Java resource loading explained. Absolute and relative names. Difference between ClassLoader and Class resource loading

By neokrates, written on July 15, 2010

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Understanding how Java finds resources is important. Resource loading in Java is “location independent”, Java also differentiates between absolute and relative resource names. And these names are processed by Java ResourceLoader differently. That last little issue may cause problems, especially because the “wrong” resource names really work under some circumstances.

1

How java locates resources

Java loads resources from the “environment”, in many cases it uses all jars in Classpath to retrieve the resource. Resource loading in java is called location independent because it is not relevant where your code is running, it just needs correct environment to find the resources.

Given all jars and paths in Classpath, java will search relatively to each tone to find the resource you specified. You specify resources using resource names.

2

Absolute and relative resource names

Resources are referred using resource name:
getResourceAsStream("/path/resource.xml");

“/path/resource.xml” is the resource name here.

The resource name can be:

  • absolute like “/path/resource.xml”;
  • relative like “path/resource.xml”;

 
Relative means, relative to the location, where the method was called. The path will be appended if needed.
Absolute will be used as is, only first / will be removed before the search.

Example:

package my.location;

class ResourceFinder {
...
public void findResources(){
   InputStream stream1 = 
getClass().getResourceAsStream("/path/resource.xml");
   InputStream stream2 = 
getClass().getResourceAsStream("path/resource.xml");
}
...
}

 

➕ In this case, stream1 will get the resource, located in classpath path/resource.xml.

stream2 will get the resource, located in classpath my/location/path/resource.xml (Relative to package where the search was initiated).

3

ClassLoader and Class apply resource names differently

There are ClassLoader.getResource() and Class.getResource() and they work differently!!!

java docu:

The methods in ClassLoader use the given String as the name of the
resource without applying any absolute/relative transformation (cf. the
methods in Class). The name should not have a leading “/”.

So, to extend our example:

package my.location;

class ResourceFinder {
...
public void findResources(){
   InputStream stream1 = 
getClass().getResourceAsStream("/path/resource.xml");
   InputStream stream2 = 
getClass().getResourceAsStream("path/resource.xml");
   InputStream stream3 = 
getClass().getClassLoader().getResourceAsStream("path/resource.xml");
   InputStream stream4 = 
getClass().getClassLoader().getResourceAsStream("/path/resource.xml");

}
...
}

 

stream3 will become resource in classpath under path/resource.xml.

stream4 resource name /path/resource.xml is wrong!

4

Where it is important. Example

I was assessing Maven3 against Maven2 build. Same big project, just call m3.

Following code has worked well in my previous environment:

InputStream exportFileInputStream = 
getClass().getClassLoader().getResourceAsStream("/com/thinkplexx/lang/de/general.xml");

 

But after I changed the environment (maven2 build -> maven3 build) the general.xml was missing. I know that resource is there.

Following worked:

InputStream exportFileInputStream = 
getClass().getClassLoader().getResourceAsStream("com/thinkplexx/lang/de/general.xml");

 
It is a speculation as of how the first snippet could have worked, but it deed. Going to new clean environment triggered the bug.

Links

1) http://download.oracle.com/docs/cd/E17476_01/javase/1.4.2/docs/guide/resources/resources.html
2) http://stackoverflow.com/questions/3238562/getresourceasstream-fails-under-new-environment
3) http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/api/java/lang/ClassLoader.html
4) http://download.oracle.com/docs/cd/E17409_01/javase/6/docs/api/java/lang/Class.html

That’s it. Have fun! ;)

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