但是当Collections.sort无论是否有unwrapped = null时,我都得到了NullPointerException.
这可能发生在我的示例源代码和您的示例源代码之间的差异,或Java版本差异.我会检查你是否告诉Java版本.

我使用java下面的版本.

java version "1.7.0_40"
Java(TM) SE Runtime Environment (build 1.7.0_40-b43)
Java HotSpot(TM) 64-Bit Server VM (build 24.0-b56,mixed mode)

如果你想欺骗JIT编译,下面的代码插入你的源代码而不是unwrapped = null(例如).然后,JIT编译不会消除未包装的代码.

long value = unwrapped.size() * unwrapped.size();
if(value * value % 3 == 1) {
  //Because value * value % 3 always is 1 or 0,this code can't reach. 
  //Insert into this the source code that use unwrapped array,for example,show unwrapped array.
}

我的考试成绩如下.

>如果JIT没有优化我对应于updateOrdering的方法,则不会发生NullPointerException.
>如果JIT优化了我的方法,那么在某些时候会发生NullPointerException.

>如果JIT优化我的方法插入上面的代码欺骗JIT编译器,那么就不会发生NullPointerException.

所以,我(和你)建议JIT optimze消除未包装的代码,然后发生NullPointerException.

顺便说一下,如果要显示JIT编译器优化,可以使用-XX：PrintCompilation调用java.
如果要显示GC,请使用-verbose：gc.

仅供参考,我的示例源代码如下.

public class WeakSampleMain {
    private static List<WeakReference<Integer>> weakList = new LinkedList<>();
    private static long sum = 0;
    public static void main(String[] args) {
        System.out.println("start");
        int size = 1_000_000;
        for(int i = 0; i < size; i++) {
            Integer value = Integer.valueOf(i);
            weakList.add(new WeakReference<Integer>(value));
        }
        for(int i = 0; i < 10; i++) {
            jitSort();
        }
        GcTask gcTask = new GcTask();
        Thread thread = new Thread(gcTask);
        thread.start();
        for(int i = 0; i < 100000; i++) {
            jitSort();
        }
        thread.interrupt();
        System.out.println(sum);
    }
 
    public static void jitSort() {
        List<Integer> unwrappedList = unwrapped();
        removeNull();
        Collections.sort(weakList,new Comparator<WeakReference<Integer>>() {
 
                    @Override
                    public int compare(WeakReference<Integer> o1,WeakReference<Integer> o2) {
                        return Integer.compare(o1.get(),o2.get());
                    }
        }
                );
        for(int i = 0; i < Math.min(weakList.size(),1000); i++) {
            sum += weakList.get(i).get();
        }
        unwrappedList = null;
//          long value = (sum + unwrappedList.size());
//          if((value * value) % 3 == 2) {
//              for(int i = 0; i < unwrappedList.size(); i++) {
//                  System.out.println(unwrappedList.get(i));
//              }
//          }
    }
 
    public static List<Integer> unwrapped() {
        ArrayList<Integer> list = new ArrayList<Integer>();
        for(WeakReference<Integer> ref : weakList) {
            Integer i = ref.get();
            if(i != null) {
                list.add(i);
            }
        }
        return list;
    }
 
    public static void removeNull() {
        Iterator<WeakReference<Integer>> itr = weakList.iterator();
        while(itr.hasNext()) {
            WeakReference<Integer> ref = itr.next();
            if(ref.get() == null) {
                itr.remove();
            }
        }
    }
 
    public static class GcTask implements Runnable {
        private volatile int result = 0;
        private List<Integer> stockList = new ArrayList<Integer>();
        public void run() {
            while(true) {
                if(Thread.interrupted()) {
                    break;
                }
                int size = 1000000;
                stockList = new ArrayList<Integer>(size);
                for(int i = 0; i < size; i++) {
                    stockList.add(new Integer(i));
                }
                if(System.currentTimeMillis() % 1000 == 0) {
                    System.out.println("size : " + stockList.size());
                }
            }
        }
 
        public int getResult() {
            return result;
        }
    }
}