GWT通信机制初探

GWT RPC：GWT提供类似Swing的界面开发模式（基于UI组件重用,事件注册及监听等机制），遵循此开发模式的客户端Java代码将被编译为Javascript代码(并优化,压缩),以运行于客户端浏览器中。然而，客户端Java代码不仅仅包含即将呈现在HTML页面中的UI元素，还包含提供服务的接口和相对应的代理接口（实现异步调用服务），服务接口声明即将被
客户端通过RPC调用的方法。服务器端实现该方法来提供具体服务。服务架构参考下图：



服务接口必须继承一个空接口RemoteService：

//提供getAddresses()服务的接口类
import java.util.List;

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

@RemoteServiceRelativePath("address")
public interface AddressService extends RemoteService{
	public List<Address> getAddresses();
}

/*--------------------------------------------------------*/
//代理接口，提供异步调用服务
import java.util.List;
import com.google.gwt.user.client.rpc.AsyncCallback;

public interface AddressServiceAsync {
	public void getAddresses(AsyncCallback<List<Address>> callback);
}

服务接口的注释@RemoteServiceRelativePath("address")指定了代理接口调用的服务的相对路径（通常为servlet访问路径）；

服务实现类

public class AddressServiceImpl extends RemoteServiceServlet implements AddressService{
	
	private static final long serialVersionUID = 7819604306802209305L;

        //实现从数据库查询地理信息的数据集
	@Override
	public List<Address> getAddresses(){
		List<Address> addr_list = new ArrayList<Address>();
		DBConnectionPool pool = new DBConnectionPool("pool01", "com.mysql.jdbc.Driver", "jdbc:mysql://localhost:3306/db?autoReconnect=true&useUnicode=true&characterEncoding=UTF-8",
				"admin", "admin123", 5);
		System.out.println("[INFO] Connection Pool created!");
		ResultSet rs = null;
		String sqlStmt = "SELECT addr_id,state,city,address,zip,description FROM address";
		try {
			Connection conn = pool.getConnection();
			System.out.println("[INFO] Connection to DB established!");
			PreparedStatement ps = conn.prepareStatement(sqlStmt);
			rs = ps.executeQuery();
			if(rs == null){
				System.out.println("[ERROR] Failed to query data from database!");
				return null;	
			}
			System.out.println("[INFO] Execution of query for address list finished!");
			Address addr = null;
			int cnt = 0;
			while(rs.next()){
				addr = new Address();
				++cnt;
				System.out.println("[INFO] No. " + cnt + " of the result records.");
				addr.setId(rs.getLong("addr_id"));
				addr.setState(rs.getString("state") == null ? "N/A" : rs.getString("state"));
				addr.setCity(rs.getString("city"));
				addr.setAddress(rs.getString("address"));
				addr.setZip(rs.getString("zip"));
				addr.setDescription(rs.getString("description") == null ? "N/A" : rs.getString("description") );
				addr_list.add(addr);
			}
		} catch (SQLException e) {
			System.out.println("[ERROR] Failed to get field value from resultSet!");
			e.printStackTrace();
		}
		return addr_list;
	}

        @Override
	public void onBeforeRequestDeserialized(String serializedRequest){
		System.out.println("[INFO] Received Serialized Request Diagram: " + serializedRequest);
        }

        @Override
	public void onAfterResponseSerialized(String serializedResponse) {
		System.out.println("[INFO] Serialized Response Diagram to be sent: " + serializedResponse);
	}
}

实现类所继承的RemoteServiceServlet是GWT提供的核心Servlet处理类，该类接收客户请求，反序列化RPC请求数据包（GWT提供了序列化及反序列化类库），将反序列化之后的数据交由开发者做业务处理，处理结果将被序列化并组织为响应对象返回客户端。以下为该Servlet处理类的对请求的处理过程：

 /**
   * Standard HttpServlet method: handle the POST.
   * 
   * This doPost method swallows ALL exceptions, logs them in the
   * ServletContext, and returns a GENERIC_FAILURE_MSG response with status code
   * 500.
   * 
   * @throws ServletException
   * @throws SerializationException
   */
  @Override
  public final void processPost(HttpServletRequest request,
      HttpServletResponse response) throws IOException, ServletException,
      SerializationException {
    // Read the request fully.
    String requestPayload = readContent(request);

    // Let subclasses see the serialized request.
    onBeforeRequestDeserialized(requestPayload);

    // Invoke the core dispatching logic, which returns the serialized
    // result.
    String responsePayload = processCall(requestPayload);

    // Let subclasses see the serialized response.
    onAfterResponseSerialized(responsePayload);

    // Write the response.
    writeResponse(request, response, responsePayload);
  }

  //本方法处理反序列化后的请求数据
  public String processCall(String payload) throws SerializationException {
    try {
      RPCRequest rpcRequest = RPC.decodeRequest(payload, this.getClass(), this);
      onAfterRequestDeserialized(rpcRequest);
      return RPC.invokeAndEncodeResponse(this, rpcRequest.getMethod(),
          rpcRequest.getParameters(), rpcRequest.getSerializationPolicy(),
          rpcRequest.getFlags());
    } catch (IncompatibleRemoteServiceException ex) {
      log(
          "An IncompatibleRemoteServiceException was thrown while processing this call.",
          ex);
      return RPC.encodeResponseForFailure(null, ex);
    }
  }

/*---------------------- RPC.java -------------------------*/
//RPC类中定义的方法invokeAndEncodeResponse，采用反射机制来调用具体的服务方法。
  public static String invokeAndEncodeResponse(Object target,
      Method serviceMethod, Object[] args,
      SerializationPolicy serializationPolicy, int flags)
      throws SerializationException {
    if (serviceMethod == null) {
      throw new NullPointerException("serviceMethod");
    }

    if (serializationPolicy == null) {
      throw new NullPointerException("serializationPolicy");
    }

