学步园

RPC交互在我看来并不是目前广泛流传的各种模式的首选，而gwt的RPC，则是把servlet变相的作为了RPC交互的一个接口终端。

Building and calling a GWT service

There are three steps involved in creating a GWT service:

1. Defining the service’s synchronous and asynchronous interface
2. Implementing the service
3. Calling the service

1. Defining a GWT service

Synchronous Interface

The first step in creating the String Reverser Service is defining the service’s client-side interface. The interface must extend the GWT RemoteService interface and contain the signatures of the service methods that should be exposed to the client. Method parameters and return types must be serializable.

The code snippet below shows the client-side class (StringReverserService) which defines the synchronous interface for the String Reverser Service.

public interface StringReverserService extends RemoteService {</p> <p> public String reverseString(String stringToReverse);</p> <p>}<br />

Asynchronous Interface

A client-side asynchronous interface, based on the synchronous interface, must be built before a service call can be made. The nature of asynchronous method calls requires the caller to pass in a callback object (AsyncCallback). This callback object is used to notify the caller when an asynchronous call completes. Asynchronous methods cannot have return types, and so they must always return void. After an asynchronous call is made, all communication back to the caller is via the passed-in callback object.

A service’s asynchronous interface must be in the same package and have the same name, but with the suffix “Async”. E.g. if a service interface is called com.example.app.client.RandomService, the asynchronous interface must be called com.example.app.client.RandomServiceAsync.

An asynchronous “sibling” method should be defined for each method in your service’s synchronous interface. The method signature for these asynchronous sibling methods is the same as the synchronous methods signature, but with the addition of an asynchronous callback. If the synchronous interface method is:

public ReturnType methodName(ParamType1 one, ParamType2 two);

the asynchronous sibling method will be:

public void methodName(ParamType1 one, ParamType2 two, AsyncCallback callback);

The code snippet below shows the client-side class (StringReverserServiceAsync) which defines the asynchronous interface for the String Reverser Service.

public interface StringReverserServiceAsync {</p> <p> void reverseString(String stringToReverse, AsyncCallback async);</p> <p>}<br />

2. Implementing a GWT service

Now that the service interfaces have been defined, the next step is to implement the service methods. These methods are implemented in a class that extends GWT’s RemoteServiceServlet class. This service implementation can be hosted in any servlet container.

For the String Reverser Service, the service implementation will take the user-entered string that was passed to the service and reverse the order of the characters. Once reversed, the reversed string is returned.

The code snippet below shows the server-side class (StringReverserServiceImpl) which implements the String Reverser Service Interface.

public class StringReverserServiceImpl extends RemoteServiceServlet implements StringReverserService {</p> <p> /**</p> <p> * This method is used to reverse a string</p> <p> * @param stringToReverse The string that you want reversed</p> <p> * @return The reversed string</p> <p> */</p> <p> public String reverseString(String stringToReverse) {</p> <p> StringBuffer reverse = new StringBuffer(stringToReverse.length());</p> <p> for (int i = (stringToReverse.length() - 1); i >= 0; i–)</p> <p> reverse.append(stringToReverse.charAt(i));</p> <p> return reverse.toString();</p> <p> }</p> <p>}<br />

3. Calling a GWT service

Once a service has been defined and implemented, it can be called from a client. The process of making a GWT RPC call from the client involves the following steps:

a. Instantiate the service interface using GWT.create().

Before we can use the service, we need to instantiate an instance of the service. The following listing demonstrates how to create an instance of StringReverserService. Note that although an instance of the synchronous service interface is being created and then cast to asynchronous version of the interface. The cast is always safe because the generated proxy implements the asynchronous interface automatically.

The code snippet below shows you how to instantiate the String Reverser Service.

1: StringReverserServiceAsync reverserService = (StringReverserServiceAsync) GWT.create(StringReverserService.class);

b.Specify a service entry point URL for the service using ServiceDefTarget.

Once an instance of the service is created, we need to specify the URL at which the service implementation is, or will be, running. The target URL for the service must reside on the same domain and port from which the host page was served. This is due to the fact that most modern browsers implement the “Single Origin Policy” (SOP) security model. The SOP policy makes it so that JavaScript code running in a currently loaded web-page can only interact with resources originating from sites with the same domain name and port as the currently loaded web-page.

The code snippet below shows you how to specify the URL for the String Reverser Service Implementation.

