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| 序号 | 网址 | 内容介绍 | 备注 |
| 1 | https://blog.csdn.net/LangLang1111111/article/details/121531354 | Keyed state can only be used on a 'keyed stream', i.e., after a 'keyBy()' operation. | |
| 2 | https://blog.csdn.net/yang_shibiao/article/details/122739655 | BroadcastState的使用教程 | |
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1 出现如下错误信息：

java.lang.NullPointerException: Keyed state can only be used on a 'keyed stream', i.e., after a 'keyBy()' operation.

at org.apache.flink.util.Preconditions.checkNotNull(Preconditions.java:76)

at org.apache.flink.streaming.api.operators.StreamingRuntimeContext.checkPreconditionsAndGetKeyedStateStore(StreamingRuntimeContext.java:238)

at org.apache.flink.streaming.api.operators.StreamingRuntimeContext.getMapState(StreamingRuntimeContext.java:230)

java.lang.NullPointerException: Keyed state can only be used on a 'keyed str

2 出现如下错误信息