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changing the classpath at runtime in code

 
 
Aryeh M. Friedman
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      10-24-2007
Is it possible for a java app to change it's classpath at runtime. I
tried:

System.setProperty("java.class.path","foo")

The reason for wanting to try is URLClassLoader will not physically
reread a class file if it is in the classpath. See
http://groups.google.com/group/comp....aa154348f6f271

 
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=?ISO-8859-1?Q?Arne_Vajh=F8j?=
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      10-25-2007
Aryeh M. Friedman wrote:
> Is it possible for a java app to change it's classpath at runtime. I
> tried:
>
> System.setProperty("java.class.path","foo")
>
> The reason for wanting to try is URLClassLoader will not physically
> reread a class file if it is in the classpath.


Create a URLClassLoader with a url that are *not* in the parent
classloaders url's.

Arne
 
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Aryeh M. Friedman
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      10-25-2007
On Oct 24, 8:16 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
> Aryeh M. Friedman wrote:
> > Is it possible for a java app to change it's classpath at runtime. I
> > tried:

>
> > System.setProperty("java.class.path","foo")

>
> > The reason for wanting to try is URLClassLoader will not physically
> > reread a class file if it is in the classpath.

>
> Create a URLClassLoader with a url that are *not* in the parent
> classloaders url's.


so just do:

URL[] url={new URL("file://foo")};
URLClassLoader laoder=new URLClassLoader(url);
?

 
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=?ISO-8859-1?Q?Arne_Vajh=F8j?=
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      10-25-2007
Aryeh M. Friedman wrote:
> On Oct 24, 8:16 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
>> Aryeh M. Friedman wrote:
>>> Is it possible for a java app to change it's classpath at runtime. I
>>> tried:
>>> System.setProperty("java.class.path","foo")
>>> The reason for wanting to try is URLClassLoader will not physically
>>> reread a class file if it is in the classpath.

>> Create a URLClassLoader with a url that are *not* in the parent
>> classloaders url's.

>
> so just do:
>
> URL[] url={new URL("file://foo")};
> URLClassLoader laoder=new URLClassLoader(url);
> ?


I have a little demo example I often use to illustrate
(subdir test is not in classpath of program).

Arne

===============================

import java.io.*;
import java.net.*;

public class DoubleDynmaic {
private static void dynno(int n) {
(new File("test")).mkdir();
try {
OutputStream os = new FileOutputStream("test/Test.java");
PrintStream ps = new PrintStream(os);
ps.println("public class Test {");
ps.println(" public Test() {");
ps.println(" System.out.println(" + n + ");");
ps.println(" }");
ps.println("}");
ps.close();
os.close();
Runtime.getRuntime().exec("javac -d test
test/Test.java").waitFor();
URL[] url = new URL[1];
url[0] = new URL("file:test/");
URLClassLoader cl = new URLClassLoader(url);
Class.forName("Test", true, cl).newInstance();
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
for(int i = 0; i < 10; i++) {
dynno(i);
}
}
}
 
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Aryeh M. Friedman
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      10-25-2007
On Oct 24, 9:13 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
> Aryeh M. Friedman wrote:
> > On Oct 24, 8:16 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
> >> Aryeh M. Friedman wrote:
> >>> Is it possible for a java app to change it's classpath at runtime. I
> >>> tried:
> >>> System.setProperty("java.class.path","foo")
> >>> The reason for wanting to try is URLClassLoader will not physically
> >>> reread a class file if it is in the classpath.
> >> Create a URLClassLoader with a url that are *not* in the parent
> >> classloaders url's.

>
> > so just do:

>
> > URL[] url={new URL("file://foo")};
> > URLClassLoader laoder=new URLClassLoader(url);
> > ?

>
> I have a little demo example I often use to illustrate
> (subdir test is not in classpath of program).


You posted this in the original thread but as noted in the linked to
thread almost all the classes I am dealing with *WILL* be in the
classpath.... so the question remains if I do the code in my previous
post will it now load from the class path or from "foo".

>
> Arne
>
> ===============================
>
> import java.io.*;
> import java.net.*;
>
> public class DoubleDynmaic {
> private static void dynno(int n) {
> (new File("test")).mkdir();
> try {
> OutputStream os = new FileOutputStream("test/Test.java");
> PrintStream ps = new PrintStream(os);
> ps.println("public class Test {");
> ps.println(" public Test() {");
> ps.println(" System.out.println(" + n + ");");
> ps.println(" }");
> ps.println("}");
> ps.close();
> os.close();
> Runtime.getRuntime().exec("javac -d test
> test/Test.java").waitFor();
> URL[] url = new URL[1];
> url[0] = new URL("file:test/");
> URLClassLoader cl = new URLClassLoader(url);
> Class.forName("Test", true, cl).newInstance();
> } catch (Exception e) {
> e.printStackTrace();
> }
> }
> public static void main(String[] args) {
> for(int i = 0; i < 10; i++) {
> dynno(i);
> }
> }
>
> }



 
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Aryeh M. Friedman
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Posts: n/a
 
      10-25-2007
On Oct 24, 9:13 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
> Aryeh M. Friedman wrote:
> > On Oct 24, 8:16 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
> >> Aryeh M. Friedman wrote:
> >>> Is it possible for a java app to change it's classpath at runtime. I
> >>> tried:
> >>> System.setProperty("java.class.path","foo")
> >>> The reason for wanting to try is URLClassLoader will not physically
> >>> reread a class file if it is in the classpath.
> >> Create a URLClassLoader with a url that are *not* in the parent
> >> classloaders url's.

