如何使用getClass（）。getResource（）方法

Question

When I create ImageIcon class objects I use the following code:

iconX = new ImageIcon (getClass().getResource("imageX.png"))

---

The above code works correctly either in an applet or a desktop app when the .png is in the same folder of the class.

The question is: how to avoid a NullPointerException when the .Png is in another folder? Or how load the image in the object ImageIcon when it is in a different location to the class?

I don't understand how this method works, if anyone can help me I appreciate it. Thanks!!

Answer 1

Take a look at this - Class#getResource(java.lang.String)

Please click the link above and read the docs and follow to understand what's going on.

It says -

If the name begins with a '/', then the absolute name of the resource is the portion of the name following the '/'.

and

Otherwise, the absolute name is of the following form:

  modified_package_name/name 

Where the modified_package_name is the package name of this object with '/' substituted for '.'.

So, if this object (where you call getResource ) is in package /pkg1 ( / meaning pkg1 is right under the root of the classpath) and you used "imageX.png" then the result would be pkg1/imageX.png which is correct because that's where the image is located at.

But, if we moved the resource (imageX.png) to some other package /pkg2 and you called the method same way then the result would still be pkg1/imageX.png but this time it would be incorrect because the resource is actually located in /pkg2 . That's when you end up with NPE.

It's good to explicitly specify the full path of the resource starting from the root of the classpath. (eg "/pkg/imageX.png").

Hope this helps.

Answer 2

Simply supply the path to the resource.

So, if you put the image in "/resources/images" within your Jar, you would simply use

iconX = new ImageIcon(getClass().getResource("/resources/images/imageX.png"))

Essentially what you're saying is, class loader, please search your class path for the following resource.

Answer 3

If the image is internal (you want a location relative to your project, or perhaps packaged into your jar), do what mad programmer said:

iconX = new ImageIcon(getClass().getResource("/path/imageX.png"))

The path is relative, so path/ will be a folder in the same folder as your project (or packaged into your jar).

If you want an external image, simply hand ImageIcon constructor the path (ex. "C:/.../file.png"). This isn't recommended though, as it's better to use it as a resource.

For more info on the ImageIcon constructor, see here . for more info on loading class resources, see here (Javadoc links)