我试着在jar里添加一个自定义项，然后试着加载。下面是我的代码片段

register 'target/warcbase-0.1.0-SNAPSHOT-fatjar.jar';

DEFINE WarcLoader org.warcbase.pig.WarcLoader();

warc = LOAD '/raw/' USING WarcLoader AS (url: chararray, date: chararray, mime: chararray, content: bytearray);
STORE warc INTO '/raw/proc/';

我有以下例外。不幸的是，它没有告诉我哪个类没有找到。下面是整个堆栈跟踪

Backend error message
---------------------
Error: java.io.IOException: java.lang.ClassNotFoundException: Class  not found
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigSplit.readFields(PigSplit.java:236)
        at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:71)
        at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:42)
        at org.apache.hadoop.mapred.MapTask.getSplitDetails(MapTask.java:372)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:751)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
        at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:163)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:415)
        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1628)
        at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:158)
Caused by: java.lang.ClassNotFoundException: Class  not found
        at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:1982)
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigSplit.readFields(PigSplit.java:224)
        ... 10 more

Pig Stack Trace
---------------
ERROR 0: java.io.IOException: java.lang.ClassNotFoundException: Class  not found

org.apache.pig.backend.executionengine.ExecException: ERROR 0: java.io.IOException: java.lang.ClassNotFoundException: Class  not found
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MapReduceLauncher.getStats(MapReduceLauncher.java:819)
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MapReduceLauncher.launchPig(MapReduceLauncher.java:452)
        at org.apache.pig.backend.hadoop.executionengine.HExecutionEngine.launchPig(HExecutionEngine.java:280)
        at org.apache.pig.PigServer.launchPlan(PigServer.java:1390)
        at org.apache.pig.PigServer.executeCompiledLogicalPlan(PigServer.java:1375)
        at org.apache.pig.PigServer.execute(PigServer.java:1364)
        at org.apache.pig.PigServer.executeBatch(PigServer.java:415)
        at org.apache.pig.PigServer.executeBatch(PigServer.java:398)
        at org.apache.pig.tools.grunt.GruntParser.executeBatch(GruntParser.java:171)
        at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:234)
        at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:205)
        at org.apache.pig.tools.grunt.Grunt.exec(Grunt.java:81)
        at org.apache.pig.Main.run(Main.java:624)
        at org.apache.pig.Main.main(Main.java:170)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.apache.hadoop.util.RunJar.run(RunJar.java:221)
        at org.apache.hadoop.util.RunJar.main(RunJar.java:136)
Caused by: java.io.IOException: java.lang.ClassNotFoundException: Class  not found
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigSplit.readFields(PigSplit.java:236)
        at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:71)
        at org.apache.hadoop.io.serializer.WritableSerialization$WritableDeserializer.deserialize(WritableSerialization.java:42)
        at org.apache.hadoop.mapred.MapTask.getSplitDetails(MapTask.java:372)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:751)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
        at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:163)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:415)
        at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1628)
        at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:158)
Caused by: java.lang.ClassNotFoundException: Class  not found
        at org.apache.hadoop.conf.Configuration.getClassByName(Configuration.java:1982)
        at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigSplit.readFields(PigSplit.java:224)

请帮助我如何从这里开始。