一对多（多对一）关系中的inverse和cascade属性

时间 2019-11-09

标签一对多多对一关系 inverse cascade 属性繁體版

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首先说一下inverse：java

"inverse" 直译过来就是"反转，使颠倒"的意思，书面化的解释为"是否将关系维护的权力交给对方"mysql

1.　　在hibernate中inverse默认是false，也就是己方拥有维护关系的权利，固然为true的话，就是把维护关系的权利交给了对方sql

2.　　在一对多的关系（多对一）中，一般将一端的inverse设置为false（一端设为true的话会多出更新语句，有性能问题，下面会讲到），而多对多的关系中，inverse的值只能有一个为true,由于若是双发都为true，那么双方都去维护关系，会形成中间关系表中出现重复的数据。(这一点之后有深入的理解，会补上解释的额)数据库

说到这里，那么问题来了，什么是关系？关系的具体体现又是什么？session

什么是关系？app

　　"关系"就是两个表之间的关系，一般为"一对多","一对一","多对多"三种关系,eclipse

关系的具体体现是什么？ide

暂且先看下面这张图，后面会详细讲解性能

这是AClazz.hbm.xml中的部分截图，在一端配置外键关系时，关系的具体体现就是column="clazzid",未来这一列会在学生表中生成（外键）

接下来谈一谈cascade：

1.　　"cascade"-直译过来就是"级联、串联"的意思，书面化的解释为"该属性会使咱们在操做主对象时，同时Hibernate帮助咱们完成从属对象相应的操做

　　　　（好比，有Customer和Order这两张表，关系为一对多，只使用JDBC删除Customer表中的一行记录时，咱们还须要手动的将 Order表中与之关联的记录全都删除，使用Hibernate的'cascade'属性后，当咱们删除一条Customer记录时，Hibernate 会帮助咱们完成相应Order表记录的删除工做，方便了咱们的工做）"。

2. 　 用"cascade"属性时，主对象(一方)通常设置为"all"，而多方不建议设置包含delete操做的选项，建议设置多方为"save-update"，这是由于你删除一方，多方已经没有存在的意义了，而删除多方不能表明一方没意义了（例如，教室和学生）

举个例子

好比： AClazz.hbm.xml -->一端

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
 3 <!-- 
 4     hbm.xml的做用
 5                将pojo中每个属性对应到表的列名
 6 -->
 7 <hibernate-mapping>
 8     <class name="com.bjsxt.hibernate.pojo.AClazz" table="a_clazz">
 9         <!-- id必须写name -->
10         <id name="id">
11             <generator class="native"/>
12         </id>
13         <property name="name"/>
14         <property name="createTime"/>
15         <!-- 一对多如何设置 -->
16         <set name="studentSet" inverse="false" cascade="all" fetch="subselect">
17             <!-- 配置外键，两张表的关联关系 -->
18             <key column="clazzid"></key>
19             
20             <!-- 另外一端对应的类 -->
21             <one-to-many class="com.bjsxt.hibernate.pojo.AStudent"/>
22         </set>
23     </class>
24 </hibernate-mapping>

AStudent.hbm.xml -->多端

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
 3 <!-- 
 4     hbm.xml的做用
 5                将pojo中每个属性对应到表的列名
 6 -->
 7 <hibernate-mapping>
 8     <class name="com.bjsxt.hibernate.pojo.AStudent" table="a_student">
 9         <!-- id必须写name -->
10         <id name="id">
11             <generator class="native"/>
12         </id>
13         <property name="name"/>
14         <property name="createTime"/>
15         <many-to-one name="clazz" column="clazzid" class="com.bjsxt.hibernate.pojo.AClazz" cascade="save-update"></many-to-one>
16     </class>
17 </hibernate-mapping>

