请问各位大大...s:optiontransferselect 怎么传值啊???

是这样的...现在在做个图书管理系统有book author 两张表,他们是个多对多的关系在book这边设置了manytomany        private Set<Author> authors = new HashSet<Author>();
        @ManyToMany(cascade=CascadeType.ALL)
@JoinTable(name="authorisbn",
joinColumns={@JoinColumn(name="isbn")},
inverseJoinColumns={@JoinColumn(name="authorId")}
)
public Set<Author> getAuthors() {
return authors;
}
public void setAuthors(Set<Author> authors) {
this.authors = authors;
}
然后jsp页面中配置
<s:optiontransferselect theme="simple" 
"list="authors" listValue="firstName" listKey="authorId"
doubleName="book.authors.authorId" doubleList="" />
这样设置之后会报这个异常不知道为什么 Error setting expression 'book.authors.authorId' with value '[Ljava.lang.String;@197d415'是doubleList doubleListkey doubleListvalue 要设置么? 
请问怎么设置?