Github user EronWright commented on the issue:
https://github.com/apache/flink/pull/3933
@tillrohrmann the issue is with code that uses the thread's context
classloader (TCCL), that may execute in `Factory.create` or later in the hook
methods. For example, the Pravega connector uses grpc, which uses the TCCL
during initialization (see
[ManagedChannelProvider](https://github.com/grpc/grpc-java/blob/v1.3.0/core/src/main/java/io/grpc/ManagedChannelProvider.java#L132)).
Looking at other areas in Flink where usercode is called, I see that the
TCCL is consistently set. For example, see
[InputFormatVertex](https://github.com/apache/flink/blob/2c68085f658873c2d5836fbad6b82be76a79f0f9/flink-runtime/src/main/java/org/apache/flink/runtime/jobgraph/InputFormatVertex.java#L81),
[ExecutionJobVertex](https://github.com/apache/flink/blob/7ad489d87281b74c53d3b1a0dd97e56b7a8ef303/flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/ExecutionJobVertex.java#L229),
and
[Task](https://github.com/apache/flink/blob/6181302f1ab741b86af357e4513f5952a5fc1531/flink-runtime/src/main/java/org/apache/flink/runtime/taskmanager/Task.java#L698).
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