array cast Java 8 vs Java 9

问题:

Does anyone have insights on why this code works on java 8 but not on java 9

String[] strings = (String[]) Arrays.asList("foo", "bar").toArray();
for (String string : strings) {
    System.out.println(string);
}

I understand we can specify the type while doing toArray instead of casting it. But I found this issue while debugging one of our dependency (hive-metastore-2.1.1 HiveMetaStoreClient line 274). So I don't have the liberty to change the code and we are running java 9. Is there a way to get around this? Is this a problem with java 9(since it seems like a breaking change) or just file a bug in the hive repo.

回答1:

Seems like it might be due to the change (coll) Arrays.asList(x).toArray().getClass() should be Object[].class

Looks like they fixed a bug that toArray could return types other than Object.

Quoting the release notes

This may cause code that was expecting the old behavior to fail with a ClassCastException...If this problem occurs, rewrite the code to use the one-arg form toArray(T[]), and provide an instance of the desired array type. This will also eliminate the need for a cast.

So it seems like you'll need to file a bug in the Hive repo to update the code to work after this change.

Looks like they actually added a configuration value in a future commit which if set with a certain value would actually avoid the code path causing the issue. https://github.com/apache/hive/commit/07492e0d2f1942c1794a3190610e10207c850cf7#diff-ca39aa4869cc58909a31c761cd7a27ccR257

Maybe you can upgrade to a version that has this and use this configuration to avoid the problem. So long as you don't care about the functionality that requires that code path. Seems like the code causing the problem is selecting which URI to use randomly instead of just picking the first one out of a list.

回答2:

The implementation of Arrays.ArrayList.toArray seems to have been changed. The old implementation was to just clone the backing array:

private final E[] a;

ArrayList(E[] array) {
    a = Objects.requireNonNull(array);
}

@Override
public Object[] toArray() {
    return a.clone();
}

The new implementation forces the returned array to be an Object[]:

@Override
public Object[] toArray() {
    return Arrays.copyOf(a, a.length, Object[].class);
}

To be clear, though, in Java 8 the cast only worked because the backing array was originally a String[], created by the asList varargs. Implicitly all that was happening was new String[] {"foo", "bar"}.clone(), but the array was passed through the asList List implementation.

---

As for fixing the broken dependency, I don't think there's a way besides either using a Java 8 run-time environment or rewriting what was introduced in that commit. Filing a bug report seems like the right thing to do.