Chapter 19. Remoting - JBoss Seam 1.2.1 正式版英文参考手册

Let's step through a simple example to see how the Seam object works. First of all, let's create a new Seam component called helloAction.

          
  @Stateless
  @Name("helloAction")
  public class HelloAction implements HelloLocal {
    public String sayHello(String name) {
      return "Hello, " + name;
    }
  }
          
        

You also need to create a local interface for our new component - take special note of the @WebRemote annotation, as it's required to make our method accessible via remoting:

          
  @Local
  public interface HelloLocal {
    @WebRemote
    public String sayHello(String name);
  }
          
        

That's all the server-side code we need to write. Now for our web page - create a new page and import the following scripts:

          
  <script type="text/javascript" src="seam/resource/remoting/resource/remote.js"></script>
  <script type="text/javascript" src="seam/resource/remoting/interface.js?helloAction"></script>
          
        

To make this a fully interactive user experience, let's add a button to our page:

          
  <button onclick="javascript:sayHello()">Say Hello</button>
          
        

We'll also need to add some more script to make our button actually do something when it's clicked:

          
  <script type="text/javascript">
    //<![CDATA[

    function sayHello() {
      var name = prompt("What is your name?");
      Seam.Component.getInstance("helloAction").sayHello(name, sayHelloCallback);
    }

    function sayHelloCallback(result) {
      alert(result);
    }

    // ]]>
  </script>
          
        

We're done! Deploy your application and browse to your page. Click the button, and enter a name when prompted. A message box will display the hello message confirming that the call was successful. If you want to save some time, you'll find the full source code for this Hello World example in Seam's /examples/remoting/helloworld directory.

So what does the code of our script actually do? Let's break it down into smaller pieces. To start with, you can see from the Javascript code listing that we have implemented two methods - the first method is responsible for prompting the user for their name and then making a remote request. Take a look at the following line:

  Seam.Component.getInstance("helloAction").sayHello(name, sayHelloCallback);
        

The first section of this line, Seam.Component.getInstance("helloAction") returns a proxy, or "stub" for our helloAction component. We can invoke the methods of our component against this stub, which is exactly what happens with the remainder of the line: sayHello(name, sayHelloCallback);.

What this line of code in its completeness does, is invoke the sayHello method of our component, passing in name as a parameter. The second parameter, sayHelloCallback isn't a parameter of our component's sayHello method, instead it tells the Seam Remoting framework that once it receives the response to our request, it should pass it to the sayHelloCallback Javascript method. This callback parameter is entirely optional, so feel free to leave it out if you're calling a method with a void return type or if you don't care about the result.

The sayHelloCallback method, once receiving the response to our remote request then pops up an alert message displaying the result of our method call.

The Seam.Component Javascript object provides a number of client-side methods for working with your Seam components. The two main methods, newInstance() and getInstance() are documented in the following sections however their main difference is that newInstance() will always create a new instance of a component type, and getInstance() will return a singleton instance.

  @Name("customer")
  @Entity
  public class Customer implements Serializable
  {
    private Integer customerId;
    private String firstName;
    private String lastName;
    
    @Column public Integer getCustomerId() { 
      return customerId; 
    }
    
    public void setCustomerId(Integer customerId} { 
      this.customerId = customerId; 
    }
    
    @Column public String getFirstName() { 
      return firstName; 
    }
    
    public void setFirstName(String firstName) {
      this.firstName = firstName; 
    }
    
    @Column public String getLastName() {
      return lastName;
    }
    
    public void setLastName(String lastName) {
      this.lastName = lastName;
    }
  }
          

  var customer = Seam.Component.newInstance("customer");
          

  customer.setFirstName("John");
  // Or you can set the fields directly
  customer.lastName = "Smith";
          

  Seam.Component.getInstance("customerAction").saveCustomer(customer);          
          

  if (Seam.Component.getComponentName(instance) == "customer")
    alert("Customer");
  else if (Seam.Component.getComponentName(instance) == "staff")
    alert("Staff member");          
          

Most of the client side functionality for Seam Remoting is contained within the Seam.Remoting object. While you shouldn't need to directly call most of its methods, there are a couple of important ones worth mentioning.

  var widget = Seam.Remoting.createType("com.acme.widgets.MyWidget");          
          

This section describes the support for basic data types. On the server side these values are generally compatible with either their primitive type or their corresponding wrapper class.

In general these will be either Seam entity or JavaBean components, or some other non-component class. Use the appropriate method (either Seam.Component.newInstance() for Seam components or Seam.Remoting.createType() for everything else) to create a new instance of the object.

It is important to note that only objects that are created by either of these two methods should be used as parameter values, where the parameter is not one of the other valid types mentioned anywhere else in this section. In some situations you may have a component method where the exact parameter type cannot be determined, such as:

  @Name("myAction")
  public class MyAction implements MyActionLocal {
    public void doSomethingWithObject(Object obj) {
      // code
    }
  }
        

In this case you might want to pass in an instance of your myWidget component, however the interface for myAction won't include myWidget as it is not directly referenced by any of its methods. To get around this, MyWidget needs to be explicitly imported:

                  
  <script type="text/javascript" src="seam/resource/remoting/interface.js?myAction&myWidget"></script>
          
        

This will then allow a myWidget object to be created with Seam.Component.newInstance("myWidget"), which can then be passed to myAction.doSomethingWithObject().

Date values are serialized into a String representation that is accurate to the millisecond. On the client side, use a Javascript Date object to work with date values. On the server side, use any java.util.Date (or descendent, such as java.sql.Date or java.sql.Timestamp class.