运用

了解了inverse，cascade 下面一对多（双向）综合运用一下

1.下面这张截图时eclipse中的项目目录

2.根据上面的目录结构首先建立pojo类

2.1 AClazz.java -- 一端

 1 package com.bjsxt.hibernate.pojo;
 2 
 3 import java.util.Date;
 4 import java.util.HashSet;
 5 import java.util.Set;
 6 
 7 
 8 public class AClazz {
 9     private int id;
10     private String name;
11     private Date createTime;
12     //一个班级能够放多个学生，一对多
13     private Set<AStudent> studentSet = new HashSet<>() ;
14     public AClazz() {
15         
16     }
17     public AClazz(int id, String name, Date createTime) {
18         super();
19         this.id = id;
20         this.name = name;
21         this.createTime = createTime;
22     }
23     public int getId() {
24         return id;
25     }
26     public void setId(int id) {
27         this.id = id;
28     }
29     public String getName() {
30         return name;
31     }
32     public void setName(String name) {
33         this.name = name;
34     }
35     public Date getCreateTime() {
36         return createTime;
37     }
38     public void setCreateTime(Date createTime) {
39         this.createTime = createTime;
40     }
41     @Override
42     public String toString() {
43         return "AClazz [id=" + id + ", name=" + name + ", createTime=" + createTime + "]";
44     }
45     @Override
46     public int hashCode() {
47         final int prime = 31;
48         int result = 1;
49         result = prime * result + ((createTime == null) ? 0 : createTime.hashCode());
50         result = prime * result + id;
51         result = prime * result + ((name == null) ? 0 : name.hashCode());
52         return result;
53     }
54     @Override
55     public boolean equals(Object obj) {
56         if (this == obj)
57             return true;
58         if (obj == null)
59             return false;
60         if (getClass() != obj.getClass())
61             return false;
62         AClazz other = (AClazz) obj;
63         if (createTime == null) {
64             if (other.createTime != null)
65                 return false;
66         } else if (!createTime.equals(other.createTime))
67             return false;
68         if (id != other.id)
69             return false;
70         if (name == null) {
71             if (other.name != null)
72                 return false;
73         } else if (!name.equals(other.name))
74             return false;
75         return true;
76     }
77     public Set<AStudent> getStudentSet() {
78         return studentSet;
79     }
80     public void setStudentSet(Set<AStudent> studentSet) {
81         this.studentSet = studentSet;
82     }
83 }

AClazz.java

2.1 AStudent.java -- 多端

 1 package com.bjsxt.hibernate.pojo;
 2 
 3 import java.util.Date;
 4 
 5 public class AStudent {
 6     private int id;
 7     private String name;
 8     private Date createTime;
 9     // 多对一，多个学生对应一个班级
10     private AClazz clazz;
11 
12     public AStudent() {
13     }
14 
15     public AStudent(int id, String name, Date createTime) {
16         super();
17         this.id = id;
18         this.name = name;
19         this.createTime = createTime;
20     }
21 
22     public int getId() {
23         return id;
24     }
25 
26     public void setId(int id) {
27         this.id = id;
28     }
29 
30     public String getName() {
31         return name;
32     }
33 
34     public void setName(String name) {
35         this.name = name;
36     }
37 
38     public Date getCreateTime() {
39         return createTime;
40     }
41 
42     public void setCreateTime(Date createTime) {
43         this.createTime = createTime;
44     }
45 
46     @Override
47     public String toString() {
48         return "AStudent [id=" + id + ", name=" + name + ", createTime=" + createTime + "]";
49     }
50 
51     @Override
52     public int hashCode() {
53         final int prime = 31;
54         int result = 1;
55         result = prime * result + ((createTime == null) ? 0 : createTime.hashCode());
56         result = prime * result + id;
57         result = prime * result + ((name == null) ? 0 : name.hashCode());
58         return result;
59     }
60 
61     @Override
62     public boolean equals(Object obj) {
63         if (this == obj)
64             return true;
65         if (obj == null)
66             return false;
67         if (getClass() != obj.getClass())
68             return false;
69         AStudent other = (AStudent) obj;
70         if (createTime == null) {
71             if (other.createTime != null)
72                 return false;
73         } else if (!createTime.equals(other.createTime))
74             return false;
75         if (id != other.id)
76             return false;
77         if (name == null) {
78             if (other.name != null)
79                 return false;
80         } else if (!name.equals(other.name))
81             return false;
82         return true;
83     }
84 
85     public AClazz getClazz() {
86         return clazz;
87     }
88 
89     public void setClazz(AClazz clazz) {
90         this.clazz = clazz;
91     }
92 }

