出现这个错误是自己的粗心大意，解决：
在配置flume-conf.properties文件时，source和channel的对应关系是：
myAgentName.sources.mySourceName.channels = myChannelName
myAgentName.sinks.mySinkName.channel = myChannelName
注意其中的后缀，带s和不带s后缀。
这也恰好说明
source可以“流向”多个channel，而sink只能接收一个channel的“流入”。
从channel的角度看：channel既可以接收多个source的“流入”，又可以“流向”多个sink
例如多对多关系：
对应的配置如下：
# example.conf: A single-node Flume configuration # Name the components on this agent
agent1.sources=sour1 s2 s3
agent1.sinks=sink1 sink12 sink21 sink3
agent1.channels=chan1 chan2 chan3 # Describe/configure the source
agent1.sources.sour1.type=netcat
agent1.sources.sour1.bind=localhost
agent1.sources.sour1.port=44444 agent1.sources.s2.type=netcat
agent1.sources.s2.bind=localhost
agent1.sources.s2.port=44445 agent1.sources.s3.type=netcat
agent1.sources.s3.bind=localhost
agent1.sources.s3.port=44446 # Describe the sink
agent1.sinks.sink1.type=logger
agent1.sinks.sink12.type=logger
agent1.sinks.sink21.type=logger
agent1.sinks.sink3.type=logger # Use a channel which buffers events in memory
agent1.channels.chan1.type=memory
agent1.channels.chan1.capacity=1000
#agent1.channels.chan1.transactionCapacity=100 agent1.channels.chan2.type=memory
agent1.channels.chan2.capacity=1000 agent1.channels.chan3.type=memory
agent1.channels.chan3.capacity=1000 # Bind the source and sink to the channel
agent1.sources.sour1.channels=chan1 chan2
agent1.sources.s2.channels=chan2
agent1.sources.s3.channels=chan2 chan3 agent1.sinks.sink1.channel=chan1
agent1.sinks.sink12.channel=chan1
agent1.sinks.sink21.channel=chan2
agent1.sinks.sink3.channel=chan3