Warn about unused declared thrown exceptions in Eclipse

[wilkinsona]

We have numerous places in Boot where we've declared that an exception is thrown when it isn't or when the exception is a RuntimeException so it doesn't need to be declared. There's a PR that removes them which I am in favour of. I think it would also be nice to try and stop them reappearing. To that end, we could change the Eclipse JDT core preferences. Instead of the following:

spring-javaformat/spring-javaformat-eclipse/io.spring.javaformat.eclipse/src/io/spring/javaformat/eclipse/projectsettings/org.eclipse.jdt.core.prefs

Lines 91 to 94 in c8835c8

	org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownException=ignore
	org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionExemptExceptionAndThrowable=enabled
	org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionIncludeDocCommentReference=enabled
	org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionWhenOverriding=disabled

We'd use the following:

org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownException=warning
org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionExemptExceptionAndThrowable=disabled
org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionIncludeDocCommentReference=disabled
org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionWhenOverriding=enabled

The changes above will result in warnings being generated for checked exceptions that are unnecessarily declared as thrown. It won't help with unchecked exceptions.

[wilkinsona]

Thinking some more, we may want to leave the following two settings unchanged:

org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionIncludeDocCommentReference=enabled
org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionWhenOverriding=disabled

This will allow an exception to be declared thrown if it's mentioned in the javadoc or if it comes from the signature of a method that's being overridden.

[weixsun]

This will allow an exception to be declared thrown if it's mentioned in the javadoc or if it comes from the signature of a method that's being overridden.

@wilkinsona yes, when I was erasing an exception that was never thrown, I deliberately kept the case you mentioned.

e.g.:

class TestFilter implements Filter {
	@Override
	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		// do something
	}
}

IOException and ServletException should not be erased even if never thrown.