为什么我有映射异常？

Question

现在我想制作一个小型的java应用程序，以学习hibernate框架。但它给了我org.hibernate.MappingException: Repeated column in mapping for entity: model.Book column: author (should be mapped with insert="false" update="false")。如果我删除entities.hbm.xml笔者列映射，那么就说明我的SQL消息“SELEC FROM ......”但在那之后，它给了我2个例外：

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

任何人都可以帮我吗？

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> 

entities.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的实体：

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

你有两个属性映射到专栏作者：

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

为了解决第二个错误的数据库名称附加到你的JDBC连接URL：

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

进一步阅读您的资源我偶然发现了您的getBooks（）方法。每次需要Hibernate会话时，都不应创建SessionFactory。 SessionFactory的创建过于昂贵（及时测量）以便每次执行此操作。最小的解决方案将是一个Singleton类你可以要求为SessionFactory：

public class SessionFactoryUtil { 

    private static SessionFactory sessionFactory; 

    private SessionFactoryUtil() {} 

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

    public static SessionFactory getInstance() { return sessionFactory; } 

} 

Answer 2

也许你应该定义服务器内部的数据库/模式名称。目前，您只指定数据库服务器。

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