Transfer Object Assembler

Context

In a J2EE application, the server-side business components are implemented using session beans, entity beans, DAOs, and so forth. Application clients frequently need to access data that is composed from multiple objects.

Problem

Application clients typically require the data for the model or parts of the model to present to the user or to use for an intermediate processing step before providing some service. The application model is an abstraction of the business data and business logic implemented on the server side as business components. A model may be expressed as a collection of objects put together in a structured manner (tree or graph). In a J2EE application, the model is a distributed collection of objects such as session beans, entity beans, or DAOs and other objects. For a client to obtain the data for the model, such as to display to the user or to perform some processing, it must access individually each distributed object that defines the model. This approach has several drawbacks:

• Because the client must access each distributed component individually, there is a tight coupling between the client and the distributed components of the model over the network

• The client accesses the distributed components via the network layer, and this can lead to performance degradation if the model is complex with numerous distributed components. Network and client performance degradation occur when a number of distributed business components implement the application model and the client directly interacts with these components to obtain model data from that component. Each such access results in a remote method call that introduces network overhead and increases the chattiness between the client and the business tier.

• The client must reconstruct the model after obtaining the model's parts from the distributed components. The client therefore needs to have the necessary business logic to construct the model. If the model construction is complex and numerous objects are involved in its definition, then there may be an additional performance overhead on the client due to the construction process. In addition, the client must contain the business logic to manage the relationships between the components, which results in a more complex, larger client. When the client constructs the application model, the construction happens on the client side. Complex model construction can result in a significant performance overhead on the client side for clients with limited resources.

• Because the client is tightly coupled to the model, changes to the model require changes to the client. Furthermore, if there are different types of clients, it is more difficult to manage the changes across all client types. When there is tight coupling between the client and model implementation, which occurs when the client has direct knowledge of the model and manages the business component relationships, then changes to the model necessitate changes to the client. There is the further problem of code duplication for model access, which occurs when an application has many types of clients. This duplication makes client (code) management difficult when the model changes.

Forces

• Separation of business logic is required between the client and the server-side components.

• Because the model consists of distributed components, access to each component is associated with a network overhead. It is desirable to minimize the number of remote method calls over the network.

• The client typically needs only to obtain the model to present it to the user. If the client must interact with multiple components to construct the model on the fly, the chattiness between the client and the application increases. Such chattiness may reduce the network performance.

• Even if the client wants to perform an update, it usually updates only certain parts of the model and not the entire model.

• Clients do not need to be aware of the intricacies and dependencies in the model implementation. It is desirable to have loose coupling between the clients and the business components that implement the application model.

• Clients do not otherwise need to have the additional business logic required to construct the model from various business components.

Solution

Use a Transfer Object Assembler to build the required model or submodel. The Transfer Object Assembler uses Transfer Objects to retrieve data from various business objects and other objects that define the model or part of the model.

The Transfer Object Assember constructs a composite Transfer Object that represents data from different business components. The Transfer Object caries the data for the model to the client in a single method call. Since the model data can be complex, it is recommended that this Transfer Object be immutable. That is, the client obtains such Transfer Objects with the sole purpose of using them for presentation and processing in a read-only manner. Clients are not allowed to make changes to the Transfer Objects.

When the client needs the model data, and if the model is represented by a single coarse-grained component (such as a Composite Entity), then the process of obtaining the model data is simple. The client simply requests the coarse-grained component for its composite Transfer Object. However, most real-world applications have a model composed of a combination of many coarse-grained and fine-grained components. In this case, the client must interact with numerous such business components to obtain all the data necessary to represent the model. The immediate drawbacks of this approach can be seen in that the clients become tightly coupled to the model implementation (model elements) and that the clients tend to make numerous remote method invocations to obtain the data from each individual component.

In some cases, a single coarse-grained component provides the model or parts of the model as a single Transfer Object (simple or composite). However, when multiple components represent the model, a single Transfer Object (simple or composite) may not represent the entire model. To represent the model, it is necessary to obtain Transfer Objects from various components and assemble them into a new composite Transfer Object. The server, not the client, should perform such "on-the-fly" construction of the model.

Structure

Figure 8.27 shows the class diagram representing the relationships for the Transfer Object Assembler pattern.

Figure 8.27 Transfer Object Assembler class diagram

Participants and Responsibilities

The sequence diagram in Figure 8.28 shows the interaction between the various participants in the Transfer Object Assembler pattern.

