用ie8時，請求json,eclipse直接死機！！！！
調試時，可以用chrome,看到無限循環的報錯。。。類似
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。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。
User 與 Role 是hibernate的多對多的關系！！！！
jackson中的@JsonBackReference和@JsonManagedReference，以及@JsonIgnore均是為了解決對象中存在雙向引用導致的無限遞歸（infinite recursion）問題。這些標注均可用在屬性或對應的get、set方法中。
@JsonBackReference和@JsonManagedReference：這兩個標注通常配對使用，通常用在父子關系中。@JsonBackReference標注的屬性在序列化（serialization，即將對象轉換為json數據）時，會被忽略（即結果中的json數據不包含該屬性的內容）。@JsonManagedReference標注的屬性則會被序列化。在序列化時，@JsonBackReference的作用相當於@JsonIgnore，此時可以沒有@JsonManagedReference。但在反序列化（deserialization，即json數據轉換為對象）時，如果沒有@JsonManagedReference，則不會自動注入@JsonBackReference標注的屬性（被忽略的父或子）；如果有@JsonManagedReference，則會自動注入自動注入@JsonBackReference標注的屬性。
@JsonIgnore：直接忽略某個屬性，以斷開無限遞歸，序列化或反序列化均忽略。當然如果標注在get、set方法中，則可以分開控制，序列化對應的是get方法，反序列化對應的是set方法。在父子關系中，當反序列化時，@JsonIgnore不會自動注入被忽略的屬性值（父或子），這是它跟@JsonBackReference和@JsonManagedReference最大的區別。