为什么我得到：java.lang.NullPointerException？

Question

I am completely new in Java and I found this script on the web (this is my modified version). I have created a Javabean (so I can use it in my application), but when I test it, I get this error. Besides, I am getting a warning "java uses or overrides a deprecated API" :

Java Plug-in 1.6.0_45
Using JRE version 1.6.0_45-b06 Java HotSpot(TM) Client VM
User home directory = C:\Users\xyz
----------------------------------------------------
c:   clear console window
f:   finalize objects on finalization queue
g:   garbage collect
h:   display this help message
l:   dump classloader list
m:   print memory usage
o:   trigger logging
q:   hide console
r:   reload policy configuration
s:   dump system and deployment properties
t:   dump thread list
v:   dump thread stack
x:   clear classloader cache
0-5: set trace level to <n>
----------------------------------------------------
proxyHost=null
proxyPort=0
connectMode=HTTP, native.
Forms Applet version is : 10.1.2.3
NO INGRESA AL PRINT
java.lang.NullPointerException
 at JavaGetUrl.setProperty(JavaGetUrl.java:53)
 at oracle.forms.handler.UICommon.setFVP(Unknown Source)
 at oracle.forms.handler.UICommon.setFVP(Unknown Source)
 at oracle.forms.handler.UICommon.onUpdate(Unknown Source)
 at oracle.forms.handler.ComponentItem.onUpdate(Unknown Source)
 at oracle.forms.handler.JavaContainer.onUpdate(Unknown Source)
 at oracle.forms.handler.UICommon.onUpdate(Unknown Source)
 at oracle.forms.handler.JavaContainer.onCreate(Unknown Source)
 at oracle.forms.engine.Runform.onCreateHandler(Unknown Source)
 at oracle.forms.engine.Runform.processMessage(Unknown Source)
 at oracle.forms.engine.Runform.processSet(Unknown Source)
 at oracle.forms.engine.Runform.onMessageReal(Unknown Source)
 at oracle.forms.engine.Runform.onMessage(Unknown Source)
 at oracle.forms.engine.Runform.sendInitialMessage(Unknown Source)
 at oracle.forms.engine.Runform.startRunform(Unknown Source)
 at oracle.forms.engine.Main.createRunform(Unknown Source)
 at oracle.forms.engine.Main.start(Unknown Source)
 at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
 at java.lang.Thread.run(Unknown Source)

What I want to do with this code is:

• Capture (to a variable) the text contained in the URL (I understand that this is actually what it does)
• If the link I pass, generates a HTML or PDF file, then create it or save it in the local machine (any suggestion how to achieve this or any other JAVA that get it done will be really appreciate it)

Here is my actual java source file:

import java.io.*;
import java.net.*;
import oracle.forms.properties.*;
import oracle.forms.ui.*;

public class JavaGetUrl  extends VTextArea {
private static final ID GEN = ID.registerProperty("cre");


      //-----------------------------------------------------//
      //  Step 1:  Start creating a few objects we'll need.
      //-----------------------------------------------------//

      URL u;
      InputStream is = null;
      DataInputStream dis;
      String s;


    public boolean setProperty(ID id, Object value) {
                boolean retorno = true;
      try {
          System.out.println("INGRESA AL TRY");  
       if (id == GEN) {

           System.out.println("INGRESA AL IF");  

         //------------------------------------------------------------//
         // Step 2:  Create the URL.                                   //
         //------------------------------------------------------------//
         // Note: Put your real URL here, or better yet, read it as a  //
         // command-line arg, or read it from a file.                  //
         //------------------------------------------------------------//

         u = new URL("http://200.210.220.1:8080/index.shtml");

        //----------------------------------------------//
         // Step 3:  Open an input stream from the url.  //
         //----------------------------------------------//

         is = u.openStream();         // throws an IOException

         //-------------------------------------------------------------//
         // Step 4:                                                     //
         //-------------------------------------------------------------//
         // Convert the InputStream to a buffered DataInputStream.      //
         // Buffering the stream makes the reading faster; the          //
         // readLine() method of the DataInputStream makes the reading  //
         // easier.                                                     //
         //-------------------------------------------------------------//

                                                    //
         dis = new DataInputStream(new BufferedInputStream(is));

         //------------------------------------------------------------//
         // Step 5:                                                    //
         //------------------------------------------------------------//
         // Now just read each record of the input stream, and print   //
         // it out.  Note that it's assumed that this problem is run   //
         // from a command-line, not from an application or applet.    //
         //------------------------------------------------------------//


                                                   //
          while ((s = dis.readLine()) != null) {
            System.out.println(s);
         }
       }
       else {
                System.out.println("NO INGRESA AL PRINT");  
                return false;
            }
      } catch (MalformedURLException mue) {

         System.out.println("Ouch - a MalformedURLException happened.");
         mue.printStackTrace();
         System.exit(1);

      } catch (IOException ioe) {
         System.out.println("Oops- an IOException happened.");
         ioe.printStackTrace();
         System.exit(1);

      } 

      finally {


         //---------------------------------//
         // Step 6:  Close the InputStream  //
         //---------------------------------//

         try {
            is.close();
         } catch (IOException ioe) {
            // just going to ignore this one
         }

      } // end of 'finally' clause
     return retorno;
    }
} // end of class definition

Answer 1

如果在该尝试的中间发生异常， is null，但是最终将被调用并尝试关闭它

Answer 2

Seems some variable is null at line# 53 If I am not wrong line#53 is following:

dis = new DataInputStream(new BufferedInputStream(is));

check if is (InputStream) is initialized before using it or BufferedInputStream is created successfully.