getClass() getResource 返回 null

Question

When I try and load a image from a resource directory in eclipse I keep getting a null pointer exception (NPE). The res folder is in the project directory. This where I get the NPE:

image.setIcon(new ImageIcon(new ImageIcon(this.getClass().getResource("res/bg.jpg")).getImage().getScaledInstance(image.getWidth(), image.getHeight(), Image.SCALE_SMOOTH)));

When I remove the getClass().getReource() the image is returned:

image.setIcon(new ImageIcon(new ImageIcon(("res/bg1.jpg")).getImage().getScaledInstance(image.getWidth(), image.getHeight(), Image.SCALE_SMOOTH)));

When I print the URL for the res directory I get null:

URL resource = this.getClass().getResource("/");
        resource.getFile(); // print me somehwere

        URL resource1 = this.getClass().getResource("/res");
        resource.getFile(); // print me as well

        URL resource2 = this.getClass().getResource("res/bg2");
        resource.getFile(); // print me as well

        System.out.println("Reource1 : " + resource);
        System.out.println("Reource1 : " + resource1);
        System.out.println("Reource1 : " + resource2);

Output:

Reource1 :file:/C:/Users/cmooney/eclipse-workspace/TextSimplifier/bin/
Reource1 : null
Reource1 : null

I have refreshed, cleaned, and built the project several times.

Directory:

Any idea why this is happening?

Thanks

Answer 1

From the official java doc: https://docs.oracle.com/javase/8/docs/api/java/lang/Class.html#getResource-java.lang.String-

Finds a resource with a given name. The rules for searching resources associated with a given class are implemented by the defining class loader of the class. This method delegates to this object's class loader. If this object was loaded by the bootstrap class loader, the method delegates to ClassLoader.getSystemResource(java.lang.String).
Before delegation, an absolute resource name is constructed from the given resource name using this algorithm:

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

• 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 '.' ('\.').

Answer 2

Replace this.getClass().getResource("res/bg.jpg") with this.getClass().getResource("../res/bg.jpg")

Since this.getClass().getResource("/") returns file:/C:/Users/cmooney/eclipse-workspace/TextSimplifier/bin/ , you need to go one level(directory) up in the directory structure so that you can enter the res directory. It's like cd ../res from the current location of file:/C:/Users/cmooney/eclipse-workspace/TextSimplifier/bin/

Note : I can't see bg.jpg in the screenshot that you have attached. Make sure, you have bg.jpg in this path.

Answer 3

I think, you got the directory structure wrong. If '/' points to the bin directory, as shown by the output of your first resource, '/res' points to the res subdirectory of bin and not to the sibling in the parent directory of bin.

You need to either move the res directory or change the way the resolution of '/' works.

The rules for searching resources associated with a given class are implemented by the defining class loader of the class.

from Java documentation

For maven based projects the resource directory is usually src/main/resources.