Restore default JUL root handler on Logback cleanup [FaaFyfxR9WAQrL7FcAgEHJvztd8cVMxvjHRS55rw1nwH]

[waterWang]

Motivation

When the JUL-to-SLF4J bridge is installed, the default java.util.logging root ConsoleHandler is removed. When the logging system is cleaned up (for example when an ApplicationFailedEvent is published), the bridge is uninstalled but the ConsoleHandler was not restored, leaving the JUL root logger with no handlers. Anything subsequently logged through JUL, such as a startup failure reported via a JUL-backed commons-logging Log, was then silently discarded.

Fixes #50969. See also #50404.

Modification

• Add a defaultRootHandler field to save the ConsoleHandler reference before it is removed.
• In removeDefaultRootHandler(), save the ConsoleHandler before removing it from the root logger.
• In cleanUp(), after uninstalling the bridge, call restoreDefaultRootHandler() to add the saved ConsoleHandler back to the root logger.

Result

After the logging system is cleaned up, the JUL root logger still has its ConsoleHandler, so JUL logging continues to work.

[snicoll]

Please do not open a PR when one already exists