極客書

GWT - RPC Communication

基於GWT的應用程式通常由客戶端模塊和伺服器端模塊組成。客戶端代碼在瀏覽器中運行，伺服器端代碼在web伺服器中運行。客戶端代碼必須通過網絡發出HTTP請求才能訪問伺服器端數據。

遠程過程調用（RPC，Remote Procedure Call）是GWT使用的一種機制，其中客戶端代碼可以直接執行伺服器端方法。

  • GWT-RPC是基於servlet的。

  • GWT-RPC是異步的，客戶機在通信期間從不被阻塞。

  • 使用GWT RPC Java對象可以直接在客戶機和伺服器之間發送（由GWT框架自動序列化）。

  • 伺服器端servlet被稱爲服務。

  • 從客戶端代碼調用伺服器端servlet方法的遠程過程調用稱爲調用服務。

GWT RPC Components

以下是GWT-RPC通信機制中使用的三個組件

  • A remote service (server-side servlet) that runs on the server.
  • Client code to invoke that service.
  • Java data objects which will be passed between client and server.

GWT客戶機和伺服器都會自動序列化和反序列化數據，因此不需要開發人員序列化/反序列化對象，數據對象可以通過HTTP進行傳輸。

下圖顯示了RPC體系結構。

GWT RPC workflow

要開始使用RPC，我們需要遵循GWT約定。

RPC Communication Workflow

Step 1 - Create a Serializable Model Class

在客戶端定義一個java模型對象，該對象應該是可序列化的。

public class Message implements Serializable {
   ...
   private String message;
   public Message(){};

   public void setMessage(String message) {
      this.message = message;
   }
   ...
}

Step 2 - Create a Service Interface

在客戶端爲服務定義一個接口，該接口擴展列出所有服務方法的RemoteService。

使用annotation@remoteservicererelativepath將服務映射爲遠程servlet相對於模塊基URL的默認路徑。

@RemoteServiceRelativePath("message")
public interface MessageService extends RemoteService {
   Message getMessage(String input);
}

Step 3 - Create a Async Service Interface

在客戶機端（與上面提到的服務位於同一位置）定義一個異步服務接口，該接口將在GWT客戶機代碼中使用。

public interface MessageServiceAsync {
   void getMessage(String input, AsyncCallback<Message> callback);
}

Step 4 - Create a Service Implementation Servlet class

在伺服器端實現接口，該類應擴展RemoteServiceServlet類。

public class MessageServiceImpl extends RemoteServiceServlet
   implements MessageService{
   ...
   public Message getMessage(String input) {
      String messageString = "Hello " + input + "!";
      Message message = new Message();
      message.setMessage(messageString);
      return message;
   }
}

Step 5 - Update Web.xml to include Servlet declaration

編輯web應用程式部署描述符（web.xml）以包含MessageServiceImpl Servlet聲明。

<web-app>
   ...
   <servlet>
      <servlet-name>messageServiceImpl</servlet-name>
      <servlet-class>com.tutorialspoint.server.MessageServiceImpl
      </servlet-class>
   </servlet>
   
   <servlet-mapping>
      <servlet-name>messageServiceImpl</servlet-name>
      <url-pattern>/helloworld/message</url-pattern>
   </servlet-mapping>
</web-app>

Step 6 - Make the remote procedure call in Application Code

創建服務代理類。

MessageServiceAsync messageService = GWT.create(MessageService.class);

創建AsyncCallback處理程序以處理伺服器將消息返回給客戶端的RPC回調

class MessageCallBack implements AsyncCallback<Message> {

   @Override
   public void onFailure(Throwable caught) {
      Window.alert("Unable to obtain server response: "
      + caught.getMessage());	
   }

   @Override
   public void onSuccess(Message result) {
      Window.alert(result.getMessage()); 
   }	   
}

當用戶與UI交互時調用遠程服務

public class HelloWorld implements EntryPoint {
   ... 
   public void onModuleLoad() {
   ...
      buttonMessage.addClickHandler(new ClickHandler() {			
         @Override
         public void onClick(ClickEvent event) {
            messageService.getMessage(txtName.getValue(), 
            new MessageCallBack());
         }
      });
   ...
   }
}

RPC Communication Complete Example

這個例子將帶您通過簡單的步驟來展示GWT中RPC通信的例子。按照以下步驟更新我們在GWT-Create application−一章中創建的GWT應用程式;

Step Description
1 Create a project with a name HelloWorld under a package com.tutorialspoint as explained in the GWT - Create Application chapter.
2 Modify HelloWorld.gwt.xml, HelloWorld.css, HelloWorld.html and HelloWorld.java as explained below. Keep rest of the files unchanged.
3 Compile and run the application to verify the result of the implemented logic.

下面是修改後的模塊描述符的內容。

<?xml version = "1.0" encoding = "UTF-8"?>
<module rename-to = 'helloworld'>
   <!-- Inherit the core Web Toolkit stuff.                        -->
   <inherits name = 'com.google.gwt.user.User'/>

   <!-- Inherit the default GWT style sheet.                       -->
   <inherits name = 'com.google.gwt.user.theme.clean.Clean'/>
   <!-- Inherit the UiBinder module.                               -->
   <inherits name = "com.google.gwt.uibinder.UiBinder"/>
   <!-- Specify the app entry point class.                         -->
   <entry-point class = 'com.tutorialspoint.client.HelloWorld'/>
  
   <!-- Specify the paths for translatable code                    -->
   <source path = 'client'/>
   <source path = 'shared'/>

