Issue #9146: upgrade minimal required jdk version to 11

[pbludov]

Issue #9146
Not to merge. This PR is about finding out how ready we are to move to java 11.

When all the issues are resolved, the last PR will have to add a new check to CI to make sure that the code in master can be built with --source=11.

[pbludov]

The underscore (_) is not a valid name for a variable since java9.

[rnveach]

@pbludov Are you saying you can't name variables like MY_EXAMPLE_HERE anymore for constants?

[pbludov]

no, just the underscore itself: _ invalid, but __ valid. Comes from http://openjdk.java.net/jeps/302

[pbludov]

New SpotBugs errors:

[ERROR] Low: JavadocDetailNodeParser.errorListener not initialized in constructor and dereferenced in com.puppycrawl.tools.checkstyle.JavadocDetailNodeParser.getFirstNonTightHtmlTag(JavadocParser) [com.puppycrawl.tools.checkstyle.JavadocDetailNodeParser] At JavadocDetailNodeParser.java:[line 534] UWF_FIELD_NOT_INITIALIZED_IN_CONSTRUCTOR
[ERROR] Low: AbstractCheck$FileContext.fileContents not initialized in constructor and dereferenced in com.puppycrawl.tools.checkstyle.api.AbstractCheck.getLine(int) [com.puppycrawl.tools.checkstyle.api.AbstractCheck] At AbstractCheck.java:[line 294] UWF_FIELD_NOT_INITIALIZED_IN_CONSTRUCTOR
[ERROR] Low: AbstractCheck$FileContext.fileContents not initialized in constructor and dereferenced in com.puppycrawl.tools.checkstyle.api.AbstractCheck.getLines() [com.puppycrawl.tools.checkstyle.api.AbstractCheck] At AbstractCheck.java:[line 284] UWF_FIELD_NOT_INITIALIZED_IN_CONSTRUCTOR
[ERROR] Low: AbstractFileSetCheck$FileContext.fileContents not initialized in constructor and dereferenced in com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck.log(int, int, String, Object[]) [com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck] At AbstractFileSetCheck.java:[line 219] UWF_FIELD_NOT_INITIALIZED_IN_CONSTRUCTOR

Why don't we get these violations for Java8 builds?

[pbludov]

TODO:

• review test inputs. All usages of sun.* packages should be removed
• Fix Idea Inspections
• Fix Pitest suppressions

[pbludov]

closing for now, will return to this issue later.