    String responsePayload;
    try {
      Object result = serviceMethod.invoke(target, args);

      responsePayload = encodeResponseForSuccess(serviceMethod, result,
          serializationPolicy, flags);
    } catch (IllegalAccessException e) {
      SecurityException securityException = new SecurityException(
          formatIllegalAccessErrorMessage(target, serviceMethod));
      securityException.initCause(e);
      throw securityException;
    } catch (IllegalArgumentException e) {
      SecurityException securityException = new SecurityException(
          formatIllegalArgumentErrorMessage(target, serviceMethod, args));
      securityException.initCause(e);
      throw securityException;
    } catch (InvocationTargetException e) {
      // Try to encode the caught exception
      //
      Throwable cause = e.getCause();

      responsePayload = encodeResponseForFailure(serviceMethod, cause,
          serializationPolicy, flags);
    }

    return responsePayload;
  }

onBeforeRequestDeserialized()和onAfterResponseSerialized()由实现类继承，以做定制化处理。

以下是从客户端获取的请求数据包及请求属性：

引用

[INFO]  Received Serialized Request Diagram: 5|0|4|http://localhost:1947/gmap/|A16261BA4B0DD6867308FEA211E4BEC2|com.wipro.gmap.client.AddressService|getAddresses|1|2|3|4|0|
[INFO]  ContentType: text/x-gwt-rpc; charset=utf-8 ServletPath: /gmap/address ContextPath: Protocol: HTTP/1.1 Remote Address: 127.0.0.1 Remote Host: 127.0.0.1:1949 Remote User: null

[Header Information]:
[Host]: localhost:1947 [Connection]: keep-alive [User-Agent]: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/532.2 (KHTML, like Gecko) Chrome/4.0.222.5 Safari/532.2 [Referer]: http://localhost:1947/gmap/hosted.html?gmap [Accept]: */* [Accept-Encoding]: gzip,deflate [Accept-Language]: en-US,en;q=0.8 [Accept-Charset]: UTF-8,*;q=0.5 [Content-Length]: 127 [Origin]: http://localhost:1947 [X-GWT-Module-Base]: http://localhost:1947/gmap/ [Content-Type]: text/x-gwt-rpc; charset=utf-8 [X-GWT-Permutation]: HostedMode

以下为处理后即将返回客户端的数据包（JSON数据格式）：

引用

[INFO]  Serialized Response Diagrm to be sent: //OK[7,13,0.0,3.0,5,12,11,2,7,10,0.0,2.0,5,9,8,2,7,6,0.0,1.0,5,4,3,2,3,1,["java.util.ArrayList/3821976829","com.wipro.gmap.client.Address/1028263900","Golden Gate Bridge","San Francisco","","california","611756","Main St","Oxford","New York","Zhonghe","Chengdu","Sichuan"],0,5]

该数据返回客户端后将触发回调方法，回调方法定义在入口类（EntryPoint）中：

//final ArrayList<Address> addressList = new ArrayList<Address>();
//final ListBox addresses = new ListBox();

AddressServiceAsync as = (AddressServiceAsync) GWT
				.create(AddressService.class);
as.getAddresses(new AsyncCallback<List<Address>>() {
	@Override
	public void onSuccess(List<Address> result) {
		Window.alert("[INFO] Succeed to retrieve data from db!");
		Iterator<Address> it = result.iterator();
		Address address = null;
		while(it.hasNext()){
			address = new Address();
			address = it.next();
			System.out.println("[INFO] Address: " + address.getAddress());
			addresses.addItem(address.getAddress());
			addressList.add(address);
		}
		System.out.println("[INFO] Data in addresses ListBox:");
		addresses.setVisibleItemCount(result.size());
		addressGrid.setAddress(addressList.get(0));
		System.out.println("[INFO] [GMap.getAddresse()] Retrieved addresses already be populated into the ListBox!");
	}

	@Override
	public void onFailure(Throwable caught) {
	        String msg = "[ERROR] Failed to retrieve data from database!";
            	Window.alert(msg);
        	System.out.println(msg);
	        GWT.log(msg, caught);
        }
});

服务接口及相应的异步接口由GWT编译至客户端Javascript中，由GWT Plugin根据服务器地址及端口，模块（Module），服务调用的页面来对服务器进行RPC调用。参考编译后的客户端JS代码（GWT Plugin负责连接服务器，调用服务端RPC服务）：

if (!plugin) {
    // try searching for a v1 plugin for backwards compatibility
    var found = false;
    for (var i = 0; i < pluginFinders.length; ++i) {
      try {
        plugin = pluginFinders[i]();
        if (plugin != null && plugin.connect($hosted, $moduleName, window)) {
          return;
        }
      } catch (e) {
      }
    }
    loadIframe("http://gwt.google.com/missing-plugin");
  } else {
    if (plugin.connect(url, topWin.__gwt_SessionID, $hosted, $moduleName,
        $hostedHtmlVersion)) {
      window.onUnload = function() {
        try {
          // wrap in try/catch since plugins are not required to supply this
          plugin.disconnect();
        } catch (e) {
        }
      };
    } else {
      if (errFn) {
        errFn(modName);
      } else {
        alert("Plugin failed to connect to hosted mode server at " + $hosted);
        loadIframe("http://code.google.com/p/google-web-toolkit/wiki/TroubleshootingOOPHM");
      }
    }
  }

评论

2 楼 tinger1 2016-07-07  

狂赞楼主！ 

1 楼 wwt18946637566 2015-03-31  

这样的文章要顶一个，讲的真明白