Problems passing class objects through GWT RPC

Answer 1

GWT needs the .java file in addition to the .class file. Additionally, Stock needs to be in the "client" location of a GWT module.

Answer 2

The GWT compiler doesn't know about Stock, because it's not in a location it looks in. You can either move it to the client folder, or if it makes more sense leave it where it is and create a ModuleName.gwt.xml that references any other classes you want, and get your Main.gwt.xml file to inherit from that.

eg: DomainGwt.gwt.xml

<module>
    <inherits name='com.google.gwt.user.User'/>
    <source path="javapackagesabovethispackagegohere"/>
</module>

and:

<module rename-to="gwt_ui">
    <inherits name="com.google.gwt.user.User"/>
    <inherits name="au.com.groundhog.groundpics.DomainGwt"/>

    <entry-point class="au.com.groundhog.groundpics.gwt.client.GPicsUIEntryPoint"/>
</module>

Answer 3

After much trial and error, I managed to find a way to do this. It might not be the best way, but it works. Hopefully this post can save someone else a lot of time and effort.

These instructions assume that you have completed both the basic StockWatcher tutorial and the Google App Engine StockWatcher modifications.

Create a Client-Side Implementation of the Stock Class

There are a couple of things to keep in mind about GWT:

1. Server-side classes can import client-side classes, but not vice-versa (usually).
2. The client-side can't import any Google App Engine libraries (i.e. com.google.appengine.api.users.User)

Due to both items above, the client can never implement the Stock class that we created in com.google.gwt.sample.stockwatcher.server. Instead, we'll create a new client-side Stock class called StockClient.

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;
  }
}

Modify Client Classes to Use StockClient[] instead of String[]

Now we make some simple modifications to the client classes so that they know that the RPC call returns StockClient[] instead of 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:

Add one import:

import com.google.gwt.sample.stockwatcher.client.StockClient;

All other code stays the same, except addStock, loadStocks, and 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());
  }
 });
}

Modify the StockServiceImpl Class to Return StockClient[]

Finally, we modify the getStocks method of the StockServiceImpl class so that it translates the server-side Stock classes into client-side StockClient classes before returning the array.

StockServiceImpl.java

import com.google.gwt.sample.stockwatcher.client.StockClient;

We need to change the addStock method slightly so that the generated ID is returned:

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();
}

All other methods stay the same, except 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]);
}

Summary

The code above works perfectly for me when deployed to Google App Engine, but triggers an error in Google Web Toolkit Hosted Mode:

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

Let me know if you encounter the same problem or not. The fact that it works in Google App Engine seems to indicate a bug in Hosted Mode.

Answer 4

Thanks it was really helpful

Answer 5

Yes, it is sure that we need to use the Serialization for getting the server objects to the client. These modile file setting wont work to use the Stock clas in the client side.

In your case you have only one class Stock and you can create a StockClient in client side. It is easy. But what will be the solution if anyone having more classes. Something like the properties of this class are also some other type of classes.

example stock.getEOD(date).getHigh();

getEOD will return another class with the given date and that class has the getHigh method.

What to do in such big cases. I dont think createting all classes implementing serialization in client side is good for that. Then we have to write code in both server and client. all classes two times.

Help me in this scenario.

Answer 6

There's a better answer here: http://stackoverflow.com/questions/2237142/gwt-simple-rpc-use-case-problem-code-included

Basically, you can add parameters to your APPNAME.gwt.xml file so the compiler to give the compiler a path to the server-side class.

Answer 7

I was getting the same issue and the "mvn gwt:compile" output was not very helpful. Instead, when I tried deploying to tomcat (via the maven tomcat plugin: mvn tomcat:deploy) I got helpful error messages.

A few things I had to fix up:

1. Make the object that is sent from the client to the server implement Serializable
2. Add an empty-arg constructor to that same object