Total Noob to gwt/gson in eclipse, error: "Unable to invoke no-args constructor for class"

Question

I'm playing around with this Google Web toolkit/GSON example that gets data from twitter.

Everything is compiling just fine, but when I debug as Web Application, everything loads just fine, but when I click the "search" button I get the following error:

>SEVERE: javax.servlet.ServletContext log: Exception while dispatching incoming RPC call
com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract java.util.List com.google.gwt.twittersearch.client.TwitterService.searchTweets(java.lang.String) throws java.io.IOException,java.lang.IllegalArgumentException' threw an unexpected exception: java.lang.RuntimeException: Unable to invoke no-args constructor for class com.google.gwt.twittersearch.server.TwitterServiceImpl$SearchResponse. Register an InstanceCreator with Gson for this type may fix this problem.
    at com.google.gwt.user.server.rpc.RPC.encodeResponseForFailure(RPC.java:385)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:588)
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:208)
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processPost(RemoteServiceServlet.java:248)
    at com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:62)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
    at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:35)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:60)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.tools.development.BackendServersFilter.doFilter(BackendServersFilter.java:97)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
    at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:94)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:370)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:326)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
    at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)
Caused by: java.lang.RuntimeException: Unable to invoke no-args constructor for class com.google.gwt.twittersearch.server.TwitterServiceImpl$SearchResponse. Register an InstanceCreator with Gson for this type may fix this problem.
    at com.google.gson.internal.ConstructorConstructor$8.construct(ConstructorConstructor.java:167)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:162)
    at com.google.gson.Gson.fromJson(Gson.java:795)
    at com.google.gson.Gson.fromJson(Gson.java:734)
    at com.google.gwt.twittersearch.server.TwitterServiceImpl.parseSearchResponse(TwitterServiceImpl.java:80)
    at com.google.gwt.twittersearch.server.TwitterServiceImpl.searchTweets(TwitterServiceImpl.java:35)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.google.appengine.tools.development.agent.runtime.Runtime.invoke(Runtime.java:115)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:569)
    ... 34 more
Caused by: java.lang.UnsupportedOperationException: Cannot allocate class com.google.gwt.twittersearch.server.TwitterServiceImpl$SearchResponse
    at com.google.gson.internal.UnsafeAllocator$4.newInstance(UnsafeAllocator.java:100)
    at com.google.gson.internal.ConstructorConstructor$8.construct(ConstructorConstructor.java:164)
    ... 45 more

Here is the TwitterServiceImpl code:

package com.google.gwt.twittersearch.server;

import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.util.List;

import com.google.gwt.twittersearch.client.Tweet;
import com.google.gwt.twittersearch.client.TwitterService;
import com.google.gson.Gson;
import com.google.gwt.user.server.rpc.RemoteServiceServlet;

@SuppressWarnings("serial") 
public class TwitterServiceImpl extends RemoteServiceServlet implements
        TwitterService {

    @Override
    public List<Tweet> searchTweets(String query) throws IllegalArgumentException, IOException {
          query = query.trim();
          if (query.isEmpty()) {
            throw new IllegalArgumentException("No search query specified.");
          }
         
          // see: https://dev.twitter.com/docs/api/1/get/search
          String q = URLEncoder.encode(query, "UTF-8");
          URL url = new URL("http://search.twitter.com/search.json?q=" + q);
          HttpURLConnection connection = (HttpURLConnection) url.openConnection();
          InputStream response = null;
          try {
            response = connection.getInputStream();
            return parseSearchResponse(response);
          } finally {
            if (response != null) {
              response.close();
            }
          }
    }
    
    @Override
    public String getPrivacyPolicy() throws IOException {
        // see: https://dev.twitter.com/docs/api/1/get/legal/privacy
        URL url = new URL("https://api.twitter.com/1/legal/privacy.json");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        InputStream response = null;
        try {
            response = connection.getInputStream();
            return parsePolicyResponse(response);
        } finally {
            if (response != null) {
                response.close();
            }
        }
    }

    /**
     * Parses the privacy policy response returned from the Twitter API.
     * @param response the response
     * @return the privacy policy
     * @throws IOException if there was a problem reading the response
     */
    private String parsePolicyResponse(InputStream response) throws IOException {
        Reader reader = new InputStreamReader(response);
        PrivacyPolicyResponse privacyPolicyResponse = new Gson().fromJson(reader, PrivacyPolicyResponse.class);
        return privacyPolicyResponse.privacy;
    }

    /**
     * Parses the search response returned from the Twitter API.
     * @param response the response
     * @return the search results
     * @throws IOException if there was a problem reading the response
     */

    private List<Tweet> parseSearchResponse(InputStream response) throws IOException {
          Reader reader = new InputStreamReader(response);
          SearchResponse searchResponse = new Gson().fromJson(reader, SearchResponse.class);
          return searchResponse.results;
        }
    
    private class PrivacyPolicyResponse {
    public String privacy;
    }

     
    private class SearchResponse {
      public List<Tweet> results;
    }
}

Here is the entry point code:

package com.google.gwt.twittersearch.client;

import java.util.List;
import com.google.gwt.safehtml.shared.SafeHtmlBuilder;
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.user.client.rpc.AsyncCallback;
import com.google.gwt.user.client.ui.Button;
import com.google.gwt.user.client.ui.HTML;
import com.google.gwt.user.client.ui.HorizontalPanel;
import com.google.gwt.user.client.ui.Image;
import com.google.gwt.user.client.ui.Label;
import com.google.gwt.user.client.ui.Panel;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.user.client.ui.TextBox;
import com.google.gwt.user.client.ui.VerticalPanel;