((ServiceDefTarget) reverserService).setServiceEntryPoint( GWT.getModuleBaseURL() +

                                                           "/StringReverser/StringReverserService");

c. Create an AsyncCallback object so that the client can be notified when the service call is completed.

The next step is to create an Asynchrounous Callback (AsyncCallback) object. This object will be passed as a parameter to the remote service when it is called. As was mentioned before, all network operation is GWT are asynchronous and non-blocking. When a call to the service is made, there is no immediate value returned. Since the service call is asynchronous and non-blocking, the GWT application will not wait for a response from the service. It will continue executing until it receives an asynchronous callback from the service. This callback informs the GWT application whether or not the service call has been executed successfully.

If the service call is unsuccessful, the onFailure method of the AsyncCallback object is called. If the service call is successful, the onSuccess method of the AsyncCallback object is called. The onSuccess method accepts a value of type java.lang.Object as a parameter. If your service returns data, this result object is it. It is always safe to downcast the parameter to the return type of the service method. Note that if the return type of the synchronous service interface method is a primitive (int, long, etc) then the parameter will be the boxed version of the primitive (for example, an int return type becomes an Integer).

The following code snippet shows you how to create an AsyncCallback object.

AsyncCallback callback = new AsyncCallback() {</p> <p> // This method will be called if the service call fails </p> <p> public void onFailure(Throwable caught) {</p> <p> // Show a message informing the user why the call failed </p> <p> Window.alert("Failed to get response from server" + caught.getMessage());</p> <p> }</p> <p> // This method will be called if the service call succeeds </p> <p> public void onSuccess(Object result) {</p> <p> // Get the service call result and cast it to the desired type and display it</p> <p> String reverse = (String) result;</p> <p> Window.alert(reverse);</p> <p> }</p> <p>};</p> <p>

d. Make the call

The final step is to make the service call. The following code snippet demonstrates how to make the call to the String Reverser Service.

1: reverserService.reverseString(stringToReverse, callback);

Putting it all together

The following code snippet shows the sections of the String Reverser widget (StringReverserWidget.java) where the call to the String Reverser Service is made. Note that AsyncCallback below is created as an Anonymous Inner Class when the String Reverser Service is called.

/**</p> <p> * Method used to get an instance of the String Reverser Service</p> <p> *</p> <p> * @return Reversed String</p> <p> */</p> <p>private StringReverserServiceAsync getReverserServiceInstance() {</p> <p> if (reverserService == null) {</p> <p> // Instantiate the service </p> <p> reverserService = (StringReverserServiceAsync) GWT.create(StringReverserService.class);</p> <p> // Specify the URL at which the service implementation is running.</p> <p> // The target URL must reside on the same domain and port from</p> <p> // which the host page was served.</p> <p> ((ServiceDefTarget) reverserService).setServiceEntryPoint( GWT.getModuleBaseURL() +</p> <p> "/StringReverser/StringReverserService");</p> <p> }</p> <p> return reverserService;</p> <p>}</p> <p>/**</p> <p> * Method used to call the String Reverser Service. The response from the service is displayed in a dialog box after</p> <p> * it is received.</p> <p> *</p> <p> * @param stringToReverse String that you want reversed</p> <p> */</p> <p>public void callStringReverserService(final String stringToReverse) {</p> <p> // Show the loading message and animation</p> <p> showLoadingMessageAndAnimation();</p> <p> // Call the String Reverser Service</p> <p> // The asynchronous callback used to handle the service response is created inline.</p> <p> // If the service call fails, a message will be displayed on the client</p> <p> // If the service call is successful, the reversed string will be displayed</p> <p> getReverserServiceInstance().reverseString(stringToReverse, new AsyncCallback() {</p> <p> public void onFailure(Throwable caught) {</p> <p> // Reset the button text and remove the loading animation</p> <p> resetWiget();</p> <p> Window.alert("Failed to get response from server");</p> <p> }</p> <p> public void onSuccess(Object result) {</p> <p> // Reset the button text and remove the loading animation</p> <p> resetWiget();</p> <p> // Get the service call response and display it</p> <p> String reverse = (String) result;</p> <p> ReversedStringDisplayDialog dlg = new ReversedStringDisplayDialog(stringToReverse, reverse);</p> <p> dlg.show();</p> <p> }</p> <p>});</p> <p>}</p> <p>

The String Reverser Application

Click on this link to see a demo of the application.

Downloads

All the source code provided on this site is distributed under the Apache Open Source License v2.0.

1. You can download the full source code used to build the String Reverser Application here.
2. You can download the WAR file to deploy the String Reverser Application here.