Cannot read an image in jar file

问题:

when i build a jar file and run it ,it shows a null pointer exception due to imageicon not found

new ImageIcon(getClass().getClassLoader().getResource("icons/exit.png")));

this is the error what i get when i run the jar file

Exception in thread "main" java.lang.NullPointerException
        at javax.swing.ImageIcon.<init>(Unknown Source)
        at mediaplayer.MediaPlayer.buildtoolbar(MediaPlayer.java:130)
        at mediaplayer.MediaPlayer.<init>(MediaPlayer.java:81)
        at mediaplayer.MediaPlayer.main(MediaPlayer.java:464)

But when i run the project in NetBeans, its working good

This is the output when i list all the files inside my jar

META-INF/
META-INF/MANIFEST.MF
helliker/
helliker/id3/
icons/
mediaplayer/
Thumbs.db
exit.png
ff1.png
helliker/id3/BinaryParser.class
helliker/id3/CorruptHeaderException.class
helliker/id3/ID3Exception.class
helliker/id3/ID3FieldDataException.class
helliker/id3/ID3Tag.class
helliker/id3/ID3v1Tag.class
helliker/id3/ID3v2ExtendedHeader.class
helliker/id3/ID3v2Footer.class
helliker/id3/ID3v2FormatException.class
helliker/id3/ID3v2Frame.class
helliker/id3/ID3v2Frames.class
helliker/id3/ID3v2Header.class
helliker/id3/ID3v2Tag.class
helliker/id3/MP3Comparator.class
helliker/id3/MP3File.class
helliker/id3/MP3FileFilter.class
helliker/id3/MPEGAudioFrameHeader.class
helliker/id3/NoMPEGFramesException.class
helliker/id3/NullsoftID3GenreTable.class
helliker/id3/Playlist.class
helliker/id3/PlaylistException.class
helliker/id3/XingVBRHeader.class
icons/Thumbs.db
icons/exit.png
icons/ff1.png
icons/label.jpg
icons/openpl.gif
icons/pause1.png
icons/play1.png
icons/playlist.png
icons/rr.png
icons/rr1.PNG
icons/stop.png
label.jpg
mediaplayer/MediaPlayer$1.class
mediaplayer/MediaPlayer$PlaylistFilter.class
mediaplayer/MediaPlayer.class
mediaplayer/PlaylistManager$1.class
mediaplayer/PlaylistManager$MP3Filter.class
mediaplayer/PlaylistManager$PlaylistFilter.class
mediaplayer/PlaylistManager.class
mediaplayer/Settings.class
mediaplayer/TagEditor.class
mediaplayer/Thumbs.db
openpl.gif
pause1.png
play1.png
playlist.png
rr.png
rr1.PNG

回答1:

There is missing some information in the question how the jar file is actually built, but with the following directory layout

├── bin
│   ├── com
│   │   └── example
│   │       └── ImageIconTest.class
│   └── icons
│       └── exit.png
└── src
    ├── MANIFEST.MF
    └── com
        └── example
            └── ImageIconTest.java

and the following code in ImageIconTest.java

package com.example;
import javax.swing.ImageIcon;

public class ImageIconTest {
   public void run() {
      ImageIcon ii = new ImageIcon(getClass().getClassLoader().getResource("icons/exit.png"));
      System.out.println(ii);
   }
   public static void main(String[] args) {
      ImageIconTest app = new ImageIconTest();
      app.run();
   }
}

you can properly run the sample from the file system with

$ java -classpath bin com.example.ImageIconTest

Using a MANIFEST.MF file with the following content:

Manifest-Version: 1.0
Main-Class: com.example.ImageIconTest

you can package it into an executable jar file and run it from the jar file:

$ jar cvfm app.jar src/MANIFEST.MF -C bin .
$ java -jar app.jar

Both approaches are working fine, the important detail is to make sure that the icon directory is included in the jar file at the proper location.

When listing the jar file contents, it should look like this:

  0 Tue Nov 06 12:27:56 CET 2012 META-INF/
107 Tue Nov 06 12:27:56 CET 2012 META-INF/MANIFEST.MF
  0 Tue Nov 06 12:27:56 CET 2012 com/
  0 Tue Nov 06 12:27:56 CET 2012 com/example/
950 Tue Nov 06 12:27:56 CET 2012 com/example/ImageIconTest.class
  0 Tue Nov 06 12:00:36 CET 2012 icons/
873 Tue Nov 06 12:00:36 CET 2012 icons/exit.png

Note the location of the icons directory.

回答2:

The Exception is occurring when MediaPlayer in package mediaplayer calls for an embedded resource at "icons/exit.png". This would resolve to a path of:

mediaplayer/icons/exit.png

I am guessing that is not the path, which is actually.

icons/exit.png

---

That is why the String needs to be "/icons/exit.png" - note the / prefix.

The / that precedes the String informs the class-loader that we mean it to search for the resource from the root of the class path, as opposed to the package of the class from which it is called.

回答3:

Thanx a lot everyone. I figured out the answer

button = new JButton(new ImageIcon(getClass().getResource("/icons/playlist.png")));

i removed the ClassLoader() and it worked,but i just dont know why. Can somebody please explain me the theory behind this.?

回答4:

This code works well:

ClassLoader cl = getClass().getClassLoader();
InputStream stream = cl.getResourceAsStream( "hpms/study/Starbucks.jpg" );
if( stream == null ) System.err.println( "resource not found" );
BufferedImage image = ImageIO.read( stream );
ImageIcon icon = new ImageIcon( image );