class.getResource在Eclipse中与在jar中不一致的行为

Question

I'm testing how to express paths to resource files. I get different results between Eclipse and command line with a jar.

Could you help me understand why there are differences?

I've two resource files:

• one is in the class package, at the same level than the class file.
• one is outside the class package, at the same level than the package.

Eclipse. This does work:

package com.example.mypackage;

/*    FileIO (project)
 *      |
 *      +---- src (source folder)
 *             |
 *             +---- com.example.mypackage (package)
 *             |              |
 *             |              +---- TestRes.java (class)
 *             |              |
 *             |              +---- users (resource file 1)
 *             |
 *             +---- test.txt (resource file 2)
 */

public class TestRes {
    public static void main(String[] args) {
        // Some resource names (relative, absolute)
        String[][] res = {
            { "users"            , "/com/example/mypackage/users" },
            { "../../../test.txt", "/test.txt" },
            { "."                , "/com/example/mypackage/" }
        };

        // Get resource URL
        String f = "Relative: %s\nAbsolute: %s\nURL: %s (%s)\n\n";
        for (String[] r : res) {
            String n0 = r[0];
            String n1 = r[1];
            try {
                String url0 = TestRes.class.getResource(n0).getPath();
                String url1 = TestRes.class.getResource(n1).getPath();
                System.out.printf(f, n0, n1, url0, (url0.equals(url1)));
            }
            catch (Exception e) { e.printStackTrace(); }
        }
    }
}

Output:

Relative: users
Absolute: /com/example/mypackage/users
URL: /C:/Users/User/workspace/FileIO/bin/com/example/mypackage/users (true)

Relative: ../../../test.txt
Absolute: /test.txt
URL: /C:/Users/User/workspace/FileIO/bin/test.txt (true)

Relative: .
Absolute: /com/example/mypackage/
URL: /C:/Users/User/workspace/FileIO/bin/com/example/mypackage/ (true)

But when I export the project to a jar (right-click on FileIO, then export ), and run the jar with java.exe at the command line, then class.getResource() returns null when trying to processing the second case ( "../../../test.txt" ) and third case ( "." ).

C:\Users\User\Desktop>java -jar TestRes.jar
Relative: users
Absolute: /com/example/mypackage/users
URL: file:/C:/Users/User/Desktop/TestRes.jar!/com/example/mypackage/users (true)

java.lang.NullPointerException
        at com.example.mypackage.TestRes.main(TestRes.java:31)
java.lang.NullPointerException
        at com.example.mypackage.TestRes.main(TestRes.java:31)

I checked the structure of the jar:

• test.txt is at the root of the jar
• there are 3 additional files at the root (classpath, project, and manifest)
• TestRes.class and users are in com\\example\\mypackage folder

Answer 1

When running from Eclipse, the IDE is simulating the runtime class loader behavior by mapping to basic file system operations over src and bin folders. The simulated behavior may not be an exact match for the real runtime behavior in all the corner cases. In this particular case, it looks like the simulator is not preventing the use of paths not supported at runtime.