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ClassLoader.getSystemResourceAsStream versus this.getClass().getResourceAsStream()

 
 
zyng
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      12-03-2012
I am sure the difference between the two have been discussed well: the first one is using the System classloader and the second one is using the classloader which has loaded this class, most likely a child of System classloader, according to what I have found on the web.

The part that is painful is that the first one, the file name cannot start with "/", while the second one must start with "/". I don't understand this.. If you can help me by shedding some light on this, I would greatly appreciate it.

For example, suppose I have a file "hello.txt" which locates in the directory "/abc/efg/hij/." And "/abc/efg" is on Java's classpath, so for the Java code to access "/abc/efg/hij/hello.txt" in the file system or access "hij/hello.txt" inside the JAR file:

//option 1:
InputStream is = ClassLoader.getSystemResourceAsStream("hij/hello.txt");

//option 2:
InputStream is = ResourceTools.class.getResourceAsStream("/hij/hello.txt");

The first one cannot start with "/" and the second one must start with "/".I found this is very confusing since the code between the two are already very similar looking.

Thank you very much.

 
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markspace
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      12-03-2012
On 12/3/2012 11:29 AM, zyng wrote:
> I am sure the difference between the two have been discussed well:
> the first one is using the System classloader and the second one is
> using the class loader which has loaded this class, most likely a
> child of System classloader, according to what I have found on the
> web.


First, note that there are two separate objects here, ClassLoader and
Class. Just because the names of their respective methods are the named
the same, doesn't mean they do the same thing.

>
> //option 1: InputStream is =
> ClassLoader.getSystemResourceAsStream("hij/hello.txt");
>
> //option 2: InputStream is =
> ResourceTools.class.getResourceAsStream("/hij/hello.txt");


Note in the documentation of the second method call:
" This method delegates to this object's class loader. If this object
was loaded by the bootstrap class loader, the method delegates to
ClassLoader.getSystemResourceAsStream(java.lang.St ring)."

So it's different because it delegates to a different method. I have to
guess that the two methods are designed to do different things, in spite
of their identical names. I suppose this could be considered a
"gotcha." My advice is to consider option 2; I've never seen the first
form in actual practice.

Why they did things this way, I have no idea. Classloading pretty
confusing except in trivial cases. I think it's best to keep things as
simple as possible when you resort to loading manually.

For extra fun, look at Thread::getContextClassLoader, which is even more
correct than option 1 or 2 above in certain circumstances (mainly web apps).

<http://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#getContextClassLoader()>


 
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Lew
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      12-03-2012
markspace wrote:
> zyng wrote:
>> I am sure the difference between the two have been discussed well:


Between the two ...?

Oh, you think the subject line is part of the post. Oops.

>> the first one is using the System classloader and the second one is


The "first one" being 'ClassLoader.getSystemResourceAsStream()'?

The "second one" being 'Class#getResourceAsStream()'?

>> using the class loader which has loaded this class, most likely a
>> child of System classloader, according to what I have found on the
>> web.

>
> First, note that there are two separate objects here, ClassLoader and
> Class. Just because the names of their respective methods are the named
> the same, doesn't mean they do the same thing.


The names of the respective methods in this case are *not* named the same!

'ClassLoader' also has a method 'getResourceAsStream()', which unlike
'getSystemResourceAsStream()' is not static and does different things.

>> //option 1: InputStream is =
>> ClassLoader.getSystemResourceAsStream("hij/hello.txt");

>
>> //option 2: InputStream is =
>> ResourceTools.class.getResourceAsStream("/hij/hello.txt");

>
> Note in the documentation of the second method call:
> " This method delegates to this object's class loader. If this object
> was loaded by the bootstrap class loader, the method delegates to
> ClassLoader.getSystemResourceAsStream(java.lang.St ring)."
>
> So it's different because it delegates to a different method. I have to


And has a different name and is documented to do different things.

> guess that the two methods are designed to do different things, in spite
> of their identical [sic] names. I suppose this could be considered a
> "gotcha." My advice is to consider option 2; I've never seen the first
> form in actual practice.
>
> Why they did things this way, I have no idea. Classloading pretty
> confusing except in trivial cases. I think it's best to keep things as
> simple as possible when you resort to loading manually.


