Hibernate关联关系映射

本文参考

谈一点我对hibernate关联关系映射的看法

Hibernate关联关系映射--级联功能

Hibernate作为持久层框架，初始实现最基本的ORM之外，最重要的就是关系关联映射。通过配置实体间的关联，如one-to-one、one-to-many等，可以实现级联保存，级联更新，级联查询，级联删除等级联操作。在建关联关系表的时候，可以通过设置@OneToOne,@@OneToMany,@@ManyToOne,@ManyToMany的cascade()属性值，来设置关系表之间的级联功能。举例如下

没有仔细研究过Hinbernate的级联（cascade）与反转（inverse），有一篇文章请参考Hibernate级联（cascade）与反转（inverse）详解 

• ALL
• PERSIST 新增时，可以参考 spring jpa 如何给外键赋值
• MERGE
• REMOVE
• REFRESH
• DETACH

ClassEntity表

import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

import lombok.Data;

@Data
@Entity
@Table(name = "tb_class")
public class ClassEntity {

    @Id
    private String id;

    @Column
    private String name;

    @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.REFRESH })
    private List<Student> students;
}

Student表

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;

import lombok.Data;

@Data
@Entity
public class Student {

    @Id
    private String id;
    
    @Column
    private String name;
    
    @Column
    private Long age;
    
    @ManyToOne
    @JoinColumn(name = "STUDENT_ID")
    private ClassEntity classEntity;   
}

/*
 * Copyright (c) 2008, 2019 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v. 2.0 which is available at
 * http://www.eclipse.org/legal/epl-2.0,
 * or the Eclipse Distribution License v. 1.0 which is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
 */

// Contributors:
//     Linda DeMichiel - 2.1
//     Linda DeMichiel - 2.0

package javax.persistence;

/**
 * Defines the set of cascadable operations that are propagated 
 * to the associated entity.
 * The value <code>cascade=ALL</code> is equivalent to 
 * <code>cascade={PERSIST, MERGE, REMOVE, REFRESH, DETACH}</code>.
 *
 * @since 1.0
 */
public enum CascadeType { 

    /** Cascade all operations */
    ALL, 

    /** Cascade persist operation */
    PERSIST, 

    /** Cascade merge operation */
    MERGE, 

    /** Cascade remove operation */
    REMOVE,

    /** Cascade refresh operation */
    REFRESH,

    /**
     * Cascade detach operation
     *
     * @since 2.0
     * 
     */   
    DETACH
}

要不要用Hibernaate的级联功能：建议不用
Hibernate的级联并不好用，而且它实现的功能我们完全可以程序里面自己控制：插入的时候，先插入父表再插入子表；删除的时候，先删除子表再删除父表。而且在代码中控制，语义更明显，效率上和hibernate也没有差异，而且实现难度也比配置级联更容易。如果使用了Hibernate作为持久层框架，又不想使用Hibernate的级联，Hibernate默认级联关系就是none，不设置就好了。
 

/*
 * Copyright (c) 2008, 2019 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v. 2.0 which is available at
 * http://www.eclipse.org/legal/epl-2.0,
 * or the Eclipse Distribution License v. 1.0 which is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
 */

// Contributors:
//     Linda DeMichiel - 2.1
//     Linda DeMichiel - 2.0

package javax.persistence;

import java.lang.annotation.Target;
import java.lang.annotation.Retention;
import javax.persistence.CascadeType;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import static javax.persistence.FetchType.EAGER;

/**
 * Specifies a single-valued association to another entity class that
 * has many-to-one multiplicity. It is not normally necessary to
 * specify the target entity explicitly since it can usually be
 * inferred from the type of the object being referenced.  If the
 * relationship is bidirectional, the non-owning
 * <code>OneToMany</code> entity side must used the
 * <code>mappedBy</code> element to specify the relationship field or
 * property of the entity that is the owner of the relationship.
 *
 * <p> The <code>ManyToOne</code> annotation may be used within an
 * embeddable class to specify a relationship from the embeddable
 * class to an entity class. If the relationship is bidirectional, the
 * non-owning <code>OneToMany</code> entity side must use the <code>mappedBy</code>
 * element of the <code>OneToMany</code> annotation to specify the
 * relationship field or property of the embeddable field or property
 * on the owning side of the relationship. The dot (".") notation
 * syntax must be used in the <code>mappedBy</code> element to indicate the
 * relationship attribute within the embedded attribute.  The value of
 * each identifier used with the dot notation is the name of the
 * respective embedded field or property.
 * <pre>
 *
 *     Example 1:
 *
 *     &#064;ManyToOne(optional=false) 
 *     &#064;JoinColumn(name="CUST_ID", nullable=false, updatable=false)
 *     public Customer getCustomer() { return customer; }
 *
 *
 *     Example 2:
 * 
 *     &#064;Entity
 *        public class Employee {
 *        &#064;Id int id;
 *        &#064;Embedded JobInfo jobInfo;
 *        ...
 *     }
 *
 *     &#064;Embeddable
 *        public class JobInfo {
 *        String jobDescription; 
 *        &#064;ManyToOne ProgramManager pm; // Bidirectional
 *     }
 *
 *     &#064;Entity
 *        public class ProgramManager {
 *        &#064;Id int id;
 *        &#064;OneToMany(mappedBy="jobInfo.pm")
 *        Collection&#060;Employee&#062; manages;
 *     }
 *
 * </pre>
 *
 * @since 1.0
 */
@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;
}