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Hibernate DataViewObjects

When working with Hibernate and nested domain object models, it can be a performance problem when trying to display several (including nested) properties in a view. Usually for such cases the use of view objects is practical.
This is the first scetch of my idea to automate population of view objects with hibernate.

Meanwhile I worked out some extended example with the use of Annotations. However, the current form is not usefull enough, as using criterions for restrictions is quite difficult. But more details on the next part of this. Here the first one:

Assume we have some nested domain model consisting of a person, an address and a country as following:

@Entity
public class Person{
	private Long id;
	private String firstname:
	private String lastName;
	private Date birthdate;
	private String comment;
	private Address address;
 
	@Id
	@GeneratedValue(strategy=GenerationType.AUTO)
	public Long getId(){
		return id;
	}
 
	// appropriate getters and setter for id,
	// firstname, lastname, birthdate and comment
 
	@ManyToOne
	public Address getAddress(){
		return address;
	}
}
 
@Entity
public class Address{
	private Long id;
	private String street;
	private Country country;
 
	// appropriate getters and setter for 
	// id, street and country
 
	@ManyToOne
	public Country getCountry(){
		return country;
	}
}
 
@Entity
public class Country{
	private String iso3166; 
	// iso 3166 2 letter code 
	// (uppercase), like DE for Germany
	private String name;
 
	@Id
	@Column(length=2)
	public String getIso3166(){
		return iso3166;
	}
 
	// appropriate getters and setter for name and iso3166
}

Now let’s assume we want to build a list displaying only the lastname and the countryname of a person. In such a case it’s unneccessary to do a query like „SELECT p FROM Person p“, since even if lazy loading is enabled, hibernate will load all properties leading to lot’s of unneeded data transported from database to application.
In such a case we would prefer to build a view object, that only will hold the information we need to display:

public class PersonLastNameCountryViewObject{
	private String lastName;
	private String countryName;
 
	// appropriate getters and setter
}

So we now have a lightweight view object, but how do we get the data into it?
One possibility would be to build a custom query for this, like:

SELECT p.lastName, c.name FROM Person p LEFT JOIN p.address AS a LEFT JOIN a.country AS c

This would give us the needed data and we only would have to map the results to our view object.
Having several views may lead to lots of custom queries and code doing the retrieval and mapping stuff. So why not automating it?

I created some classes to handle this work, but there are few informations that have to be given to it, especially about which view object properties to map to which domain model properties.
In the above view object example, I decided to use a map of view object properties and define a domain model path for each, just like in commons-beanutils.
The mapping for the above example would be:

lastName -> lastName
countryName -> address.country.name

The path here starts with the initial domain class, in this case the Person class. Since Hibernate does not support nested object paths like in

SELECT p.name, p.address.country.name FROM Person p

I built some code to split the path and join the appropriate domain models into the query just to have the working query denoted previously.
So my view object mapping helpers contain of following classes:

// Just an interface that will create a 
// view object for us for each resultset row
public interface ViewObjectFactory{
  public Object createNewViewObject();
}
 
// a cimple class to hold all ,eta data needed for building our view objects
public class ViewObjectMetaData{
  // This will hold the mappings like
  // lastName -> lastName
  // countryName -> address.country.name
  private final HashMap<String, String> 
    propertiesToObjectPaths = new HashMap<String, String>();
 
  // this will hold our initial domain class, like Person.class.
  // we need this to create a criteria in hibernate later on
  private Class domainClass;
 
  // the factory we will use to create the view objects
  private ViewObjectFactory voFactory;
 
  // as default, if no object paths are given, we assume that each view object property
  // will be mapped non-nestingly to the domain class propertys with the same name.
  // so the mapping will be something like:
  // propertyA -> propertyA
  // propertyB -> propertyB
  // will need BeanUtils to work, or make some own code with reflection
  public ViewObjectMetaData(ViewObjectFactory voFactory, 
    Class domainClass){
    this.voFactory = voFactory;
    this.domainClass = domainClass;
    try {
      Map desc = BeanUtils.describe(dtoFactory.createDto());
      desc.remove("class");
      final Collection<String> properties = 
        (Collection<String>) desc.keySet();
      for (final String property:properties){
        propertiesToObjectPaths.put(property, property);
      }
    }
    catch (Exception e) {
      // just make a runtime exception from it...
      if (e instanceof RuntimeException){
        throw (RuntimeException) e;
      }
      else{
        final RuntimeException r = new RuntimeException(e);
        r.setStackTrace(e.getStackTrace());
        throw r;
      }
    }
  }
 
  public ViewObjectMetaData(ViewObjectFactory voFactory, 
    Class domainClass, 
    Map<String, String> propertiesToObjectPaths){
    this.voFactory = voFactory;
    this.domainClass = domainClass;
    this.propertiesToObjectPaths.putAll(propertiesToObjectPaths); 
  }
 
