SpringBoot 从入门到入门（八 ORM-JPA-4 Example查询）

// select userentity0_.id as id1_0_, userentity0_.email as email2_0_, userentity0_.name as name3_0_ from user userentity0_ where userentity0_.name=? and userentity0_.id=0
        UserEntity userEntity = new UserEntity();
        userEntity.setName("admin");
        Example<UserEntity> example = Example.of(userEntity);
        List<UserEntity> list = userRepository.findAll(example);

以findAll方法为例，可接收Example对象

	/*
	 * (non-Javadoc)
	 * @see org.springframework.data.repository.query.QueryByExampleExecutor#findAll(org.springframework.data.domain.Example)
	 */
	@Override
	<S extends T> List<S> findAll(Example<S> example);

	/*
	 * (non-Javadoc)
	 * @see org.springframework.data.repository.query.QueryByExampleExecutor#findAll(org.springframework.data.domain.Example, org.springframework.data.domain.Sort)
	 */
	@Override
	<S extends T> List<S> findAll(Example<S> example, Sort sort);

Example接口结构如下，可使用of方法获取实例

of(T probe) 等同于  of(T probe, ExampleMatcher.matching()) 

	/**
	 * Create a new {@link Example} including all non-null properties by default.
	 *
	 * @param probe must not be {@literal null}.
	 * @return
	 */
	static <T> Example<T> of(T probe) {
		return new TypedExample<>(probe, ExampleMatcher.matching());
	}

	/**
	 * Create a new {@link Example} using the given {@link ExampleMatcher}.
	 *
	 * @param probe must not be {@literal null}.
	 * @param matcher must not be {@literal null}.
	 * @return
	 */
	static <T> Example<T> of(T probe, ExampleMatcher matcher) {
		return new TypedExample<>(probe, matcher);
	}

ExampleMatcher接口结构如下

 示例

        // select userentity0_.id as id1_0_, userentity0_.email as email2_0_, userentity0_.name as name3_0_ from user userentity0_ where userentity0_.id=1
        UserEntity userEntity = new UserEntity();
        userEntity.setId(1);
        Example<UserEntity> example = Example.of(userEntity);
        List<UserEntity> list = userRepository.findAll(example);

ExampleMatcher.matching() 创建一个新的ExampleMatcher，默认情况下包含所有非NULL属性，以匹配从该示例派生的所有谓词。由于UserEntity中id初始值为0，不为NULL，所以下方示例中会有id字段的查询

// select userentity0_.id as id1_0_, userentity0_.email as email2_0_, userentity0_.name as name3_0_ from user userentity0_ where userentity0_.name=? and userentity0_.id=0
        UserEntity userEntity = new UserEntity();
        userEntity.setName("admin");
        Example<UserEntity> example = Example.of(userEntity);
        List<UserEntity> list = userRepository.findAll(example);

 可以使用ExampleMatcher指定不参与查询的字段

        // select userentity0_.id as id1_0_, userentity0_.email as email2_0_, userentity0_.name as name3_0_ from user userentity0_ where userentity0_.name=?
        UserEntity userEntity = new UserEntity();
        userEntity.setName("admin");
        ExampleMatcher exampleMatcher = ExampleMatcher.matching().withIgnorePaths("id");
        Example<UserEntity> example = Example.of(userEntity,exampleMatcher);
        List<UserEntity> list = userRepository.findAll(example);

可以使用ExampleMatcher的其他方法来实现稍微复杂的查询