经过多次尝试和错误，我设法找到了一种方法来做到这一点。这可能不是最好的方法，但它确实有效。希望这篇文章可以为其他人节省大量时间和精力。
这些说明假设您已完成basic StockWatcher tutorial 和Google App Engine StockWatcher modifications。
创建 Stock 类的客户端实现
关于 GWT 有几点需要牢记：
- 服务器端类可以导入客户端类，但反之亦然（通常）。
- 客户端无法导入任何 Google App Engine 库（即 com.google.appengine.api.users.User）
由于上述两项，客户端永远无法实现我们在 com.google.gwt.sample.stockwatcher.server 中创建的 Stock 类。相反，我们将创建一个名为 StockClient 的新客户端 Stock 类。
StockClient.java：
package com.google.gwt.sample.stockwatcher.client;
import java.io.Serializable;
import java.util.Date;
public class StockClient implements Serializable {
private Long id;
private String symbol;
private Date createDate;
public StockClient() {
this.createDate = new Date();
}
public StockClient(String symbol) {
this.symbol = symbol;
this.createDate = new Date();
}
public StockClient(Long id, String symbol, Date createDate) {
this();
this.id = id;
this.symbol = symbol;
this.createDate = createDate;
}
public Long getId() {
return this.id;
}
public String getSymbol() {
return this.symbol;
}
public Date getCreateDate() {
return this.createDate;
}
public void setId(Long id) {
this.id = id;
}
public void setSymbol(String symbol) {
this.symbol = symbol;
}
}
修改客户端类以使用 StockClient[] 而不是 String[]
现在我们对客户端类进行一些简单的修改，以便他们知道 RPC 调用返回的是 StockClient[] 而不是 String[]。
StockService.java：
package com.google.gwt.sample.stockwatcher.client;
import com.google.gwt.sample.stockwatcher.client.NotLoggedInException;
import com.google.gwt.user.client.rpc.RemoteService;
import com.google.gwt.user.client.rpc.RemoteServiceRelativePath;
@RemoteServiceRelativePath("stock")
public interface StockService extends RemoteService {
public Long addStock(String symbol) throws NotLoggedInException;
public void removeStock(String symbol) throws NotLoggedInException;
public StockClient[] getStocks() throws NotLoggedInException;
}
StockServiceAsync.java：
package com.google.gwt.sample.stockwatcher.client;
import com.google.gwt.sample.stockwatcher.client.StockClient;
import com.google.gwt.user.client.rpc.AsyncCallback;
public interface StockServiceAsync {
public void addStock(String symbol, AsyncCallback<Long> async);
public void removeStock(String symbol, AsyncCallback<Void> async);
public void getStocks(AsyncCallback<StockClient[]> async);
}
StockWatcher.java：
添加一个导入：
import com.google.gwt.sample.stockwatcher.client.StockClient;
所有其他代码保持不变，除了 addStock、loadStocks 和 displayStocks：
private void loadStocks() {
stockService = GWT.create(StockService.class);
stockService.getStocks(new AsyncCallback<String[]>() {
public void onFailure(Throwable error) {
handleError(error);
}
public void onSuccess(String[] symbols) {
displayStocks(symbols);
}
});
}
private void displayStocks(String[] symbols) {
for (String symbol : symbols) {
displayStock(symbol);
}
}
private void addStock() {
final String symbol = newSymbolTextBox.getText().toUpperCase().trim();
newSymbolTextBox.setFocus(true);
// Stock code must be between 1 and 10 chars that are numbers, letters,
// or dots.
if (!symbol.matches("^[0-9a-zA-Z\\.]{1,10}$")) {
Window.alert("'" + symbol + "' is not a valid symbol.");
newSymbolTextBox.selectAll();
return;
}
newSymbolTextBox.setText("");
// Don't add the stock if it's already in the table.
if (stocks.contains(symbol))
return;
addStock(new StockClient(symbol));
}
private void addStock(final StockClient stock) {
stockService.addStock(stock.getSymbol(), new AsyncCallback<Long>() {
public void onFailure(Throwable error) {
handleError(error);
}
public void onSuccess(Long id) {
stock.setId(id);
displayStock(stock.getSymbol());
}
});
}
修改 StockServiceImpl 类以返回 StockClient[]
最后，我们修改 StockServiceImpl 类的 getStocks 方法，使其在返回数组之前将服务器端 Stock 类转换为客户端 StockClient 类。
StockServiceImpl.java
import com.google.gwt.sample.stockwatcher.client.StockClient;
我们需要稍微改变一下 addStock 方法，以便返回生成的 ID：
public Long addStock(String symbol) throws NotLoggedInException {
Stock stock = new Stock(getUser(), symbol);
checkLoggedIn();
PersistenceManager pm = getPersistenceManager();
try {
pm.makePersistent(stock);
} finally {
pm.close();
}
return stock.getId();
}
所有其他方法保持不变，除了 getStocks：
public StockClient[] getStocks() throws NotLoggedInException {
checkLoggedIn();
PersistenceManager pm = getPersistenceManager();
List<StockClient> stockclients = new ArrayList<StockClient>();
try {
Query q = pm.newQuery(Stock.class, "user == u");
q.declareParameters("com.google.appengine.api.users.User u");
q.setOrdering("createDate");
List<Stock> stocks = (List<Stock>) q.execute(getUser());
for (Stock stock : stocks)
{
stockclients.add(new StockClient(stock.getId(), stock.getSymbol(), stock.getCreateDate()));
}
} finally {
pm.close();
}
return (StockClient[]) stockclients.toArray(new StockClient[0]);
}
总结
上面的代码在部署到 Google App Engine 时非常适合我，但在 Google Web Toolkit Hosted Mode 中触发错误：
SEVERE: [1244408678890000] javax.servlet.ServletContext log: Exception while dispatching incoming RPC call
com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract com.google.gwt.sample.stockwatcher.client.StockClient[] com.google.gwt.sample.stockwatcher.client.StockService.getStocks() throws com.google.gwt.sample.stockwatcher.client.NotLoggedInException' threw an unexpected exception: java.lang.NullPointerException: Name is null
如果您遇到同样的问题，请告诉我。它在 Google App Engine 中工作的事实似乎表明托管模式存在错误。