How Do `ClassLoader.getResourceAsStream()` and `Class.getResourceAsStream()` Differ When Loading Files as InputStreams?

different ways to load a file as an InputStream

There are subtle differences in the interpretation of the passed file name. Basically there are two different methods: ClassLoader.getResourceAsStream() and Class.getResourceAsStream(). These two methods find the resource in different ways.

In Class.getResourceAsStream(path), the path is interpreted as a path within the package of the class from which you are calling it. For example, calling String.class.getResourceAsStream("myfile.txt") will look for a file in your classpath at the following location: "java/lang/myfile.txt". If your path starts with a /, it will be considered an absolute path and the search will start from the root of the classpath. So when you call String.class.getResourceAsStream("/myfile.txt"), it looks at the following location in the classpath: ./myfile.txt.

ClassLoader.getResourceAsStream(path) considers all paths as absolute Paths. So if you call String.class.getClassLoader().getResourceAsStream("myfile.txt") and String.class.getClassLoader().getResourceAsStream("/myfile.txt"), in both cases it will be in the classpath at the following location looking for a file: ./myfile.txt.

Whenever I mention a location in this post, it may be one Location in the file system itself or in the corresponding JAR file, depending on the class and/or ClassLoader from which you are loading the resource.

In your case, you are loading the class from an application server, so you should Use Thread.currentThread().getContextClassLoader().getResourceAsStream(filename) instead of this.getClass().getClassLoader().getResourceAsStream(filename). this.getClass().getResourceAsStream() also works.

Read this article for more information on this specific issue.

Note for users of Tomcat 7 and below

One of the answers to this question states that my explanation for Tomcat 7 appears to be incorrect. I was trying to figure out why this might be.

So I looked at the source code of Tomcat's WebAppClassLoader for different Tomcat versions. The implementation of findResource(String name) (which is ultimately responsible for the URL to the requested resource) is almost identical in Tomcat 6 and Tomcat 7, but different in Tomcat 8.

In versions 6 and 7, the implementation does not attempt to normalize the resource name. This means that in these versions classLoader.getResourceAsStream("/resource.txt") may not give the same result as classLoader.getResourceAsStream("resource.txt"), even though it should (as stated in the Javadoc). [Source code]

However, in version 8, the resource name is normalized to ensure that the absolute version of the resource name is used. Therefore, the two calls described above should always produce the same result in Tomcat 8. [Source code]

Therefore, you need to be extra careful when using ClassLoader.getResourceAsStream() or Class.getResourceAsStream() on Tomcat versions before 8. And you also have to keep in mind that class.getResourceAsStream("/resource.txt") actually calls classLoader.getResourceAsStream("resource.txt") (the leading / is removed).

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