  // appropriate getters
}

Now we have all information needed to build up queries and populate the view objects. The main work will consist of building up the query from the object path and mapping the resultset to the view objects.
For building the query, I used the criteria API of hibernate, which allows to add further restrictions on the query later on. The algorithm for transforming the object path to a criteria is not that complicated. We have to think about two different cases in the object path:

1. object path consists of one part, like „lastName“. This has to be mapped to a projection, like:

SELECT p.lastName FROM Person p

2. the object consists of multiple parts, like „address.street“ or „address.country.name“. In this case we have to create a hibernate alias (which internally maps to a join) for each part except the first (which is the root domain class) and the last, which has to be added as a projection as it is the property we want to have. So we have for the two part object path example:

SELECT a.street FROM Person p LEFT JOIN p.address AS a

and the three part object path example:

SELECT c.name FROM Person p LEFT JOIN p.address AS a LEFT JOIN a.country AS c

Also we’ll keep track of the indices of the projected properties in the resultset for the mapping between resultset and viewobject

public class ViewObjectCriteriaBuilder{
 
  // just a helper class to transport 
  // criteria and columnmappings
  public static class ViewObjectCriteria{
    private Criteria criteria;
    private HashMap<String, Integer> propertyToColumnMappings = 
      new HashMap<String, Integer>();
 
    // apropriate getters/setters
  }
 
  private SessionFactory sessionFactory;
 
  // appropriate setter for the sessionFactory 
  // or just pass a current session to the next method
 
  public ViewObjectCriteria buildCriteria(ViewObjectMetaData voMetaData){
    final ViewObjectCriteria voCriteria = new ViewObjectCriteria();
 
    final Criteria criteria = sessionFactory.
      getCurrentSession().
      createCriteria("d", voMetaData.getDomainClass());
    voCriteria.setCriteria(criteria);
 
    final Map<String, String> propertiesToObjectPaths = 
      voMetaData.getPropertiesToObjectPaths();
 
    // our projections
    final ProjectionList projections = Projections.projectionList();
 
    // max. amount of aggregations in object path
    int maxAggregationDeepth = 0;
    // index of property in resultset
    int propertyColumnIndex = 0;
 
    String objectPath; // objectpath of current vo property
    // amount of aggregations in current object path
    int aggregationDeepth; 
    final ArrayList<String> aggregationParts = 
      new ArrayList<String>(5); // parts of the object path, splitted
 
 
    // in the first step we only collect information 
    // about the needed aggregations,
    // the joins will be build later on
    for (final String property:propertiesToObjectPaths.keySet()){
      objectPath = propertiesToObjectPaths.get(property);
 
      // this helper just counts the 
      // occurences of '.' in the objectPath
      aggregationDeepth = StringUtil.occurences(objectPath, '.');
 
      if (aggregationDeepth<1){
        // no aggregation, like
        // lastName -> d.lastName
        projections.add(Projections.property("d."+objectPath));
      }
      else{
        maxAggregationDeepth = 
          Math.max(maxAggregationDeepth, aggregationDeepth);
        // aggregations exist
 
        // first split the object path
        aggregationParts.clear();
        // this helper splits the objectPath 
        // on "\\." and adds all parts to the 
        // aggregationParts-list. Works the same 
        // way as String.split
        CollectionSplitter.toList(aggregationParts, objectPath, "\\.");
 
        // last aggregations alias
        String lastAggregationName = null;
        String currentAggregationPart, currentAggregationName;
 
        // when we're here, there are at least two parts in the object path
        // first part is an property of the 
        // root domain class, e. g. address -> d.address
        for (int aggregationIndex=0; 
          aggregationIndex<=aggregationDeepth; aggregationIndex++){
 
          currentAggregationPart = aggregationParts.get(aggregationIndex);
          if (aggregationIndex==0){
            // equals to "LEFT JOIN d.address AS ag_address"
            lastAggregationName = "ag_"+currentAggregationPart;
            aggregations.put(
              "d."+currentAggregationPart, 
              lastAggregationName);
          }
          else if (aggregationIndex==aggregationDeepth){
            // last part in object path, which 
            // denotes a property we want to have projected
            // equals to "SELECT ... ag_address_country.name FROM ..."
            projections.add(
              Projections.property(
                lastAggregationName+"."+currentAggregationPart));
          }
          else{
            // somewhere in the middle
            // equals to 
            // "LEFT JOIN ag_address.country AS ag_address_country"
            currentAggregationName = 
              lastAggregationName+"_"+currentAggregationPart;
            aggregations.put(
              lastAggregationName+"."+currentAggregationPart, 
              currentAggregationName);
            lastAggregationName = currentAggregationName;
          }
        }
      }
 