Figure 8.28 Transfer Object Assembler sequence diagram

TransferObjectAssembler

The TransferObjectAssembler is the main class of this pattern. The TransferObjectAssembler constructs a new Transfer Object based on the requirements of the application when the client requests a composite Transfer Object. The TransferObjectAssembler then locates the required BusinessObject instances to retrieve data to build the composite Transfer Object. BusinessObjects are business-tier components such as entity beans and session beans, DAOs, and so forth.

Client

If the TransferObjectAssembler is implemented as an arbitrary Java object, then the client is typically a Session Facade that provides the controller layer to the business tier. If the TransferObjectAssembler is implemented as a session bean, then the client can be a Session Facade or a Business Delegate.

BusinessObject

The BusinessObject participates in the construction of the new Transfer Object by providing the required data to the TransferObjectAssembler. Therefore, the BusinessObject is a role that can be fulfilled by a session bean, an entity bean, a DAO, or a regular Java object.

TransferObject

The TransferObject is a composite Transfer Object that is constructed by the TransferObjectAssembler and returned to the client. This represents the complex data from various components that define the application model.

BusinessObject

BusinessObject is a role that can be fulfilled by a session bean, entity bean, or DAO. When the assembler needs to obtain data directly from the persistent storage to build the Transfer Object, it can use a DAO. This is shown as the DataAccessObject object in the diagrams.

Strategies

This section explains different strategies for implementing a Transfer Object Assembler pattern.

Java Object Strategy

The TransferObjectAssembler can be an arbitrary Java object and need not be an enterprise bean. In such implementations, a session bean usually fronts the TransferObjectAssembler. This session bean is typically a Session Facade that performs its other duties related to providing business services. The TransferObjectAssembler runs in the business tier, regardless of the implementation strategies. The motivation for this is to prevent the remote invocations from the TransferObjectAssembler to the source objects from crossing the tier.

Session Bean Strategy

This strategy implements the TransferObjectAssembler as a session bean (as shown in the class diagram). If a session bean implementation is preferred to provide the TransferObjectAssembler as a business service, it is typically implemented as a stateless session bean. The business components that make up the application model are constantly involved in transactions with various clients. As a result, when a TransferObjectAssembler constructs a new composite Transfer Object from various business components, it produces a snapshot of the model at the time of construction. The model could change immediately thereafter if another client changes one or more business components, effectively changing the business application model.

Therefore, implementing TransferObjectAssembler as a stateful session bean provides no benefits over implementing it as a stateless session bean, as preserving the state of the composite model data value when the underlying model is changing is futile. If the underlying model changes, it causes the Transfer Object held by the assembler to become stale. The TransferObjectAssembler, when next asked for the Transfer Object, either returns a stale state or reconstructs the Transfer Object to obtain the most recent snapshot. Therefore, it is recommended that the assembler be a stateless session bean to leverage the benefits of stateless over stateful session beans.

However, if the underlying model rarely changes, then the assembler may be a stateful session bean and retain the newly constructed Transfer Object. In this case, the TransferObjectAssembler must include mechanisms to recognize changes to the underlying model and to reconstruct the model for the next client request.

Business Object Strategy

The BusinessObject role in this pattern can be supported by different types of objects, as explained below.

• The BusinessObject can be a session bean. The Transfer Object Assembler may use a Service Locator (see "Service Locator" on page 368) to locate the required session bean. The Transfer Object Assembler requests this session bean to provide the data to construct the composite Transfer Object.

• The BusinessObject can be an entity bean. The Transfer Object Assembler may use a Service Locator to locate the required entity bean. The Transfer Object Assembler requests this entity bean to provide the data to construct the composite Transfer Object.

• The BusinessObject can be a DAO. The Transfer Object Assembler requests this DAO to provide the data to construct the composite Transfer Object.

• The BusinessObject can be an arbitrary Java object. The Transfer Object Assembler requests this Java object to provide the data to construct the composite Transfer Object.

• The BusinessObject can be another Transfer Object Assembler. The first Transfer Object Assembler requests the second Transfer Object Assembler to provide the data to construct the composite Transfer Object.

