProviderContext context) {
     
        return select(entity, context) + " limit " + pageStart + ", " + pageSize;
    }

    private String generateWhereSql(T entity, Class<?> clazz){
     
        StringBuilder sql = new StringBuilder();
        Field[] fields = handler.getAllFields(clazz);
        int whereNum = 0;
        for (Field field : fields) {
     
            ColumnName columnNameAnnotation = field.getAnnotation(ColumnName.class);
            String fieldName;
            if (null != columnNameAnnotation) {
     
                fieldName = columnNameAnnotation.value();
            } else {
     
                fieldName = field.getName();
            }
            if (ObjectUtil.isNotNull(handler.getFieldValue(entity, field.getName()))) {
     
                sql.append(fieldName).append(" = #{").append(field.getName()).append("} and ");
                whereNum++;
            }
        }
        if (whereNum == 0) {
     
            return null;
        } else {
     
            return sql.substring(0, sql.length() - 5);
        }
    }

}

使用：
只要继承这个接口，传递实体类，就可以简单使用了。

@Mapper
public interface customerInfoMapper extends BaseMapper<customerInfo> {
     
}