System.gc()之后未释放Java弱引用

Integer integer = 2;
WeakReference<Integer> wi = new WeakReference<>(integer);
WeakReference<Integer> sr = new WeakReference<>(new Integer(3));
System.out.println(wi.get());
System.out.println(sr.get());
System.gc();
System.out.println("step 1 wi = " + wi.get());
System.out.println("step 1 sr =: " + sr.get());
integer = null;
System.gc();
try {
    Thread.sleep(1000);
} catch (InterruptedException e) {
    e.printStackTrace();
}
System.out.println("step 1 wi = " + wi.get());
System.out.println("step 1 sr =: " + sr.get());

但即使设置了“integer=null”,wi也拒绝为null。而sr在“System.gc()”之后正好为null。它打印:

2
3
step 1 wi = 2
step 1 sr =: null
step 1 wi = 2
step 1 sr =: null

(1) wi和sr的核心区别是什么?

谢谢。