GWT

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TABLE OF CONTENTS

RequestFactory is an alternative to GWT-RPC for creating data-oriented services. RequestFactory and its related interfaces (RequestContext and EntityProxy) make it easy to build data-oriented (CRUD) apps with an ORM-like interface on the client. It is designed to be used with an ORM layer like JDO or JPA on the server, although this is not required.

Overview

Benefits

RequestFactory makes it easy to implement a data access layer on both client and server. It allows you to structure your server-side code in a data-centric way and provides a higher level of abstraction than GWT-RPC, which is service-oriented rather than data-oriented. On the client side, RequestFactory keeps track of objects that have been modified and sends only changes to the server, which results in very lightweight network payloads. In addition, RequestFactory provides a solid foundation for automatic batching and caching of requests in the future.

How does it relate to GWT-RPC?

RequestFactory uses its own servlet, RequestFactoryServlet, and implements its own protocol for data exchange between client and server. RequestFactory takes a more prescriptive approach to the client-server programming model than GWT-RPC does. GWT-RPC uses remote methods as its basic building blocks (similar to Java RMI), whereas RequestFactory uses entities (data with a persistent identity) and services. See this external site for a discussion on RequestFactory versus GWT-RPC.

Coding with RequestFactory

Let's take a look at the moving parts in an application that uses RequestFactory.

  1. Entity
  2. Entity Proxies
  3. Value Proxies
  4. RequestFactory Interface
  5. Transportable types
  6. Server Implementations
  7. Implementing a service in an entity class
  8. Using Locator and ServiceLocator

Then we'll take a look at how to put it all together.

  1. Wiring
  2. Using RequestFactory
  3. Entity Relationships
  4. Validating Entities

Entities

An entity is a domain class in your application that has concept of a persistent identity. Generally speaking, an entity can be persisted to a data store such as a relational database or the Google App Engine Datastore. In persistence frameworks like JDO and JPA, entities are annotated with @Entity. RequestFactory does not require the use of any particular framework or annotations on your domain classes. Here's part of an entity definition from the Expenses sample application found in the GWT distribution.

package com.google.gwt.sample.expenses.server.domain;

/**
 * The Employee domain object.
 */
@Entity
public class Employee {

  @Size(min = 3, max = 30)
  private String userName;

  private String department;

  @NotNull
  private String displayName;

  private String password;

  @JoinColumn
  private Long supervisorKey;

  @Id
  @Column(name = "id")
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  @Version
  @Column(name = "version")
  private Integer version;

  @Transient
  private Employee supervisor;

  public String getDepartment() {
    return department;
  }

  public String getDisplayName() {
    return this.displayName;
  }
  ...
}

Entity Proxies

An entity proxy is a client-side representation of a server-side entity. The proxy interfaces are implemented by RequestFactory and they fill the role of a DTO (Data Transfer Object) in GWT-RPC. RequestFactory automatically propagates bean-style properties between entities on the server and the corresponding EntityProxy on the client. Furthermore, the EntityProxy interface enables RequestFactory to compute and send only changes ("deltas") to the server. Here's the EntityProxy corresponding to the Employee shown above.

@ProxyFor(Employee.class)
public interface EmployeeProxy extends EntityProxy {

  String getDepartment();

  String getDisplayName();

  Long getId();

  String getPassword();

  EmployeeProxy getSupervisor();

  String getUserName();

  void setDepartment(String department);

  void setDisplayName(String displayName);

  void setPassword(String password);

  void setSupervisor(EmployeeProxy supervisor);

  void setUserName(String userName);
}

Entity proxies simply extend the EntityProxy interface and use the @ProxyFor or @ProxyForName annotation to reference the server-side entity being represented. It is not necessary to represent every property and method from the server-side entity in the EntityProxy, only getters and setters for properties that should be exposed to the client. Note that while getId() is shown in this example, most client code will want to refer to EntityProxy.stableId() instead, as the EntityProxyId returned by this method is used throughout RequestFactory-related classes.

Also note that the getSupervisor() method returns another proxy class (EmployeeProxy). All client-side code must reference EntityProxy subclasses. RequestFactory automatically converts proxy types to their corresponding entity types on the server.

Value Proxies

A value proxy can be used to represent any type. Unlike an EntityProxy, a ValueProxy is not required to expose an ID and version. ValueProxy is often used to represent embedded object types within entities. For example, a Person entity in a contact management application might represent Address as an embedded type so it will be persisted as a serialized object within the person entity.

