Which parser is used? Is it possible that some warnings do not have a category?

This is the Javac parser. And I don't know. I would hope that if it is possible for a warning not to have a category, that Jenkins would report that number, so that I can add all the numbers up and get a consistent result.

Do you have a log file that I can use for a test?

Hmm, not one which I'll be allowed to post.

I can provide extracts which might be relevant but I have to manually censor them.

/opt/jenkins/jobs/trunk-compile/workspace/my-module/src/java/com/acme/script/impl/itemtype/ItemTypeUtils.java:25: warning: no comment
    private static final Predicate<MimeType> activeMimeTypePredicate = mimeType -> {
                                                                       ^
/opt/jenkins/jobs/trunk-compile/workspace/my-module/src/java/com/acme/script/impl/itemtype/ItemTypeUtils.java:26: warning: no comment
        for (SupportedLevelInfo supportLevel : mimeType.getSupportLevel())
                                ^
/opt/jenkins/jobs/trunk-compile/workspace/my-module/src/java/com/acme/script/impl/processor/DefaultProcessor.java:332: warning: [deprecation] exists(Path,LinkOption...) in FileUtils has been deprecated
            if (com.acme.common.io.FileUtils.exists(persistentDir))
                                            ^
/opt/jenkins/jobs/trunk-compile/workspace/my-module/src/java/com/acme/script/impl/processor/DefaultProcessor.java:591: warning: [unchecked] unchecked cast
        container.setCustomMetadata((Map<String, String>) normalisedMap.get("customMetadata"));
                                                                           ^
  required: Map<String,String>
  found:    Object
/opt/jenkins/jobs/trunk-compile/workspace/my-module/src/java/com/acme/script/impl/processor/DefaultProcessor.java:631: warning: [deprecation] incorrectCollator() in Collators has been deprecated
            if (Collators.incorrectCollator().compare(existingContainer.getName(), evidenceContainer.getName()) == 0)
                         ^
/opt/jenkins/jobs/trunk-compile/workspace/my-module/src/java/com/acme/script/impl/processor/DefaultProcessor.java:646: warning: [deprecation] incorrectCollator() in Collators has been deprecated
            if (Collators.incorrectCollator().compare(container.getName(), evidenceContainer.getName()) == 0)
                         ^
/opt/jenkins/jobs/trunk-compile/workspace/my-module/src/java/com/acme/script/impl/processor/DefaultProcessor.java:835: warning: [rawtypes] found raw type: SyncDifference
            public void changeDetected(SyncDifference syncDifference)

I am guessing these lines with nothing in square brackets are the ones with no category?

Ok, thanks that is sufficient to reproduce the bug!

Is it ok to use the string "-" as empty category in the UI? Or do you have a better idea?

I think it's probably fine. Something like (no category) would probably be ideal, but I don't know how Jenkins is for localisations.

Code changed in jenkins
User: Ulli Hafner
Path:
src/main/java/hudson/plugins/analysis/util/model/AbstractAnnotation.java
http://jenkins-ci.org/commit/analysis-core-plugin/f210dee1f6f77ca485bf695376f330e997dc3505
Log:
[FIXED JENKINS-38557] Don' use an empty category or type anymore.

Code changed in jenkins
User: Ulli Hafner
Path:
pom.xml
src/test/java/hudson/plugins/warnings/parser/AbstractEclipseParserTest.java
src/test/java/hudson/plugins/warnings/parser/AcuCobolParserTest.java
src/test/java/hudson/plugins/warnings/parser/AjcParserTest.java
src/test/java/hudson/plugins/warnings/parser/AntJavacParserTest.java
src/test/java/hudson/plugins/warnings/parser/Armcc5CompilerParserTest.java
src/test/java/hudson/plugins/warnings/parser/ArmccCompilerParserTest.java
src/test/java/hudson/plugins/warnings/parser/BuckminsterParserTest.java
src/test/java/hudson/plugins/warnings/parser/ClangParserTest.java
src/test/java/hudson/plugins/warnings/parser/CoolfluxChessccParserTest.java
src/test/java/hudson/plugins/warnings/parser/DoxygenParserTest.java
src/test/java/hudson/plugins/warnings/parser/EclipseParserTest.java
src/test/java/hudson/plugins/warnings/parser/FlexSDKParserTest.java
src/test/java/hudson/plugins/warnings/parser/GoLintParserTest.java
src/test/java/hudson/plugins/warnings/parser/GoVetParserTest.java
src/test/java/hudson/plugins/warnings/parser/JavaDocParserTest.java
src/test/java/hudson/plugins/warnings/parser/JavacParserTest.java
src/test/java/hudson/plugins/warnings/parser/ParserRegistryIntegrationTest.java
src/test/java/hudson/plugins/warnings/parser/ParserTester.java
src/test/java/hudson/plugins/warnings/parser/SbtScalacParserTest.java
src/test/java/hudson/plugins/warnings/parser/SunCParserTest.java
src/test/java/hudson/plugins/warnings/parser/TiCcsParserTest.java
src/test/resources/hudson/plugins/warnings/parser/issue38557.txt
warnings.iml
http://jenkins-ci.org/commit/warnings-plugin/ec25a5f01a868eef89f43094d69578b50c69400e
Log:
[FIXED JENKINS-38557] Don' use an empty category or type anymore.

This still can be localized in the UI. I'll add that later on.

Code changed in jenkins
User: Ulli Hafner
Path:
src/main/resources/hudson/plugins/analysis/views/TabDetail/categories.jelly
src/main/resources/hudson/plugins/analysis/views/TabDetail/categories.properties
src/main/resources/hudson/plugins/analysis/views/TabDetail/categories_de.properties
src/main/resources/hudson/plugins/analysis/views/TabDetail/types.jelly
src/main/resources/hudson/plugins/analysis/views/TabDetail/types.properties
src/main/resources/hudson/plugins/analysis/views/TabDetail/types_de.properties
http://jenkins-ci.org/commit/analysis-core-plugin/a2a98c51ce7e45f5ddffe5d773a7982bd53ddd43
Log:
JENKINS-38557 Localized empty category and type.