Issue when mapping Collection of List using ModelMapper in Spring Boot

I have a Project domain which I want to map on a ProjectDTO using modelmapper. The Project domain contains a List of Locations. In the DTO, I don’t want the Location domains, just the ids of the locations.

public class Project {
   private List<Location> locations
}

public class Location {
   private Long id;
   private String name;
}

public class ProjectDTO {
   private List<Long> assignedLocations;   
}

So I followed the tutorial of Link and created a custom converter:

public Converter<List<Location>, List<Long>> locationToLongConverter = new Converter<List<Location>, List<Long>>() {

        @Override
        public List<Long> convert(MappingContext<List<Location>, List<Long>> context) {
            List<Location> source = context.getSource();
            List<Long> destination = context.getDestination();
            
            for (Location location : source) {
                destination.add(location.getId());
            }
            
            return destination;
        }       
    
    };

Then, I created the PropertyMap to tell modelmapp to use the converter for that field:

PropertyMap<Project, ProjectDTO> itemDTOMap = new PropertyMap<Project, ProjectDTO>() {

    @Override
    protected void configure() {
        using(locationToLongConverter).map(source.getLocations()).setAssignedLocations(null);
    }
};

Finally, I created a bean in my SpringBootApplication class:

@Bean
public ModelMapper modelMapper() {
    ModelMapper mapper =  new ModelMapper();
    mapper.addMappings(projectConverter.getItemDTOMap());
    return mapper;
}

But it is not working, I get errors when starting the application.

nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.modelmapper.ModelMapper]: Factory method 'modelMapper' threw exception; nested exception is org.modelmapper.ConfigurationException: ModelMapper configuration errors:

1) Failed to configure mappings

1 error
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:798) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:228) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1358) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1204) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:882) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:878) ~[spring-context-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:141) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:747) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) ~[spring-boot-2.2.6.RELEASE.jar:2.2.6.RELEASE]
    at de.qedcon.moscowmule.MoscowMuleApplication.main(MoscowMuleApplication.java:25) ~[classes/:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:566) ~[na:na]
    at org.springframework.boot.devtools.restart.RestartLauncher.run(RestartLauncher.java:49) ~[spring-boot-devtools-2.2.6.RELEASE.jar:2.2.6.RELEASE]
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'locationManagementServiceImpl' defined in file [D:DevelopmentqedconmoscowmuletargetclassesdeqedconmoscowmulelocationmanagementserviceLocationManagementServiceImpl.class]: Unsatisfied dependency expressed through constructor parameter 5; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userManagementServiceImpl' defined in file [D:DevelopmentqedconmoscowmuletargetclassesdeqedconmoscowmuleusermanagementserviceUserManagementServiceImpl.class]: Unsatisfied dependency expressed through constructor parameter 3; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'modelMapper' defined in de.qedcon.moscowmule.MoscowMuleApplication: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.modelmapper.ModelMapper]: Factory method 'modelMapper' threw exception; nested exception is org.modelmapper.ConfigurationException: ModelMapper configuration errors:

1) Failed to configure mappings

1 error
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:798) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:228) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1358) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1204) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1290) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1210) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:885) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:789) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    ... 24 common frames omitted
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'userManagementServiceImpl' defined in file [D:DevelopmentqedconmoscowmuletargetclassesdeqedconmoscowmuleusermanagementserviceUserManagementServiceImpl.class]: Unsatisfied dependency expressed through constructor parameter 3; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'modelMapper' defined in de.qedcon.moscowmule.MoscowMuleApplication: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.modelmapper.ModelMapper]: Factory method 'modelMapper' threw exception; nested exception is org.modelmapper.ConfigurationException: ModelMapper configuration errors:

1) Failed to configure mappings

1 error
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:798) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:228) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1358) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1204) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1290) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1210) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:885) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:789) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    ... 38 common frames omitted
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'modelMapper' defined in de.qedcon.moscowmule.MoscowMuleApplication: Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.modelmapper.ModelMapper]: Factory method 'modelMapper' threw exception; nested exception is org.modelmapper.ConfigurationException: ModelMapper configuration errors:

1) Failed to configure mappings

1 error
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:656) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:484) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1338) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1177) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:557) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:517) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:323) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:321) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:276) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1290) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1210) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:885) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:789) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    ... 52 common frames omitted
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.modelmapper.ModelMapper]: Factory method 'modelMapper' threw exception; nested exception is org.modelmapper.ConfigurationException: ModelMapper configuration errors:

