sqoop导出错误。我搜索了许多链接，但找不到解决方法。

enter code here18/12/02 22:55:46 ERROR tool.ExportTool: Encountered IOException running export job: 
java.io.IOException: No columns to generate for ClassWriter
    at org.apache.sqoop.orm.ClassWriter.generate(ClassWriter.java:1663)
    at org.apache.sqoop.tool.CodeGenTool.generateORM(CodeGenTool.java:106)
    at org.apache.sqoop.tool.ExportTool.exportTable(ExportTool.java:63)
    at org.apache.sqoop.tool.ExportTool.run(ExportTool.java:99)
    at org.apache.sqoop.Sqoop.run(Sqoop.java:147)
    at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:70)
    at org.apache.sqoop.Sqoop.runSqoop(Sqoop.java:183)
    at org.apache.sqoop.Sqoop.runTool(Sqoop.java:234)
    at org.apache.sqoop.Sqoop.runTool(Sqoop.java:243)
    at org.apache.sqoop.Sqoop.main(Sqoop.java:252)