GWT and SpringBoot - Is there a smart way to deal with conflicting dependencies?

Question

In an ideal world, you have your GWT app compiled to javascript, you serve it as static resource and you behind the scenes you have your back end code running on a JVM, and life goes well. But that ideal world is called production during runtime.

However, during development time, when you would like to make use of the gwt code server...

You're GWT compile time dependencies are needed during runtime (sources + classes), for debugging and recompilation purposes of the GWT module.

At the same time, you may wish to have back-end supported by something like spring-boot 1.3.5.RELEASE.

In this case, spring boot, suffering multiple frequent releases, is at this point in time wanting to add as managed dependency, for example:

<hibernate-validator.version>5.2.4.Final</hibernate-validator.version>

Which, of course, is a very good thing. It is one of the many good features of spring, in fact.

Gwt, on the other hand, see link bellow, http://www.gwtproject.org/doc/latest/DevGuideValidation.html

is still requiring you to use:

        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-validator</artifactId>
            <version>4.1.0.Final</version>
            <scope>runtime</scope>              
        </dependency>

Now, if you were to say: I do not care about productive development, just compile me something that works properly in production.

Then indeed, you can trivially solve the above problem by structuring your project in such manner:

ROOT
--- Backend-api
--- Backend-Impl
--- Gwt-Front-end
---- War or stand alone Spring jar module to glue it all together

Where you make the Gwt-Front end depend on the back-end APIs, for some RPC services and DTOs. The pom.xml dependencies you can for the most part manage only in your front-end module, independently of whichver dependencies you have in the back-end. Ultimately, you make a war, or a spring boot runable jar, that carries your gwt code as static resources and carries your back-end code with all of its dependencies, namely Hirbernate validator latest version.

However, when you are trying to get a pom that works for development purposes, as far as i can see, you are stuck having to globally to manage the dependencies that are common between the back-end and the front-end layer in the ROOT pom.xml, and downgrading your dependencies to the version required by gwt.

That is, in the ideal world scenario. You have your ROOT pom.xml simply declaring your modules and the order by which they get build. And you get your back-end-impl to have the power to state it wants to inherit dependencies from the spring-boot-starter pom.xml, etc...

In contrast to the ideal scenario, on the pom.xml configuration that actually helps you during the development time... Well, you can have to revisit the Root pom.xml. And you have to add in managed dependencies on this root pom.xml, so that for all those common conflicting dependencies between your GWT front-end and spring boot back-end, you always have to hammer the version of the GWT and put a downgrade in place.

This will ensure that when you do gwt:run, dev mode recompile, etc... you do not end up having the gwt compiler trying to javascript compile your Hibernate version 5, but instead the version that is indeed supported by GWT 4.1 final. Of course you might end up having a surprise in back-end code, one of these days by puting in place such a hack...

Does anybody have an idea of how to properly organize a multi module project where back-end and gwt based front-end are allowed to have conflicting depency requirements?

Ultimately, if the answer is no, I believe I will prefer to have network wastage and added communication delay, by having a pure stand-alone spring boot backend that integrates with a pure gwt stand alone jetty, where the back-end on this jetty does nothing more than dumbly kick the requests to the actual spring-boot backend. It is kind of pathetic to have gwt jetty back-end that gets called to do 1+1 by the UI and which forwards the computation to a second back-end running sprin boot that actual knows how to do the 1+1... but if this is the most productive way of working, ensuring that development is productive and the production will run without surprises, so be it.

Thanks for any feedback, and Ideally, I would like to see a multi-pom project that you can import as existing maven project into eclipse, and demonstrates how this can be achieved.

Answer 1

Figure out where the transitive dependency is that you want to exclude when compiling GWT code.

    <dependency>
        <groupId>spring-dependency-with-hibernate</groupId>
        <artifactId>some-spring-boot-starter</artifactId>
        <exclusions>
            <exclusion>
               <groupId>org.hibernate</groupId>
               <artifactId>hibernate-validator</artifactId>
            </exclusion>
        </exclusions>
    </dependency>

This way only the GWT provided hibernate version will be available at runtime.

Answer 2

Yes, what I wanted to do can in fact be done, though not trivially... not at all.

To setup a maven multi module pom that can be used productively in the sense that you can get code swapping both for back-end and front-end code without needing to choose either the gwt validator dependencies, or the springboot validator dependencies, essentially I organize my project as follows.

You have:

LEVEL 1: myproject-root
----- LEVEL 2: myproject-backend
---------- LEVEL 3: myproject-backend-api
---------- LEVEL 3: myproject-backend-impl
------ LEVEL 2: myproject-frontend
------ LEVEL 2: myproject-runner
-----------LEVEL 3: myproject-runner-jar
-----------LEVEL 3: myproject-runner-war

In myproject-root, you declare the submodules, naturally. You do dependency management on some inocuous plugins such as surefire plugin. You control java source and target compile language. You do very little more than that. If you have some plugins that you can configure cross cutingly, you might as well put those in the plugin management.

In the myproject-backend, you essentially make it inherit into the dependency management the springboot depencies, by adding to your dependency management:

<dependencyManagement>
        <dependencies>
            <!-- Inherit dependencies from spring boot dependency management pom -->
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-parent</artifactId>
                <version>${version.spring.boot}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

You declare the two submodules, and you're out. You do not declare any cross cuting dependencies here. You only use the dependency management. The front-end component will be hiting the api submodule, and you do not want to be creating a dependency magnet in the backend pom, so here you are conservative as much as possible.

