通過Bag一對多對映範例（使用xml檔案）

如果持久化類具有包含實體參照的列表物件，則需要使用一對多關聯來對映列表元素。 我們可以通過列表(list)或包(bag)來對映這個列表物件。

請注意，bag不是基於索引的，而list是基於索引的。

在這裡，我們使用論壇的場景：論壇中一個問題有多個答案。

下面來看看看具有列表物件的持久化類。 在這種情況下，一個問題可以有多個答案，每個答案可能有自己的資訊，這就是為什麼這裡要使用列表(list)元素(包含答案物件)代表一個答案集合。
Question類程式碼如下 -

package com.yiibai;

import java.util.List;

public class Question {
    private int id;
    private String qname;
    private List<String> answers;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getQname() {
        return qname;
    }

    public void setQname(String qname) {
        this.qname = qname;
    }

    public List<String> getAnswers() {
        return answers;
    }

    public void setAnswers(List<String> answers) {
        this.answers = answers;
    }

}

Question類有自己的資訊，如id，answername，postedBy等。

package com.yiibai;

public class Answer {
    private int id;
    private String answername;
    private String postedBy;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getAnswername() {
        return answername;
    }
    public void setAnswername(String answername) {
        this.answername = answername;
    }
    public String getPostedBy() {
        return postedBy;
    }
    public void setPostedBy(String postedBy) {
        this.postedBy = postedBy;
    }

}

Question類具有包含實體參照的列表物件(即Answer類物件)。 在這種情況下，我們需要使用一對多的bag標籤來對映此物件。 下面來看看看我們如何對映它。

<bag name="answers" cascade="all">  
    <key column="qid"></key>  
    <one-to-many class="com.yiibai.Answer"/>  
</bag>

通過一對多關聯在集合對映中對映包的範例

在這個例子中，我們將看到包含實體參照的對映列表的完整範例。建立一個Java專案：bagonetomany，其完整的目錄結構如下 -

1)建立持久化類

這個持久化類定義了類的屬性，包括List。

Question.java 程式碼如下所示 -

package com.yiibai;

import java.util.List;

public class Question {
    private int id;
    private String qname;
    private List<String> answers;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getQname() {
        return qname;
    }

    public void setQname(String qname) {
        this.qname = qname;
    }

    public List<String> getAnswers() {
        return answers;
    }

    public void setAnswers(List<String> answers) {
        this.answers = answers;
    }

}

Answer.java 程式碼如下所示 -

package com.yiibai;

public class Answer {
    private int id;
    private String answername;
    private String postedBy;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getAnswername() {
        return answername;
    }
    public void setAnswername(String answername) {
        this.answername = answername;
    }
    public String getPostedBy() {
        return postedBy;
    }
    public void setPostedBy(String postedBy) {
        this.postedBy = postedBy;
    }

    public String toString(){
        return this.answername+", PostedBy "+this.postedBy;
    }
}

2)建立持久化類的對映檔案

在這裡，我們建立了用於定義列表的question.hbm.xml檔案。

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-mapping PUBLIC
          "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
          "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="com.yiibai.Question" table="q501">
        <id name="id">
            <generator class="increment"></generator>
        </id>
        <property name="qname"></property>

        <bag name="answers" cascade="all">
            <key column="type"></key>
            <one-to-many class="com.yiibai.Answer" />
        </bag>

    </class>

    <class name="com.yiibai.Answer" table="ans501">
        <id name="id">
            <generator class="increment"></generator>
        </id>
        <property name="answername"></property>
        <property name="postedBy"></property>
    </class>

</hibernate-mapping>

3)建立組態檔案

此檔案包含有關資料庫和對映檔案的資訊。hibernate.cfg.xml檔案的程式碼如下所示 -

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

<!-- Generated by MyEclipse Hibernate Tools. -->
<hibernate-configuration>

    <session-factory>
        <property name="hbm2ddl.auto">update</property>

        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/test</property>
        <property name="connection.username">root</property>
        <property name="connection.password">123456</property>
        <property name="dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property>
        <property name="show_sql">true</property>

