resources from JAR files

[bob smith]

I have some code that reads an image from its JAR file like so:

img = ImageIO.read(frame.getClass().getResource("whatever.jpg"));

I basically just picked the "frame" object at random to call the getClass() method on it.

Is there a way to do this without picking an arbitrary object? It seems so wrong.

 

[markspace]

I have some code that reads an image from its JAR file like so:

img = ImageIO.read(frame.getClass().getResource("whatever.jpg"));

I basically just picked the "frame" object at random to call the getClass() method on it.

Is there a way to do this without picking an arbitrary object? It seems so wrong.

Yes, normally I use the object the line of code is contained it.

class MyBusinessObject {

void someMethod() {
Image img ImageIO.read( getClass().getResource( "whatever.jpg" ));
// etc...
}
}

You can also use a constant. MyBusinessObject.class.getResource(...)
instead of getClass().

You can also use
Thread.currentThread().getContextClassLoader().getResource(...), but
that's usually only useful in situations where thread contexts are used,
i.e. web apps.

 

[Roedy Green]

I have some code that reads an image from its JAR file like so:

img = ImageIO.read(frame.getClass().getResource("whatever.jpg"));

I basically just picked the "frame" object at random to call the getClass() method on it.

Is there a way to do this without picking an arbitrary object? It seems so wrong.

It is really just looking for a package name to find the resource. You
can name the main class explicitly MyProg.class or this.getClass()
There is no Package class, so they use Class. You want something in
YOUR package.

see http://mindprod.com/jgloss/resource.html
--
Roedy Green Canadian Mind Products http://mindprod.com
The first 90% of the code accounts for the first 90% of the development time.
The remaining 10% of the code accounts for the other 90% of the development
time.
~ Tom Cargill Ninety-ninety Law

 

[Arne Vajhøj]

I have some code that reads an image from its JAR file like so:

img = ImageIO.read(frame.getClass().getResource("whatever.jpg"));

I basically just picked the "frame" object at random to call the getClass() method on it.

Is there a way to do this without picking an arbitrary object? It seems so wrong.

You need to pick either an object or a class.

I think it is relative common to pick either this or the class.

Arne

 

[Lew]

There is no Package class, ...

http://docs.oracle.com/javase/7/docs/api/java/lang/Package.html

 

[Roedy Green]

http://docs.oracle.com/javase/7/docs/api/java/lang/Package.html

That came in 1.5, a fair bit after getResource.
The one advantage of using class is it unambiguously specifies a jar,
even a jar whose name is unknown.
--
Roedy Green Canadian Mind Products http://mindprod.com
The first 90% of the code accounts for the first 90% of the development time.
The remaining 10% of the code accounts for the other 90% of the development
time.
~ Tom Cargill Ninety-ninety Law

 

[Arne Vajhøj]

That came in 1.5, a fair bit after getResource.

But 1.5 came out in 2004.

A fair bit of time ago.

Arne

 

[BGB]

You need to pick either an object or a class.

I think it is relative common to pick either this or the class.

FWIW, AFIAK:
it can also be noted that it does matter which class you pick, like
generally you want the class and resource file to be in the same package
and JAR and similar (the class basically telling the JVM where to look,
otherwise the resource may not be found).

so, while a person can pick an arbitrary class, it may not necessarily
find the resource.

so, generally, picking 'this' or similar makes sense, since normally a
person will package the resources along with their own code.


or such...

 

[Arne Vajhøj]

FWIW, AFIAK:
it can also be noted that it does matter which class you pick, like
generally you want the class and resource file to be in the same package
and JAR and similar (the class basically telling the JVM where to look,
otherwise the resource may not be found).

so, while a person can pick an arbitrary class, it may not necessarily
find the resource.

so, generally, picking 'this' or similar makes sense, since normally a
person will package the resources along with their own code.


or such...

I would use "the class" of this, so ...

And the class method can be used in static context.

Arne

 

[Lew]

That's partly correct, as five minutes' reading of the 'getResource()' method docs will
reveal. What actually matters is what classloader you pick; any number of classes (and
frequently all the ones you have access to) are loaded by one classloader.

As for the seeming arbitrariness of the idiom that started this thread, perhaps the
programmer picked a class known to have the correct classloader, is all.

That's not true.

That's not true either, and that's not true.

You can have the resource in any package that makes sense. Common conventions
are 'resource', 'resources', 'res', or those relative to the "official" package root of your
application, such as 'com.lewscanon.slicedbread.resources'.

And as stated, it's "basically" the classloader telling the JVM where to look.

The resource will be found if it's in the location specified by the argument to the call.

That can be the default package, the same package, a different package, or whatever.

Same JAR, different JAR, remote URL, whatever.

You should never pick an arbitrary class, however you may pick an arbitrary
class from the set of those that use the appropriate classloader, or even use
the classloader directly. It's all good. If you read the Javadocs, you willnot guess
but know that the call is correct.

Because generally you want the same classloader as the caller's.

What is "or similar"?

Normally by what metric?

You put the resources where the architecture of the system mandates. Again,
that can be remote - quite common for applets in their day - from a JAR,
from anywhere accessible to a classloader. You are correct to the extent that
the default classloader is often the one you want, so 'this' or 'Type.class' do just
fine. But that's an arbitrary choice.

False analogy.

I would use "the class" of this, so ...
And the class method can be used in static context.

Note: He means "method" in the English sense here, as in "means of getting to the classloader",
not as in "the class literal is a Java method".

Bear in mind that the 'Class' version of 'getResource()' is a convenience method - the
'ClassLoader' version is the workhorse.

Don't use these calls by rule of thumb as BGB suggests. Breaking into 'Class' methods and more so
'ClassLoader' methods is of the world of reflection, and classpaths, and package-to-real-world
connections, and stuff that that breaks type safety. This is part of the heart of what makes Java Java.
This is stuff you need to actually know, not do by cargo-cult programming.