hibernate 多对多

Category                                 Item

类别                                         商品

单向多对多。

package com.hibernate.n2n;

import java.util.HashSet;
import java.util.Set;

public class Category {
    private Integer cid;
    private String cname;
    private Set<Item> items=new HashSet<Item>();
    public Integer getCid() {
        return cid;
    }
    public void setCid(Integer cid) {
        this.cid = cid;
    }
    public String getCname() {
        return cname;
    }
    public void setCname(String cname) {
        this.cname = cname;
    }
    public Set<Item> getItems() {
        return items;
    }
    public void setItems(Set<Item> items) {
        this.items = items;
    }
    
}

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.hibernate.n2n">

    <class name="Category" table="categories">
        
        <id name="cid" type="java.lang.Integer">
            <column name="c_id" />
            <!-- 指定主键的生成方式, native: 使用数据库本地方式 -->
            <generator class="native" />
        </id>
    
        <property name="cname" 
            type="java.lang.String" column="c_name" >
        </property>
        <!-- name  对应items的属性名。  table  是一个中间表-->
        <set name="items" table="Category_Items">
            <key column="c_id"></key>    设置category在中间表的外键
            <many-to-many class="Item" column="i_id" ></many-to-many>   设置item在中间表的外键。
        </set>
        
    </class>
    
</hibernate-mapping>

package com.hibernate.n2n;

public class Item {
    private Integer iid;
    private String iname;
    public Integer getIid() {
        return iid;
    }
    public void setIid(Integer iid) {
        this.iid = iid;
    }
    public String getIname() {
        return iname;
    }
    public void setIname(String iname) {
        this.iname = iname;
    }
    
}

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.hibernate.n2n">

    <class name="Item" table="item" >
        
        <id name="iid" type="java.lang.Integer">
            <column name="i_id" />
            <!-- 指定主键的生成方式, native: 使用数据库本地方式 -->
            <generator class="native" />
        </id>
    
        <property name="iname"
            type="java.lang.String" column="i_name" >
        </property>
        
     </class>
    
</hibernate-mapping>

测试添加：

@org.junit.Test
    public void testSave(){
        Category category1=new Category();
        category1.setCname("CA1");
        Category category2=new Category();
        category2.setCname("CA1");
        
        Item item1=new Item();
        item1.setIname("IA1");
        Item item2=new Item();
        item2.setIname("IA2");
        //维护关系.
        category1.getItems().add(item1);
        category1.getItems().add(item2);
        
        category2.getItems().add(item1);
        category2.getItems().add(item2);
        
        session.save(category1);
        session.save(category2);
        session.save(item1);
        session.save(item2);
        
    }

也是支持懒加载的。

查询item的时候。是查询的中间表。

双向N---N

只是在对象中加上set集合。

配置文件加上：set

<set name="categorys" table="Category_Items">   在item对象中的category属性。
            <key column="i_id"></key>   item在中间表中的外键
            <many-to-many class="Category" column="c_id" ></many-to-many>   category在中间表中的外键。
        </set>

执行保存的时候会出现错误。 这是因为双方都维护了关系。导致主键发生冲突。

解决方案：在其中一个set 加上 inverse="true"

org.hibernate.exception.ConstraintViolationException: could not execute statement
    at org.hibernate.exception.internal.SQLExceptionTypeDelegate.convert(SQLExceptionTypeDelegate.java:74)
    at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:49)
    at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:125)
    at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:110)
    at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:136)
    at org.hibernate.engine.jdbc.batch.internal.NonBatchingBatch.addToBatch(NonBatchingBatch.java:58)
    at org.hibernate.persister.collection.AbstractCollectionPersister.recreate(AbstractCollectionPersister.java:1256)
    at org.hibernate.action.internal.CollectionRecreateAction.execute(CollectionRecreateAction.java:58)
    at org.hibernate.engine.spi.ActionQueue.execute(ActionQueue.java:377)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:369)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:292)
    at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:339)
    at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:52)
    at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1234)
    at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:404)
    at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:101)
    at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:175)
    at com.hibernate.n2n.Test.destroy(Test.java:34)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:37)
    at org.junit.runners.BlockJUnit4ClassRunner.runNotIgnored(BlockJUnit4ClassRunner.java:79)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:71)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Duplicate entry '8-5' for key 'PRIMARY'
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
    at com.mysql.jdbc.Util.getInstance(Util.java:381)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1015)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3515)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3447)
    at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1951)
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2101)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2554)
    at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1761)
    at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2046)
    at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1964)
    at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1949)
    at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeUpdate(NewProxyPreparedStatement.java:147)
    at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:133)
    ... 36 more