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Posted to users@tomcat.apache.org by Konstantin Preißer <ve...@t-online.de> on 2013/05/10 18:41:04 UTC
[OT] Querstion about Class.forName() re. ClassLoaders
Hi all,
I apologize for being completely off-topic (this question has nothing to do with Tomcat), but I thought there may be some guys here that are experts in class loading and are able to answer my question.
You probably know the method java.lang.Class.forName(String name) which returns a class object from the given name. The JavaDoc (Java 7) of this method says:
Returns the Class object associated with the class or interface with the given string name. Invoking this method is equivalent to:
Class.forName(className, true, currentLoader)
where currentLoader denotes the defining class loader of the current class.
While this description may be a bit ambiguous about what "current class" means here, the JavaDoc of Class.forName(String name, boolean initialize, ClassLoader loader) makes it clearer by stating:
For example, in an instance method the expression:
Class.forName("Foo")
is equivalent to:
Class.forName("Foo", true, this.getClass().getClassLoader())
So, shouldn't this mean that I get the same result/behavior when writing this code:
public void doSomething() throws ClassNotFoundException {
Class<?> clazz = Class.forName("foo.Bar");
// do something...
}
Instead of this one:
public void doSomething() throws ClassNotFoundException {
Class<?> clazz = Class.forName("foo.Bar", true, this.getClass().getClassLoader());
// do something...
}
or did I misunderstand something?
Thanks!
Regards,
Konstantin Preißer
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RE: [OT] Querstion about Class.forName() re. ClassLoaders
Posted by Konstantin Preißer <ve...@t-online.de>.
Hi Konstantin,
> -----Original Message-----
> From: Konstantin Kolinko [mailto:knst.kolinko@gmail.com]
> Sent: Saturday, May 11, 2013 2:46 PM
>
> Nice catch.
> But I think it is just a documentation issue.
>
> I think documentation should be better here: Looking at 7u21, it uses two
> different wordings
> a) in description of Class.forName(String, boolean, ClassLoader):
> an example with "this.getClass().getClassloader()"
> This one is wrong.
>
> b) in description of Class.forName(String) "the defining class loader of the
> current class."
> This is correct, but one would better clarify what "current class"
> means here, as it is ambiguous.
>
> Java Language Specification (3rd edition) uses the term "the defining class
> loader" (of a class) in several places, e.g. chapter 15.8.2 Class Literals.
>
> Two points:
> 1. I expect Class.forName("Foo").newInstance() to give the same result as
> "new Foo();"
>
> I think the current behaviour is more consistent. (Relying on the class which
> bytecode is executed, instead of this.getClass()).
Yes, when I use "new Foo()" instead of Class.forName("Foo").newInstance() then I get the same result: It uses the ClassLoader of the class that implements the method.
I agree that in this case it is a documentation issue.
> 2. Implementation relies on method ClassLoader.getCallerClassLoader(),
> which looks up stack frames in JVM.
>
> You cannot change this method itself, as it is used in security checks in many
> places. To change the behaviour one would need to create a different
> method.
>
>
> BTW, if you look for oddities, Class.newInstance() is a legacy method that
> can throw a non-declared checked exception "as is", without wrapping it in
> InvocationTargetException. (The method is there @since
> 1.0 and InvocationTargetException is @since 1.1)
>
> Best regards,
> Konstantin Kolinko
Thanks!
Regards,
Konstantin Preißer
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Re: [OT] Querstion about Class.forName() re. ClassLoaders
Posted by Konstantin Kolinko <kn...@gmail.com>.
2013/5/11 Konstantin Preißer <ve...@t-online.de>:
> Hi Konstantin,
>
>> -----Original Message-----
>> From: Konstantin Kolinko [mailto:knst.kolinko@gmail.com]
>> Sent: Friday, May 10, 2013 11:46 PM
>>
>> Yes, the same.
>>
>> BTW, Oracle JDKs come with source code for their public classes, On
>> Windows that is %JAVA_HOME%/src.zip. Do you have such file?
>
> Thank you for your answer. Yes, I have that file and I looked there, but that Class.forName(String) method calls a native method for which I couldn't find the source.
>
> I asked this question because the above is not what is implemented by Sun/Oracles JRE (I tested with Windows 32-bit versions of JRE 1.7.0_21, Java 8 EA (b88) and Java 5).
>
> It seems that in Oracles Java, Class.forName(String) uses the ClassLoader of the class which implements the method that makes the call to Class.forName(), not the ClassLoader of the class on whose instance the method is called (which is the one used when calling getClass().getClassLoader()). This can make a difference e.g. if the Class with that method has a subclass and you call the method on an instance of the subclass (which is what I experienced with a Eclipse RCP app which uses a ClassLoader hierarchy for plugins).