Consequences

• Separates Business Logic
  When the client includes logic to manage the interactions with distributed components, it becomes difficult to clearly separate business logic from the client tier. The Transfer Object Assembler contains the business logic to maintain the object relationships and to construct the composite Transfer Object representing the model. The client needs no knowledge of how to construct the model or the different components that provide data to assemble the model.

• Reduces Coupling Between Clients and the Application Model
  The Transfer Object Assembler hides the complexity of the construction of model data from the clients and establishes a loose coupling between clients and the model. With loose coupling, if the model changes, then the Transfer Object Assembler requires a corresponding change. However, the client is not dependent on the model construction and interrelationships between model business components, so model changes do not directly affect the client. In general, loose coupling is preferred to tight coupling.

• Improves Network Performance
  The Transfer Object Assembler drastically reduces the network overhead of remote method calls and chattiness. The client can request the data for the application model from the Transfer Object Assembler in a single remote method call. The assembler constructs and returns the composite Transfer Object for the model. However, the composite Transfer Object may contain a large amount of data. Thus, while use of the Transfer Object Assembler reduces the number of network calls, there is an increase in the amount of data transported in a single call. This trade-off should be considered in applying this pattern.

• Improves Client Performance
  The server-side Transfer Object Assembler constructs the model as a composite Transfer Object without using any client resources. The client spends no time assembling the model.

• Improves Transaction Performance
  Typically, updates are isolated to a very small part of the model and can be performed by fine-grained transactions. These transactions focus on isolated parts of the model instead of locking up the coarse-grained object (model). After the client obtains the model and displays or processes it locally, the user (or the client) may need to update or otherwise modify the model. The client can interact directly with a Session Facade to accomplish this at a suitable granularity level. The Transfer Object Assembler is not involved in the transaction to update or modify the model. There is better performance control because transactional work with the model happens at the appropriate level of granularity.

• May Introduce Stale Transfer Objects
  The Transfer Object Assembler constructs Transfer Objects on demand. These Transfer Objects are snapshots of the current state of the model, represented by various business components. Once the client obtains a Transfer Object from the assembler, that Transfer Object is entirely local to the client. Since the Transfer Objects are not network-aware, other changes made to the business components used to construct the Transfer Object are not reflected in the Transfer Objects. Therefore, after the Transfer Object is obtained, it can quickly become stale if there are transactions on the business components.

Sample Code

Implementing the Transfer Object Assembler

Consider a Project Management application where a number of business-tier components define the complex model. Suppose a client wants to obtain the model data composed of data from various business objects, such as:

• Project Information from the Project component

• Project Manager information from the ProjectManager component

• List of Project Tasks from the Project component

• Resource Information from the Resource component

A composite Transfer Object to contain this data can be defined as shown in Example 8.24. A Transfer Object Assembler pattern can be implemented to assemble this composite Transfer Object. The Transfer Object Assembler sample code is listed in Example 8.28.

Example 8.24 Composite Transfer Object Class

 
public class ProjectDetailsData {
  public ProjectTO projectData;
  public ProjectManagerTO projectManagerData;
  public Collection listOfTasks;
  ...
}

The list of tasks in the ProjectDetailsData is a collection of TaskResourceTO objects. The TaskResourceTO is a combination of TaskTO and ResourceTO. These classes are shown in Example 8.25, Example 8.26, and Example 8.27.

Example 8.25 TaskResourceTO Class

public class TaskResourceTO {
  public String projectId;
  public String taskId;
  public String name;
  public String description;
  public Date startDate;
  public Date endDate;
  public ResourceTO assignedResource;
  ...

  public TaskResourceTO(String projectId, 
    String taskId, String name, String description, 
    Date startDate, Date endDate, ResourceTO 
    assignedResource) {
        this.projectId = projectId;
        this.taskId = taskId;
        ...
        this.assignedResource = assignedResource;
    }
    ...
}

Example 8.26 TaskTO Class

public class TaskTO {
  public String projectId;
  public String taskId;
  public String name;
  public String description;
  public Date startDate;
  public Date endDate;
  public assignedResourceId;

  public TaskTO(String projectId, String taskId, 
      String name, String description, Date startDate, 
      Date endDate, String assignedResourceId) {
        this.projectId = projectId;
        this.taskId = taskId;
        ...
        this.assignedResource = assignedResource;
    }
    ...
}

Example 8.27 ResourceTO Class

public class ResourceTO {
  public String resourceId;
  public String resourceName;
  public String resourceEmail;
  ...
  
  public ResourceTO (String resourceId, String 
    resourceName, String resourceEmail, ...) {
      this.resourceId = resourceId;
      this.resourceName = resourceName;
      this.resourceEmail = resourceEmail;
      ...
  }
}

The ProjectDetailsAssembler class that assembles the ProjectDetailsData object is listed in Example 8.28.

