errors collection when study hadoop(1)

1. hadoop jar : Not a valid JAR

at first ,I run the following code:

bin/hadoop jar /test/wordcount.jar WordCount /test/input /test/output

but errors come: Not a valid JAR

when i use the local jar package ,nothing goes wrong:

bin/hadoop jar /home/twq/Desktop/wordcount.jar WordCount /test/input /test/output

so, remember that jar packeages needn't be on hdfs .