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Everybody is encouraged to submit fixes for OpenJDK 11 by creating a pull request to the jdk11u-dev Git repository. Established community members will help new developers without commit access in getting their patch reviewed. For details on the process involved, continue reading these backport instructions.
Should you not be willing or not be able to drive a fix into JDK 11 updates, you can still suggest changes by dropping a mail to the jdk-updates-dev mailing list. But by only doing that, you are at the grace of the community to pick up your suggestion.
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