I just got the info from one of out customers that an error in the export function of a Grails application happend. The stacktrace looked like:
Error Details Error 500: Executing action [export] of controller [ExportController] caused exception: Runtime error executing action Servlet: grails URI: /app/grails/export/export.dispatch Exception Message: Comparison method violates its general contract! Caused by: Comparison method violates its general contract! Class: ExportPrintController At Line: [105] Code Snippet: Stack Trace java.lang.IllegalArgumentException: Comparison method violates its general contract! at java.util.TimSort.mergeHi(TimSort.java:868) at java.util.TimSort.mergeAt(TimSort.java:485) at java.util.TimSort.mergeCollapse(TimSort.java:408) at java.util.TimSort.sort(TimSort.java:214) at java.util.TimSort.sort(TimSort.java:173) at java.util.Arrays.sort(Arrays.java:659) at java.util.Collections.sort(Collections.java:217) at com.troii.app.web.ExportController$_closure2.doCall(ExportController.groovy:105) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603) at java.lang.Thread.run(Thread.java:722)
I did some web search and came up with this interesting discussion on stackoverflow. Which links to the compatibility information page of JDK7 and a specific change they made in Java 7: changing the default sort algorithm from MergeSort to TimSort in the java.util.Arrays.sort method.
So a quick workaround for this issue was to add -Djava.util.Arrays.useLegacyMergeSort=true to the tomcat JVM environment – which forces the app to use the old algorithm.
To really fix the issue I had to make a change to the code because it really violated the Comparable contract. Our code looked like:
int compare(ExportTerm e1, ExportTerm e2) {
if (e1.sortLabel == null) {
return -1
}
if (e2.sortLabel == null) {
return 1
}
return collator.compare(e1.sortLabel, e2.sortLabel)
}
which is wrong, because if you compare two elements where the sortLabel is null on both, the return value would be -1 or 1, depending on the order you compare them – which violates the contract.
Correct it has to look like:
int compare(ExportTerm e1, ExportTerm e2) {
if (e1.sortLabel == e2.sortLabel) {
return 0
}
if (e1.sortLabel == null) {
return -1
}
if (e2.sortLabel == null) {
return 1
}
return collator.compare(e1.sortLabel, e2.sortLabel)
}
public int compare(ExportTerm o1, ExportTerm o2) {
if (o1 == o2)
return 0;
if (o1 == null)
return -1;
if (o2 == null)
return 1;
// Before sending the result as such after compare just check once’s whether the compare result returns 0.
int result = option.compare(o1, o2);
// So we will send the result only when != 0 so that order in the list is maintained in the same order as such
if (result != 0) {
return result;
}
// if nothing is returned from the method this is returned by default
return 0;
}
I don’t see why this (“if you compare two elements where the sortLabel is null on both, the return value would be -1 or 1, depending on the order you compare them”) violates the contract. Could you explain it a bit more, please? Thanks in advance
If both e1.sortLabel and e2.sortLabel are null the compare method should return 0 because they are equal. In the case of my first implementation it would return -1 which is wrong.
Thanks a lot! Your solution worked for me.