[英]Why do I have to deal with Throwable if I invoke getCause() on an Exception
[英]Why do i have a mapping exception?
现在,我想制作一个小型Java应用程序,以学习休眠框架。 但这给了我org.hibernate.MappingException: Repeated column in mapping for entity: model.Book column: author (should be mapped with insert="false" update="false")
。 并且,如果我从entitys.hbm.xml中删除了作者列映射,那么它向我显示了SQL消息“ SELEC FROM ...”,但是在那之后,它给了我两个例外:
org.hibernate.exception.GenericJDBCException: could not execute query
java.sql.SQLException: No database selected.
谁能帮我?
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;
}
您有两个属性映射到列作者:
<property name="author" column="author" type="string"/>
<property name="publisher" column="author" type="string"/>
要解决第二个错误,请将数据库名称附加到您的JDBC连接URL:
<property name="hibernate.connection.url">
jdbc:mysql://127.0.0.1:3306/dbname
</property>
进一步阅读您的资料后,我偶然发现了您的getBooks()方法。 您不应在每次需要休眠会话时都创建一个SessionFactory。 SessionFactory的创建过于昂贵(按时间衡量),因此每次都无法执行此操作。 最小的解决方案是您可以要求SessionFactory的Singleton类:
public class SessionFactoryUtil {
private static SessionFactory sessionFactory;
private SessionFactoryUtil() {}
static {
sessionFactory = new Configuration().configure().buildSessionFactory();
}
public static SessionFactory getInstance() { return sessionFactory; }
}
也许您应该在服务器内部定义数据库/方案名称。 当前,您仅指定数据库服务器。
<property name = "hibernate.connection.url">jdbc:mysql://127.0.0.1:3306/DATABASENAME</property>
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