CODEWITHSUNDEEP
javajava-native-interfacejavacpp
milan
milan

Reputation: 2515

Data consistency and native callback from java

I have Java code that calls a native function which fills results by invoking a callback method multiple times before it returns. The native code doesn't spawn any threads on its own. All java-native is handled by javacpp. The outline looks like this:

class Scanner {
    private final LinkedList<Long> matchedIds = new LinkedList<>();

    private final Handler handler = new Handler() {
        public void call(int id) {
            matchedIds.add(id);
        }
    };

    public void scan() {
      callToNativeLib(handler);
      matchedIds.forEach(System.out::println)  // throws sometimes
    }
}


new Scanner().scan();

The problem is it sometimes (very rarely) throws:

java.lang.NullPointerException: Cannot read field "next" because "this.next" is null
    at java.base/java.util.LinkedList$ListItr.next(LinkedList.java:897)
    at java.base/java.lang.Iterable.forEach(Iterable.java:74)

Why could it be? Almost like the list is made broken by callbacks coming from native code.

EDIT: Got a ConcurrentModificationException. My assumptions must be wrong.

Upvotes: 0

Views: 68

Answers (0)

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