问题
In my GWT web app I am keeping all my jar files outside of my project and referencing them using classpath variables. This allows me to link to jars from other projects/teams without having to put a copy of the jar in my web app lib directory. Hosted mode kindly looks up the classes in this system classpath and then adds them to the web-app classpath warning me that it is doing so. When I deploy my build system pulls in only the jars I need to ship in my web app and is not a problem.
The issue I have is that some code uses dynamic lookups, searching the classpath for implementations. If a jar has not yet been added to the web app classpath beacuse no classes have yet benn loaded from the jar it is not included in the search.
The particular problem I am having is with Persistence - it looks up EntityManagerFactory implementations by searching for META-INF/services files. I have also had a similar problem with Rome and its module extensions.
I have got a workaround, in the dev/hosted mode I simply refer to a class I know is in a jar I want and this causes it to be added to my web app classpath. I do this my by calling
private void devModeClassPathHack() {
Class<?> gwtDevModeHack1 = EntityManagerImpl.class;
}
from my development mode Guice module.
My question is simple - is there a "nicer" way of doing this?
回答1:
I don't think, there's a nicer way - using this makeshift "self-healing" mechanism of the GWT Jetty/dev mode is already a hack :) If you ever have the need to run the server side code on e. g. JBoss during development (together with GWT dev mode for the client side), this will stop working.
(As an additional problem, the order of classloading - and consequently the precedence in cases when there are multiple versions of a class in different jars - will depend on program flow. This can be very nasty to debug.)
You mention, that you pull in all the jars from outside. What I would do in that case, is to set .svnignore (or .gitignore/...) to "*.jar" in the WEB-INF/lib dir. Then I'd run the same build step which is used to create the production build to copy the jars into the lib dir before starting the server.
来源：https://stackoverflow.com/questions/7442391/gwt-web-app-vs-system-classpath-for-dynamic-loaded-classes