SpringBoot之Jpa的getOne(ID id)方法

• 使用該方法根據主鍵ID獲取對象時

public News find(Long id){
    return newsRepository.getOne(id);
}

• 服務器端會報錯

{
    "timestamp": "2019-07-30T06:29:40.915+0000",
    "status": 500,
    "error": "Internal Server Error",
    "message": "Cannot call isReadOnlyBeforeAttachedToSession when isReadOnlySettingAvailable == true [cn.maidaotech.springbootdemo.news.model.News#10001]",
    "path": "/news/news"
}

• 查看getOne方法的源碼

/**
 * Returns a reference to the entity with the given identifier. Depending on how the JPA persistence provider is
 * implemented this is very likely to always return an instance and throw an
 * {@link javax.persistence.EntityNotFoundException} on first access. Some of them will reject invalid identifiers
 * immediately.
 *
 * @param id must not be {@literal null}.
 * @return a reference to the entity with the given identifier.
 * @see EntityManager#getReference(Class, Object) for details on when an exception is thrown.
 */
T getOne(ID id);

• 發現該方法返回的是實體對象的代理對象（a reference）。
• 怎么辦呢？CrudRepository接口中有一個findById()方法，源碼如下：

/**
 * Retrieves an entity by its id.
 *
 * @param id must not be {@literal null}.
 * @return the entity with the given id or {@literal Optional#empty()} if none found
 * @throws IllegalArgumentException if {@code id} is {@literal null}.
 */
Optional<T> findById(ID id);

• 該方法返回的是Optional ，在Optional類中有個get()方法，返回的是當前對象/值。
• 我們的代碼修改為

public News find(Long id){
    return newsRepository.findById(id).get();
}