AStudent.java

3.配置映射文件

3.1 AClazz.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<!-- 
    hbm.xml的做用
               将pojo中每个属性对应到表的列名
-->
<hibernate-mapping>
    <class name="com.bjsxt.hibernate.pojo.AClazz" table="a_clazz">
        <!-- id必须写name -->
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
        <property name="createTime"/>
        <!-- 一对多如何设置 -->
        <set name="studentSet" inverse="false" cascade="all">
            <!-- 配置外键，两张表的关联关系 -->
            <key column="clazzid"></key>
            
            <!-- 另外一端对应的类 -->
            <one-to-many class="com.bjsxt.hibernate.pojo.AStudent"/>
        </set>
    </class>
</hibernate-mapping>

以上的<set>....</set>它就至关于一个纽带，比如一我的牵了多只牛

3.2 AStudent.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<!-- 
    hbm.xml的做用
               将pojo中每个属性对应到表的列名
-->
<hibernate-mapping>
    <class name="com.bjsxt.hibernate.pojo.AStudent" table="a_student">
        <!-- id必须写name -->
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
        <property name="createTime"/>
        <many-to-one name="clazz" column="clazzid" class="com.bjsxt.hibernate.pojo.AClazz" cascade="save-update"></many-to-one>
    </class>
</hibernate-mapping>

别忘了在hibernate.cfg.xml中添加上面的两个映射文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<!--
     session-factory == DriverManager
     session == Connection    
 -->
<hibernate-configuration>
     <session-factory>
         <!-- 驱动包名 -->
         <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
         
         <!-- JDBC URL -->
         <property name="hibernate.connection.url">jdbc:mysql://127.0.0.1:3306/hibernate</property>
         
         <!-- JDBC 用户名 -->
         <property name="hibernate.connection.username">root</property>
         
         <!-- JDBC 密码 -->
         <property name="hibernate.connection.password">admin</property>
         
         <!-- 官方语言，告诉hibernate链接的是哪一个数据库 -->
         <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
         
         <!-- 是否显示SQL语句 -->
        <property name="hibernate.show_sql">true</property>
        
        <!-- 格式化SQL语句 -->
        <property name="hibernate.format_sql">true</property>
        
         <!-- 
            create:每次执行,都建立一张新表,将原来的记录删除
            update:每次执行,若是映射文件有修改,仅仅修改表结构
         -->
         <property name="hibernate.hbm2ddl.auto">update</property>
         
         <!-- 告诉hibernate核心配置文件，加载哪些映射文件 -->
         <mapping resource="com/bjsxt/hibernate/pojo/AStudent.hbm.xml"/>
         <mapping resource="com/bjsxt/hibernate/pojo/AClazz.hbm.xml"/>
     </session-factory>
</hibernate-configuration>

注意测试一之后的全部测试，都是基于测试一上所作的修改

4.JUnit测试

　　保持 AClazz.hbm.xml 和 AStudent.hbm.xml 中级联关系不变，只修改AClazz.hbm.xml 中的inverse属性

　　建立一个班级，多个学生，由于配置了级联关系，因此我把session.save(student)注释了，这样只须要保存班级便可

    @Test
    public void inserClazz() {
        Session session = null;
        Transaction transaction = null;
        try {
            // 获取Session==Connection
            session = sessionFactory.openSession();

            // 手动开启事务
            transaction = session.beginTransaction();

            // 建立班级
            AClazz clazz = new AClazz();
            clazz.setName("405");
            clazz.setCreateTime(new Date());

            Set<AStudent> studentSet = new HashSet<>();

            // 一个班级有多个学生
            for (int i = 0; i < 3; i++) {
                AStudent student = new AStudent();
                student.setName("张三_" + i);
                student.setCreateTime(new Date());
                studentSet.add(student);

