Different ways of loading a file as an InputStream

Question

What s the difference between   InputStream is   this getClass   getClassLoader   getResourceAsStream fileName    and  InputStream is   Thread currentThread   getContextClassLoader   getResourceAsStream fileName    and  InputStream is   this getClass   getResourceAsStream fileName    When are each one more appropriate to use than the others   The file that I want to read is in the classpath as my class that reads the file   My class and the file are in the same jar and packaged up in an EAR file  and deployed in WebSphere 6 1

User · Accepted Answer

There are subtle differences as to how the fileName you are passing is interpreted. Basically, you have 2 different methods: ClassLoader.getResourceAsStream() and Class.getResourceAsStream(). These two methods will locate the resource differently.

In Class.getResourceAsStream(path), the path is interpreted as a path local to the package of the class you are calling it from. 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 /, then it will be considered an absolute path, and will start searching from the root of the classpath. So calling String.class.getResourceAsStream("/myfile.txt") will look at the following location in your class path ./myfile.txt.

ClassLoader.getResourceAsStream(path) will consider all paths to be absolute paths. So calling String.class.getClassLoader().getResourceAsStream("myfile.txt") and String.class.getClassLoader().getResourceAsStream("/myfile.txt") will both look for a file in your classpath at the following location: ./myfile.txt.

Everytime I mention a location in this post, it could be a location in your filesystem itself, or inside the corresponding jar file, depending on the Class and/or ClassLoader you are loading the resource from.

In your case, you are loading the class from an Application Server, so your should use Thread.currentThread().getContextClassLoader().getResourceAsStream(fileName) instead of this.getClass().getClassLoader().getResourceAsStream(fileName). this.getClass().getResourceAsStream() will also work.

Read this article for more detailed information about that particular problem.

Warning for users of Tomcat 7 and below

One of the answers to this question states that my explanation seems to be incorrect for Tomcat 7. I've tried to look around to see why that would be the case.

So I've looked at the source code of Tomcat's WebAppClassLoader for several versions of Tomcat. The implementation of findResource(String name) (which is utimately responsible for producing the URL to the requested resource) is virtually identical in Tomcat 6 and Tomcat 7, but is 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 produce the same result as classLoader.getResourceAsStream("resource.txt") event though it should (since that what the Javadoc specifies). [source code]

In version 8 though, the resource name is normalized to guarantee that the absolute version of the resource name is the one that is used. Therefore, in Tomcat 8, the two calls described above should always return the same result. [source code]

As a result, you have to be extra careful when using ClassLoader.getResourceAsStream() or Class.getResourceAsStream() on Tomcat versions earlier than 8. And you must also keep in mind that class.getResourceAsStream("/resource.txt") actually calls classLoader.getResourceAsStream("resource.txt") (the leading / is stripped).

User · Answer

Use MyClass class getClassLoader   getResourceAsStream path  to load resource associated with your code  Use MyClass class getResourceAsStream path  as a shortcut  and for resources packaged within your class  package   Use Thread currentThread   getContextClassLoader   getResourceAsStream path  to get resources that are part of client code  not tightly bounds to the calling code  You should be careful with this as the thread context class loader could be pointing at anything

User · Answer

It Works   try out this     InputStream in s1     TopBrandData class getResourceAsStream   assets TopBrands xml

User · Answer

Plain old Java on plain old Java 7 and no other dependencies demonstrates the difference     I put file txt in c  temp  and I put c  temp  on the classpath   There is only one case where there is a difference between the two call   class J     public static void main String   a           as  absolute          ok        System err println J class getResourceAsStream   file txt      null            pop                 System err println J class getClassLoader   getResourceAsStream   file txt      null            as relative         ok     System err println J class getResourceAsStream    file txt      null            ok     System err println J class getClassLoader   getResourceAsStream    file txt      null            no path         ok     System err println J class getResourceAsStream  file txt      null           ok    System err println J class getClassLoader   getResourceAsStream  file txt      null

User · Answer

After trying some ways to load the file with no success  I remembered I could use FileInputStream  which worked perfectly   InputStream is   new FileInputStream  file txt      This is another way to read a file into an InputStream  it reads the file from the currently running folder

User · Answer

All these answers around here  as well as the answers in this question  suggest that loading absolute URLs  like   foo bar properties  treated the same by class getResourceAsStream String  and class getClassLoader   getResourceAsStream String   This is NOT the case  at least not in my Tomcat configuration version  currently 7 0 40    MyClass class getResourceAsStream   foo bar properties       works    MyClass class getClassLoader   getResourceAsStream   foo bar properties       does NOT work    Sorry  I have absolutely no satisfying explanation  but I guess that tomcat does dirty tricks and his black magic with the classloaders and cause the difference  I always used class getResourceAsStream String  in the past and haven t had any problems   PS  I also posted this over here

[java] Different ways of loading a file as an InputStream

Warning for users of Tomcat 7 and below

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