      // update index of property in resultset
      voCriteria.getPropertyToColumnMappings().
        put(property, propertyColumnIndex++);
    }
 
    // if there are any aggregations, we have
    // to add an alias for each to the criteria
    if (maxAggregationDeepth>0){
      // sort aggregatios (joins) by their alias length
      // this is to enshure that ag_address is 
      // added before ag_address_country
 
      final List<String> aggregationsSizeSorted = 
        new ArrayList<String>(aggregations.keySet());
 
      Collections.sort(aggregationsSizeSorted, 
        // the string length comparator just compares
        // the length of two strings
        new StringLengthComparator());
      for (final String fromAggregation:aggregationsSizeSorted){
        // here we also may use LEFT JOIN or some other join type
        criteria.createAlias(
          fromAggregation, 
          aggregations.get(fromAggregation));
      }
    }
 
    // add projections to criteria
    criteria.setProjection(projections);
 
    return voCriteria;    
  }
}

Now we have build a criteria which will load all neccessary data from the database. Meanwhile we could add some restrictions or limitations to the query to only retrieve a portion of all rows. Somewhere all along the code we will need to have a valid session/transaction, but I’ll skip that, as this is just an example.

Next we build a class that will execute the criteria and map the columns from the resultset to the viewobjects. Here are several possibilites. For example using a special hibernate rowsetmapper, but the way I did it below also works fine.

public List buildViewObjects(ViewObjectCriteria voCriteria, 
  ViewObjectMetaData voMetaData){
 
  ScrollableResults result = null;
  try{
    Criteria criteria = voCriteria.getCriteria();
    Map<String, Integer> propertyColumnMappings = 
      voCriteria.getPropertyColumnMappings();
    // some cache method or scroll lock method here, if needed
    result = criteria.scroll(ScrollMode.FORWARD_ONLY);
    Object [] rowdata;
    Object resultObject;
    final ArrayList viewObjects = new ArrayList();
    while (result.next()){
      rowdata = result.get();
      resultObject = voMetaData.getViewObjectFactory().
        createNewViewObject();
      for (final String property:
        propertyColumnMapping.keySet()){
        try {
          // also possible via reflection
          PropertyUtils.setProperty(resultObject, 
            property, rowdata[propertyColumnMappings.get(property)]);
        }
        catch (Exception e) {
          if (LOG.isErrorEnabled()){
            LOG.error("While setting property \""+
              property+"\" from column \""+
              propertyColumnMapping.get(property)+"\"", e);
          }
          resultObject = null;
        }
      }
      if (resultObject!=null){
        viewObjects.add(resultObject);
      }
    }
    return viewObjects;
  }
  finally{
    if (result!=null){
      result.close();
    }
  }
}

That’s it (for now).

Limitations:

  • the datatype of the setter in the view object has to be the same as in the corresponding domain property
  • restricting the query is quite difficult, as you don’t know the aliases of the generated aggregations a priori
  • Further improvements could be:

  • some annotations so that we just annotate our view object and rest is done in background
  • make a full working open-source API out of it
  • optimizations on the queries. if we only would like to select the id of a nested domain class, like „address.id“, which already is known to the Person table (since this is the referencing foreign key), it would be great not to have to join the addresses
  • automatic mapping between differring property types of view object and domain object (eg. long to string convertions)
  • improve the possibility to restrict the generated query
  • 2 Responses to “Hibernate DataViewObjects” »»

    1. Comment by Marcus | 10:26 18.03.10|X

      Hello Secco,

      thank you for this article. Stumbled on it, while i was searching for information, how to write dataviewobjects within hibernate. Really enjoyed reading it, since u have a nice spelling style.

      Well done!
      Marcus

    2. Comment by Secco | 14:15 26.03.10|X

      Hi Marcus,
      I appreciate your commendation which motivates me to continue and will post my further workouts on the data view objects asap.

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