How to customize ModelMapper

I've been using it from last 6 months, I'm going to explain some of my thoughts about that:

First of all, it is recommended to use it as an unique instance (singleton, spring bean,...), that's explained in the manual, and I think all agree with that.

ModelMapper is a great mapping library and wide flexible. Due to its flexibility, there are many ways to get the same result, and that's why it should be in the manual of best practices of when to use one or other way to do the same thing.

Starting with ModelMapper is a little bit difficult, it has a very tight learning curve and sometimes it is not easy to understand the best ways to do something, or how to do some other thing. So, to start it is required to read and understand the manual precisely.

You can configure your mapping as you want using the next settings:

Access level
Field matching
Naming convention
Name transformer
Name tokenizer 
Matching strategy

The default configuration is simply the best (http://modelmapper.org/user-manual/configuration/), but if you want to customise it you are able to do it.

Just one thing related to the Matching Strategy configuration, I think this is the most important configuration and is need to be careful with it. I would use the Strict or Standard but never the Loose, why?

  • Due Loose is the most flexible and intelligent mapper it could be map some properties you can not expect. So, definitively, be careful with it. I think is better to create your own PropertyMap and use Converters if it is needed instead of configuring it as Loose.

Otherwise, it is important to validate all property matches, you verify all it works, and with ModelMapper it's more need due with intelligent mapping it is done via reflection so you will not have the compiler help, it will continue compiling but the mapping will fail without realising it. That's one of the things I least like, but it needs to avoid boilerplate and manual mapping.

Finally, if you are sure to use ModelMapper in your project you should use it using the way it proposes, don't mix it with manual mappings (for example), just use ModelMapper, if you don't know how to do something be sure is possible (investigate,...). Sometimes is hard to do it with model mapper (I also don't like it) as doing by hand but is the price you should pay to avoid boilerplate mappings in other POJOs.


I faced a problem while mapping with ModelMapper. Not only properties but also My source and destination type were different. I solved this problem by doing this ->

if the source and destination type are different. For example,

@Entity
class Student {
    private Long id;
    
    @OneToOne
    @JoinColumn(name = "laptop_id")
    private Laptop laptop;
}

And Dto ->

class StudentDto {
    private Long id;
    private LaptopDto laptopDto;
}

Here, the source and destination types are different. So, if your MatchingStrategies are STRICT, you won't able to map between these two different types. Now to solve this, Just simply put this below code in the constructor of your controller class or any class where you want to use ModelMapper->

private ModelMapper modelMapper;

public StudentController(ModelMapper modelMapper) {
    this.modelMapper = modelMapper;
    this.modelMapper.typeMap(Student.class, StudentDto.class).addMapping(Student::getLaptop, StudentDto::setLaptopDto);
}

That's it. Now you can use ModelMapper.map(source, destination) easily. It will map automatically

modelMapper.map(student, studentDto);

First here are some links

  • modelmapper getting started
  • api doc
  • blog post
  • random code examples

My impression of mm is that it is very well engineered. The code is solid and a pleasure to read. However, the documentation is very terse, with very few examples. Also the api is confusing because there seems to be 10 ways to do anything, and no indication of why you’d do it one way or another.

There are two alternatives: Dozer is the most popular, and Orika gets good reviews for ease of use.

Assuming you still want to use mm, here’s what I’ve learned about it.

The main class, ModelMapper, should be a singleton in your app. For me, that meant a @Bean using Spring. It works out of the box for simple cases. For example, suppose you have two classes:

class DogData
{
    private String name;
    private int mass;
}

class DogInfo
{
    private String name;
    private boolean large;
}

with appropriate getters/setters. You can do this:

    ModelMapper mm = new ModelMapper();
    DogData dd = new DogData();
    dd.setName("fido");
    dd.setMass(70);
    DogInfo di = mm.map(dd, DogInfo.class);

and the "name" will be copied from dd to di.

There are many ways to customize mm, but first you need to understand how it works.

The mm object contains a TypeMap for each ordered pair of types, such as <DogInfo, DogData> and <DogData, DogInfo> would be two TypeMaps.

