flink&kafka报错Caused by: org.apache.flink.api.common.functions.InvalidTypesException: Type of TypeVariable 'OUT' in 'interface xxx.api.functions.source.ParallelSourceFunction' could not be determined

flink消费kafka，使用自定义实现kafka的消息反序列化，报错

org.apache.flink.api.common.functions.InvalidTypesException: The return type of function 'Custom Source' could not be determined automatically, due to type erasure. You can give type information hints by using the returns(...) method on the result of the transformation call, or by letting your function implement the 'ResultTypeQueryable' interface.
    at org.apache.flink.api.dag.Transformation.getOutputType(Transformation.java:484)
    at org.apache.flink.streaming.api.datastream.DataStream.getType(DataStream.java:190)
    at org.apache.flink.streaming.api.datastream.DataStream.flatMap(DataStream.java:638)
    at com.liujun.flink.demo.QR_LOAD_Kafka_TO_ORACLE.main(QR_LOAD_Kafka_TO_ORACLE.java:65)
Caused by: org.apache.flink.api.common.functions.InvalidTypesException: Type of TypeVariable 'OUT' in 'interface org.apache.flink.streaming.api.functions.source.ParallelSourceFunction' could not be determined. This is most likely a type erasure problem. The type extraction currently supports types with generic variables only in cases where all variables in the return type can be deduced from the input type(s). Otherwise the type has to be specified explicitly using type information.
    at org.apache.flink.api.java.typeutils.TypeExtractor.createTypeInfoWithTypeHierarchy(TypeExtractor.java:827)
    at org.apache.flink.api.java.typeutils.TypeExtractor.privateCreateTypeInfo(TypeExtractor.java:747)
    at org.apache.flink.api.java.typeutils.TypeExtractor.createTypeInfo(TypeExtractor.java:713)
    at org.apache.flink.streaming.api.environment.StreamExecutionEnvironment.getTypeInfo(StreamExecutionEnvironment.java:2347)
    at org.apache.flink.streaming.api.environment.StreamExecutionEnvironment.addSource(StreamExecutionEnvironment.java:1713)
    at org.apache.flink.streaming.api.environment.StreamExecutionEnvironment.addSource(StreamExecutionEnvironment.java:1701)
    at org.apache.flink.streaming.api.environment.StreamExecutionEnvironment.addSource(StreamExecutionEnvironment.java:1663)
    at org.apache.flink.streaming.api.environment.StreamExecutionEnvironment.addSource(StreamExecutionEnvironment.java:1645)
    at com.liujun.flink.demo.QR_LOAD_Kafka_TO_ORACLE.main(QR_LOAD_Kafka_TO_ORACLE.java:60)

解决办法：以下方法默认null改为：BasicTypeInfo.STRING_TYPE_INFO

@Override
    public TypeInformation<String> getProducedType() {
        return BasicTypeInfo.STRING_TYPE_INFO;
    }