The OP also said,
> The first one cannot start with "/" and the second one must start with "/".


Not true.

> I found this is very confusing since the code between the two are already very similar looking.


Even more confusing since it isn't true.

And the two aren't all that similar, in that they have different names and are in different classes.

If you think that's confusing, wait until you discover 'ArrayType', 'Attribute', 'BasicAttribute' vs.
'BasicAttributes' (even more similar than the two that confused you), 'Binding', 'Bindings', 'Boolean' vs.
'boolean' (also more similar than what confused you), 'Bounds', 'Certificate', ...

What about the fact that the 'close()' method of 'Closeable' implementations differ from each other
yet are all part of the same type? Ore isn't the same as 'ResultSet#close()' prior to Java 7? And is
not at all the 'close()' of 'StreamHandler'?

'List#get()' vs. 'Map#get()' - very different.

'Date#getTime()' vs. 'Calendar#getTime()'

This is fun.

--
Lew
 
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markspace
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      12-03-2012
On 12/3/2012 2:04 PM, Lew wrote:

> The names of the respective methods in this case are *not* named the
> same!
>



Huh, I totally misread that somehow. I guess I need to pay closer
attention.

 
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Lew
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      12-04-2012
markspace wrote:
> Lew wrote:
>> The names of the respective methods in this case are *not* named the
>> same!

>
>
> Huh, I totally misread that somehow. I guess I need to pay closer
> attention.


Well, the two classes do, in fact, sport a method of the same name, 'getResourceAsStream()'.

Your reply applies to those methods.

But the basis of the question is flawed - why should the same-named method in a different type
behave the same? Examples abound where they don't. Heck, two different 'Runnable' types don't
even do the same things in their 'run()' methods! And that's in the same type!

(To those who don't know me, the exclamation points are hints that I'm up to some
rhetorical mischief.)

The fact that the original question didn't even match up same-named methods when there were
such, and ones that do nearly the same thing at that, is bit interesting but doesn't materially
alter the situation. Method names are not determinant of behavior in and of themselves, although
in the case of interfaces like 'Runnable' they are indicative of a formal contract.

One really needs to turn to the API docs to resolve the original question. To make matters worse,
there's another 'getResourceAsStream()' in the Java EE API
<http://docs.oracle.com/javaee/6/api/javax/servlet/ServletContext.html#getResourceAsStream(java.lang. String)>
and it doesn't use 'ClassLoader' at all!

What's to explain, OP? The Javadocs tell all you need to know.

--
Lew
 
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zyng
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      12-04-2012
I have gone through the ClassLoader API doc(http://docs.oracle.com/javase/6/docs...va.lang.String)) several times and still don't get a clue about it.

This post cited ClassLoader API(http://www.xyzws.com/Servletfaq/what...rceasstream/21). However, in the API doc, I cannot find any information about leading slash. I guess Sun(or Oracle) has modified the API for new version. Anyway, I cannot get the answer from ClassLoader API.

I found this post which has answered me most part:
http://www.dtumanov.com/post/3355401...source-loading
 
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Lew
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      12-04-2012
zyng wrote:
> I have gone through the ClassLoader API doc

(http://docs.oracle.com/javase/6/docs...va.lang.String))
> several times and still don't get a clue about it.


Gee, less than five minutes' online search turned up this link:
http://littletutorials.com/2008/03/2...urces-in-java/

Did that article's assertions check out for you?

> This post cited ClassLoader API(http://www.xyzws.com/Servletfaq/what...rceasstream/21).
> However, in the API doc, I cannot find any information about leading slash. I guess Sun(or Oracle) has modified the API for new version.


You guess based on what? There has to be *evidence* of such a change to make such a claim.

> Anyway, I cannot get the answer from ClassLoader API.


That does seem to be a hole in the API docs, but surely you have your answer by now.

> I found this post which has answered me most part:
> http://www.dtumanov.com/post/3355401...source-loading


In the end, a quick-and-dirty code example will answer your questions for you.

--
Lew
 
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