1) Failed to configure mappings

1 error
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:185) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:651) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    ... 66 common frames omitted
Caused by: org.modelmapper.ConfigurationException: ModelMapper configuration errors:

1) Failed to configure mappings

1 error
    at org.modelmapper.internal.Errors.throwConfigurationExceptionIfErrorsExist(Errors.java:241) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.internal.ExplicitMappingBuilder.build(ExplicitMappingBuilder.java:244) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.internal.ExplicitMappingBuilder.build(ExplicitMappingBuilder.java:96) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.internal.TypeMapImpl.addMappings(TypeMapImpl.java:92) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.internal.TypeMapStore.getOrCreate(TypeMapStore.java:124) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.ModelMapper.addMappings(ModelMapper.java:113) ~[modelmapper-2.3.0.jar:na]
    at de.qedcon.moscowmule.MoscowMuleApplication.modelMapper(MoscowMuleApplication.java:38) ~[classes/:na]
    at de.qedcon.moscowmule.MoscowMuleApplication$$EnhancerBySpringCGLIB$$1c0b08e7.CGLIB$modelMapper$0(<generated>) ~[classes/:na]
    at de.qedcon.moscowmule.MoscowMuleApplication$$EnhancerBySpringCGLIB$$1c0b08e7$$FastClassBySpringCGLIB$$8262a926.invoke(<generated>) ~[classes/:na]
    at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:244) ~[spring-core-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:331) ~[spring-context-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    at de.qedcon.moscowmule.MoscowMuleApplication$$EnhancerBySpringCGLIB$$1c0b08e7.modelMapper(<generated>) ~[classes/:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:566) ~[na:na]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:154) ~[spring-beans-5.2.5.RELEASE.jar:5.2.5.RELEASE]
    ... 67 common frames omitted
Caused by: java.lang.UnsupportedOperationException: null
    at org.modelmapper.internal.asm.ClassVisitor.visitNestHostExperimental(ClassVisitor.java:158) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.internal.asm.ClassReader.accept(ClassReader.java:541) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.internal.asm.ClassReader.accept(ClassReader.java:391) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.internal.ExplicitMappingBuilder.visitPropertyMap(ExplicitMappingBuilder.java:259) ~[modelmapper-2.3.0.jar:na]
    at org.modelmapper.PropertyMap.configure(PropertyMap.java:386) ~[modelmapper-2.3.0.jar:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:566) ~[na:na]
    at org.modelmapper.internal.ExplicitMappingBuilder.build(ExplicitMappingBuilder.java:227) ~[modelmapper-2.3.0.jar:na]
    ... 82 common frames omitted

Here the full ProjectConverter class:

@Service
@Getter @RequiredArgsConstructor
public class ProjectConverter {
    
    
    public Converter<List<Location>, List<Long>> locationToLongConverter = new Converter<List<Location>, List<Long>>() {

        @Override
        public List<Long> convert(MappingContext<List<Location>, List<Long>> context) {
            List<Location> source = context.getSource();
            List<Long> destination = context.getDestination();
            
            for (Location location : source) {
                destination.add(location.getId());
            }
            
            return destination;
        }       
    
    };  
    
    PropertyMap<Project, ProjectDTO> itemDTOMap = new PropertyMap<Project, ProjectDTO>() {

        @Override
        protected void configure() {
            using(locationToLongConverter).map(Project::getLocations, ProjectDTO::setAssignedLocations);
        }
    };  

}

Any ideas what I do wrong?

Answer

This is a bug of ModelMapper, fixed on version 2.3.2 or later:

• UnsupportedOperationException when adding PropertyMap #404

In addition, NPE problem on locationToLongConverter#convert() should be fixed:

final List<Long> destination = new ArrayList<>();

---

By the way, this code could be written more simply:

    @Bean
    public ModelMapper modelMapper() {
        final Converter<List<Location>, List<Long>> locationConverter = ctx -> ctx.getSource().stream()
            .map(Location::getId).collect(Collectors.toList());

        final ModelMapper mapper = new ModelMapper();
        mapper.typeMap(Project.class, ProjectDTO.class)
            .addMappings(m -> {
                m.using(locationConverter).map(Project::getLocations, ProjectDTO::setAssignedLocations);
            });
        return mapper;
    }

(ProjectConverter.java is not required.)

refs:

• http://modelmapper.org/user-manual/property-mapping/