In the myproject-backend-api, you start by letting this submodule be a jar sub-module. Down the line, you will want to branch the api component further, by subdividing api-s by entrypoint nature. To begin with the -api component will have apis needed for the communication between server and frontend, kind of like the typically seen "shared" in the documentation. Here you are as careful as possible to have as little as possible in what regards to elements in the dependencies element. The front-end will be using these api's and you do not want your api package depending on any spring boot apis.

In the backend-impl, you write your actual business logic and you are free to depend on the full spring boot stack. The rule is, make sure no one ever points to this dependency magnet, especially, never let the gwt component even smell the existence of springboot libraries. Do not refer it ever.

In the myproject-frontend, you do not branch it into different sub-modules, as you might have done with the api component. Here you make a monolitic component for gwt code so that you can more easily have the gwt maven plugin working for you doing your hot code recompilation. In this component you depend on the myproject-api, as well as on any relevant gwt library, such as gwt-user, hibernate validator with the old version needed by gwt, etc... The rule for the myproject-frontend is to declare every single dependency in the component as optional. You do not want to take any transitive depency on the component. In production all you need is the myproject-frontend.jar/META-INF/resources/MyProjectModule.html... And all such static resources that springboot will have to detect and serve. While you are in development mode, however, what you want to do in this front end component is to: (a) tune your gwt-plugin to not run the jetty server

<noServer>true</noServer>

The point being, if run this jetty server the only thing it would have in its classpath would be the front end code and the api, and definitely not the impl code. And second, you do not want to use the gwt jetty, you either want to use the springboot tomcat or the springboot jetty. Much better to use the embedded containers of springboot with the appropriate websocket libraries and such.

(b) With springboot you want to server your static resources from the META-INF/resources/ or the public folder. I prefer the resources as it would be where you would normally put JSF components as well. In any case, because the place static resources come from is special, you want to tune your gwt plugin to compile your java sources into the write folder.

<webappDirectory>${project.build.directory}/classes/META-INF/resources</webappDirectory>

So something like above.

Finally, and this was my main mistake, you do not run your code from where you write your front-end code. That is wrong, because that forces your front-end code to need springboot depencies, which will cause you extreme pain due to conflicting libraries.

So for dodging this problem, you create the myproject-runner component, and if you want to be very detailed, you make that one a pom parent component so that you can fork your packaging into either jar or war. I personally like the executable jar, much more to the point, than the war option. I will never want to deploy a sprinboot application in something like a heavy weight weblogic... but whatever rocks your boat.

In any case, the spring boot runner is your ultimate dependency magenet. This component will depend on about every module you have and their transitive dependencies. But because you separate your runner component from the front-end code, and you make all the dependencies in the front-end code optinal... Well, you essentially only bundle springboot, and the your gwt static resources.

While you are developing, what you do is .... once you know it ... simple. (A) you start your spring boot jar application by triggering the main file. If you really want to deploy a war file, that-s your choice you can also build a war file and deploy it to a tomcat or whatever...

(b) you go to your frontend component, rirght click it, chooose run as Maven Build, and gwt:run. The gwt run will start up the gwt code server, that will be completely blind to your backend code, the only thing it will see in front of its eyes is the code your have in the frontend component as well as all the gwt dependencies you've added in as optinal.

You can finally hot swap code in the back-end if you-re using springboot dev. You can finally hot swap code in that monolitic frontend component, once you-ve started teh gwt plugin.

Conclusion, it-s possible, but its a hell of a mess. Glad this problem is out of the way.

Answer 3

I believe I had similar problems and discovered the following while using Eclipse IDE, GWT Google Plugin, SpringBoot and Maven. The recipe is not simple but it worked for my team.

GWT-dev and Jetty server doesn't work with SpringBoot. You have to fix the classpath due to Jetty and gwt-dev hardcoded dependencies, only the dependencies gwt-dev and Jetty were built against will work.

I had to split the GWT UI project into two Maven artifacts. One artifact was a WebApp capable of running only within Eclipse, with all SpringBoot libraries excluded from the classpath. The other artifact was a deployable SpringBoot JAR with "war" maven packaging type and a WAR overlay of the Eclipse dedicated artifact, including only GWT specicic content.

I hope this helps

Answer 4

Just create dedicated maven profile for running GWT codeserver and add 'gwt-dev' dependency ONLY there

<profiles>
    <profile>
        <id>gwt-dev</id>
        <dependencies>
            <dependency>
                <groupId>com.google.gwt</groupId>
                <artifactId>gwt-dev</artifactId>
            </dependency>
        </dependencies>
        <build>
            <plugins>
                <plugin>
                    <groupId>org.codehaus.mojo</groupId>
                    <artifactId>gwt-maven-plugin</artifactId>
                    <version>${gwt.version}</version>
                    <executions>
                        <execution>
                            <id>run-codeserver</id>
                            <goals>
                                <goal>run-codeserver</goal>
                            </goals>
                        </execution>
                    </executions>
                    <configuration>
                        <sourceLevel>${java.version}</sourceLevel>
                        <module>kn.iopm.documentserver.IDocServer</module>
                        <extraJvmArgs>-Xmx1G -Xms512M -Xss1G -Dlog.root=${project.build.directory}/log
                        </extraJvmArgs>
                        <runTarget>index.html</runTarget>
                        <persistentunitcachedir>${project.build.directory}</persistentunitcachedir>
                        <webappDirectory>${project.build.outputDirectory}/public</webappDirectory>
                        <deploy>${project.build.directory}/gwt-deploy</deploy>
                        <codeServerWorkDir>${project.build.directory}/gwt</codeServerWorkDir>
                        <launcherDir>${project.build.outputDirectory}/static</launcherDir>
                    </configuration>
                </plugin>
            </plugins>
        </build>
    </profile>
</profiles>