>
> However, since the bug exists in current Java 1.7.0_21 (from 2013) and even in Java 1.5.0 (2004), I wondered if nobody else noticed this discrepancy between the JavaDoc and the actual implementation or maybe if Sun/Oracle just doesn't fix such bugs. (I already submitted a bug report on December 1, 2012, but did not yet receive a response - so I wanted to make sure my understanding was correct).
>
Nice catch.
But I think it is just a documentation issue.
I think documentation should be better here: Looking at 7u21, it uses
two different wordings
a) in description of Class.forName(String, boolean, ClassLoader):
an example with "this.getClass().getClassloader()"
This one is wrong.
b) in description of Class.forName(String)
"the defining class loader of the current class."
This is correct, but one would better clarify what "current class"
means here, as it is ambiguous.
Java Language Specification (3rd edition) uses the term "the defining
class loader" (of a class) in several places, e.g. chapter 15.8.2
Class Literals.
Two points:
1. I expect Class.forName("Foo").newInstance() to give the same
result as "new Foo();"
I think the current behaviour is more consistent. (Relying on the
class which bytecode is executed, instead of this.getClass()).
2. Implementation relies on method ClassLoader.getCallerClassLoader(),
which looks up stack frames in JVM.
You cannot change this method itself, as it is used in security checks
in many places. To change the behaviour one would need to create a
different method.
BTW, if you look for oddities, Class.newInstance() is a legacy method
that can throw a non-declared checked exception "as is", without
wrapping it in InvocationTargetException. (The method is there @since
1.0 and InvocationTargetException is @since 1.1)
Best regards,
Konstantin Kolinko
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RE: [OT] Querstion about Class.forName() re. ClassLoaders
Posted by Konstantin Preißer <ve...@t-online.de>.
Hi Konstantin,
> -----Original Message-----
> From: Konstantin Kolinko [mailto:knst.kolinko@gmail.com]
> Sent: Friday, May 10, 2013 11:46 PM
>
> Yes, the same.
>
> BTW, Oracle JDKs come with source code for their public classes, On
> Windows that is %JAVA_HOME%/src.zip. Do you have such file?
Thank you for your answer. Yes, I have that file and I looked there, but that Class.forName(String) method calls a native method for which I couldn't find the source.
I asked this question because the above is not what is implemented by Sun/Oracles JRE (I tested with Windows 32-bit versions of JRE 1.7.0_21, Java 8 EA (b88) and Java 5).
It seems that in Oracles Java, Class.forName(String) uses the ClassLoader of the class which implements the method that makes the call to Class.forName(), not the ClassLoader of the class on whose instance the method is called (which is the one used when calling getClass().getClassLoader()). This can make a difference e.g. if the Class with that method has a subclass and you call the method on an instance of the subclass (which is what I experienced with a Eclipse RCP app which uses a ClassLoader hierarchy for plugins).
However, since the bug exists in current Java 1.7.0_21 (from 2013) and even in Java 1.5.0 (2004), I wondered if nobody else noticed this discrepancy between the JavaDoc and the actual implementation or maybe if Sun/Oracle just doesn't fix such bugs. (I already submitted a bug report on December 1, 2012, but did not yet receive a response - so I wanted to make sure my understanding was correct).
Thanks!
Regards,
Konstantin Preißer
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Re: [OT] Querstion about Class.forName() re. ClassLoaders
Posted by Konstantin Kolinko <kn...@gmail.com>.
2013/5/10 Konstantin Preißer <ve...@t-online.de>:
> Hi all,
>
> I apologize for being completely off-topic (this question has nothing to do with Tomcat), but I thought there may be some guys here that are experts in class loading and are able to answer my question.
>
>
> You probably know the method java.lang.Class.forName(String name) which returns a class object from the given name. The JavaDoc (Java 7) of this method says:
>
> Returns the Class object associated with the class or interface with the given string name. Invoking this method is equivalent to:
> Class.forName(className, true, currentLoader)
> where currentLoader denotes the defining class loader of the current class.
>
> While this description may be a bit ambiguous about what "current class" means here, the JavaDoc of Class.forName(String name, boolean initialize, ClassLoader loader) makes it clearer by stating:
>
> For example, in an instance method the expression:
> Class.forName("Foo")
> is equivalent to:
> Class.forName("Foo", true, this.getClass().getClassLoader())
>
>
> So, shouldn't this mean that I get the same result/behavior when writing this code:
>
> public void doSomething() throws ClassNotFoundException {
> Class<?> clazz = Class.forName("foo.Bar");
> // do something...
> }
>
> Instead of this one:
>
> public void doSomething() throws ClassNotFoundException {
> Class<?> clazz = Class.forName("foo.Bar", true, this.getClass().getClassLoader());
> // do something...
> }
>
> or did I misunderstand something?
>
Yes, the same.
BTW, Oracle JDKs come with source code for their public classes,
On Windows that is %JAVA_HOME%/src.zip. Do you have such file?
Best regards,
Konstantin Kolinko
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