Provider io.swagger.scala.converter.SwaggerScalaModelConverter could not be instantiated

Question

When trying to incorporate swagger into my API I'm running into some issues.

My API is using Jersey 2.22 and the language is Scala. I've added the following dependencies to my pom:

<dependency>
    <groupId>io.swagger</groupId>
    <artifactId>swagger-annotations</artifactId>
    <version>1.5.12</version>
</dependency>

<dependency>
    <groupId>io.swagger</groupId>
    <artifactId>swagger-scala-module_2.11</artifactId>
    <version>1.0.3</version>
</dependency> 

My plugin is configured as follows:

<plugin>
    <groupId>com.github.kongchen</groupId>
    <artifactId>swagger-maven-plugin</artifactId>
    <version>${kongchen.plugin.version}</version>
    <executions>
        <execution>
            <phase>compile</phase>
            <goals>
                <goal>generate</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <apiSources>
            <apiSource>
                <locations>${swagger.locations}</locations>
                <springmvc>false</springmvc>
                <host>${swagger.host}</host>
                <basePath>${base.path}</basePath>
                <info>
                    <title>${swagger.title}</title>
                    <version>${project.version}</version>
                    <description>${swagger.description}</description>
                </info>
                <swaggerDirectory>${project.build.directory}/</swaggerDirectory>
                <outputFormats>yaml,json</outputFormats>
            </apiSource>
        </apiSources>
    </configuration>
</plugin>

When I'm running mvn compile, the following error occurs:

Exception in thread "main" java.util.ServiceConfigurationError: io.swagger.converter.ModelConverter: Provider io.swagger.scala.converter.SwaggerScalaModelConverter could not be instantiated
        at java.util.ServiceLoader.fail(ServiceLoader.java:232)
        at java.util.ServiceLoader.access$100(ServiceLoader.java:185)
        at java.util.ServiceLoader$LazyIterator.nextService(ServiceLoader.java:384)
        at java.util.ServiceLoader$LazyIterator.next(ServiceLoader.java:404)
        at java.util.ServiceLoader$1.next(ServiceLoader.java:480)
        at io.swagger.converter.ModelConverters.<clinit>(ModelConverters.java:114)
        at com.github.kongchen.swagger.docgen.AbstractDocumentSource.loadModelModifier(AbstractDocumentSource.java:179)
        at com.github.kongchen.swagger.docgen.mavenplugin.ApiDocumentMojo.execute(ApiDocumentMojo.java:71)
        at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:207)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
        at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
        at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
        at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
        at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:307)
        at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:193)
        at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:106)
        at org.apache.maven.cli.MavenCli.execute(MavenCli.java:863)
        at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:288)
        at org.apache.maven.cli.MavenCli.main(MavenCli.java:199)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:497)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
        at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
        at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
        at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
Caused by: java.lang.VerifyError: Cannot inherit from final class

If I leave out the scala-swagger module, compile works fine and I'm getting a valid json (all resource descriptions are generated perfectly), but the fields in the scala case classes don't get generated:

@ApiModel("some model")
case class HelloModel(@(ApiModelProperty @field)(dataType = "Int", value = "some int", name = "tries") tries: Int)

Results in:

"definitions" : {
    "some model" : {
      "type" : "object"
    }
  }

Anyone got a clue?

Answer 1

Well I figured it out after some heavy debugging. It appears the swagger-maven-plugin which I was using (3.1.1) was using swagger-core 1.5.4. This swagger-core dependency had a dependency to Jackson 2.4.5.

The swagger-scala-module_2.11 is using Jackson 2.8.7. The "Cannot inherit from final class" error came from the TypeReference class, which apparently changed/got introduced in 2.8.7.

The solution was including the swagger-scala-module_2.11 inside of the swagger-maven-plugin by using the dependencies block:

<build>
    <plugins>
        <plugin>
            <groupId>com.github.kongchen</groupId>
            <artifactId>swagger-maven-plugin</artifactId>
            <version>${swagger-scala-maven-plugin}</version>
            <executions>
                <execution>
                    <phase>compile</phase>
                    <goals>
                        <goal>generate</goal>
                    </goals>
                </execution>
            </executions>
            <dependencies>
                <dependency>
                    <groupId>io.swagger</groupId>
                    <artifactId>swagger-scala-module_2.11</artifactId>
                    <version>1.0.3</version>
                </dependency>
            </dependencies>
            <configuration>
                <apiSources>
                    <apiSource>
                        <locations>
                            <location>${swagger.locations}</location>
                        </locations>
                        <springmvc>false</springmvc>
                        <host>${swagger.host}</host>
                        <basePath>${base.path}</basePath>
                        <info>
                            <title>${swagger.title}</title>
                            <version>${project.version}</version>
                            <description>${swagger.description}</description>
                        </info>
                        <swaggerDirectory>${project.build.directory}/</swaggerDirectory>
                        <outputFormats>json</outputFormats>
                    </apiSource>
                </apiSources>
            </configuration>
        </plugin>
    </plugins>
</build>

This will make the swagger-maven-plugin use swagger core 1.5.12 instead of the provided 1.5.9.

Answer 2

I had the same problem using SBT in a Play Framework 2.4.6 project. The answer provided by GieJay has proven correct in my case as well, I just wanted to point out my solution in an SBT environment; in my libraryDependencies I had

"io.swagger" %% "swagger-play2" % "1.5.1"

which works fine in another project using Play 2.4.8 but I encountered this issue within an older Play version. All I had to do was add

"io.swagger" % "swagger-scala-module_2.11" % "1.0.3"

to the dependencies and all works like a charm.