Wednesday, July 17, 2019

Re: Using Java 11 on the server to implement GWT-RPC services

This is again a case where splitting client and server code into separate Maven modules will help (see
The problem here is (probably) that GWT tries to load the compiled classes, and can't because it doesn't understand Java 11 bytecode.
So you'll want to compile the shared and client code to Java 8 bytecode, and the server-only code to Java 11 bytecode.

On Wednesday, July 17, 2019 at 10:56:58 AM UTC+2, Adrian Smith wrote:
I understand that GWT 2.8.2 supports translation of Java 8 syntax, and not Java 11.

But I thought there might be a good chance that could use Java 11 features on the server (e.g. Jetty) that responds to GWT-RPC requests.

This works fine in deployment mode: Mark the project as Java 11 in Maven, compile the WAR with Maven, GWT creates the JS fine, and then the Java 11 servlet container executes the WAR fine, and the server code can make use of Java 11 features.

However in "super dev mode" it doesn't work, and also doesn't really give an errors (neither on the console in Maven, nor in the "Jetty" tab of the "super dev mode" console.)

With a Java 11 JVM but specifying Java 8 in Maven (as is the default from the Maven archtetype), everything works fine:

$ java -version
openjdk version "11.0.2" 2019-01-15
$ ~/Downloads/gwt-2.8.2/webAppCreator -templates maven,sample -out Foo com.mycompany.Foo
$ cd Foo
$ mvn clean war:exploded gwt:devmode

That all works fine.

However, when I change, in the pom.xml


Then when I execute 

$ mvn clean war:exploded gwt:devmode

The "super dev mode" console opens, however when I click on "Launch Default Browser" I see "503 Service Unavailable" in the browser. That error message comes from Jetty.

There is a note in the "Jetty" tab of the "super dev mode" console about the 503 but just request/response headers and no further information. There is also nothing printed to the terminal console where I executed the "mvn" command.

Note: I have not changed the source code at all. It's the default Maven archetype source code, which doesn't use any Java 11 syntax.

I have also tried changing the <sourceLevel>1.8</sourceLevel> in Maven's GWT plugin section, however, as I expected, that failed immediately with a clear error message that the GWT compiler doesn't support > 1.8, which is fine. I know that's a lot of work, but I figured I could at least use Java 11 features on the server.

Am I missing something simple, or is this more complex than I imagine and thus is not supported?

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