@Entity
public class Person {
  @Id
  private Long id;
  private Integer version = 0;
  private String firstName, lastName;
  @Embedded
  private Address address;
  ...
}

The Address type is just a POJO with no persistence annotations:

public class Address {
  private String street1;
  private String street2;
  private String city;
  private String st;
  private String zip;
  ...
}

In the client, Address is represented as a ValueProxy and referenced by the containing EntityProxy:

public interface AddressProxy extends ValueProxy {
  public String getStreet1();
  public String getStreet2();
  public String getCity();
  public String getSt();
  public String getZip();
  ...
}

public interface PersonProxy extends EntityProxy {
  Long getId();
  Integer getVersion();
  String getFirstName();
  String getLastName();
  AddressProxy getAddress();
   ...
}

ValueProxy can be used to pass any bean-like type to and from the server with RequestFactory.

RequestFactory Interface

As with GWT-RPC, you define the interface between your client and server code by extending an interface. You define one RequestFactory interface for your application, and it consists of methods that return service stubs. Here's an example from the Expenses sample app:

public interface ExpensesRequestFactory extends RequestFactory {

  EmployeeRequest employeeRequest();

  ExpenseRequest expenseRequest();

  ReportRequest reportRequest();

}

The EmployeeRequest service stub looks like this:

@Service(Employee.class)
public interface EmployeeRequest extends RequestContext {

  Request<Long> countEmployees();

  Request<Long> countEmployeesByDepartment(
      String department);

  Request<List<EmployeeProxy>> findAllEmployees();

  Request<EmployeeProxy> findEmployee(Long id);

  Request<List<EmployeeProxy>> findEmployeeEntries(int firstResult,
      int maxResults);

  Request<List<EmployeeProxy>> findEmployeeEntriesByDepartment(
      String department, int firstResult, int maxResults);

  InstanceRequest<EmployeeProxy, Void> persist();

  InstanceRequest<EmployeeProxy, Void> remove();

}

The RequestFactory service stubs must extend RequestContext and use the @Service or @ServiceName annotation to name the associated service implementation class on the server. The methods in a service stub do not return entities directly, but rather return subclasses of com.google.web.bindery.requestfactory.shared.Request. This allows the methods on the interface to be invoked asynchronously with Request.fire() similar to passing an AsyncCallback object to each service method in GWT-RPC.

requestFactory.employeeRequest().findEmployee(employeeId).fire(
    new Receiver<EmployeeProxy>() {
      @Override
      public void onSuccess(EmployeeProxy employee) {
      ...
      }
    });

Just like GWT-RPC callers pass an AsyncCallback that implements onSuccess() and onFailure(), Request.fire() takes a Receiver which must implement onSuccess(). Note that Receiver is an abstract class having a default implementation of onFailure() so that you don't have to handle the failure case each time a Request is invoked. To change the default implementation, which simply throws a RuntimeException, you can extend Receiver and override onFailure() with a suitable default implementation for your application. You may also want to override the default implementation of onConstraintViolation(), which returns any constraint violations on the server (see Validating Entities below).

The Request type returned from each method is parameterized with the return type of the service method. The type parameter becomes the type expected by the Receiver's onSuccess() method as in the example above. Methods that have no return value should return type Request<Void>. Requests can be parameterized with the following types:

  • Built-in value types: BigDecimal, BigInteger, Boolean, Byte, Enum, Character, Date, Double, Float, Integer, Long, Short, String, Void
  • Custom value types: any subclass of ValueProxy
  • Entity types: any subclass of EntityProxy
  • Collections: List<T> or Set<T>, where T is one of the above value or entity types, or Map<K,V> where K and V are one of the above value or entity types

Instance methods that operate on an entity itself, like persist() and remove(), return objects of type InstanceRequest rather than Request. This is explained further in the next section.

Transportable types

RequestFactory restricts the types that may be used as proxy properties and service method parameters. Collectively, these types are referred to as transportable types. Each client-side transportable type is mapped to a server-side domain type. The mapping rules are as follows:

Mapping of transportable types to domain types
Client type Domain type
Primitive type (e.g. int) Primitive type
Boxed primitive type (e.g. Integer) Boxed primitive type
Other value types: Enums, BigInteger, BigDecimal, Date Other value type
@ProxyFor(Foo.class) FooProxy extends EntityProxy A Foo entity
@ProxyFor(Bar.class) BarProxy extends ValueProxy A Bar value object
Set<T> or List<T> where T is a transportable type Set<T> or List<T>
Map<K,V> where K and V are transportable types Map<K,V>

In determining whether or not a client-side method defined in a proxy or context type is a valid mapping for a domain method, the client types are converted to ther domain equivalent and regular Java type assignability rules are considered.

