这似乎应该相当简单,但我很难想出一个优雅的解决方案。
让我们使用一个基本文件系统的示例,它由两个带 hibernate 注释的类File和Folder组成。
我们将两个类的共同属性抽象为FileSystemObject接口:
public interface FileSystemObject {
public String getName();
public URI getWhere();
}
还有两个类:
文件:
@Entity
@Table(name = "FILE")
public class File implements FileSystemObject, Serializable {
private static final long serialVersionUID = 1L;
@Id
@Column(name = "FILE_ID")
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;
@Column(name = "NAME", nullable = false)
@Index(name = "FILE_NAME", columnNames={"NAME"})
private String name;
@Column(nullable = false)
private URI where;
public File() {}
public File(String name, URI where) {
this.name = name;
this.where = where;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public URI getWhere() {
return where;
}
public void setWhere(URI where) {
this.where = where;
}
}
文件夹:
@Entity
@Table(name = "FOLDER")
public class Folder implements FileSystemObject, Serializable {
private static final long serialVersionUID = 2L;
@Id
@Column(name = "FOLDER_ID")
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;
@Column(name = "NAME", nullable = false)
@Index(name = "FOLDER_NAME", columnNames={"NAME"})
private String name;
@Column(nullable = false)
private URI where;
@CollectionOfElements
@JoinTable(name = "FOLDER_CONTENTS",
joinColumns = @JoinColumn(name = "FOLDER_ID"))
private List<FileSystemObject> contents;
public Folder() {}
public Folder(String name, URI where, List<FileSystemObject> contents) {
this.name = name;
this.where = where;
this.contents = contents;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public URI getWhere() {
return where;
}
public void setWhere(URI where) {
this.where = where;
}
public List<FileSystemObject> getContents() {
return contents;
}
public void setContents(List<FileSystemObject> contents) {
this.contents = contents;
}
}
从逻辑上讲,Folder的内容可以是File或Folder ,因此内容 List 的类型为FileSystemObject是有意义的。
现在,假设一切都在 pom.xml 中正确设置,当我尝试生成一些模式时:
mvn hibernate3:hbm2ddl
它抛出以下错误:
Failed to execute goal org.codehaus.mojo:hibernate3-maven-plugin:2.0:hbm2ddl (generate-ddl) on
project foo: Execution generate-ddl of goal org.codehaus.mojo:hibernate3-maven
plugin:2.0:hbm2ddl failed: Could not determine type for: com.foo.data.FileSystemObject, for
columns: [org.hibernate.mapping.Column(element)] -> [Help 1]
希望有人可以对此有所了解!