                // session.save(student);
            }
            // 将学生集合放到班级中
            clazz.setStudentSet(studentSet);
            session.save(clazz);

            // 手动提交事务
            transaction.commit();
        } catch (Exception e) {
            e.printStackTrace();
            // 事务回滚
            transaction.rollback();
        } finally {
            if (session != null && session.isOpen()) {
                session.close();
            }
        }
    }

如今保持 AClazz.hbm.xml 和 AStudent.hbm.xml 中级联关系不变，只修改AClazz.hbm.xml 中的inverse属性

测试一：当AClazz.hbm.xml 中inverse属性为false时

　　运行inserClazz() ，控制台语句输出结果以下：

Hibernate: 
    
    create table a_clazz (
        id integer not null auto_increment,
        name varchar(255),
        createTime datetime,
        primary key (id)
    )
2016-11-10 18:16:20,238 INFO  [main] internal.InformationExtractorJdbcDatabaseMetaDataImpl (InformationExtractorJdbcDatabaseMetaDataImpl.java:393) - HHH000262: Table not found: a_student
2016-11-10 18:16:20,239 INFO  [main] internal.InformationExtractorJdbcDatabaseMetaDataImpl (InformationExtractorJdbcDatabaseMetaDataImpl.java:393) - HHH000262: Table not found: a_student
Hibernate: 
    
    create table a_student (
        id integer not null auto_increment,
        name varchar(255),
        createTime datetime,
        clazzid integer,
        primary key (id)
    )
Hibernate: 
    
    alter table a_student 
        add constraint FK9jjuqt3yl34nmy329k44uje4j 
        foreign key (clazzid) 
        references a_clazz (id)
2016-11-10 18:16:20,291 INFO  [main] fetch.FetchTest (FetchTest.java:48) - --init--sessionFactory:org.hibernate.internal.SessionFactoryImpl@3ff57625
Hibernate: 
    insert 
    into
        a_clazz
        (name, createTime) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)
Hibernate: 
    update
        a_student 
    set
        clazzid=? 
    where
        id=?
Hibernate: 
    update
        a_student 
    set
        clazzid=? 
    where
        id=?
Hibernate: 
    update
        a_student 
    set
        clazzid=? 
    where
        id=?

View Code

mysql数据库截图以下：

a_clazz

a_student

解析：咱们能够看到，先是建立了两个表，教室表和学生表，由于我配置了一对多的关系one-to-many（我这里是双向配置，即一对多双向配置，只配一端的也能够），因此会为a_stuent表设置外键，

　　　由于我再AClazz.hbm.xml中配置了级联all，因此我再保存班级的时候会自动将学生保存到数据库中。

　　　又由于我将AClazz.hbm.xml中的inverse属性设置成了false，表明着教师表能够维护它与学生表之间的关系，也就是在保存班级的时候，能够将学生表中的外键clazzid设置上，因此最后也就出现了三条update语句，为a_student的外键更新值，正是由于在一端将inverse设置为false（inverse不设置的话，默认值false）,因此再保存完学生以后，一端还要为多端设置外键，如今是插入了三个学生，若是我要是插入了10000个学生呢？There is no doubt that 最后会多出10000条更新语句，因此说咱们通常将一端的inverse设置为false,将维护关系的权利交给多方，而后配置级联（一端为all,多端为save-update）,这样的话，咱们只须要保存多端的对象，即学生，那么最后就不会生成update语句了。（测试四验证这句话）

固然若是我把AClazz.hbm.xml中的inverse属性设置成true,表明着教室表失去了维护关系的权利，也就是说在保存班级的时候，不能够将学生表中的外键clazzid设置上，因此最后天然也就不会出现三条update语句，那么a_student中外键那一列也就变成了null