Each TypeMap contains a PropertyMap with a list of mappings. So in the example the mm will automatically create a TypeMap<DogData, DogInfo> that contains a PropertyMap that has a single mapping.

We can write this 

    TypeMap<DogData, DogInfo> tm = mm.getTypeMap(DogData.class, DogInfo.class);
    List<Mapping> list = tm.getMappings();
    for (Mapping m : list)
    {
        System.out.println(m);
    }

and it will output

PropertyMapping[DogData.name -> DogInfo.name]

When you call mm.map() this is what it does,

  1. see if the TypeMap exists yet, if not create the TypeMap for the <S, D> source/destination types
  2. call the TypeMap Condition, if it returns FALSE, do nothing and STOP
  3. call the TypeMap Provider to construct a new destination object if necessary
  4. call the TypeMap PreConverter if it has one
  5. do one of the following:
    • if the TypeMap has a custom Converter, call it
    • or, generate a PropertyMap (based on Configuration flags plus any custom mappings that were added), and use it (Note: the TypeMap also has optional custom Pre/PostPropertyConverters that I think will run at this point before and after each mapping.)
  6. call the TypeMap PostConverter if it has one

Caveat: This flowchart is sort of documented but I had to guess a lot, so it might not be all correct!

You can customize every single step of this process. But the two most common are

  • step 5a. – write custom TypeMap Converter, or
  • step 5b. – write custom Property Mapping.

Here is a sample of a custom TypeMap Converter:

    Converter<DogData, DogInfo> myConverter = new Converter<DogData, DogInfo>()
    {
        public DogInfo convert(MappingContext<DogData, DogInfo> context)
        {
            DogData s = context.getSource();
            DogInfo d = context.getDestination();
            d.setName(s.getName());
            d.setLarge(s.getMass() > 25);
            return d;
        }
    };

    mm.addConverter(myConverter);

Note the converter is one-way. You have to write another if you want to customize DogInfo to DogData.

Here is a sample of a custom PropertyMap:

    Converter<Integer, Boolean> convertMassToLarge = new Converter<Integer, Boolean>()
    {
        public Boolean convert(MappingContext<Integer, Boolean> context)
        {
            // If the dog weighs more than 25, then it must be large
            return context.getSource() > 25;
        }
    };

    PropertyMap<DogData, DogInfo> mymap = new PropertyMap<DogData, DogInfo>()
    {
        protected void configure()
        {
            // Note: this is not normal code. It is "EDSL" so don't get confused
            map(source.getName()).setName(null);
            using(convertMassToLarge).map(source.getMass()).setLarge(false);
        }
    };

    mm.addMappings(mymap);

The pm.configure function is really funky. It’s not actual code. It is dummy EDSL code that gets interpreted somehow. For instance the parameter to the setter is not relevant, it is just a placeholder. You can do lots of stuff in here, such as

  • when(condition).map(getter).setter
  • when(condition).skip().setter – safely ignore field.
  • using(converter).map(getter).setter – custom field converter
  • with(provider).map(getter).setter – custom field constructor

Note the custom mappings are added to the default mappings, so you do not need, for example, to specify

            map(source.getName()).setName(null);

in your custom PropertyMap.configure().

In this example, I had to write a Converter to map Integer to Boolean. In most cases this will not be necessary because mm will automatically convert Integer to String, etc.

I'm told you can also create mappings using Java 8 lambda expressions. I tried, but I could not figure it out.

Final Recommendations and Best Practice

By default mm uses MatchingStrategies.STANDARD which is dangerous. It can easily choose the wrong mapping and cause strange, hard to find bugs. And what if next year someone else adds a new column to the database? So don't do it. Make sure you use STRICT mode:

    mm.getConfiguration().setMatchingStrategy(MatchingStrategies.STRICT);

Always write unit tests and ensure that all mappings are validated.

    DogInfo di = mm.map(dd, DogInfo.class);
    mm.validate();   // make sure nothing in the destination is accidentally skipped

Fix any validation failures with mm.addMappings() as shown above.

Put all your mappings in a central place, where the mm singleton is created.

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Java

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Modelmapper

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