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

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)

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