        <mapping resource="question.hbm.xml" />

    </session-factory>

</hibernate-configuration>

4)建立儲存資料的類

在這個類中，我們儲存Question類的資料。MainTest.java檔案中的程式碼如下所示 -

package com.yiibai;

import java.util.ArrayList;

import org.hibernate.*;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.*;

public class MainTest {
    public static void main(String[] args) {
        // 但在5.1.0版本匯總，hibernate則採用如下新方式獲取：
        // 1. 組態型別安全的準服務註冊類，這是當前應用的單例物件，不作修改，所以宣告為final
        // 在configure("cfg/hibernate.cfg.xml")方法中，如果不指定資源路徑，預設在類路徑下尋找名為hibernate.cfg.xml的檔案
        final StandardServiceRegistry registry = new StandardServiceRegistryBuilder()
                .configure("hibernate.cfg.xml").build();
        // 2. 根據服務註冊類建立一個後設資料資源集，同時構建後設資料並生成應用一般唯一的的session工廠
        SessionFactory sessionFactory = new MetadataSources(registry)
                .buildMetadata().buildSessionFactory();

        /**** 上面是組態準備，下面開始我們的資料庫操作 ******/
        Session session = sessionFactory.openSession();// 從對談工廠獲取一個session

        // creating transaction object
        Transaction t = session.beginTransaction();

        Answer ans1 = new Answer();
        ans1.setAnswername("java is a programming language");
        ans1.setPostedBy("Ravi Su");

        Answer ans2 = new Answer();
        ans2.setAnswername("java is a platform");
        ans2.setPostedBy("Sudhir Lee");

        Answer ans3 = new Answer();
        ans3.setAnswername("Servlet is an Interface");
        ans3.setPostedBy("Jai Wong");

        Answer ans4 = new Answer();
        ans4.setAnswername("Servlet is an API");
        ans4.setPostedBy("Arun");

        ArrayList<Answer> list1 = new ArrayList<Answer>();
        list1.add(ans1);
        list1.add(ans2);

        ArrayList<Answer> list2 = new ArrayList<Answer>();
        list2.add(ans3);
        list2.add(ans4);

        Question question1 = new Question();
        question1.setQname("What is Java?");
        question1.setAnswers(list1);

        Question question2 = new Question();
        question2.setQname("What is Servlet?");
        question2.setAnswers(list2);

        session.persist(question1);
        session.persist(question2);

        t.commit();
        session.close();

        System.out.println("success");

    }
}

如何獲取列表的資料

在這裡，我們使用HQL來獲取Question類的所有記錄，包括答案。 在這種情況下，它從功能相關的兩個表中獲取資料。 在這裡，我們直接列印答案類的物件，但是我們已經在Answer類中覆蓋了返回 answername 和 postername 的toString()方法。 所以它列印answername 和 postername 而不是參考ID。

FetchData.java 程式碼如下 -

package com.yiibai;

import java.util.*;

import org.hibernate.*;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.*;

public class FetchData {
    public static void main(String[] args) {

        // 但在5.1.0版本匯總，hibernate則採用如下新方式獲取：
        // 1. 組態型別安全的準服務註冊類，這是當前應用的單例物件，不作修改，所以宣告為final
        // 在configure("cfg/hibernate.cfg.xml")方法中，如果不指定資源路徑，預設在類路徑下尋找名為hibernate.cfg.xml的檔案
        final StandardServiceRegistry registry = new StandardServiceRegistryBuilder()
                .configure("hibernate.cfg.xml").build();
        // 2. 根據服務註冊類建立一個後設資料資源集，同時構建後設資料並生成應用一般唯一的的session工廠
        SessionFactory sessionFactory = new MetadataSources(registry)
                .buildMetadata().buildSessionFactory();

