getClassLoader().getResource() returns null

问题

I have this test app:

import java.applet.*;
import java.awt.*;
import java.net.URL;
public class Test extends Applet
{

    public void init()
    {
        URL some=Test.class.getClass().getClassLoader().getResource("/assets/pacman.png");
        System.out.println(some.toString());
        System.out.println(some.getFile());
        System.out.println(some.getPath());

    }
}

When I run it from Eclipse, I get the error:

java.lang.NullPointerException
    at Test.init(Test.java:9)
    at sun.applet.AppletPanel.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Classpath (from .CLASSPATH file)

<classpathentry kind="src" path="src"/>

In my c:\project\src folder, I have only the Test.java file and the 'assets' directory which contains pacman.png.

What am I doing wrong and how to resolve it?

回答1:

I would do it this way:

final InputStream stream;

stream = Test.class.getResourceAsStream("assets/pacman.png");
System.out.println("Stream = " + stream);

"/assets/pacman.png" is an absolute location whle "assets/pacman.png" is a relative location.

回答2:

You don't need the slash at the start when getting a resource from a ClassLoader, because there's no idea of a "relative" part to start with. You only need it when you're getting a resource from a Class where relative paths go from the class's package level.

In addition, you don't want Test.class.getClass() as that gets the class of Test.class, which will be Class<Class>.

In other words, try either of these lines:

URL viaClass=Test.class.getResource("/assets/pacman.png");
URL viaLoader=Test.class.getClassLoader().getResource("assets/pacman.png");

回答3:

Click Upvote,

When you use .getClass().getResource(fileName) it considers the location of the fileName is the same location of the of the calling class.
When you use .getClass().getClassLoader().getResource(fileName) it considers the location of the fileName is the root - in other words bin folder

It hits NullPointerException if the file is actually not exist there.

Source:

package Sound;
public class ResourceTest {
    public static void main(String[] args) {
        String fileName = "Kalimba.mp3";
        System.out.println(fileName);
        System.out.println(new ResourceTest().getClass().getResource(fileName));
        System.out.println(new ResourceTest().getClass().getClassLoader().getResource(fileName));

OutPut ;

Kalimba.mp3
file:/C:/Users/User/Workspaces/MyEclipse%208.5/JMplayer/bin/Sound/Kalimba.mp3
file:/C:/Users/User/Workspaces/MyEclipse%208.5/JMplayer/bin/Kalimba.mp3
    }
}

回答4:

This works for me:

URL viaClass=Test.class.getResource("assets/test.html");

which assets in the same folder with Test.class output file (after a miserable checking and debugging)

来源：https://stackoverflow.com/questions/625687/getclassloader-getresource-returns-null

标签

java

applet