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Bug
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Resolution: Fixed
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Medium
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5.9.6
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4
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Severity 2 - Major
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Code to reproduce
Use a unit test below to reproduce this by running the comparator:
Collections.sort(fullList, (o1, o2) -> {
// Sorting by name is not as good as sorting by start time (unavailable) but better than anything
// else we can think of this iteration. Changing this order later is unable to raise any blockers
// in production.
Message o1Name = o1.getName();
if (o1Name == null || o1Name.getKey() == null)
return 1; // push o1 to bottom
Message o2Name = o2.getName();
if (o2Name == null || o2Name.getKey() == null)
return -1; // push o2 to bottom
return o1Name.getKey().compareTo(o2Name.getKey());
});
Which is copied from Confluence code.
Confluence will throw the following error message:
Comparison method violates its general contract!
That basically means that the comparator is not written correctly.
Environments
Attached a test class to reproduce the error: ContractViolationTest.java
- on Windows 10,
- java version "1.8.0_111" Java(TM) SE Runtime Environment (build 1.8.0_111-b14), Java HotSpot(TM) Client VM (build 25.111-b14, mixed mode)
- Important: it only breaks if collection size is >= 32
A possible fix in Confluence
Switching the +1 and -1 return values in the comparator's two if blocks fixes the problem.
Note that this change makes "null key" cases appear before "not-null key" cases in the sorted list. It does not affect the original order of objects where the key is not-null.
Workaround
Adding this JVM system property fixes the issue:
-Djava.util.Arrays.useLegacyMergeSort=true
References
- TimSort has been introduced in Java 7, it is a stable sort that was designed for relatively expensive comparison operations. Only used for sorting objects, not primitives. Its problematic code path is only reached for collections with size >= 32. See http://stackoverflow.com/questions/4018332/is-java-7-using-tim-sort-for-the-method-arrays-sort
- Exception might be caused by non-transitive comparators: http://stackoverflow.com/questions/8327514/comparison-method-violates-its-general-contract
- Atlassian has seen something similar, too: https://confluence.atlassian.com/fishkb/comparison-method-violates-its-general-contract-387941517.html
- Oracle has open issues related to this, promised to be fixed in Java 9: http://bugs.java.com/bugdatabase/view_bug.do?bug_id=7075600
- See: com.atlassian.confluence.api.impl.service.longtasks.LongTaskFactory#buildStatus
- is related to
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- Long Term Backlog
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[CONFSERVER-45910] Confluence throws "Comparison method violates its general contract!" when trying to sort multiple long-running tasks
| Link | New: This issue is related to CONFSERVER-58059 [ CONFSERVER-58059 ] |
| Workflow | Original: JAC Bug Workflow v3 [ 2888370 ] | New: CONFSERVER Bug Workflow v4 [ 2999520 ] |
| Workflow | Original: JAC Bug Workflow v2 [ 2798013 ] | New: JAC Bug Workflow v3 [ 2888370 ] |
| Status | Original: Resolved [ 5 ] | New: Closed [ 6 ] |
| Workflow | Original: JAC Bug Workflow [ 2728129 ] | New: JAC Bug Workflow v2 [ 2798013 ] |
| Symptom Severity | Original: Major [ 14431 ] | New: Severity 2 - Major [ 15831 ] |
| Workflow | Original: Confluence Workflow - Public Facing - Restricted v5 - TEMP [ 2385180 ] | New: JAC Bug Workflow [ 2728129 ] |
| Remote Link | New: This issue links to "Page (Confluence)" [ 368516 ] |
| Remote Link | New: This issue links to "Page (Confluence)" [ 367239 ] |
| Labels | Original: affects-server bugfix long-running-task loyalty pse-request | New: affects-server long-running-task loyalty pse-request |
| Labels | Original: affects-server bugfix long-running-task pse-request | New: affects-server bugfix long-running-task loyalty pse-request |