this.getClass().getClassLoader().getResource("...") and NullPointerException

Question

I have created a minimal maven project with a single child module in eclipse helios.

In the src/test/resources folder I have put a single file "install.xml". In the folder src/test/java I have created a single package with a single class that does:

  @Test
  public void doit() throws Exception {
    URL url = this.getClass().getClassLoader().getResource("install.xml");
    System.out.println(url.getPath());

  }

but when I run the code as a junit 4 unit test I just get a NullPointerException. This has worked fine a million of times before. Any ideas?

I have followed this guide:

http://www.fuyun.org/2009/11/how-to-read-input-files-in-maven-junit/

but still get the same error.

score 104 · Accepted Answer · edited Nov 22 '16 at 09:24

104

When you use

this.getClass().getResource("myFile.ext")

getResource will try to find the resource relative to the package. If you use:

this.getClass().getResource("/myFile.ext")

getResource will treat it as an absolute path and simply call the classloader like you would have if you'd done.

this.getClass().getClassLoader().getResource("myFile.ext")

The reason you can't use a leading / in the ClassLoader path is because all ClassLoader paths are absolute and so / is not a valid first character in the path.

edited Nov 22 '16 at 09:24

Behrang

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answered Aug 17 '11 at 19:44

AmishDave

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It turned out that the parent pom my team uses only includes *.xml and *.properties. Now...why this ran on my windows workstation I am unsure. I even looked in my local maven repo to verify that it had the same pom as what was out on the linux box. – AmishDave Aug 17 '11 at 21:20
2

Hi @AmishDave, your follow-up question (if you still need an answer) should be posted as a new question. Feel free to link back to this question and/or this answer, but having a follow-up question in an answer is not conventionally how Stack Overflow works. Thanks! :-) – Platinum Azure Jun 26 '12 at 21:28

score 39 · Answer 2 · answered Nov 29 '11 at 00:41

tul,

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.

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

What should I do to have Maven resources from resources folder in bin? I see that even when I run Maven install, they don't appear there. But probably I understand something just wrong :) — Line, Aug 02 '17 at 13:52

score 21 · Answer 3 · answered Sep 27 '10 at 11:58

21

It should be getResource("/install.xml");

The resource names are relative to where the getClass() class resides, e.g. if your test is org/example/foo/MyTest.class then getResource("install.xml") will look in org/example/foo/install.xml.

If your install.xml is in src/test/resources, it's in the root of the classpath, hence you need to prepend the resource name with /.

Also, if it works only sometimes, then it might be because Eclipse has cleaned the output directory (e.g. target/test-classes) and the resource is simply missing from the runtime classpath. Verify that using the Navigator view of Eclipse instead of the Package explorer. If the files is missing, run the mvn package goal.

answered Sep 27 '10 at 11:58

mhaller

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I have also tried: this.getClass().getClassLoader().getResource("/install.xml"); does not help (All my other project that reads resources do not use the "/" prefix and they work fine). I have also tried to run mvn clean install (both from commandline and from eclipse) on the parent and child but still the same problem. When I check the child folder: target/test-classes in the navigator view its empty - guess this is bad. – u123 Sep 27 '10 at 12:15
yes, that's bad. Post the part of your pom where you specified (if any) the tags. Perhaps you're doing some or in there? mvn install should definitely copy your resource files to the classes resp. test-classes folder. Are you using M2Eclipse? – mhaller Sep 27 '10 at 12:46
I have not touched any include/exclude statements. I found this: https://issues.sonatype.org/browse/MNGECLIPSE-823 which might deal with this issue. I have created a new project and it works again. – u123 Sep 27 '10 at 12:50
running mvn package worked for me. Thanks a lot. – NickyPatel Jun 02 '21 at 14:08

score 6 · Answer 4 · answered Oct 30 '13 at 06:41

When eclipse runs the test case it will look for the file in target/classes not src/test/resources. When the resource is saved eclipse should copy it from src/test/resources to target/classes if it has changed but if for some reason this has not happened then you will get this error. Check that the file exists in target/classes to see if this is the problem.

score 5 · Answer 5 · answered May 22 '12 at 14:47

I had the same issue with the following conditions:

The resource files are in the same package as the java source files, in the java source folder (src/test/java).
I build the project with maven on the command line and the build failed on the tests with the NullPointerException.
The command line build did not copy the resource files to the test-classes folder, which explained the build failure.
When going to eclipse after the command line build and rerun the tests in eclipse I also got the NullPointerException in eclipse.
When I cleaned the project (deleted the content of the target folder) and rebuild the project in Eclipse the test did run correctly. This explains why it runs when you start with a clean project.

I fixed this by placing the resource files in the resources folder in test: src/test/resources using the same package structure as the source class.

BTW I used getClass().getResource(...)

score 1 · Answer 6 · answered May 16 '13 at 12:24

I think I did encounter the same issue as yours. I created a simple mvn project and used "mvn eclipse:eclipse" to setup a eclipse project.

For example, my source file "Router.java" locates in "java/main/org/jhoh/mvc". And Router.java wants to read file "routes" which locates in "java/main/org/jhoh/mvc/resources"

I run "Router.java" in eclipse, and eclipse's console got NullPointerExeption. I set pom.xml with this setting to make all *.class java bytecode files locate in build directory.

<build>
    <defaultGoal>package</defaultGoal>
    <directory>${basedir}/build</directory>
<build>

I went to directory "build/classes/org/jhoh/mvc/resources", and there is no "routes". Eclipse DID NOT copy "routes" to "build/classes/org/jhoh/mvc/resources"

I think you can copy your "install.xml" to your *.class bytecode directory, NOT in your source code directory.

score 0 · Answer 7 · answered Jun 18 '12 at 05:45

I had the same issue working on a project with Maven. Here how I fixed it: I just put the sources (images, musics and other stuffs) in the resources directory:

src/main/resources

I created the same structure for the packages in the resources directory too. For example:

If my class is on

com.package1.main

In the resources directory I put one package with the same name

com.package1.main

So I use

getClass().getResource("resource.png");

score 0 · Answer 8 · answered Feb 02 '15 at 21:27

One other thing to look at that solved it for me :

In an Eclipse / Maven project, I had Java classes in src/test/java in which I was using the this.getClass().getResource("someFile.ext"); pattern to look for resources in src/test/resources where the resource file was in the same package location in the resources source folder as the test class was in the the test source folder. It still failed to locate them.

Right click on the src/test/resources source folder, Build Path, then "configure inclusion / exclusion filters"; I added a new inclusion filter of **/*.ext to make sure my files weren't getting scrubbed; my tests now can find their resource files.

score 0 · Answer 9 · answered Jul 25 '21 at 14:20

If you are using a Java Maven project in Netbeans (not Ant), then place your file as following:

src
  test
    java
    resources
      install.xml

Notice the location of the resources folder. You can also have nested folders within the resources. Then you can access your file as following,

URL url = this.getClass().getResource("/install.xml");

More details: https://stackoverflow.com/a/56008648/1942069

this.getClass().getClassLoader().getResource("...") and NullPointerException

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