相关疑难解决方法(0)

有时,当我运行我的应用程序时,它会给我一个错误,如下所示:

Exception in thread "main" java.lang.NullPointerException
        at com.example.myproject.Book.getTitle(Book.java:16)
        at com.example.myproject.Author.getBookTitles(Author.java:25)
        at com.example.myproject.Bootstrap.main(Bootstrap.java:14)

人们将此称为"堆栈跟踪".什么是堆栈跟踪？有什么能告诉我程序中发生的错误？

关于这个问题 - 我经常看到一个问题,一个新手程序员"得到一个错误",他们只是粘贴他们的堆栈跟踪和一些随机的代码块,而不了解堆栈跟踪是什么或如何使用它.这个问题旨在作为新手程序员的参考,他们可能需要帮助来理解堆栈跟踪的价值.

我已成功使用SimpleBroker 从这里运行示例.

我的配置设置如下:

@Override
public void configureMessageBroker(MessageBrokerRegistry registry) {
    registry.enableSimpleBroker("/topic");
    //registry.enableStompBrokerRelay("/queue/","/topic/").setRelayHost("localhost").setRelayPort(61614);
    registry.setApplicationDestinationPrefixes("/app");
}

当我注释掉SimpleBroker并尝试使用StompBroker时,我收到以下错误.

ERROR: org.springframework.web.servlet.DispatcherServlet - Context initialization failed
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'stompBrokerRelayMessageHandler' defined in class path resource   [org/springframework/web/socket/config/annotation/DelegatingWebSocketMessageBrokerConfigurat    ion.class]: Instantiation of bean failed; nested exception is org.springframework.beans.factory.BeanDefinitionStoreException: Factory method [public org.springframework.messaging.simp.broker.AbstractBrokerMessageHandler org.springframework.messaging.simp.config.AbstractMessageBrokerConfiguration.stompBrokerRelayMessageHandler()] threw exception; nested exception is java.lang.NoClassDefFoundError: reactor/tcp/encoding/Codec
at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:592)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1094)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:989)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:504)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:475)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:304)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:300)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:195)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:700)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:760)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:482) …

我一直试图让本教程工作: 链接 我正在使用Apache Tomcat 7.0和Jersey 2.0库.这是我的服务:

package org.arpit.javapostsforlearning.webservice;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

@Path("ConversionService")  
public class FeetToInchAndInchToFeetConversionService {  
 @GET  
 @Path("/InchToFeet/{i}")  
  @Produces(MediaType.TEXT_XML)  
  public String convertInchToFeet(@PathParam("i") int i) {  

    int inch=i;  
    double feet = 0;  
    feet =(double) inch/12;  

    return "<InchToFeetService>"  
    + "<Inch>" + inch + "</Inch>"  
      + "<Feet>" + feet + "</Feet>"  
     + "</InchToFeetService>";  
  }  

  @Path("/FeetToInch/{f}")  
  @GET  
  @Produces(MediaType.TEXT_XML)  
  public String convertFeetToInch(@PathParam("f") int f) {  
   int inch=0;  
      int feet = f;  
      inch = 12*feet;  

      return "<FeetToInchService>"  
        + "<Feet>" …

我已经初始化了板,我正在尝试打印它,但它总是给我空指针异常.这是我的Checkers.java类:

 package specialCheckers;
/*There is another class: 
    Checkers which contains a Cell[][] board and a String message. 
    Checkers should have mutators and accessors for these.*/
public class Checkers {
    Cell[][] board;
    String message;
    /**********Setters and getters Started*****/
    public void setBoard(Cell[][] board){
        this.board = board;
    }
    public Cell[][] getBoard(){
        return board;
    }
    public void setMessage(String message){
        this.message = message;
    }
    public String getMessage(){
        return message;
    }
    /**********Setters and getters Ended*****/

    /*Checkers should also have a public void init() method that sets …

我想将json数据转换为字符串

import java.io.BufferedReader;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import org.json.JSONArray;
import org.json.JSONObject;
public static void main(String[] args) throws Exception
{

    URL url = new URL("http://192.168.1.13/test/ProductWb.php?productId=9");
    HttpURLConnection conn ;
    conn = (HttpURLConnection) url.openConnection();
    conn.setRequestMethod("GET");
    conn.setReadTimeout(60);
    conn.setRequestProperty("Accept", "application/json");
    String json="";

    json = readUrl(conn);
           System.out.println(json);
           JSONObject jsonObject=new JSONObject(json);
           JSONArray jarray=jsonObject.getJSONArray("modeles");
           JSONObject  modele= jarray.getJSONObject("modele");
           for (int i=0;i<modele.length();i++) {
               System.out.println(modele(i).getString("id_product"));
                System.out.println(modele(i).getString("meta_title"));
                System.out.println("*********");
              }


}

它告诉我json数据,但给我这个错误:

{"modeles":[{"modele":{"id_product":"9","id_shop":"1","id_lang":"4","description":null,"description_short":"<pre>Peugeot 208<\/pre>","info_prix":"","info_1":null,"info_2":null,"info_3":null,"info_4":null,"info_5":null,"link_rewrite":"208","meta_description":"Peugeot 208","meta_keywords":"peugeot 208","meta_title":"Peugeot 208","name":"208","available_now":"","available_later":""}}]}
Exception in thread "main" java.lang.IllegalStateException: This is not a JSON Array.
at …