Google API 错误：空指针异常（Google API 最近是否被 google 禁止使用？）

Question

我有一个使用 eclipse IDE 3.4 在 Jsf 1.2、Richfaces 3.3、Jdk 1.7、Tomcat Server 7.0.55 和 gson 2.2.1.jar 中开发的 Web 项目。我在 Eclipse 中使用来自 here 的google api 示例来检索结果。我的应用程序在检索结果时运行正常。但是，突然间我开始收到错误。我的代码sn-p如下：-

public class SearchBean  {
.
.
.

public void search() throws IOException {   


    for (int i = 0; i < 20; i = i + 4) 
              {
        String address = "http://ajax.googleapis.com/ajax/services/search/web?v=1.0&start="+i+"&q=";
        String query = "Semantics";
        String charset = "UTF-8";


        URL url = new URL(address + URLEncoder.encode(query, charset));
        Reader reader = new InputStreamReader(url.openStream(), charset);
        GoogleResults results = new Gson().fromJson(reader, GoogleResults.class);
        GoogleList gList = new GoogleList();




        for (int m = 0; m <= 3; m++) 
          {
            try {
                      System.out.println(results.getResponseData().getResults().get(m).getTitle()));     // line no 2076 generating error
                  System.out.println(results.getResponseData().getResults().get(m).getUrl()));

                }
            catch (Exception e)
            {
                e.printStackTrace();

            }

          }


         }



}   


class GoogleResults{

private ResponseData responseData;
public ResponseData getResponseData() { return responseData; }
public void setResponseData(ResponseData responseData) { this.responseData = responseData; }
public String toString() { return "ResponseData[" + responseData + "]"; }

static class ResponseData {
    private List<Result> results;
    public List<Result> getResults() { return results; }
    public void setResults(List<Result> results) { this.results = results; }
    public String toString() { return "Results[" + results + "]"; }
}

static class Result {
    private String url;
    private String title;
    public String getUrl() { return url; }
    public String getTitle() { return title; }
    public void setUrl(String url) { this.url = url; }
    public void setTitle(String title) { this.title = title; }
    public String toString() { return "Result[url:" + url +",title:" + title + "]"; }
}

}

我收到的错误是：-

java.lang.NullPointerException
at org.bean.SearchBean.search(SearchBean.java:2076)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.el.parser.AstValue.invoke(AstValue.java:278)
at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:273)
at org.apache.jasper.el.JspMethodExpression.invoke(JspMethodExpression.java:70)
at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:88)
at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
at javax.faces.component.UICommand.broadcast(UICommand.java:387)
at org.ajax4jsf.component.UIDataAdaptor.broadcast(UIDataAdaptor.java:1364)
at org.ajax4jsf.component.AjaxViewRoot.processEvents(AjaxViewRoot.java:324)
at org.ajax4jsf.component.AjaxViewRoot.broadcastEvents(AjaxViewRoot.java:299)
at org.ajax4jsf.component.AjaxViewRoot.processPhase(AjaxViewRoot.java:256)
at org.ajax4jsf.component.AjaxViewRoot.processDecodes(AjaxViewRoot.java:412)
at com.sun.faces.lifecycle.ApplyRequestValuesPhase.execute(ApplyRequestValuesPhase.java:78)
at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:100)
at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:265)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.ajax4jsf.webapp.BaseXMLFilter.doXmlFilter(BaseXMLFilter.java:178)
at org.ajax4jsf.webapp.BaseFilter.handleRequest(BaseFilter.java:290)
at org.ajax4jsf.webapp.BaseFilter.processUploadsAndHandleRequest(BaseFilter.java:388)
at org.ajax4jsf.webapp.BaseFilter.doFilter(BaseFilter.java:515)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)

突然产生此错误的可能原因是什么。 我是 Java 新手。所以请放弃任何天真。

Answer 1

所以，如果你从这样的一行得到 NullPointerException：

System.out.println(results.getResponseData().getResults().get(m).getTitle()));

您可以看到results、.getResponseData()、.getResults() 或.get(m) 为空。

然后你必须检查其中哪一个是空的，并确保它正在被初始化。