The current Java Persistence API (JPA) standard does not mandate JPA provider to support data type conversions through annotations, not even a with simple boolean field. For readers who are unfamiliar with JPA, what I mean is, to persist a boolean field, JPA expects the database data type to be integer, where value of "1" means true, and value of "0" means false.

@boolean(trueValue="Yes", falseValue="No")
private boolean enabled;

For me, this is a very annoying limitation, espeically when you have to deliver applications on an existing database with hundreds of tables.

After some research, I decided to create my own Java Annotation Processing Tool (APT) Compile Time Annotation, called @BooleanMagic, with a compile time Java APT preprocessing factory, which will automatically generate additional code to work around the issue.

I've also made some changes on the original code:

• Fixed the bug of Null pointer exception, when JPA return null on the annotated field.
• Introduce a new properties call ifNull, which allows user to configure what to return if JPA returns null, it expect enum of org.jbpcc.util.jpa.ReturnType, which have values of ReturnType.True, ReturnType.FALSE, and ReturnType.Null,. The default value of ifNull is ReturnType.Null

So here is an  example, assuming we have model class defined as below:

package org.jbpcc.domain.model;
import javax.persistence.Entity;
import javax.persistence.Id;
import org.jbpcc.util.jpa.BooleanMagic;
import org.jbpcc.util.jpa.BooleanMagic.ReturnType;

@Entitypublic class SomeVO 
{   
 @Id   private Integer id;   
 @BooleanMagic(trueValue = "Yes", falseValue = "No",                
                       columnName = "OVERDUED", ifNull = ReturnType.FALSE)   
 private transient Boolean overdued;   
 public Boolean isOverdued() 
 {       
   return overdued;   
 }   
 public void setOverdued(Boolean overdued) 
 {       
   this.overdued = overdued;   
 }
}

Using Java APT with JPABooleanMagicConverter factory, the code above will be now be converted to:

@Entitypublic class SomeVO 
{     
     @Id   private Integer id;   
      private transient Boolean overdued;   
        //--- Lines below are generated by JBPCC BooleanMagicConvertor PROCESSOR   
        //--- START :   
       @Column(name="OVERDUED")   
       private String magicBooleanOverdued;   
       public Boolean isOverdued() 
      {      
         if (this.magicBooleanOverdued == null)          
              return false;       
          return this.magicBooleanOverdued.equals("Yes") ? Boolean.TRUE : Boolean.FALSE;   
      }   
      public Boolean getOverdued() 
     {       
          if (this.magicBooleanOverdued == null)            
               return false;       
             return this.magicBooleanOverdued.equals("Yes") ? Boolean.TRUE : Boolean.FALSE;   
      }   
       public void setOverdued(Boolean trueFlag) 
      {        
          this.magicBooleanOverdued = trueFlag ? "Yes" : "No";   
       }  
      //--- END  
       //--- GENERATED BY JBPCC BooleanMagicConvertor PROCESSOR
 }

I have put the annotation with it compile time process factory under my open source project - Java Batch Process control center, at http://code.google.com/p/jbpcc, I also posted an article at my blog detailing the usage of the annotation. I hope some readers will find the annotation and the APT preprocessing factory useful.

Do share your thoughts and suggestions.