Investigate if there is a way to auto-detect main class and correct classloader

[dzou]

Right now we must explicitly set the classloader and also ask users to specify the MAIN_CLASS:
https://github.com/dmitry-s/spring-cloud-function/blob/master/spring-cloud-function-adapters/spring-cloud-function-adapter-gcp/src/main/java/org/springframework/cloud/function/adapter/gcloud/GcfSpringBootHttpRequestHandler.java#L64

AWS however seems to be able to auto-detect these settings. Oleg recommended that we look at the following resources:

https://github.com/spring-cloud/spring-cloud-function/blob/master/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/utils/FunctionClassUtils.java

https://github.com/spring-cloud/spring-cloud-function/blob/master/spring-cloud-function-adapters/spring-cloud-function-adapter-aws/src/main/java/org/springframework/cloud/function/adapter/aws/FunctionInvoker.java

[dzou]

I think what we're doing is fine.

Main Class Setting
Regarding Main-class detection, asking users to provide the manifest.mf is fine.

Classloader detection
Our method of setting the Context Classloader appears to be one of three valid ways to set the classloader.
See SpringApplication.java:700.

/**
 * Either the ClassLoader that will be used in the ApplicationContext (if
 * {@link #setResourceLoader(ResourceLoader) resourceLoader} is set, or the context
 * class loader (if not null), or the loader of the Spring {@link ClassUtils} class.
 * @return a ClassLoader (never null)
 */
public ClassLoader getClassLoader() {
	if (this.resourceLoader != null) {
		return this.resourceLoader.getClassLoader();
	}
	return ClassUtils.getDefaultClassLoader();
}

Think this is OK. spring-boot-maven-plugin appears to do something similar when running a spring application from the plugin: https://github.com/spring-projects/spring-boot/blob/master/spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/StartMojo.java#L145

To me it makes sense that we would need to devise a similar solution as we are not starting our Spring application through the spring-boot-maven-plugin anymore.

The alternative solution is to call setResourceLoader(..).

Overall, it is probably not worth the time to look into this further; the current solution is satisfactory and we can look to improve it further if it turns out to be problematic.