A proxy type will be available on the client if it is:

  • Referenced from a RequestContext as a Request parameter or return type.
  • Referenced from a referenced proxy.
  • A supertype of a referenced proxy that is also a proxy (i.e. assignable to EntityProxy or ValueProxy and has an @ProxyFor(Name) annotation).
  • Referenced via an @ExtraTypes annotation placed on the RequestFactory, RequestContext, or a referenced proxy. Adding an @ExtraTypes annotation on the RequestFactory or RequestContext allows you to add subtypes to "some else's" proxy types.

Polymorphic type-mapping rules:

  • All properties defined in a proxy type or inherited from super-interfaces must be available on the domain type. This allows a proxy interface to extend a "mix-in" interface.
  • All proxies must map to a single domain type via a @ProxyFor(Name) annotation.
  • The @ProxyFor of the proxy instance is used to determine which concrete type on the server to instantiate.
  • Any supertypes of a proxy interface that are assignable to EntityProxy or ValueProxy and have an @ProxyFor(Name) annotation must be valid proxies. Given BProxy extends AProxy: if only BProxy is referenced (e.g. via @ExtraTypes), it is still permissible to create an AProxy.
  • Type relationships between proxy interfaces do not require any particular type relationship between the mapped domain types. Given BProxy extends AProxy: it is allowable for BEntity not to be a subclass of AEntity. This allows for duck-type-mapping of domain objects to proxy interfaces.
  • To return a domain object via a proxy interface, the declared proxy return type must map to a domain type assignable to the returned domain object.
  • The specific returned proxy type will be the most-derived type assignable to the declared proxy type that also maps to the returned domain type or one of its supertypes.

Server Implementations

Services can be implemented on the server in one of two ways: as static methods in a type or as instance methods in a service class accompanied by a ServiceLocator.

In either case, the methods defined in a service interface are implemented in the class named in the @Service or @ServiceName annotation. Unlike GWT-RPC, service implementations do not directly implement the RequestContext interface. The server-side service must implement each method defined in the service's RequestContext interface even though the implementation does not formally implement the RequestContext interface. The name and argument list for each method is the same on client and server, with the following mapping rules:

  • Client side methods that return Request<T> return only T on the server. For example, a method that returns Request<String> in the client interface simply returns String on the server.
  • EntityProxy types become the domain entity type on the server, so a method that returns Request<List<EmployeeProxy>> will just return List<Employee> on the server.
  • Methods that return a Request object in the client interface are implemented as static methods in the service class. Alternatively, they may be implemented as instance methods in a service object returned by a ServiceLocator.
  • Methods that operate on an instance of an entity, like persist() and remove(), return an InstanceRequest object in the client interface. Instance methods do not pass the instance directly, but rather via the using() method on the InstanceRequest. On the server, instance methods must be implemented as non-static methods in the entity type.

The RequestFactory servlet requires four special methods for all entities. They may be implemented either in the entity itself or in a default-instantiable type that implements the Locator interface. The required methods are summarized in the table below.

Entity locator methods can be implemented in the entity or a Locator class
Method Directly in entity Locator<T,I> impl Description
Constructor no-arg constructor T create(Class<? extends T> clazz) Returns a new entity instance
getId id_type getId() I getId(T domainObject) Returns an entity's persisted ID, which may be any transportable type. The ID is typically auto-generated by the persistence engine (JDO, JPA, Objectify, etc.)
find by ID static findEntity(id_type id) T find(Class<? extends T> clazz, I id) Returns the persisted entity for an ID. When implemented directly in the entity, this method has a special naming convention. On the client side, there is a find() method defined in the RequestFactory interface which service interfaces extend. On the server, the method is named "find" plus the type's simple name, like findEmployee, and takes an argument of the entity's ID type.
getVersion Integer getVersion() Object getVersion(T domainObject) Used by RequestFactory to infer if an entity has changed. The backing store (JDO, JPA, etc.) is responsible for updating the version each time the object is persisted, and RequestFactory calls getVersion() to learn of changes. This information is used in two places. First, the RequestFactoryServlet sends an UPDATE event to the client if an entity changes as a result of the method invocation on the server, for example, when a call to persist an editable entity results in an updated version on the server. Second, the client maintains a version cache of recently seen entities. Whenever it sees an entity whose version has changed, it fires UPDATE events on the event bus so that listeners can update the view.

