在Apache Flink中使用集合$ UnmodifiableCollection

Question

使用Apache Flink时使用以下代码:

DataStream<List<String>> result = source.window(Time.of(1, TimeUnit.SECONDS)).mapWindow(new WindowMapFunction<String, List<String>>() {

    @Override
    public void mapWindow(Iterable<String> iterable, Collector<List<String>> collector) throws Exception {
        List<String> top5 = Ordering.natural().greatestOf(iterable, 5);
        collector.collect(top5);
    }
}).flatten();

我得到了这个例外

Caused by: java.lang.UnsupportedOperationException
    at java.util.Collections$UnmodifiableCollection.add(Collections.java:1055)
    at com.esotericsoftware.kryo.serializers.CollectionSerializer.read(CollectionSerializer.java:116)
    at com.esotericsoftware.kryo.serializers.CollectionSerializer.read(CollectionSerializer.java:22)
    at com.esotericsoftware.kryo.Kryo.readClassAndObject(Kryo.java:761)
    at org.apache.flink.api.java.typeutils.runtime.kryo.KryoSerializer.deserialize(KryoSerializer.java:211)
    at org.apache.flink.streaming.runtime.streamrecord.StreamRecordSerializer.deserialize(StreamRecordSerializer.java:110)
    at org.apache.flink.streaming.runtime.streamrecord.StreamRecordSerializer.deserialize(StreamRecordSerializer.java:41)
    at org.apache.flink.runtime.plugable.NonReusingDeserializationDelegate.read(NonReusingDeserializationDelegate.java:55)
    at org.apache.flink.runtime.io.network.api.serialization.SpillingAdaptiveSpanningRecordDeserializer.getNextRecord(SpillingAdaptiveSpanningRecordDeserializer.java:125)
    at org.apache.flink.streaming.runtime.io.StreamInputProcessor.processInput(StreamInputProcessor.java:127)
    at org.apache.flink.streaming.runtime.tasks.OneInputStreamTask.run(OneInputStreamTask.java:56)
    at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:172)
    at org.apache.flink.runtime.taskmanager.Task.run(Task.java:581)
    at java.lang.Thread.run(Thread.java:745)

我怎么UnmodifiableCollection能跟Flink一起玩？

Answer 1

问题是CollectionSerializerKryo 的默认值不能再次反序列化集合,因为它不可修改(.add()调用失败).

要解决这个问题,我们可以使用UnmodifiableCollectionsSerializer从KRYO-串行项目.Flink过渡依赖于项目,因此不需要将其添加为依赖项.

接下来,我们必须使用Flink的Kryo实例注册序列化程序.

StreamExecutionEnvironment see = StreamExecutionEnvironment.getExecutionEnvironment();
Class<?> unmodColl = Class.forName("java.util.Collections$UnmodifiableCollection");
see.getConfig().addDefaultKryoSerializer(unmodColl, UnmodifiableCollectionsSerializer.class);

通常,我们不必调用Class.forName()注册序列化程序,但在这种情况下,java.util.Collections$UnmodifiableCollection包是可见的,因此我们无法直接访问该类.