对象寿命

Question

class Beta{}
class Alpha {
   static Beta b1;
   Beta b2;
}

void main (..){
 Beta b1 = new Beta();
 Beta b2 = new Beta();
 Alpha a1 = new Alpha();
 Alpha a2 = new Alpha();
 a1.b1=b1;
 a1.b2=b1;
 a2.b2=b2;
 a1=null; b1=null; b2 = null;
 // do stuff
}

这是一个示例Java证书问题，我似乎并不理解。这个问题询问有多少符合GC条件的对象在第//行做什么，答案是1个对象，而我正在考虑4.有人可以解释一下吗？ 因此，b1和a1引用的第一个Beta对象应该符合GC条件，因为b1 = null且a1 = null。不应该由a1引用的Alpha对象是否有资格获得GC，因为a1 = null并且没有其他任何引用它？或者，如果实例Beta b1变量不是静态的，它是否符合条件？

Answer 1

只有a1符合GC条件。

b1由Alpha的静态成员引用（由于a1.b1 = b1;） b2由a2的非静态成员引用（由于a2.b2 = b2;） a2引用未设置为null。

Answer 2

只有一个对象符合GC条件。设1A，2A，1B，2B分别表示A和B的第一和第二对象实例（不是参考）。我在每个步骤后用注释注释了您的代码，以反映当前对这四个对象的引用状态。

class Beta{}
class Alpha {
   static Beta b1;
   Beta b2;
}

void main (..){
 Beta b1 = new Beta();
 Beta b2 = new Beta();
 Alpha a1 = new Alpha();
 Alpha a2 = new Alpha();
 //1A <- pointed to by a1
 //2A <- pointed to by a2
 //1B <- pointed to by b1
 //2B <- pointed to by b2 

 a1.b1=b1;
 //1A <- pointed to by a1
 //2A <- pointed to by a2
 //1B <- pointed to by b1, Alpha.b1
 //2B <- pointed to by b2 

 a1.b2=b1;
 //1A <- pointed to by a1
 //2A <- pointed to by a2
 //1B <- pointed to by b1, Alpha.b1, a1.b2
 //2B <- pointed to by b2 

 a2.b2=b2;
 //1A <- pointed to by a1
 //2A <- pointed to by a2
 //1B <- pointed to by b1, Alpha.b1
 //2B <- pointed to by b2, a2.b2 

 a1=null; b1=null; b2 = null;
 //1A <- Eligible for GC
 //2A <- pointed to by a2
 //1B <- pointed to by Alpha.b1
 //2B <- pointed to by a2.b2 

// do stuff
}