Provide support for custom class loader to bridge into LaunchedURLClassLoader

[aantono]

This class provides ability to specify a custom JVM System Class Loader to enable loading classes provided inside nested jars of the Spring Boot uber-jar for use cases such as setting custom policy.provider or java.security.manager or java.rmi.server.RMIClassLoaderSpi. All those classes are being loaded by JVM from the System Class Loader, so this enables packaging custom implementations inside the distribution jar as nested artifact dependencies.
-Djava.system.class.loader=org.springframework.boot.loader.SpringBootSystemClassLoader can be provided as a startup argument to enable use of custom system class loader

[aantono]

Here is the question to the Spring Boot guys - For the SpringBootSystemClassLoader, does it make sense from the actual/real usage scenarios to support War/PropertiesLauncher, or the usage of SpringBootSystemClassLoader is really only applicable with the JarLauncher configuration?

[philwebb]

See PR #1669

[snicoll]

See #1669 (comment)