下面验证我刚才的说法：

测试二：当AClazz.hbm.xml 中inverse属性为true时

　　运行inserClazz() ，控制台语句输出结果以下：

    create table a_student (
        id integer not null auto_increment,
        name varchar(255),
        createTime datetime,
        clazzid integer,
        primary key (id)
    )
Hibernate: 
    
    alter table a_student 
        add constraint FK9jjuqt3yl34nmy329k44uje4j 
        foreign key (clazzid) 
        references a_clazz (id)
2016-11-10 18:33:10,940 INFO  [main] fetch.FetchTest (FetchTest.java:48) - --init--sessionFactory:org.hibernate.internal.SessionFactoryImpl@3ff57625
Hibernate: 
    insert 
    into
        a_clazz
        (name, createTime) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)

View Code

mysql数据库截图以下：

a_clazz

a_student

　　看见了吧，以上的结果证明了我刚才的说法是正确的

测试三：如今将AClazz.hbm.xml 中的级联关系删除，而且中inverse属性为true，其余保持不变

　　运行inserClazz() ，控制台语句输出结果以下：

Hibernate: 
    
    create table a_clazz (
        id integer not null auto_increment,
        name varchar(255),
        createTime datetime,
        primary key (id)
    )
2016-11-10 18:37:12,045 INFO  [main] internal.InformationExtractorJdbcDatabaseMetaDataImpl (InformationExtractorJdbcDatabaseMetaDataImpl.java:393) - HHH000262: Table not found: a_student
2016-11-10 18:37:12,046 INFO  [main] internal.InformationExtractorJdbcDatabaseMetaDataImpl (InformationExtractorJdbcDatabaseMetaDataImpl.java:393) - HHH000262: Table not found: a_student
Hibernate: 
    
    create table a_student (
        id integer not null auto_increment,
        name varchar(255),
        createTime datetime,
        clazzid integer,
        primary key (id)
    )
Hibernate: 
    
    alter table a_student 
        add constraint FK9jjuqt3yl34nmy329k44uje4j 
        foreign key (clazzid) 
        references a_clazz (id)
2016-11-10 18:37:12,107 INFO  [main] fetch.FetchTest (FetchTest.java:48) - --init--sessionFactory:org.hibernate.internal.SessionFactoryImpl@187eb9a8
Hibernate: 
    insert 
    into
        a_clazz
        (name, createTime) 
    values
        (?, ?)
Hibernate: 
    update
        a_student 
    set
        clazzid=? 
    where
        id=?
2016-11-10 18:37:12,552 ERROR [main] internal.ExceptionMapperStandardImpl (ExceptionMapperStandardImpl.java:39) - HHH000346: Error during managed flush [org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance beforeQuery flushing: com.bjsxt.hibernate.pojo.AStudent]
2016-11-10 18:37:12,565 INFO  [main] internal.AbstractBatchImpl (AbstractBatchImpl.java:193) - HHH000010: On release of batch it still contained JDBC statements
java.lang.IllegalStateException: org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance beforeQuery flushing: com.bjsxt.hibernate.pojo.AStudent
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:144)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:155)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:162)
    at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1411)
    at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:475)
    at org.hibernate.internal.SessionImpl.flushBeforeTransactionCompletion(SessionImpl.java:3168)
    at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:2382)
    at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:467)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:146)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.access$100(JdbcResourceLocalTransactionCoordinatorImpl.java:38)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:220)
    at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:68)
    at com.bjsxt.hibernate.fetch.FetchTest.inserClazz(FetchTest.java:86)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance beforeQuery flushing: com.bjsxt.hibernate.pojo.AStudent
    at org.hibernate.engine.internal.ForeignKeys.getEntityIdentifierIfNotUnsaved(ForeignKeys.java:279)
    at org.hibernate.type.EntityType.getIdentifier(EntityType.java:462)
    at org.hibernate.type.ManyToOneType.nullSafeSet(ManyToOneType.java:151)
    at org.hibernate.persister.collection.AbstractCollectionPersister.writeElement(AbstractCollectionPersister.java:894)
    at org.hibernate.persister.collection.AbstractCollectionPersister.recreate(AbstractCollectionPersister.java:1317)
    at org.hibernate.persister.collection.OneToManyPersister.recreate(OneToManyPersister.java:170)
    at org.hibernate.action.internal.CollectionRecreateAction.execute(CollectionRecreateAction.java:50)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:582)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:456)
    at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:337)
    at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:39)
    at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1405)
    ... 33 more