</module>

下面是修改後的樣式表文件war/HelloWorld.css的內容。

body {
   text-align: center;
   font-family: verdana, sans-serif;
}

h1 {
   font-size: 2em;
   font-weight: bold;
   color: #777777;
   margin: 40px 0px 70px;
   text-align: center;
}

下面是修改後的HTML宿主文件war/HelloWorld.HTML的內容。

<html>
   <head>
      <title>Hello World</title>
      <link rel = "stylesheet" href = "HelloWorld.css"/>
      <script language = "javascript" src = "helloworld/helloworld.nocache.js">
      </script>
   </head>

   <body>
      <h1>RPC Communication Demonstration</h1>
      <div id = "gwtContainer"></div>
   </body>
</html>


package com.tutorialspoint.client;

import java.io.Serializable;

public class Message implements Serializable {
 
   private static final long serialVersionUID = 1L;
   private String message;
   public Message(){};

   public void setMessage(String message) {
      this.message = message;
   }

   public String getMessage() {
      return message;
   }
}


package com.tutorialspoint.client;

import com.google.gwt.user.client.rpc.RemoteService;
import com.google.gwt.user.client.rpc.RemoteServiceRelativePath;

@RemoteServiceRelativePath("message")
public interface MessageService extends RemoteService {
   Message getMessage(String input);
}


package com.tutorialspoint.client;

import com.google.gwt.user.client.rpc.AsyncCallback;

public interface MessageServiceAsync {
   void getMessage(String input, AsyncCallback<Message> callback);
}


package com.tutorialspoint.server;

import com.google.gwt.user.server.rpc.RemoteServiceServlet;
import com.tutorialspoint.client.Message;
import com.tutorialspoint.client.MessageService;

public class MessageServiceImpl extends RemoteServiceServlet 
   implements MessageService{

   private static final long serialVersionUID = 1L;

   public Message getMessage(String input) {
      String messageString = "Hello " + input + "!";
      Message message = new Message();
      message.setMessage(messageString);
      return message;
   }   
}

更新修改後的web應用程式部署描述符的內容，使其包含MessageServiceImpl Servlet聲明。

<?xml version = "1.0" encoding = "UTF-8"?>
<!DOCTYPE web-app
   PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
   "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
   <!-- Default page to serve -->
   <welcome-file-list>
      <welcome-file>HelloWorld.html</welcome-file>
   </welcome-file-list>
   
   <servlet>
      <servlet-name>messageServiceImpl</servlet-name>
      <servlet-class>com.tutorialspoint.server.MessageServiceImpl
      </servlet-class>
   </servlet>

   <servlet-mapping>
      <servlet-name>messageServiceImpl</servlet-name>
      <url-pattern>/helloworld/message</url-pattern>
   </servlet-mapping>
</web-app>


package com.tutorialspoint.client;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;
import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.event.dom.client.KeyCodes;
import com.google.gwt.event.dom.client.KeyUpEvent;
import com.google.gwt.event.dom.client.KeyUpHandler;
import com.google.gwt.user.client.Window;
import com.google.gwt.user.client.rpc.AsyncCallback;
import com.google.gwt.user.client.ui.Button;
import com.google.gwt.user.client.ui.DecoratorPanel;
import com.google.gwt.user.client.ui.HasHorizontalAlignment;
import com.google.gwt.user.client.ui.HorizontalPanel;
import com.google.gwt.user.client.ui.Label;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.TextBox;
import com.google.gwt.user.client.ui.VerticalPanel;

public class HelloWorld implements EntryPoint {
	
   private MessageServiceAsync messageService = 
   GWT.create(MessageService.class);

   private class MessageCallBack implements AsyncCallback<Message> {
      @Override
      public void onFailure(Throwable caught) {
         /* server side error occured */
         Window.alert("Unable to obtain server response: " + caught.getMessage());	
      }
      @Override
      public void onSuccess(Message result) {
          /* server returned result, show user the message */
         Window.alert(result.getMessage());
      }	   
   }

   public void onModuleLoad() {
      /*create UI */
      final TextBox txtName = new TextBox(); 
      txtName.setWidth("200");
      txtName.addKeyUpHandler(new KeyUpHandler() {
         @Override
         public void onKeyUp(KeyUpEvent event) {
            if(event.getNativeKeyCode() == KeyCodes.KEY_ENTER){
               /* make remote call to server to get the message */
               messageService.getMessage(txtName.getValue(), 
               new MessageCallBack());
            }				
         }
      });
      Label lblName = new Label("Enter your name: ");

      Button buttonMessage = new Button("Click Me!");

      buttonMessage.addClickHandler(new ClickHandler() {			
         @Override
         public void onClick(ClickEvent event) {
            /* make remote call to server to get the message */
            messageService.getMessage(txtName.getValue(), 
            new MessageCallBack());
         }
      });

      HorizontalPanel hPanel = new HorizontalPanel();	
      hPanel.add(lblName);
      hPanel.add(txtName);
      hPanel.setCellWidth(lblName, "130");

      VerticalPanel vPanel = new VerticalPanel();
      vPanel.setSpacing(10);
      vPanel.add(hPanel);
      vPanel.add(buttonMessage);
      vPanel.setCellHorizontalAlignment(buttonMessage, 
      HasHorizontalAlignment.ALIGN_RIGHT);

      DecoratorPanel panel = new DecoratorPanel();
      panel.add(vPanel);

      // Add widgets to the root panel.
      RootPanel.get("gwtContainer").add(panel);
   }    
}

完成所有更改後，讓我們以開發模式編譯並運行應用程式，就像我們在「gwt-創建應用程式」一章中所做的那樣。如果你的申請一切順利，這將產生以下結果;