        /**** 上面是組態準備，下面開始我們的資料庫操作 ******/
        Session session = sessionFactory.openSession();// 從對談工廠獲取一個session

        // creating transaction object
        Transaction t = session.beginTransaction();

        Query query = session.createQuery("from Question");
        List<Question> list = query.list();

        Iterator<Question> itr = list.iterator();
        while (itr.hasNext()) {
            Question q = itr.next();
            System.out.println("Question Name: " + q.getQname());

            // printing answers
            List<Answer> list2 = q.getAnswers();
            Iterator<Answer> itr2 = list2.iterator();
            while (itr2.hasNext()) {
                System.out.println(itr2.next());
            }

        }
        session.close();
        System.out.println("success");

    }
}

執行範例

首先執行 MainTest.java 等到以下結果 -

log4j:WARN No appenders could be found for logger (org.jboss.logging).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Sun Mar 26 21:54:43 CST 2017 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
Hibernate: alter table ans501 add constraint FKtppk9gq3v1rge6ntclgp1jucm foreign key (type) references q501 (id)
Hibernate: select max(id) from q501
Hibernate: select max(id) from ans501
Hibernate: insert into q501 (qname, id) values (?, ?)
Hibernate: insert into ans501 (answername, postedBy, id) values (?, ?, ?)
Hibernate: insert into ans501 (answername, postedBy, id) values (?, ?, ?)
Hibernate: insert into q501 (qname, id) values (?, ?)
Hibernate: insert into ans501 (answername, postedBy, id) values (?, ?, ?)
Hibernate: insert into ans501 (answername, postedBy, id) values (?, ?, ?)
Hibernate: update ans501 set type=? where id=?
Hibernate: update ans501 set type=? where id=?
Hibernate: update ans501 set type=? where id=?
Hibernate: update ans501 set type=? where id=?
success

再讀取上一步中儲存的資訊，執行 FetchData.java 得到以下結果 -

log4j:WARN No appenders could be found for logger (org.jboss.logging).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Sun Mar 26 21:51:56 CST 2017 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
Hibernate: alter table ans501 add constraint FKtppk9gq3v1rge6ntclgp1jucm foreign key (type) references q501 (id)
Hibernate: select question0_.id as id1_1_, question0_.qname as qname2_1_ from q501 question0_
Question Name: What is Java?
Hibernate: select answers0_.type as type4_0_0_, answers0_.id as id1_0_0_, answers0_.id as id1_0_1_, answers0_.answername as answerna2_0_1_, answers0_.postedBy as postedBy3_0_1_ from ans501 answers0_ where answers0_.type=?
java is a platform, PostedBy Sudhir Wong
Servlet is an API, PostedBy Arun
Question Name: What is Servlet?
Hibernate: select answers0_.type as type4_0_0_, answers0_.id as id1_0_0_, answers0_.id as id1_0_1_, answers0_.answername as answerna2_0_1_, answers0_.postedBy as postedBy3_0_1_ from ans501 answers0_ where answers0_.type=?
Question Name: What is Java?
Hibernate: select answers0_.type as type4_0_0_, answers0_.id as id1_0_0_, answers0_.id as id1_0_1_, answers0_.answername as answerna2_0_1_, answers0_.postedBy as postedBy3_0_1_ from ans501 answers0_ where answers0_.type=?
java is a programming language, PostedBy Ravi Su
java is a platform, PostedBy Sudhir Lee
Question Name: What is Servlet?
Hibernate: select answers0_.type as type4_0_0_, answers0_.id as id1_0_0_, answers0_.id as id1_0_1_, answers0_.answername as answerna2_0_1_, answers0_.postedBy as postedBy3_0_1_ from ans501 answers0_ where answers0_.type=?
Servlet is an Interface, PostedBy Jai Wong
Servlet is an API, PostedBy Arun
success