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Domain Searching Using Visitors
Pages: 1, 2

SQL Visitor

The SQL visitor traverses the collection of search criteria objects and constructs a prepared statement. This prepared statement is then executed and a search result constructed. The various visit methods are used to accumulate the tables to include in the search, and the constraints on the search. The doSearch method visits all of the search criteria and combines the information yielded by the visitor methods to create a prepared statement that is executed. The result set is then traversed and a search result created. The following section shows the accumulation of information from the search criteria by visitor methods. After that, the actual query execution and result handling is explained.



Visitor Methods

The visitor methods accumulate information in four instance variables; the prepared statement to be built will have the form:

SELECT <selects> FROM <tables> WHERE <constraints> 

The SQL visitor uses collections to accumulate the values used to populate this statement. The collection used for <selects> is a list, since the order is significant; the order provides the basis for interpretation of the result set. Order is not significant for <tables>, so here a set is used. In the case of constraints, in order to be able to match actual values to the placeholders created in the prepared statement, two lists are used. The first contains the actual string values to be conjoined in the prepared statement; the second contains the values to be used to populate the placeholders.

This leads to the following instance variable declarations:

public class SQLSearchVisitor implements ISearchVisitor {
  private List selects = new ArrayList();
  private Set tables = new HashSet();
  private List criteria = new ArrayList();
  private List parameters = new ArrayList();
  ...
}

Having previously defined what the query should yield, it is possible to add some values to the collection, such as the names of the tables and columns, that need to be included for all searches. This is done by the method addFixedInformation:

private void addFixedInformation() {
  selects.add("VOLUME_TB.NAME");
  selects.add("FILE_TB.NAME");
  selects.add("FILE_TB.SIZE");
  selects.add("FILE_TB.LASTMODIFIED");
  tables.add("VOLUME_TB");
  tables.add("FILE_TB");
  criteria.add("FILE_TB.VOLUMEFK = VOLUME_TB.ID");
}

Each visitor method then adds information to the instance variables. For instance, consider visitVolumeName from the class SQLSearchVisitor. The visitor method adds the criterion that any matched volumes must include the submitted volume name as a substring. visitKeyword provides a more interesting example as in this case, extra tables have to be added.

public void visitKeyword(SearchKeyword keyword) {
  tables.add("KEYWORD_TB");
  tables.add("KEYWORD_VOLUME_REL");
  addCriterion("KEYWORD_TB.KEYWORD LIKE ?",
  keyword.getKeyword());
}

In this case, it is also necessary to constrain the tables so that the relationship between the keyword and volume represented by KEYWORD_VOLUME_REL is used. This is consigned to a separate method to avoid repetition in the criterion list:

private void addJoins(){
  if (tables.contains("KEYWORD_VOLUME_REL")){
    // Must also contain KEYWORD_TB and VOLUME_TB
    criteria.add("KEYWORD_VOLUME_REL.KEYWORDFK =
    KEYWORD_TB.ID");
    criteria.add("KEYWORD_VOLUME_REL.VOLUMEFK = VOLUME_TB.ID");
  }
}

The remaining visitor methods can be seen in the source code. These methods are glued together by the method buildQuery, from SQLSearchVisitor, which ensures that the instance variables are populated. Having populated these instance variables, it is possible to create and execute a prepared statement.

Query Construction and Execution

Query construction falls into two phases: creating the SQL text, and then inserting the parameters into the prepared statement, corresponding to the placeholders in the text. This is captured by getPreparedStatement, from SQLSearchVisitor, which uses the instance variable conn, which is an instance of java.sql.Connection.

Creating the SQL statement is just a question of iterating over the collections and concatenating the values to form a string. A helper method, addItems from SQLSearchVisitor, is used to exploit the similarity in the iterations. For example, if a search is submitted for files matching the pattern *.jpg, this would result in the following SQL statement:

SELECT VOLUME_TB.NAME, FILE_TB.NAME, FILE_TB.SIZE,
FILE_TB.LASTMODIFIED FROM FILE_TB, VOLUME_TB WHERE
FILE_TB.VOLUMEFK = VOLUME_TB.ID AND FILE_TB.NAME LIKE ?

And the parameters list would contain the single value "%.jpg%".

Once the prepared statement has been created, it can be executed. This yields a ResultSet object that can be traversed and a SearchResult constructed. This SearchResult object is returned as the result of the search to the presentation tier for rendering in a suitable form. Details can be found in the source code provided.

