Flink键控流键为空

Question

我试图在Flink中的KeyedStream上执行映射操作：

stream.map(new JsonToMessageObjectMapper())
                    .keyBy("keyfield")
                    .map(new MessageProcessorStateful())

JsonToObjectMapper运算符的输出是 MessageObject 类的POJO，它具有String字段“ keyfield
”。然后，将流键入此字段。

MessageProcessorStateful是一个RichMapFunction，如下所示：

public class MessageAdProcessorStateful extends RichMapFunction<MessageObject, Tuple2<String, String>> {

    private transient MapState<String, Tuple2<Tuple3<String, String, String>, Tuple2<Double, Long>>> state;
    ...
    @Override
    public void open(Configuration config) throws Exception {
        MapStateDescriptor<String, Tuple2<Tuple3<String, String, String>, Tuple2<Double, Long>>> descriptor =
                    new MapStateDescriptor<>(
                        "state",                                                                                     // the state name
                            TypeInformation.of(new TypeHint<String>() {}),
                            TypeInformation.of(new TypeHint<Tuple2<Tuple3<String, String, String>, Tuple2<Double, Long>>>() {}) ); // type information
                    state = getRuntimeContext().getMapState(descriptor);

        state.put(...); // Insert a key, value here. Exception here!

    }
}

该代码引发NullPointer异常：

Caused by: java.lang.NullPointerException: No key set. This method should not be called outside of a keyed context.
    at org.apache.flink.util.Preconditions.checkNotNull(Preconditions.java:75)
    at org.apache.flink.runtime.state.heap.CopyOnWriteStateTable.checkKeyNamespacePreconditions(CopyOnWriteStateTable.java:528)
    at org.apache.flink.runtime.state.heap.CopyOnWriteStateTable.computeHashForOperationAndDoIncrementalRehash(CopyOnWriteStateTable.java:722)
    at org.apache.flink.runtime.state.heap.CopyOnWriteStateTable.get(CopyOnWriteStateTable.java:265)
    at org.apache.flink.runtime.state.heap.CopyOnWriteStateTable.get(CopyOnWriteStateTable.java:306)
    at org.apache.flink.runtime.state.heap.HeapMapState.put(HeapMapState.java:75)
    at org.apache.flink.runtime.state.UserFacingMapState.put(UserFacingMapState.java:52)
    at org.myorg.quickstart.MessageStreamProcessor$MessageAdProcessorStateful.open(MessageStreamProcessor.java:226)
    at org.apache.flink.api.common.functions.util.FunctionUtils.openFunction(FunctionUtils.java:36)
    at org.apache.flink.streaming.api.operators.AbstractUdfStreamOperator.open(AbstractUdfStreamOperator.java:102)
    at org.apache.flink.streaming.runtime.tasks.StreamTask.openAllOperators(StreamTask.java:393)
    at org.apache.flink.streaming.runtime.tasks.StreamTask.invoke(StreamTask.java:254)
    at org.apache.flink.runtime.taskmanager.Task.run(Task.java:718)
    at java.lang.Thread.run(Thread.java:748)

尽管我已经验证了’keyfield’始终是有效的字符串，但KeyedStream之一的keyedState中的键似乎为null。根据Flink文档，休息似乎是正确的。知道发生了什么吗？

Answer 1

问题是您尝试访问方法中的键控状态open()。

键控状态为每个键维护一个状态实例。在您的示例中，您正在使用MapState。因此MapState，每个密钥都有一个实例。访问状态时，您将始终获得与当前处理记录的键对应的状态实例。在一个MapFunction（如您的示例中）这将是传递给该map()方法的记录。

由于open()未与记录一起调用，因此当前键open()为null，因此无法访问键控状态。