On the client side, enums are treated the same as Strings. When setting the value for an enum parameter, simply use the String representation of the enum. Take the following component as an example:

  @Name("paintAction")
  public class paintAction implements paintLocal {
    public enum Color {red, green, blue, yellow, orange, purple};

    public void paint(Color color) {
      // code
    }    
  }            
        

To call the paint() method with the color red, pass the parameter value as a String literal:

  Seam.Component.getInstance("paintAction").paint("red");
        

The inverse is also true - that is, if a component method returns an enum parameter (or contains an enum field anywhere in the returned object graph) then on the client-side it will be represented as a String.

To change the message from the default "Please Wait..." to something different, set the value of Seam.Remoting.loadingMessage:

  Seam.Remoting.loadingMessage = "Loading...";        
        

To completely suppress the display of the loading message, override the implementation of displayLoadingMessage() and hideLoadingMessage() with functions that instead do nothing:

  // don't display the loading indicator
  Seam.Remoting.displayLoadingMessage = function() {};
  Seam.Remoting.hideLoadingMessage = function() {};        
        

It is also possible to override the loading indicator to display an animated icon, or anything else that you want. To do this override the displayLoadingMessage() and hideLoadingMessage() messages with your own implementation:

  Seam.Remoting.displayLoadingMessage = function() {
    // Write code here to display the indicator
  };
  
  Seam.Remoting.hideLoadingMessage = function() {
    // Write code here to hide the indicator
  };
        

If your remote method returns an instance of Widget, but you don't want to expose the secret field because it contains sensitive information, you would constrain it like this:

  @WebRemote(exclude = {"secret"})
  public Widget getWidget();      
        

The value "secret" refers to the secret field of the returned object. Now, suppose that we don't care about exposing this particular field to the client. Instead, notice that the Widget value that is returned has a field child that is also a Widget. What if we want to hide the child's secret value instead? We can do this by using dot notation to specify this field's path within the result's object graph:

  @WebRemote(exclude = {"child.secret"})
  public Widget getWidget();      
        

The other place that objects can exist within an object graph are within a Map or some kind of collection (List, Set, Array, etc). Collections are easy, and are treated like any other field. For example, if our Widget contained a list of other Widgets in its widgetList field, to constrain the secret field of the Widgets in this list the annotation would look like this:

  @WebRemote(exclude = {"widgetList.secret"})
  public Widget getWidget();      
        

To constrain a Map's key or value, the notation is slightly different. Appending [key] after the Map's field name will constrain the Map's key object values, while [value] will constrain the value object values. The following example demonstrates how the values of the widgetMap field have their secret field constrained:

  @WebRemote(exclude = {"widgetMap[value].secret"})
  public Widget getWidget();      
        

There is one last notation that can be used to constrain the fields of a type of object no matter where in the result's object graph it appears. This notation uses either the name of the component (if the object is a Seam component) or the fully qualified class name (only if the object is not a Seam component) and is expressed using square brackets:

  @WebRemote(exclude = {"[widget].secret"})
  public Widget getWidget();      
        

Constraints can also be combined, to filter objects from multiple paths within the object graph:

  @WebRemote(exclude = {"widgetList.secret", "widgetMap[value].secret"})
  public Widget getWidget();      
        

Before you can subscribe to a JMS topic, you must first configure a list of the topics that can be subscribed to by Seam Remoting. List the topics under org.jboss.seam.remoting.messaging.subscriptionRegistry.allowedTopics in seam.properties, web.xml or components.xml.

<remoting:remoting poll-timeout="5" poll-interval="1"/>

The following example demonstrates how to subscribe to a JMS Topic:

  function subscriptionCallback(message)
  {
    if (message instanceof Seam.Remoting.TextMessage)
      alert("Received message: " + message.getText());
  }        
        
  Seam.Remoting.subscribe("topicName", subscriptionCallback);
        

The Seam.Remoting.subscribe() method accepts two parameters, the first being the name of the JMS Topic to subscribe to, the second being the callback function to invoke when a message is received.

There are two types of messages supported, Text messages and Object messages. If you need to test for the type of message that is passed to your callback function you can use the instanceof operator to test whether the message is a Seam.Remoting.TextMessage or Seam.Remoting.ObjectMessage. A TextMessage contains the text value in its text field (or alternatively call getText() on it), while an ObjectMessage contains its object value in its object field (or call its getObject() method).

To unsubscribe from a topic, call Seam.Remoting.unsubscribe() and pass in the topic name:

  Seam.Remoting.unsubscribe("topicName");        
        

There are two parameters which you can modify to control how polling occurs. The first one is Seam.Remoting.pollInterval, which controls how long to wait between subsequent polls for new messages. This parameter is expressed in seconds, and its default setting is 10.

The second parameter is Seam.Remoting.pollTimeout, and is also expressed as seconds. It controls how long a request to the server should wait for a new message before timing out and sending an empty response. Its default is 0 seconds, which means that when the server is polled, if there are no messages ready for delivery then an empty response will be immediately returned.

Caution should be used when setting a high pollTimeout value; each request that has to wait for a message means that a server thread is tied up until a message is received, or until the request times out. If many such requests are being served simultaneously, it could mean a large number of threads become tied up because of this reason.

It is recommended that you set these options via components.xml, however they can be overridden via Javascript if desired. The following example demonstrates how to configure the polling to occur much more aggressively. You should set these parameters to suitable values for your application:

Via components.xml:

<remoting:remoting poll-timeout="5" poll-interval="1"/>

Via JavaScript:

  // Only wait 1 second between receiving a poll response and sending the next poll request.
  Seam.Remoting.pollInterval = 1;
  
  // Wait up to 5 seconds on the server for new messages
  Seam.Remoting.pollTimeout = 5;