構造器注入時，因為實例化(構造器方法)時需要初始化依賴的對象，所以無法解決循環依賴問題 ``` public?class?A?{ ????private?B?b; ????public?A(B?b)?{ ????????this.b?=?b; ????} } public?class?B?{ ????private?A?a; ????public?B(A?a)?{ ????????this.a?=?a; ????} } ```