Mapping Immutable Value-Objects with Dozer

2 good things happened to me this week (well actually 3, but I will probably blog about the 3rd one later):

The first thing is that I finally managed to convince Dozer to map immutable objects, and the second one is that I found something interesting to blog about ;)

I mentioned in a previous post about Dozer’s lack of support for constructor arguments. In general Dozer is aimed at supporting JavaBean to JavaBean mapping, and other usage scenarios seem to be hard to implement. It turns out that the problem can be solved using design-patterns, and a little bit of trickery.

The first step towards the solution, is introducing the Builder Pattern. Actually a form of the Builder Pattern that introduced by Joshua Bloch at Java One. The pattern solves the problem of too many constructors, too many constructor arguments, and the verbosity of object creation while using setters. The pattern is described in detail here: http://ow.ly/1L2JV.

Let’s suppose we are about to map an Address JavaBean to an Immutable Address object. Here are the Address classes:

public class Coordinate {
 private double longitude;
 private double latitude;
 // getters, setters, c'tors, equals(), hashCode(), toString(), etc...
}

public class Address {
 private String country;
 private String state;
 private String city;
 private String street;
 private String zipcode;
 private Coordinate coordinate;
 // getters, setters, c'tors, equals(), hashCode(), toString(), etc...
}

And here’s the immutable address:

public class ImmutableCoordinate {
 private final double longitude;
 private final double latitude;
 
 private ImmutableCoordinate(Builder builder) {
  this.latitude = builder.latitude;
  this.longitude = builder.longitude;
 }

 public double getLongitude() {
  return longitude;
 }
 
 public double getLatitude() {
  return latitude;
 }
  
 public static class Builder {
  private double longitude;
  private double latitude;
  
  public Builder longitude(double longitude) {
   this.longitude = longitude;
   return this;
  }
  
  public Builder latitude(double latitude) {
   this.latitude = latitude;
   return this;
  }
  
  public ImmutableCoordinate build() {
   return new ImmutableCoordinate(this);
  }
 }
}

public class ImmutableAddress {
 private final String country;
 private final String state;
 private final String city;
 private final String street;
 private final String zipcode;
 private final ImmutableCoordinate coordinate;
 
 private ImmutableAddress(Builder builder) {
  this.country = builder.country;
  this.state = builder.state;
  this.city = builder.city;
  this.street = builder.street;
  this.zipcode = builder.zipcode;
  this.coordinate = builder.coordinate;  
 }

 public String getCountry() {
  return country;
 }
 
 public String getState() {
  return state;
 }
  
 public String getCity() {
  return city;
 }
 
 public String getStreet() {
  return street;
 }
  
 public String getZipcode() {
  return zipcode;
 }
 
 public ImmutableCoordinate getCoordinate() {
  return coordinate;
 }
  
 public static class Builder {
  private String country;
  private String state;
  private String city;
  private String street;
  private String zipcode;
  private ImmutableCoordinate coordinate;
    
  public Builder country(String country) {
   this.country = country;
   return this;
  }

  public Builder state(String state) {
   this.state = state;
   return this;
  }

  public Builder city(String city) {
   this.city = city;
   return this;
  }

  public Builder street(String street) {
   this.street = street;
   return this;
  }

  public Builder zipcode(String zipcode) {
   this.zipcode = zipcode;
   return this;
  }

  public Builder coordinate(ImmutableCoordinate coordinate) {
   this.coordinate = coordinate;
   return this;
  }
  
  public ImmutableCoordinate getCoordinate() {
   return coordinate;
  }

  public ImmutableAddress build() {
   return new ImmutableAddress(this);
  }
 }
}

Now, by we can map our mutable class to the Builder of the immutable class, and throw in a custom DozerConverter where nested properties are involved. Below is the mapping for the Address classes:

<?xml version="1.0" encoding="UTF-8"?>
<mappings xmlns="http://dozer.sourceforge.net"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://dozer.sourceforge.net
          http://dozer.sourceforge.net/schema/beanmapping.xsd">

 <configuration>
  <stop-on-errors>true</stop-on-errors>
  <date-format>MM/dd/yyyy HH:mm</date-format>
  <wildcard>true</wildcard>
 </configuration>
 
