com.google.gdata.util.PreconditionFailedException on ListEntry.update() in Google Docs Java API

Question

I have an application which manipulates the rows of a google spreadsheet. Occasionally, when I call ListEntry.update(), I receive the following stack trace:

Exception in thread "main" java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.eclipse.jdt.internal.jarinjarloader.JarRsrcLoader.main(JarRsrcLoader.java:58)
Caused by: com.google.gdata.util.PreconditionFailedException: Precondition Failed
Mismatch: etags = ["E10QemAgYit7ImA-CEFaShYM"], version = [2ag9hk74om621l]
at com.google.gdata.client.http.HttpGDataRequest.handleErrorResponse(HttpGDataRequest.java:614)
at com.google.gdata.client.http.GoogleGDataRequest.handleErrorResponse(GoogleGDataRequest.java:564)
at com.google.gdata.client.http.HttpGDataRequest.checkResponse(HttpGDataRequest.java:560)
at com.google.gdata.client.http.HttpGDataRequest.execute(HttpGDataRequest.java:538)
at com.google.gdata.client.http.GoogleGDataRequest.execute(GoogleGDataRequest.java:536)
at com.google.gdata.client.Service.update(Service.java:1563)
at com.google.gdata.client.Service.update(Service.java:1530)
at com.google.gdata.client.GoogleService.update(GoogleService.java:597)
at com.google.gdata.data.BaseEntry.update(BaseEntry.java:639)
at feedProcessor.ProcessClientFeed.UpdateRow(ProcessClientFeed.java:466)
at feedProcessor.ProcessClientFeed.updateGoogleSpreadsheet(ProcessClientFeed.java:404)
at feedProcessor.ProcessClientFeed.processFeed(ProcessClientFeed.java:318)
at feedProcessor.ProcessClientFeed.main(ProcessClientFeed.java:61)
... 5 more

Answer 1

Here is the relevant documentation:

https://developers.google.com/gdata/javadoc/com/google/gdata/data/spreadsheet/ListEntry

https://developers.google.com/gdata/javadoc/com/google/gdata/data/BaseEntry#update()

According to these docs, the update() function is not even capable of throwing a PreconditionFailedException, so the docs are essentially useless here. Testing the issue has shown that this exception is thrown when you try to call the update() function on the same row more than once in a session. Exactly what defines a 'session' is still unclear, but if you loop through all your rows more than once, and call update() on each row in each iteration, you will get this error. The only resolution I am aware of is to write your software such that each row (ListEntry) has update() called only once.