JPA @OneToOne @OneToMany @ManyToOne @ManyToMany 描述及说明_(optional) whether the association should be lazil

@OneToOne

@Target({METHOD, FIELD}) 
@Retention(RUNTIME)

public @interface OneToOne {

    /** 
     * (Optional) The entity class that is the target of 
     * the association. 
     *
     * <p> Defaults to the type of the field or property 
     * that stores the association. 
     */
    Class targetEntity() default void.class;

    /**
     * (Optional) The operations that must be cascaded to 
     * the target of the association.
     *
     * <p> By default no operations are cascaded.
     */
    CascadeType[] cascade() default {};

    /** 
     * (Optional) Whether the association should be lazily 
     * loaded or must be eagerly fetched. The EAGER 
     * strategy is a requirement on the persistence provider runtime that 
     * the associated entity must be eagerly fetched. The LAZY 
     * strategy is a hint to the persistence provider runtime.
     */
    FetchType fetch() default EAGER;

    /** 
     * 如果设置为false时，其值必须不能为null
     * (Optional) Whether the association is optional. If set 
     * to false then a non-null relationship must always exist.
     */
    boolean optional() default true;

    /**
      *  拥有mappedBy注解的实体类为关系被维护端
      * (Optional) The field that owns the relationship. This 
      * element is only specified on the inverse (non-owning) 
      * side of the association.
     */
    String mappedBy() default "";


    /**
     * (Optional) Whether to apply the remove operation to entities that have
     * been removed from the relationship and to cascade the remove operation to
     * those entities.
     * @since 2.0
     */
    boolean orphanRemoval() default false;
}
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@OneToMany

注：

1. 在JPA规范中，一对多的双向关系由多端来维护，负责关系的增删改查；而一端为关系被维护端，不能维护关系
2. 关于级联一定要在关系的维护端设置，不能在被维护端设置

@Target({METHOD, FIELD}) 
@Retention(RUNTIME)

public @interface OneToMany {

    /**
     * (Optional) The entity class that is the target
     * of the association. Optional only if the collection
     * property is defined using Java generics.
     * Must be specified otherwise.
     *
     * <p> Defaults to the parameterized type of
     * the collection when defined using generics.
     */
    Class targetEntity() default void.class;

    /** 
     * (Optional) The operations that must be cascaded to 
     * the target of the association.
     * <p> Defaults to no operations being cascaded.
     *
     * <p> When the target collection is a {@link java.util.Map
     * java.util.Map}, the <code>cascade</code> element applies to the
     * map value.
     */
    CascadeType[] cascade() default {};

    /** (Optional) Whether the association should be lazily loaded or
     * must be eagerly fetched. The EAGER strategy is a requirement on
     * the persistence provider runtime that the associated entities
     * must be eagerly fetched.  The LAZY strategy is a hint to the
     * persistence provider runtime.
     */
    FetchType fetch() default LAZY;

    /** 
     * The field that owns the relationship. Required unless 
     * the relationship is unidirectional.
     */
    String mappedBy() default "";

    /**
     * (Optional) Whether to apply the remove operation to entities that have
     * been removed from the relationship and to cascade the remove operation to
     * those entities.
     * @since 2.0
     */
    boolean orphanRemoval() default false;
}
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@ManyToOne

@Target({METHOD, FIELD}) 
@Retention(RUNTIME)

public @interface ManyToOne {

    /** 
     * (Optional) The entity class that is the target of 
     * the association. 
     *
     * <p> Defaults to the type of the field or property 
     * that stores the association. 
     */
    Class targetEntity() default void.class;

    /**
     * (Optional) The operations that must be cascaded to 
     * the target of the association.
     *
     * <p> By default no operations are cascaded.
     */
    CascadeType[] cascade() default {};

    /** 
     * (Optional) Whether the association should be lazily 
     * loaded or must be eagerly fetched. The EAGER
     * strategy is a requirement on the persistence provider runtime that 
     * the associated entity must be eagerly fetched. The LAZY 
     * strategy is a hint to the persistence provider runtime.
     */
    FetchType fetch() default EAGER;

    /** 
     * (Optional) Whether the association is optional. If set 
     * to false then a non-null relationship must always exist.
     */
    boolean optional() default true;
}
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@ManyToMany

@Target({METHOD, FIELD}) 
@Retention(RUNTIME)
public @interface ManyToMany {

    /**
     * (Optional) The entity class that is the target of the
     * association. Optional only if the collection-valued
     * relationship property is defined using Java generics.  Must be
     * specified otherwise.
     *
     * <p> Defaults to the parameterized type of
     * the collection when defined using generics.
     */
    Class targetEntity() default void.class;

    /** 
     * (Optional) The operations that must be cascaded to the target
     * of the association.  
     *
     * <p> When the target collection is a {@link java.util.Map
     * java.util.Map}, the <code>cascade</code> element applies to the
     * map value.
     *
     * <p> Defaults to no operations being cascaded.
     */
    CascadeType[] cascade() default {};

    /** (Optional) Whether the association should be lazily loaded or
     * must be eagerly fetched. The EAGER strategy is a requirement on
     * the persistence provider runtime that the associated entities
     * must be eagerly fetched.  The LAZY strategy is a hint to the
     * persistence provider runtime.
     */
    FetchType fetch() default LAZY;

    /** 
     * The field that owns the relationship. Required unless 
     * the relationship is unidirectional.
     */
    String mappedBy() default "";
}
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