Hibernate Visitor

Hibernate is an object-relational persistence and query service for Java. It supports transparent persistence by allowing persistent data to be defined as plain old Java objects (POJOs). Runtime configuration is then used to map these objects to database tables, giving a clean separation between the domain object model and persistence management. Extensive literature about Hibernate is available elsewhere (see the Resources section), so I don't propose to provide a thorough introduction here.

Domain Objects and Persistence

The domain object model for the example is straightforward and is shown in Figure 5. I will briefly describe the mapping of one domain object to the database in order to give a flavor of how this is done.

Application Domain Object Model
Figure 5: Application domain object model

The domain class File represents the information stored about a backed-up file by the application. It contains a number of instance variables (listed in the table below) with associated getter and setter methods.

Name Description
id : Integer A unique identifier for the file.
lastModified : Date The date of the last modification of the file prior to backup.
name : String The name of the file.
size : Integer The size of the file in bytes.
volume : Volume The volume used to back up the file. If this File object is returned as the result of a search, this will be null.
volumeName : String The name of the volume. This is only used if the volume is null.

Instances of this class correspond to rows in the FILE_TB table shown in Figure 2; the VOLUME_FK column in this table is used to provide the volume and volumeName instance variables as needed.

The mapping between the File class and the FILE_TB table is provided by File.hbm.xml. Similar mapping files for the other domain classes can be defined. These can be seen in the accompanying source code.

Searching Using Hibernate

Having established the object-relational mapping, Hibernate offers a number of different methods of querying. It is possible to perform a query using the underlying JDBC connection, in a similar manner to the SQL search visitor described previously. However, this approach bypasses the persistence service and is provided only if the persistence service does not support the desired behavior, which is not the case here. A second method provided by Hibernate is Hibernate Query Language, which allows SQL-style queries to be phrased in the language of domain objects. This is a very powerful approach, and for complex queries is the recommended strategy. The third approach is to use Hibernate criteria, which are objects that constrain properties of the domain object model. For simple searches, such as those needed for the example, this approach is ideal and is therefore the one I have used.

The query to be executed by Hibernate is specified using a Criteria object. This object is built up by the visitor as it traverses the supplied search criteria. This object is therefore implemented as an instance variable by the visitor and is initialized in the constructor from a supplied Hibernate Session object:

public class HibernateSearchVisitor implements ISearchVisitor {
  private Criteria criteria;
  public HibernateSearchVisitor(Session session)
    throws SearchException {
    this.criteria = session.createCriteria(File.class);
  }
  ...
}

The use of File.class here tells Hibernate that instances of File are to be yielded by the search. Each visitor then adds to the criteria object. Dependent domain objects are tied in by creating child criteria. For example, the method visitVolumeName from HibernateSearchVisitor constrains the name of the dependent Volume object by creating a child criteria for the volume property of File and constraining it.

The other visitor methods constrain the properties of File in a similar manner. The overall search is performed by the doSearch method from the class HibernateSearchVisitor, which builds the query using the buildQuery method. The actual query is performed by the list method of the criteria object, and the result is then used to populate a SearchResult object. Compared to the SQL visitor, the simplicity and elegance of Hibernate is illustrated perfectly in this example.

Closing Remarks

In the preceding sections, I have described a design for domain searching that is based around defining domain objects to represent the search, and then using the Visitor pattern to build the actual search to be performed. I have shown two specific implementations of the visitor to illustrate the approach.

The main strengths of the design are:

  • Clean separation between domain objects and persistence-service-specific implementation of the search.
  • Physical localization of query-building code for a specific search criterion. That is, I know exactly where to go to see where the query is being built for a specific search criterion.
  • Flexible support for different persistence approaches.

Note with this last point that this isn't just a question of supporting multiple databases (in my experience, switching databases during a project is rare). It might be that a product is being developed and the flexibility to support other persistence approaches in the future needs to be built in now. Using the visitor design provides this flexibility at very little cost. Moreover, it might be that in the future the application might need to search an external information system, say, using web services. This design makes support for such functionality straightforward to incorporate.

The main weakness of the design is essentially that inherited from the use of the visitor: if the structure of the domain objects changes frequently, then all of the visitor implementations need to be updated to reflect these changes. However, in my experience domain objects change infrequently compared to the pace of change of other parts of an application.

Whatever your approach to persistence, next time you have to implement domain searching, I hope this article has added some weapons to your armory.

Resources

Paul Mukherjee works as a technical architect for Systematic Software Engineering Limited in Britain, and is a Sun Certified Enterprise Architect.


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