Endpoints API generation error when using Guava

Question

We are using Appengine Endpoints Java with Guava and when Function is used inside an endpoint method the API generator returns an exception.

Function is NOT part of the method signature. Its just used inside the method to transform a list. Comment out this body part and it generates OK.

Guava sure is in the classpath, and other parts of the application uses it normally.

I'm not sure its related to Guava or any outside API would give the same error.

The method:

@ApiMethod(
    httpMethod = "GET",
    name = "ledgers.accountgroups.get",
    path="ledgers/{ledgerId}/accountgroups")
public Collection<IAccountGroup> listAccountGroups(@Named("ledgerId") String ledgerId, User user) throws Exception {
collaboratorDAO.assertCollaboratorOn(ledgerId, user);
List<Group> groups = accountGroupsDAO.getAccountGroups(ledgerId);
List<IAccountGroup> transformedGroups = Lists.transform(groups, new Function<Group, IAccountGroup>() {
    @Override
    public IAccountGroup apply(Group group) {
        return group.createClient();
    }
});
return transformedGroups;

}

The exception:

INFO: Successfully processed ./war/WEB-INF/appengine-web.xml
interface com.google.api.server.spi.config.Api
interface com.google.api.server.spi.config.Api
Exception in thread "main" java.lang.NoClassDefFoundError: com/google/common/base/Function
    at java.lang.Class.getDeclaredMethods0(Native Method)
    at java.lang.Class.privateGetDeclaredMethods(Class.java:2436)
    at java.lang.Class.getDeclaredMethods(Class.java:1793)
    at com.google.api.server.spi.MethodHierarchyReader.addServiceMethods(MethodHierarchyReader.java:174)
    at com.google.api.server.spi.MethodHierarchyReader.readMethodHierarchyIfNecessary(MethodHierarchyReader.java:44)
    at com.google.api.server.spi.MethodHierarchyReader.getEndpointOverrides(MethodHierarchyReader.java:99)
    at com.google.api.server.spi.config.annotationreader.ApiConfigAnnotationReader.readEndpointMethods(ApiConfigAnnotationReader.java:215)
    at com.google.api.server.spi.config.annotationreader.ApiConfigAnnotationReader.loadEndpointMethods(ApiConfigAnnotationReader.java:92)
    at com.google.api.server.spi.config.ApiConfigLoader.loadConfiguration(ApiConfigLoader.java:55)
    at com.google.api.server.spi.tools.AnnotationApiConfigGenerator.generateConfigObjects(AnnotationApiConfigGenerator.java:237)
    at com.google.api.server.spi.tools.AnnotationApiConfigGenerator.generateConfig(AnnotationApiConfigGenerator.java:185)
    at com.google.api.server.spi.tools.GenApiConfigAction.genApiConfig(GenApiConfigAction.java:78)
    at com.google.api.server.spi.tools.GetClientLibAction.getClientLib(GetClientLibAction.java:66)
    at com.google.api.server.spi.tools.GetClientLibAction.execute(GetClientLibAction.java:49)
    at com.google.api.server.spi.tools.EndpointsTool.execute(EndpointsTool.java:66)
    at com.google.api.server.spi.tools.EndpointsTool.main(EndpointsTool.java:92)
Caused by: java.lang.ClassNotFoundException: com.google.common.base.Function
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    ... 16 more

It's very simple not to use Guava at this point, and it works. I just want to know why this is happening as I need a more have use of Guava in other parts.

Update:

I think it the same problem as:

Google Cloud Endpoints doesn't know about the Work class from Objectify 4 Transaction, causing ClassNotFoundException

Any tips?

Thanks!