关于4大引用的疑惑点（待解决）

同样的代码，将引用的对象置为null,为啥main方法里可以回收，android里button click触发的无法回收？

没有回收

 private void methodWeakTest() {
        String str = new String("value");

        WeakReference weakReference = new WeakReference<>(str);

        str = null;

        System.out.println("弱引用：WeakReference == GC回收前：" + weakReference.get());

        //系统GC垃圾回收
        System.gc();
        System.runFinalization();

        System.out.println("弱引用：WeakReference == GC回收后：" + weakReference.get());
    }

回收

public static  void main(String[] args){
        String str = new String("value");

        WeakReference weakReference = new WeakReference<>(str);

        str = null;

        System.out.println("弱引用：WeakReference == GC回收前：" + weakReference.get());

        //系统GC垃圾回收
        System.gc();
        System.runFinalization();

        System.out.println("弱引用：WeakReference == GC回收后：" + weakReference.get());
    }