java - @Embedded 对象如果没有基本数据类型字段则不会自动实例化

Question

基本问题:为什么不总是实例化@Embedded 对象？

有趣的观察是，如果 @Embedded 对象不包含基本数据类型(int、boolean...)或之前未接触过，Ebean 不会实例化这些对象。示例:

@Entity
public class Embedder {
    // getNotAutoInstantiated() will return null if this field was not touched before
    @Embedded
    private NotAutoInstantiated notAutoInstantiated = new NotAutoInstantiated();
    // getAutoInstantiated() will always return an instance!
    @Embedded
    private AutoInstantiated autoInstantiated = new AutoInstantiated();
}

@Embeddable
public class AutoInstantiated {
    // theKey is why this embedded object is always instantiated
    private int theKey; 
    private String field1;      
}

@Embeddable
public class NotAutoInstantiated {
    private String field2;      
}

Answer 1

对于 Hibernate，您可能需要查看问题 HHH-7610 .

特别是，从 5.1 开始，有一个实验性功能可以改变这种行为。请注意，此功能存在已知问题，在稳定之前不应在生产中使用。 org.hibernate.cfg.AvailableSettings 的 Javadocs 对此进行了详细说明):

/**
 * [EXPERIMENTAL] Enable instantiation of composite/embedded objects when all of its attribute values are {@code null}.
 * The default (and historical) behavior is that a {@code null} reference will be used to represent the
 * composite when all of its attributes are {@code null}
 * <p/>
 * This is an experimental feature that has known issues. It should not be used in production
 * until it is stabilized. See Hibernate Jira issue HHH-11936 for details.
 *
 * @since 5.1
 */
String CREATE_EMPTY_COMPOSITES_ENABLED = "hibernate.create_empty_composites.enabled";

将 hibernate.create_empty_composites.enabled 属性设置为 true，瞧瞧!