Wednesday, May 31, 2017

Re: Code references between client/server/shared folders

If there are no visual classes, functionality will normally occur. Regarding your comment, javac will always be applied in all classes, regardless of what directory they are in. The classes destined to the sharing between client and server, it is sufficient to implement them without using any graphical interface.

sorry my english.


Em 30/05/2017 12:43, Thomas Broyer escreveu:

On Tuesday, May 30, 2017 at 5:36:01 PM UTC+2, Magnus wrote:

my understanding of the typical folders in a GWT project is this:
  • client
    only client-side code here
  • server
    only server-side code here
  • shared
    code that may be used on client-side and server-side
I believe that the code in the client folder gets compiled by the GWT compiler into JS only, while the code in the server folder gets compiled by javac into class files only.
Besides that, code in the shared folder gets compiled twice, once by the GWT compiler and once from javac.

You'll probably want to use javac on client code too; if only to get faster feedback for compilation errors. There are a few cases where GWT will look for the compiled classes too, so better have them.
My understanding always was that the code in the shared folder must be "pure" java, so that it can be compiled in both worlds, without any references into pure client-side code.

As Jens said, those are only conventions. For example, you'll find in GWT that the RemoteService interface is in the "client" package (this is legacy, because it predates the convention of using a "shared" package)
Also note that it's not about compilation (all your client code will compile using javac) but runtime (client-side code won't work in a JVM, i.e. on the server)
So far so good. But what happens, when code in the shared folder references some classes in the client folder?
I have just found such an example in my projects:
  • There is a class Move that represents a move in a chess game.
    It's located in the shared folder, since it's used by the server and the client.
  • The class Move uses a class TimeFormatter (to format timestamps as "yyyy-mm-dd hh:mm:ss").
    It's located in the client code and in turn uses the class
What happens, when the javac compiler sees the Move class?

It depends how you invoke javac.
If the folders you're talking about are subpackages in the same source tree, then just pass everything at once to javac and GWT (there's a reason GWT has those *.gwt.xml files with <source path="…"/> elements in there)
What does it do with when compiling server-side code?

If it's in the classpath (gwt-user.jar or gwt-servlet.jar), it'll compile.
You should probably be more concerned about runtime behavior though: if you call that code on the server, it'll throw.
Should I move TimeFormatter into the shared folder, too?
And one more step: Could you use the same time formatting code in client and server code?
(At the moment I use at the client and SimpleDateFormat.format at the client.)

Jens suggested c.g.g.i.shared.DateTimeFormat; I wouldn't recommend it though (needs some additional setup on server-side, almost nobody uses it on server-side so you're basically on your own if you need help).

The best practice is to abstract both approaches behind an interface (or abstract class), that you'd put in the "shared" package, then have concrete implementations in the "client" and "server" packages, and arrange your code to use an instance of the appropriate implementation depending on the context (using the dependency injection pattern helps here). This means your Move class will receive a TimeFormatter (interface) as argument and will never instantiate one (ClientTimeFormatter or ServerTimeFormatter).
Super-source, as explained by Jens, also works, but is IMO more complex (to understand and maintain).
BTW: What's the "public" folder for? I saw it in some projects but not in the docs...

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