Implementing a service in an entity class

When mapping a service, methods that return a Request object in the client interface are implemented as static methods in a service class, like Employee.findAllEmployees() in the example below. Here is more of the Employee entity in the Expenses sample project:

// The Employee domain object
@Entity public class Employee {

// properties, getters, and setters omitted
  public static List<Employee> findAllEmployees() {
    EntityManager em = entityManager();
    try {
      List<Employee> list = em.createQuery("select o from Employee o").getResultList();
      // force to get all the employees
      list.size();
      return list;
    } finally {
      em.close();
    }
  }

  public static Employee findEmployee(Long id) {
    if (id == null) {
      return null;
    }
    EntityManager em = entityManager();
    try {
      Employee employee = em.find(Employee.class, id);
      return employee;
    } finally {
      em.close();
    }
  }

  public static final EntityManager entityManager() {
    return EMF.get().createEntityManager();
  }

  public void persist() {
    EntityManager em = entityManager();
    try {
      em.persist(this);
    } finally {
      em.close();
    }
  }

  public void remove() {
    EntityManager em = entityManager();
    try {
      Employee attached = em.find(Employee.class, this.id);
      em.remove(attached);
    } finally {
      em.close();
    }
  }

  ...

}

Using Locator and ServiceLocator

What if you don't want to implement persistence code in an entity itself? To implement the required entity locator methods, create an entity locator class that extends Locator<T,I>:

public class EmployeeLocator extends Locator<Employee, Long> {
  @Override
  public Employee create(Class<? extends Employee> clazz)
  {
    return new Employee();
  }
    ...
}

Then associate it with the entity in the @ProxyFor annotation:

@ProxyFor(value = Employee.class, locator = EmployeeLocator.class)
  public interface EmployeeProxy extends EntityProxy {
    ...
  }

Since many persistence frameworks offer generic find/get/query methods, it's also possible to create a generic Locator class and specify it in the @ProxyFor annotation for each entity type. To do this, all your entities can extend a base class that provides getId() and getVersion(). Alternatively, the generic Locator can use reflection to call getId() and getVersion() when needed.

Many persistence frameworks also make it possible to utilize a generic DAO class where entity DAOs extend the generic DAO and override methods as needed. One of the benefits of RequestFactory is that it's possible to expose DAO classes directly as services. However, inheriting service methods from a base class doesn't work if services are implemented as static methods in an entity class. Fortunately, you can use a ServiceLocator to tell RequestFactory how to obtain an instance of a service. When using a ServiceLocator, RequestFactory will invoke methods that return a Request type as instance methods instead of static methods.

To use a ServiceLocator, simply implement the ServiceLocator interface. It may be as simple as this:

public class MyServiceLocator implements ServiceLocator {
  @Override
  public Object getInstance(Class<?> clazz) {
    try {
      return clazz.newInstance();
    } catch (InstantiationException e) {
      throw new RuntimeException(e);
    } catch (IllegalAccessException e) {
      throw new RuntimeException(e);
    }
  }
}

Then annotate the service interface with the name of the service and locator class:

@Service(value = EmployeeDao.class, locator = MyServiceLocator.class)
interface EmployeeRequestContext extends RequestContext

Note: RequestFactory caches ServiceLocator and service instances, so make sure both are thread-safe.

Putting it all together

Wiring

In order to use RequestFactory, add the following line to your .gwt.xml:

<inherits name='com.google.web.bindery.requestfactory.RequestFactory' />

Add the following jars to your WEB-INF/lib directory:

  • requestfactory-server.jar
  • javax/validation/validator-api-1.0.0.GA.jar
  • A JSR 303 Validator of your choice, such as hibernate-validator

Map RequestFactoryServlet in web.xml:

<servlet>
	<servlet-name>requestFactoryServlet</servlet-name>
	<servlet-class>com.google.web.bindery.requestfactory.server.RequestFactoryServlet</servlet-class>
	<init-param>
		<param-name>symbolMapsDirectory</param-name>
		<!-- You'll need to compile with -extras and move the symbolMaps directory 
			to this location if you want stack trace deobfuscation to work -->
		<param-value>WEB-INF/classes/symbolMaps/</param-value>
	</init-param>
</servlet>

<servlet-mapping>
	<servlet-name>requestFactoryServlet</servlet-name>
	<url-pattern>/gwtRequest</url-pattern>
</servlet-mapping>

