[VALIDATOR]Immutable objects that always fulfill constraints

eirirlar · **Posted:** Thu Nov 06, 2008 4:59 am

Hi all,

I've been fiddling around with the builder pattern and have combined it with hibernate validator to create immutable value objects. See my example for a typical Address class, it should be pretty self explanatory.

I believe this is a safe and powerful way to build your domain: the mutable inner builder class is used whenever you create objects incrementally from an untrusted source, for example user input from a web form, reading from the db, recieving objects from a web service etc. The immutable outer class is used for all kind of object sharing inside and outside the domain. Additionally it is always in a valid state.

Comments appreciated.

Code:

package org.schwin;

import org.hibernate.validator.ClassValidator;
import org.hibernate.validator.Length;
import org.hibernate.validator.Min;
import org.hibernate.validator.NotNull;
import org.hibernate.validator.Pattern;

public final class Address {
   private final String streetname;
   private final int streetnumber;
   private final String postalcode;
   private final String postalplace;

   private Address(final String streetname, final int streetnumber,
         final String postalcode, final String postalplace) {
      super();
      this.streetname = streetname;
      this.streetnumber = streetnumber;
      this.postalcode = postalcode;
      this.postalplace = postalplace;
   }

   public final Builder builder() {
      return new Builder(this);
   }

   public final String getStreetname() {
      return streetname;
   }

   public final int getStreetnumber() {
      return streetnumber;
   }

   public final String getPostalcode() {
      return postalcode;
   }

   public final String getPostalplace() {
      return postalplace;
   }

   public static final class Builder implements org.schwin.Builder<Address>,
         Validator {
      private String streetname;
      private int streetnumber;
      private String postalcode;
      private String postalplace;

      private boolean valid = false;
      private boolean validated = false;
      private static final ClassValidator<Builder> validator = new ClassValidator<Builder>(
            Builder.class);

      public Builder() {
         super();
      }

      private Builder(final Address address) {
         super();
         this.streetname = address.streetname;
         this.streetnumber = address.streetnumber;
         this.postalcode = address.postalcode;
         this.postalplace = address.postalplace;
      }

      public final void setStreetname(final String streetname) {
         invalidate();
         this.streetname = streetname;
      }

      @Length(min = 1)
      public final String getStreetname() {
         return streetname;
      }

      public final void setStreetnumber(final int streetnumber) {
         invalidate();
         this.streetnumber = streetnumber;
      }

      @Min(1)
      public final int getStreetnumber() {
         return streetnumber;
      }

      @NotNull
      @Length(min = 4, max = 8)
      @Pattern(regex = "^\\d*$")
      public final String getPostalcode() {
         return postalcode;
      }

      public void setPostalcode(final String postalcode) {
         invalidate();
         this.postalcode = postalcode;
      }

      @NotNull
      @Length(min = 1, max = 50)
      public final String getPostalplace() {
         return postalplace;
      }

      public void setPostalplace(final String postalplace) {
         invalidate();
         this.postalplace = postalplace;
      }

      private void invalidate() {
         validated = false;
      }

      private void validate() {
         valid = 0 == validator.getInvalidValues(this).length;
         validated = true;
      }

      @Override
      public final boolean isValid() {
         if (!validated)
            validate();
         return valid;
      }

      @Override
      public final Address build() {
         if (!isValid())
            throw DEFAULT_INVALID_EXCEPTION;
         return new Address(streetname, streetnumber, postalcode,
               postalplace);
      }
   }
}

Code:

package org.schwin;

public interface Builder<T> {
   public final static IllegalStateException DEFAULT_INVALID_EXCEPTION = new IllegalStateException(
         "build() called while builder was in an invalid state");

   public abstract T build() throws IllegalStateException;
}

Code:

package org.schwin;

public interface Validator {
   public abstract boolean isValid();
}

eirirlar · **Posted:** Thu Nov 06, 2008 5:17 am

Here's a test that demonstrates some of the functionality:

Code:

package org.schwin;

import static org.junit.Assert.assertFalse;

import org.junit.Test;

public class AddressTest {
   @Test(expected = IllegalStateException.class)
   public void testInvalid() {
      Builder<Address> builder = new Address.Builder();
      Address address = builder.build();
   }

   @Test
   public void testValid() {
      Address.Builder mutableAddress = new Address.Builder();
      mutableAddress.setStreetname("Thorvald Meyers gate");
      mutableAddress.setStreetnumber(2);
      mutableAddress.setPostalcode("Something illegal");
      mutableAddress.setPostalplace("OSLO");
      assertFalse(mutableAddress.isValid());
      mutableAddress.setPostalcode("1234");
      Address immutableAddress = mutableAddress.build();
      Address.Builder mutableCopy = immutableAddress.builder();
      mutableCopy.setStreetname("Karl Johans gate");
      Address immutableAddress2 = mutableCopy.build();
   }
}

sanne.grinovero · **Posted:** Thu Nov 06, 2008 12:40 pm

hey this is a cool pattern,
you should blog about it.

What about having all setters return "this", so you can write
it like

Code:

Address immutableAddress = new Address.Builder()
      .setStreetname("Thorvald Meyers gate")
      .setStreetnumber(2)
      .setPostalcode("Something illegal")
      .setPostalplace("OSLO")
      .build();

this is how I know the builder pattern.

eirirlar · **Posted:** Thu Nov 06, 2008 3:00 pm

s.grinovero wrote:

hey this is a cool pattern,
you should blog about it.

What about having all setters return "this", so you can write
it like

Code:

Address immutableAddress = new Address.Builder()
      .setStreetname("Thorvald Meyers gate")
      .setStreetnumber(2)
      .setPostalcode("Something illegal")
      .setPostalplace("OSLO")
      .build();

this is how I know the builder pattern.

Thanks for the feedback. Yeah, I should have mentioned that: some frameworks (JSF) don't like the setters to return stuff, they like the JavaBeans to follow the JavaBeans standard, otherwise exceptions might be thrown (don't know why, but anyway it's Good Practice to follow the standard).

There's a simple workaround to get the chaining you describe above to work with JSF though, just use builder.streetname("Tha Str33t") like Josh Bloch describes in More Effective Java 2nd edition.

I'm creating a small demo app (seam, jsf, hibernate validator) to show more of the concepts I'm thinking about. I'll provide a link to it in a while.

sanne.grinovero · **Posted:** Sat Nov 08, 2008 10:50 am

looking forward for you demo,
I'm curious to see how immutable objects could be useful to Seam/JSF,
it doesn't look natural but could be an interesting reading.

eirirlar · **Posted:** Thu Nov 27, 2008 7:35 pm

s.grinovero wrote:

looking forward for you demo,
I'm curious to see how immutable objects could be useful to Seam/JSF,
it doesn't look natural but could be an interesting reading.

I have a demo about this on my blog now. Check it out and let me know what you think :)

http://larsen.blogsite.org

Cheers, Eirik