问题三：循环引用
还是先来看一个例子。下面是WebService的接口：
1 @WebService
2 public interface IHello {
3
4 @WebMethod
5 public String sayHello(Teacher teacher);
6
7 }
8
9 public class Teacher {
10 private Student[] students;
11
12 //getters and setters
13 ...
14 }
15
16 public class Student {
17 private Teacher teacher;
18
19 //getters and setters
20 ...
21 }
2 public interface IHello {
3
4 @WebMethod
5 public String sayHello(Teacher teacher);
6
7 }
8
9 public class Teacher {
10 private Student[] students;
11
12 //getters and setters
13 ...
14 }
15
16 public class Student {
17 private Teacher teacher;
18
19 //getters and setters
20 ...
21 }
请注意，Teacher和Student是一对多的“双向”关系。在这种情况下，我们可以想一下如何将一个Teacher对象转换成一段XML?
您可能想到下面的答案：
1 <teacher>
2 <students>
3 <teacher>
4 <students>
5 <teacher>
6 <students>
7 ...
8 </students>
9 ...
10 </teacher>
11 </students>
12 ...
13 </teacher>
14 </students>
15
16 <students>
17 ...
18 </students>
19 ...
20 </teacher>
2 <students>
3 <teacher>
4 <students>
5 <teacher>
6 <students>
7 ...
8 </students>
9 ...
10 </teacher>
11 </students>
12 ...
13 </teacher>
14 </students>
15
16 <students>
17 ...
18 </students>
19 ...
20 </teacher>
看到了吧，XML竟然也会进入“死循环”。问题的根源在于对象之间的循环引用。这种问题通常在客户端发送WebService请求之前就会抛出异常，因为无法将这个对象转换为可传输的XML。