Once you've created your entities, EntityProxy types, and RequestFactory with its service interfaces, you bring it to life with GWT.create() and initialize it with your application's EventBus:

final EventBus eventBus = new SimpleEventBus();
requestFactory = GWT.create(ExpensesRequestFactory.class);
requestFactory.initialize(eventBus);

Using RequestFactory

Now we're ready to put RequestFactory to work. To create a new entity on the client, call RequestContext.create() on the EntityProxy type, then persist it using a method defined in the entity's service interface. Using the example code above to create a new Employee object and persist it to the database, we would write:

EmployeeRequest request = requestFactory.employeeRequest();
EmployeeProxy newEmployee = request.create(EmployeeProxy.class);
newEmployee.setDisplayName(...);
newEmployee.setDepartment(...);
...
Request<Void> createReq = request.persist().using(newEmployee);

All client-side code should use the EmployeeProxy, not the Employee entity itself. This way, the domain object need not be GWT-compatible, unlike GWT-RPC, where the same concrete type is used on both client and server. EmployeeProxy has no constructor because it's an interface, not a class, so you must instantiate it with requestContext.create(EmployeeProxy.class). The benefit of this approach is that it allows RequestFactory to track object creation and modification so it can send only changes to the server.

Note that the persist() method has no arguments, which is consistent with the method's implementation in the Employee entity. In the EmployeeRequest interface, it returns type InstanceRequest, which in turn gets the instance on which the method will be invoked from the using() method as shown above. Alternatively, if using a ServiceLocator, the persist() method could be declared as Request<Void> persist(Employee emp), in which case newEmployee would be passed as an argument to the persist() method in place of the using() method.

Now let's add the code to save the newly created employee to the server:

createReq.fire(new Receiver<Void>()
{
  @Override
    public void onSuccess(Void arg0)
    {
        // Update display
    }
});

We fire the request with a Receiver, which calls us back when the request has completed.

Any objects not returned from RequestContext.create(), such as those received from the server, must be enabled for changes by calling the RequestFactory's edit() method. Any EntityProxies returned from the getters of an editable proxy are also editable.

EmployeeProxy editableEmployee = request.edit(returnedEmployee);
editableEmployee.setDepartment(newDepartment);
...
Request<Void> updateReq = request.persist().using(editableEmployee);

The edit() method returns a new copy of the immutable object, and the original can be discarded. To send changes to the server, you create a new server request using a method defined in the service interface method and fire it as shown above. All edits occur with a particular context, and when the context is fired, all of those edits are sent, so any method invocation will result in changes being sent to the server.

Entity Relationships

Changes to related entities can be persisted in a single request. For example, this code from the DynatableRF sample app in GWT trunk creates a new Person and Address at the same time:

PersonRequest context = requestFactory.personRequest();
AddressProxy address = context.create(AddressProxy.class);
PersonProxy person = context.create(PersonProxy.class);
person.setAddress(address);
context.persist().using(person).fire(...);

RequestFactory automatically sends the whole object graph in a single request. In this case, the implementation of Person.persist() on the server is responsible for persisting the related Address also, which may or may not happen automatically, depending on the ORM framework and how the relationship is defined. Note that RequestFactory does not currently support embedded objects (@Embedded in various ORM frameworks) because it expects every entity to exist independently with its own ID.

When querying the server, RequestFactory does not automatically populate relations in the object graph. To do this, use the with() method on a request and specify the related property name as a String:

Request findReq = requestFactory.personRequest().find(personId).with("address");

It is also necessary to use the with() method to retrieve any properties with types extending ValueProxy. The with() method takes multiple String arguments, so you can specify multiple property names at once. To specify nested properties, use dot notation. Putting it all together, you might have

Request findReq = find(personId).with("phone","address.city","address.zip")

Validating Entities

RequestFactory supports JSR 303 bean validation. This makes it possible to keep validation rules on the server and notify the client when an entity cannot be persisted due to validation failures. To use it, ensure that a JSR 303 implementation is available in the server classpath and annotate your entities with the java.validation annotations like @Size and @NotNull as shown in the Employee entity above. Prior to invoking a service method on the server, RequestFactory will call the validation framework and send any ConstraintViolations from the server to the client, which will then call the onViolation() method on the Receiver for the request.

Conclusion

RequestFactory is the heart of the new "Bindery" features in GWT 2.1. In future articles, we'll look at integration with cell widgets, Editors, the event bus, and Activities and Places.