Example 8.28 Implementing the Transfer Object Assembler

public class ProjectDetailsAssembler 
  implements javax.ejb.SessionBean {

  ...

  public ProjectDetailsData getData(String projectId){ 

    // Construct the composite Transfer Object
    ProjectDetailsData pData = new 
                      ProjectDetailsData();

    //get the project details; 
    ProjectHome projectHome = 
        ServiceLocator.getInstance().getHome(
          "Project", ProjectEntityHome.class);
    ProjectEntity project = 
      projectHome.findByPrimaryKey(projectId);
    ProjectTO projTO = project.getData();
  
    // Add Project Info to ProjectDetailsData
    pData.projectData = projTO;

    //get the project manager details;
    ProjectManagerHome projectManagerHome =
      ServiceLocator.getInstance().getHome( 
        "ProjectManager", ProjectEntityHome.class);
  
    ProjectManagerEntity projectManager = 
      projectManagerHome.findByPrimaryKey(
        projTO.managerId);

    ProjectManagerTO projMgrTO = 
      projectManager.getData();
  
    // Add ProjectManager info to ProjectDetailsData
    pData.projectManagerData = projMgrTO;

    // Get list of TaskTOs from the Project
    Collection projTaskList = project.getTasksList();

    // construct a list of TaskResourceTOs
    ArrayList listOfTasks = new ArrayList();
  
    Iterator taskIter = projTaskList.iterator();
    while (taskIter.hasNext()) {
      TaskTO task = (TaskTO) taskIter.next();

      //get the Resource details; 
      ResourceHome resourceHome = 
      ServiceLocator.getInstance().getHome( 
        "Resource",         ResourceEntityHome.class);
  
      ResourceEntity resource = 
        resourceHome.findByPrimaryKey(
          task.assignedResourceId);

      ResourceTO resTO = resource.getResourceData();

      // construct a new TaskResourceTO using Task
      // and Resource data
      TaskResourceTO trTO = new TaskResourceTO( 
              task.projectId, task.taskId,
              task.name, task.description,
              task.startDate, task.endDate,
              resTO);

      // add TaskResourceTO to the list
      listOfTasks.add(trTO);
    }
    
    // add list of tasks to ProjectDetailsData
    pData.listOfTasks = listOfTasks;

    // add any other data to the Transfer Object
    ...
  
    // return the composite Transfer Object
    return pData;

  }

  ...
}

Related Patterns

• Transfer Object
  The Transfer Object Assembler uses the Transfer Object pattern in order to create and transport Transfer Objects to the client. The Transfer Objects created carry the data representing the application model from the business tier to the clients requesting the data.

• Composite Entity
  The Composite Entity pattern promotes a coarse-grained entity bean design, where entities can produce composite Transfer Objects similar to the one produced by the Transfer Object Assembler. However, the Transfer Object Assembler is more applicable when the composite Transfer Object constructed is derived from a number of components (session beans, entity beans, DAOs, and so forth), whereas the Composite Entity pattern constructs the Transfer Object from its own data (that is, a single entity bean).

• Session Facade
  The Transfer Object Assembler is typically implemented as a stateless session bean. As such, it could be viewed as a limited special application of the Session Facade pattern. More importantly, Transfer Object Assembler constructs composite Transfer Objects that are immutable. Therefore, the client receiving this composite Transfer Object can only use the data for its presentation and processing purposes. The client cannot update the Transfer Object. If the client needs to update the business objects that derive the composite Transfer Object, it may have to access the Session Facade (session bean) that provides that business service.

• Data Access Object
  A possible strategy for the Transfer Object Assembler involves obtaining data for the composite Transfer Object from the persistent store without enterprise bean involvement. The Data Access Object pattern can be applied, thus leveraging its benefits to provide persistent storage access to the Transfer Object Assembler.

• Service Locator
  The Transfer Object Assembler needs to locate and use various business objects. The Service Locator pattern can be used in conjunction with the Transfer Object Assembler pattern whenever a business object or a service needs to be located.