save() 方法

Spring Data JPA 写入数据可以通过CrudRepository的save()方法来实现。

public interface CrudRepository<T, ID> extends Repository<T, ID> {

    /**
     * Saves a given entity. Use the returned instance for further operations as the save operation might have changed the
     * entity instance completely.
     *
     * @param entity must not be {@literal null}.
     * @return the saved entity; will never be {@literal null}.
     * @throws IllegalArgumentException in case the given {@literal entity} is {@literal null}.
     */
    <S extends T> S save(S entity);

    ...
}

CrudRepository.save(...)通过调用EntityManager.persist(...)来实现插入一条新数据，通过调用EntityManager.merge(...)来实现更新旧数据。那么，Spring Data 如何判断要插入新数据还是更新旧数据呢？

Entity 的保存策略

Spring Data JPA 通过以下几个策略来判断新增数据还是更新数据。

@Id 属性（主键）

• 如果 @Id 为 null，则会新增一条数据
• 如果 @Id 为一个已存在的主键，则会修改该主键对应数据
• 如果 @Id 为一个不存在的主键值，则会抛出ConstraintViolationException异常

继承 Persistable

package org.springframework.data.domain;

import org.springframework.lang.Nullable;

/**
 * Simple interface for entities.
 * <p>
 * Note that methods declared in this interface ({@link #getId()} and {@link #isNew()}) become property accessors when
 * implementing this interface in combination with
 * {@link org.springframework.data.annotation.AccessType @AccessType(PROPERTY)}. Either of these can be marked as
 * transient when annotated with {@link org.springframework.data.annotation.Transient @Transient}.
 *
 * @param <ID> the type of the identifier
 * @author Oliver Gierke
 * @author Christoph Strobl
 * @author Mark Paluch
 */
public interface Persistable<ID> {

	/**
	 * Returns the id of the entity.
	 *
	 * @return the id. Can be {@literal null}.
	 */
	@Nullable
	ID getId();

	/**
	 * Returns if the {@code Persistable} is new or was persisted already.
	 *
	 * @return if {@literal true} the object is new.
	 */
	boolean isNew();
}

如果isNew()返回了true，则会插入一条新数据，反之，修改ID对应的旧数据。

实现 EntityInformation

不推荐。

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