為什么會有映射異常？

Question

現在，我想制作一個小型Java應用程序，以學習休眠框架。 但這給了我org.hibernate.MappingException: Repeated column in mapping for entity: model.Book column: author (should be mapped with insert="false" update="false") 。 並且，如果我從entitys.hbm.xml中刪除了作者列映射，那么它向我顯示了SQL消息“ SELEC FROM ...”，但是在那之后，它給了我兩個例外：

• org.hibernate.exception.GenericJDBCException: could not execute query
• java.sql.SQLException: No database selected.

誰能幫我？

hibernate.cfg.xml文件：

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

<hibernate-configuration>
<session-factory>

    <property name = "hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name = "hibernate.connection.url">jdbc:mysql://127.0.0.1:3306</property>
    <property name = "hibernate.connection.username">root</property>
    <property name = "hibernate.connection.password"></property>
    <property name = "hibernate.connection.pool_size">10</property>
    <property name = "dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name = "hibernate.hbm2ddl.auto">update</property>
    <property name = "show_sql">true</property>

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

</session-factory>
</hibernate-configuration>

Entitys.hbm.xml文件：

<?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 = "model">

   <class name = "Genre" table = "virtual bookcase.genres">
      <id name = "id" column = "idGenres" type = "long">
        <generator class = "increment"/>
      </id>   
      <property name = "name" column = "name" type = "string"/>
      <set name = "books" table = "books" cascade = "all-delete-orphan">
        <key column = "idGenres" not-null = "true" />
        <one-to-many class = "Book"/>
      </set>
   </class>

   <class name = "Book" table = "virtual bookcase.books">
        <id name = "id" column = "idBooks" type = "long">
         <generator class = "increment"/>
        </id>
        <property name = "title" column = "title" type = "string"/>
        <property name = "author" column = "author" type = "string"/>
        <property name = "publisher" column = "author" type = "string"/>
        <property name = "pages" column = "pages" type = "short"/>
        <property name = "borrowed" column = "borrowed" type = "byte"/>
        <property name = "borrowedTo" column = "borrowedTo" type = "string"/>
   </class>

</hibernate-mapping>

Java實體：

public class Genre
{
    private long id;
    private String name;
    private Set<Book> books;

    public long getId()
    {
        return id;
    }

    public String getName()
    {
        return name;
    }

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

    public void setName(String name)
    {
        this.name = name;
    }

    public Set<Book> getBooks()
    {
        return books;
    }

    public void setBooks(Set<Book> books)
    {
        this.books = books;
    }

    @Override
    public String toString()
    {
        return name;
    }
}

getBooks（）方法：

public Set<Book> getBooks()
    {
        Set<Book> books = null;

        connect();

        SessionFactory sf = new Configuration().configure().buildSessionFactory();
        Session s = sf.openSession();

        Query q = s.createQuery("FROM Book");
        books = new TreeSet<Book>(q.list());

        for (Book b : books)
            System.out.println(b);

        s.close();
        sf.close();

        disconnect();

        return books;
    }

Answer 1

您有兩個屬性映射到列作者：

<property name="author" column="author" type="string"/>
<property name="publisher" column="author" type="string"/>

要解決第二個錯誤，請將數據庫名稱附加到您的JDBC連接URL：

<property name="hibernate.connection.url">
    jdbc:mysql://127.0.0.1:3306/dbname
</property>

進一步閱讀您的資料后，我偶然發現了您的getBooks（）方法。 您不應在每次需要休眠會話時都創建一個SessionFactory。 SessionFactory的創建過於昂貴（按時間衡量），因此每次都無法執行此操作。 最小的解決方案是您可以要求SessionFactory的Singleton類：

public class SessionFactoryUtil {

    private static SessionFactory sessionFactory;

    private SessionFactoryUtil() {}

    static {
       sessionFactory = new Configuration().configure().buildSessionFactory();
    }

    public static SessionFactory getInstance() { return sessionFactory; }

}

Answer 2

也許您應該在服務器內部定義數據庫/方案名稱。 當前，您僅指定數據庫服務器。

<property name = "hibernate.connection.url">jdbc:mysql://127.0.0.1:3306/DATABASENAME</property>