Bean Mapping of Transfer Objects

In the past years I have been working on multiple projects where the so-called Data Transfer Object (short DTO) pattern has been heavily used. This is a pattern that has even been a core pattern in the JEE world. This pattern certainly has its justification for the right cases. But in many cases I have seen it applied inappropriately. This blog posting by Adam Bien, a JEE advocate, is outlining the case where it should be considered useful. However when applied, this pattern comes at the cost of additional code to maintain and some extra CPU cycles for doing the mapping.

In this post we take a brief look at some mapping frameworks (just enough to do simple bean mapping). Finally we do some simple benchmarking just to get an idea what the performance costs of bean mapping are.

Below we see a typical implementation of mapping two objects. It becomes quite evident that every time we add/remove an attribute to the source or target object we need to update the mapping implementation. So a complex domain model and over-use of the mapping pattern can keep a developer fairly busy.

A hand crafted bean mapping implementation

public class PersonNameMapperImpl implements PersonNameMapper {

    @Override
    public PersonNameResource map(PersonName person) {
        if ( person == null ) {
            return null;
        }

        PersonNameResource personNameResource = new PersonNameResource();

        personNameResource.setNamePrefix( person.getNamePrefix() );
        personNameResource.setFirstName( person.getFirstName() );
        personNameResource.setLastName( person.getLastName() );
        personNameResource.setMiddleName( person.getMiddleName() );

        return personNameResource;
    }
}

As a workaround quite a number of mapping frameworks have been put into life. They mostly work based on heavy use of the Java Reflection API in order to dynamically map between two objects (or even complex object networks). These frameworks let us describe the rules for mapping and/or rely on sensible defaults for deriving a mapping (e.g. based on name matching).

The most popular frameworks are probably Dozer or Model Mapper.

Bean mapping based on Dozer

PersonName name = new PersonNameFactory().newEntity();
Mapper dozerMapper = new DozerBeanMapper();
dozerMapper.map(name, PersonNameResource.class);

The above mapping code using Dozer simply uses name matching for generating property mapping rules using the Reflection API. It is quite evident that it is required to have good test coverage for such code to ensure that all properties are properly mapped.

Bean mapping based on ModelMapper

PersonName name = new PersonNameFactory().newEntity();
ModelMapper modelMapper = new ModelMapper();
modelMapper.map(name, PersonNameResource.class);

Using ModelMapper the code is almost identical. Most of the well established mapping frameworks are doing an excellent job providing their mapping capabilities. Certainly they differ in the way the are configured through an API and/or annotations (or even XML). So choosing may be just a matter of taste.

During one of my last project assignments I had an argument with a colleague. The more mapping code we had implemented the more I felt uneasy. Yes, I had a pretty old development workstation with a CPU not as powerful compared to my team mates.

I started arguing that we are putting a lot of work into code for configuring the mapping framework and it was hard to debug (It’s all Reflection code you need to work through when debugging). But also with my poor hardware I was experiencing that automated tests would run quite slow (and extremely slow when in debugger mode) and to me it was pretty clear that it’s the mapping framework. Granted, it definitely helped us to avoid writing tons of boring mapping code but I still feared that it would come at some performance cost. Though, having no reliable numbers for the execution costs, I knew it would be hard to find good arguments against the mapping framework we had chosen.

Today, the project is almost done, the mapping approach using ModelMapper was continued. Still that questionable performance continued to puzzle me. So, a few days ago I sat down and setup a sample project to compare a few mapping frameworks and run some benchmarks. It is quite clear that a hand-crafted mapping code would outnumber any reflection based mapping implementation. Yet I have to admit that hand-crafted mapping code is not a viable option. The improvement in performance would be neutralized by unmaintainable code.

So, first I did a research to find a mapping framework that would not use the Reflection API at runtime but rather generate the mapping code. I was successful and stumbled over a fairly new project: mapstruct. It is still missing some features (like field access) and it has its problems with Lombok (which we use for reducing boiler plate code).

Bean mapping based on mapstruct

@Mapper
public interface PersonNameMapper {
    PersonNameMapper INSTANCE = Mappers.getMapper(PersonNameMapper.class);

    PersonNameResource map(PersonName person);
}

Mapstruct works with an annotation processor (apt) to actually generate Java code for the mapping. As a developer you just declare the mapper interface, annotate it with @Mapper and mapstruct generates the implementation based on an convention over configuration approach at compile time. The "hand-crafted" mapping code we saw earlier is in fact generated by mapstruct.

PersonName name = new PersonNameFactory().newEntity();
PersonNameMapper mapstructMapper = PersonNameMapper.INSTANCE;
mapstructMapper.map(name);

So this aproach would get me the best of both worlds - good performance and no need to write mapping code. Next up, we have to get some numbers and arm our arguments towards non-reflection based mapping.

The OpenJDK project has published their JMH micro-benchmarking framework which they use for their performance improvements.

JMH benchmark

public class PersonNameMappingBenchmark {

    private static final PersonName name = new PersonNameFactory().newEntity();

    private static final PersonNameMapper mapstructMapper = PersonNameMapper.INSTANCE;

    @Benchmark
    @BenchmarkMode(Mode.SampleTime)
    @OutputTimeUnit(TimeUnit.MICROSECONDS)
    public void mapstruct() throws InterruptedException {
        mapstructMapper.map(name);
    }

    private static ModelMapper modelMapper = new ModelMapper();

    @Benchmark
    @BenchmarkMode(Mode.SampleTime)
    @OutputTimeUnit(TimeUnit.MICROSECONDS)
    public void modelmapper() throws InterruptedException {
        modelMapper.map(name, PersonNameResource.class);
    }

    private static Mapper dozerMapper = new DozerBeanMapper();

    @Benchmark
    @BenchmarkMode(Mode.SampleTime)
    @OutputTimeUnit(TimeUnit.MICROSECONDS)
    public void dozer() throws InterruptedException {
        dozerMapper.map(name, PersonNameResource.class);
    }

    public static void main(String[] args) throws RunnerException {
        Options opt = new OptionsBuilder()
                .include(PersonNameMappingBenchmark.class.getSimpleName())
                .warmupIterations(5)
                .measurementIterations(5)
                .forks(1)
                .build();

        new Runner(opt).run();
    }
}

As I don’t want to pick on any of the Reflection API based frameworks I don’t publish the results here. I suggest that you checkout the project at GitHub und get your numbers yourself.

Obviously the solution based on explicit mapping code will outperform their Reflection API based sisters - that should come with no surprise. Also, the results from this should not be an argument towards blindly applying the DTO pattern.