 <mapping>
  <class-a>location.Coordinate</class-a>
  <class-b>location.ImmutableCoordinate$Builder</class-b>
  
  <field>
   <a set-method="setLongitude">longitude</a>
   <b set-method="longitude">longitude</b>
  </field>
  <field>
   <a set-method="setLatitude">latitude</a>
   <b set-method="latitude">latitude</b>
  </field>
 </mapping>

 <mapping>
  <class-a>location.Address</class-a>
  <class-b>location.ImmutableAddress$Builder</class-b>
  
  <field>
   <a set-method="setCountry">country</a>
   <b set-method="country">country</b>
  </field>
  <field>
   <a set-method="setState">state</a>
   <b set-method="state">state</b>
  </field>
  <field>
   <a set-method="setCity">city</a>
   <b set-method="city">city</b>
  </field>
  <field>
   <a set-method="street">street</a>
   <b set-method="street">street</b>
  </field>
  <field>
   <a set-method="setZipcode">zipcode</a>
   <b set-method="zipcode">zipcode</b>   
  </field>
  <field custom-converter-id="coordConverter">
   <a set-method="setCoordinate">coordinate</a>
   <b set-method="coordinate">coordinate</b>
  </field>
 </mapping>
</mappings>

And the DozerConverter is a fairly straight forward implementation (I actually use Dozer to do its own job…):

public class CoordinateConverter extends DozerConverter {

 private final Mapper mapper;
  
 public CoordinateConverter(Mapper mapper) {
  super(Coordinate.class, ImmutableCoordinate.class);
  this.mapper = mapper;
 } 
 
 @Override
 public Coordinate convertFrom(ImmutableCoordinate source, Coordinate destination) {  
  return mapper.map(source, Coordinate.class);
 }
 
 @Override
 public ImmutableCoordinate convertTo(Coordinate source, ImmutableCoordinate destination) {
  return source == null ? null : mapper.map(source, ImmutableCoordinate.Builder.class).build();
 }
}

Now mapping between the classes is a matter of a single line:

Address address = new Address();
// set set set...
  
ImmutableAddress immutableAddress = mapper.map(address, ImmutableAddress.Builder.class).build();

And it even works in the opposite direction :D

Although this solution is not as clean as it should have been – there’s still some over verbosity, and an obscure need for a getter in some cases, it is still preferable over the piles of code you get when messing with object mapping. This technique may also be easier to sneak into the Dozer code-base than constructor arguments support.

Value Objects Mapping

My company decided to break its monolithic build and make our life easier.  The main idea was to move from Jini services to Protocol-Buffers over RESTful API.

The main reasons were to introduce loose coupling between our services, adopt open standards, and become cool ;-).

I will probably discuss all this in a later post. The reason I am mentioning all this is that it raised the issue of transforming value objects back and forth.  We are all faced with this issue when developing enterprise software. For example, you create objects in your DAO layer. These are passed to domain objects that massage them a little. Later these objects are passed to your service layer, controller, and presentation layer.

Some of you are probably saying right now, “Are you really coupling all these layers”?  Well, no… Most of us will go and transform the Value Objects (VOs, or DTOs) to another form when passed between the layers. All this copying and transforming is extremely repetitive and time consuming.

When we started working with protobuf, I quickly noticed that transforming protobuf messages into our domain model is going to be an issue. To avoid decoupling between services, each service has to write its own transformation code. This requirement poses a high risk of code duplication (I know some will argue you can’t really call it duplication). My idea was to introduce XML object to object mapping rules.

In comes Dozer. Dozer is a Java Bean to Java Bean mapper that recursively copies data from one object to another. It has many cool features, but lacks the ability to transform your objects into immutable objects – Dozer has no support for constructor arguments. Making your VO classes immutable is desired when you are not administrating them, and when these objects are going to be placed in caches.

Unfortunately, I don’t have a solution to the immutability issue (but more will come - I promise). I have started a private project that will either extend Dozer to support constructor args (hard…), or utilize for its features while extending its abilities to do so (somewhat easier).

I just wonder how many of you stumbled into this…