View Code

　　能够看到hibernate表帮咱们建立了，外键也帮咱们加上了，可是却报出了异常，

　　报的是一个临时对象异常：对象的引用是一个为保存的持久化实例，说白了就是没有设置级联

可是hibernate帮咱们把表建好了，只不过是空的

a_clazz

a_student

测试四：如今将AClazz.hbm.xml 中的级联关系依旧设置为all，而且inverse属性为true，

单元测试更改以下：（注意我把set集合放学生，以及最后的保存班级都注释了，下面的测试代码中只保存了学生）

    /**
     * 保存方法
     */
    @Test
    public void inserClazz() {
        Session session = null;
        Transaction transaction = null;
        try {
            // 获取Session==Connection
            session = sessionFactory.openSession();

            // 手动开启事务
            transaction = session.beginTransaction();

            // 建立班级
            AClazz clazz = new AClazz();
            clazz.setName("405");
            clazz.setCreateTime(new Date());

            Set<AStudent> studentSet = new HashSet<>();

            // 一个班级有多个学生
            for (int i = 0; i < 3; i++) {
                AStudent student = new AStudent();
                student.setName("张三_" + i);
                student.setCreateTime(new Date());
                student.setClazz(clazz);
                //studentSet.add(student);
 session.save(student);//只保存学生，由于多端配置了级联关系，save-update，因此保存学生的时候先保存班级
            }
            /*// 将学生集合放到班级中
            clazz.setStudentSet(studentSet);
            session.save(clazz);*/

            // 手动提交事务
            transaction.commit();
        } catch (Exception e) {
            e.printStackTrace();
            // 事务回滚
            transaction.rollback();
        } finally {
            if (session != null && session.isOpen()) {
                session.close();
            }
        }
    }

　　运行inserClazz() ，控制台语句输出结果以下：

Hibernate: 
    
    create table a_clazz (
        id integer not null auto_increment,
        name varchar(255),
        createTime datetime,
        primary key (id)
    )
2016-11-10 18:55:25,462 INFO  [main] internal.InformationExtractorJdbcDatabaseMetaDataImpl (InformationExtractorJdbcDatabaseMetaDataImpl.java:393) - HHH000262: Table not found: a_student
2016-11-10 18:55:25,467 INFO  [main] internal.InformationExtractorJdbcDatabaseMetaDataImpl (InformationExtractorJdbcDatabaseMetaDataImpl.java:393) - HHH000262: Table not found: a_student
Hibernate: 
    
    create table a_student (
        id integer not null auto_increment,
        name varchar(255),
        createTime datetime,
        clazzid integer,
        primary key (id)
    )
Hibernate: 
    
    alter table a_student 
        add constraint FK9jjuqt3yl34nmy329k44uje4j 
        foreign key (clazzid) 
        references a_clazz (id)
2016-11-10 18:55:25,549 INFO  [main] fetch.FetchTest (FetchTest.java:48) - --init--sessionFactory:org.hibernate.internal.SessionFactoryImpl@3ff57625
Hibernate: 
    insert 
    into
        a_clazz
        (name, createTime) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)
Hibernate: 
    insert 
    into
        a_student
        (name, createTime, clazzid) 
    values
        (?, ?, ?)

View Code

mysql数据库截图以下：

a_clazz

a_student

解析：从测试四的运行结果，咱们能够清楚的看到，在保存学生的时候（多端），由于学生映射xml中配置了级联，因此先保存班级（一端），这样保存班级的时候返回该班级的主键，而后保存学生的时候就有了外键，因此最后并无三条更新语句，从而提升了性能

参考：http://www.cnblogs.com/o-andy-o/archive/2012/03/26/2418235.html

以上是我对hibernate中inverse和cascade属性的理解，若有不对的地方，欢迎你们及时指正！！！