下面这个类是一个树结构的节点，我想用Hibernate的OneToMany来实现树的结构，报错信息在后面，实在弄不明白了，各位帮帮忙...
- Java code
package model;import java.util.Set;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.ManyToOne;import javax.persistence.OneToMany;import org.hibernate.annotations.GenericGenerator;import com.sun.istack.internal.NotNull;@Entitypublic class Node { private int nodeid;//主键 private String nodename;//节点名 private int nodelevel;//节点层级// private int nodeSerialNo;//节点序号 private Node parentnode;//父节点 private Set<Node> childnodes;//子节点 public Node(){ } @Id @GeneratedValue() @GenericGenerator(name="EAM",strategy="native") public int getNodeid() { return nodeid; } public void setNodeid(int nodeid) { this.nodeid = nodeid; } @NotNull public String getNodename() { return nodename; } public void setNodename(String nodename) { this.nodename = nodename; } @NotNull public int getNodelevel() { return nodelevel; } public void setNodelevel(int nodelevel) { this.nodelevel = nodelevel; } @OneToMany(mappedBy="parentnode",fetch=FetchType.EAGER) public Node getParentnode() { return parentnode; } public void setParentnode(Node parentnode) { this.parentnode = parentnode; } @ManyToOne public Set<Node> getChildnodes() { return childnodes; } public void setChildnodes(Set<Node> childnodes) { this.childnodes = childnodes; }}报错信息：
org.hibernate.AnnotationException: Illegal attempt to map a non collection as a @OneToMany, @ManyToMany or @CollectionOfElements: model.Node.parentnode
org.hibernate.cfg.annotations.CollectionBinder.getCollectionBinder(CollectionBinder.java:324)
org.hibernate.cfg.AnnotationBinder.processElementAnnotations(AnnotationBinder.java:1723)
org.hibernate.cfg.AnnotationBinder.processIdPropertiesIfNotAlready(AnnotationBinder.java:796)
org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:707)
org.hibernate.cfg.Configuration$MetadataSourceQueue.processAnnotatedClassesQueue(Configuration.java:4035)
org.hibernate.cfg.Configuration$MetadataSourceQueue.processMetadata(Configuration.java:3989)
org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1398)
org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1856)
controller.TheStructure.enter(TheStructure.java:51)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:452)
com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:291)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:254)
com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:263)
org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)