/**
 * Entry point classes define <code>onModuleLoad()</code>.
 */
public class TwitterSearch implements EntryPoint {
  private Button privacyPolicyButton;
  private Button searchButton;
  private TextBox searchQueryTextBox;
  private Panel resultsPanel;
  private Label errorLabel;
  private Image loadingImage;
  private final TwitterServiceAsync service = GWT.create(TwitterService.class);

/**
 * This is the entry point method.
 */
public void onModuleLoad() {
    createWidgets();
    layoutWidgets();
    
}

private void createWidgets() {
    searchQueryTextBox = new TextBox();

    searchButton = new Button("Search");
    searchButton.addClickHandler(new ClickHandler() {
        @Override
        public void onClick(ClickEvent event) {
            setLoading(true);
            String query = searchQueryTextBox.getText();
            service.searchTweets(query, new AsyncCallback<List<Tweet>>() {
                @Override
                public void onFailure(Throwable caught) {
                    errorLabel.setText(caught.getMessage());
                    errorLabel.setVisible(true);
                    setLoading(false);
                }

                @Override
                public void onSuccess(List<Tweet> result) {
                    resultsPanel.clear();
                    for (Tweet tweet : result) {
                        SafeHtmlBuilder builder = new SafeHtmlBuilder();
                        builder.appendHtmlConstant("<b>User: </b>");
                        builder.appendEscaped(tweet.getFrom_user());
                        builder.appendHtmlConstant("<br /><b>Created: </b>");
                        builder.appendEscaped(tweet.getCreated_at());
                        builder.appendHtmlConstant("<br /><b>Tweet: </b>");
                        builder.appendEscaped(tweet.getText());
                        builder.appendHtmlConstant("<br /><br />");
                        resultsPanel.add(new HTML(builder.toSafeHtml()));
                    }
                    setLoading(false);
                }
            });
        }
    });

    privacyPolicyButton = new Button("Privacy Policy");
    privacyPolicyButton.addClickHandler(new ClickHandler() {
        @Override
        public void onClick(ClickEvent event) {
            setLoading(true);
            service.getPrivacyPolicy(new AsyncCallback<String>() {
                @Override
                public void onFailure(Throwable caught) {
                    errorLabel.setText(caught.getMessage());
                    errorLabel.setVisible(true);
                    setLoading(false);
                }

                @Override
                public void onSuccess(String result) {
                    resultsPanel.clear();

                    // convert newlines to <br />
                    SafeHtmlBuilder builder = new SafeHtmlBuilder();
                    builder.appendEscapedLines(result);

                    resultsPanel.add(new HTML(builder.toSafeHtml()));

                    setLoading(false);
                }
            });
        }
    });

    resultsPanel = new VerticalPanel();

    errorLabel = new Label();
    errorLabel.addStyleName("errorLabel");
    errorLabel.setVisible(false);

    //image from http://loadinfo.net/
    loadingImage = new Image("loading.gif");
    loadingImage.setVisible(false);
}

private void layoutWidgets() {
    Panel panel = new VerticalPanel();

    panel.add(errorLabel);

    Panel horizPanel = new HorizontalPanel();
    horizPanel.add(searchQueryTextBox);
    horizPanel.add(searchButton);
    horizPanel.add(privacyPolicyButton);
    horizPanel.add(loadingImage);
    panel.add(horizPanel);

    panel.add(resultsPanel);

    RootPanel.get().add(panel);
}

/**
 * Updates the UI for when a RPC call is made.
 * @param loading true if an RPC call is being sent, false if not
 */
private void setLoading(boolean loading) {
    if (loading) {
        errorLabel.setVisible(false);
    }
    searchQueryTextBox.setEnabled(!loading);
    searchButton.setEnabled(!loading);
    privacyPolicyButton.setEnabled(!loading);
    loadingImage.setVisible(loading);
}
}

Here is the tweet code:

package com.google.gwt.twittersearch.client;

import java.io.Serializable;

@SuppressWarnings("serial")
public class Tweet implements Serializable{
private String id;
  private String from_user;
  private String created_at;
  private String text;
 
  public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getFrom_user() {
        return from_user;
    }

    public void setFrom_user(String from_user) {
        this.from_user = from_user;
    }

    public String getCreated_at() {
        return created_at;
    }

    public void setCreated_at(String created_at) {
        this.created_at = created_at;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }


}

Any insight into this error or where I might go from here would be greatly appreciated!

Answer 1

You need to add a no-args constructor such as:

public class Tweet implements Serializable{

  public Tweet(){
  }

}

EDIT

You will also need one here too

private class SearchResponse {

  public SearchResponse(){
  }
  public List<Tweet> results;
}

to deal with your error java.lang.RuntimeException: Unable to invoke no-args constructor for class

A user defined class is serializable if:

1. the class is assignable to IsSerializable or java.io.Serializable, either because it implements one of these interfaces, or because it is derived from a superclass that implements one of these interfaces.

2. all the class’s non-final, non-transient instance fields are serializable

3. the class has a public default (zero argument) constructor

above is from this tutorial