>
> > so just do:

>
> > URL[] url={new URL("file://foo")};
> > URLClassLoader laoder=new URLClassLoader(url);
> > ?

>
> I have a little demo example I often use to illustrate
> (subdir test is not in classpath of program).


Perhaps google ate my previous response.... but as I said several
places (including the linked to thread) almost all my classes *WILL*
be in the classpath... btw as mentioned in an other thread your demo
doesn't quite work once divorced from the code to rewrite the class
(see orginial thread)... so the question remains does the code above
load from "foo" or from the classpath when I do loader.loadClass(...)

 
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=?ISO-8859-1?Q?Arne_Vajh=F8j?=
Guest
Posts: n/a
 
      10-25-2007
Aryeh M. Friedman wrote:
> On Oct 24, 9:13 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
>> Aryeh M. Friedman wrote:
>>> On Oct 24, 8:16 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
>>>> Aryeh M. Friedman wrote:
>>>>> Is it possible for a java app to change it's classpath at runtime. I
>>>>> tried:
>>>>> System.setProperty("java.class.path","foo")
>>>>> The reason for wanting to try is URLClassLoader will not physically
>>>>> reread a class file if it is in the classpath.
>>>> Create a URLClassLoader with a url that are *not* in the parent
>>>> classloaders url's.
>>> so just do:
>>> URL[] url={new URL("file://foo")};
>>> URLClassLoader laoder=new URLClassLoader(url);
>>> ?

>> I have a little demo example I often use to illustrate
>> (subdir test is not in classpath of program).

>
> You posted this in the original thread but as noted in the linked to
> thread almost all the classes I am dealing with *WILL* be in the
> classpath.... so the question remains if I do the code in my previous
> post will it now load from the class path or from "foo".


If the class get loaded by the parent classloader because it is
in the apps general classpath, then you can (as far as I know)
neither get it unloaded or reloaded.

You need to get those classes of the classpath.

Arne
 
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Daniel Pitts
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Posts: n/a
 
      10-25-2007
Aryeh M. Friedman wrote:
> Is it possible for a java app to change it's classpath at runtime. I
> tried:
>
> System.setProperty("java.class.path","foo")
>
> The reason for wanting to try is URLClassLoader will not physically
> reread a class file if it is in the classpath. See
> http://groups.google.com/group/comp....aa154348f6f271
>

Once a class is loaded by any class loader, it can not be reloaded by
any other class loader in that same branch of the class-loader tree.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>
 
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Daniel Pitts
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Posts: n/a
 
      10-25-2007
Aryeh M. Friedman wrote:
> On Oct 24, 9:13 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
>> Aryeh M. Friedman wrote:
>>> On Oct 24, 8:16 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
>>>> Aryeh M. Friedman wrote:
>>>>> Is it possible for a java app to change it's classpath at runtime. I
>>>>> tried:
>>>>> System.setProperty("java.class.path","foo")
>>>>> The reason for wanting to try is URLClassLoader will not physically
>>>>> reread a class file if it is in the classpath.
>>>> Create a URLClassLoader with a url that are *not* in the parent
>>>> classloaders url's.
>>> so just do:
>>> URL[] url={new URL("file://foo")};
>>> URLClassLoader laoder=new URLClassLoader(url);
>>> ?

>> I have a little demo example I often use to illustrate
>> (subdir test is not in classpath of program).

>
> Perhaps google ate my previous response.... but as I said several
> places (including the linked to thread) almost all my classes *WILL*
> be in the classpath... btw as mentioned in an other thread your demo
> doesn't quite work once divorced from the code to rewrite the class
> (see orginial thread)... so the question remains does the code above
> load from "foo" or from the classpath when I do loader.loadClass(...)
>

Here's the scoop.
If its ever on the class path, you can't reload it. What you're trying
to do is not possible to do without restricting what's on the class
path. That is not a bad restriction! Just make it happen.

Changing the class path after the fact isn't going to help either.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>
 
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Aryeh M. Friedman
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Posts: n/a
 
      10-25-2007
On Oct 24, 9:31 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
> Aryeh M. Friedman wrote:
> > On Oct 24, 9:13 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
> >> Aryeh M. Friedman wrote:
> >>> On Oct 24, 8:16 pm, Arne Vajh°j <(E-Mail Removed)> wrote:
> >>>> Aryeh M. Friedman wrote:
> >>>>> Is it possible for a java app to change it's classpath at runtime. I
> >>>>> tried:
> >>>>> System.setProperty("java.class.path","foo")
> >>>>> The reason for wanting to try is URLClassLoader will not physically
> >>>>> reread a class file if it is in the classpath.
> >>>> Create a URLClassLoader with a url that are *not* in the parent
> >>>> classloaders url's.
> >>> so just do:
> >>> URL[] url={new URL("file://foo")};
> >>> URLClassLoader laoder=new URLClassLoader(url);
> >>> ?
> >> I have a little demo example I often use to illustrate
> >> (subdir test is not in classpath of program).

>
> > You posted this in the original thread but as noted in the linked to
> > thread almost all the classes I am dealing with *WILL* be in the
> > classpath.... so the question remains if I do the code in my previous
> > post will it now load from the class path or from "foo".

>
> If the class get loaded by the parent classloader because it is
> in the apps general classpath, then you can (as far as I know)
> neither get it unloaded or reloaded.
>
> You need to get those classes of the classpath.


Since this is for a commercial unit testing product this is not
possible in practice (it is not safe to make assumptions about where
people place things). Some one off line suggested forking a process
to do it.... if I can't find a ClassLoader based solution I will have
to do that but it is down right a) not portable b) evil


 
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