absolute .class file name to java class name

[Fu Bo Xia]

the java.lang.Objec t.forName method takes a java class name and returns a
Class object associated with that class.
eg. Class myClass = Object.forName( "java.lang.Stri ng");

by if i only know the absolute file name of a .class file eg.
C:\myJava\myApp .java, then how do i translate this file name to a java class
name the Object.forName method would accept has it's parameter?
thanks,
fu bo

[Raymond DeCampo]

Fu Bo Xia wrote:

the java.lang.Objec t.forName method takes a java class name and returns a
Class object associated with that class.
eg. Class myClass = Object.forName( "java.lang.Stri ng");

by if i only know the absolute file name of a .class file eg.
C:\myJava\myApp .java, then how do i translate this file name to a java class
name the Object.forName method would accept has it's parameter?

Fu Bo,

Do you have a class file or a java source file? If you have the java
source file, there were just a couple of Java Tech tips from Sun
concerning how to compile and load a class. Check their web site.

If you really have a class file, then you need for the directory
containing the class file to be in the classpath (set by the CLASSPATH
variable or by the -classpath option to the java executable) assuming
the class is in the default package. If the class is in a package, e.g.
com.xxx.MyClass , and the class file is in
C:\myJava\com\x xx\MyClass.clas s, then the directory C:\myJava should be
added to the classpath.

If you do not have the option of modifying the classpath, you can load
the file into an array of bytes and the Java API provides a means of
loading the bytes as a class. See the java.lang.Class and
java.lang.Class Loader APIs.

HTH,
Ray

[Fu Bo Xia]

thanks ray,

sorry i meant .class files not .java files.

let clearify my problem:

my application is given the absolute file name of a .class file eg.
C:\myJava\myApp .class during run time (via command line or file selection
dialog box.)
how can i use this class? ie. instantiate it or use java.lang.refec t tools
on it.
thanks,
fu bo

Fu Bo,

Do you have a class file or a java source file? If you have the java
source file, there were just a couple of Java Tech tips from Sun
concerning how to compile and load a class. Check their web site.

If you really have a class file, then you need for the directory
containing the class file to be in the classpath (set by the CLASSPATH
variable or by the -classpath option to the java executable) assuming
the class is in the default package. If the class is in a package, e.g.
com.xxx.MyClass , and the class file is in
C:\myJava\com\x xx\MyClass.clas s, then the directory C:\myJava should be
added to the classpath.

If you do not have the option of modifying the classpath, you can load
the file into an array of bytes and the Java API provides a means of
loading the bytes as a class. See the java.lang.Class and
java.lang.Class Loader APIs.

HTH,
Ray

[Neomorph]

On Mon, 1 Sep 2003 12:07:09 +1000, "Fu Bo Xia" <fu****@optusho me.com.au>
two-finger typed:

thanks ray,

sorry i meant .class files not .java files.

let clearify my problem:

my application is given the absolute file name of a .class file eg.
C:\myJava\myAp p.class during run time (via command line or file selection
dialog box.)
how can i use this class? ie. instantiate it or use java.lang.refec t tools
on it.
Reflection is the only way, since you have the class name only in a string.

Since the class file needs to be found through the classpath, you could try
setting the java.class.path system property, and then use reflection to
load the class, the byte loading method explained below.

Cheers.


thanks,
fu bo

Fu Bo,

Do you have a class file or a java source file? If you have the java
source file, there were just a couple of Java Tech tips from Sun
concerning how to compile and load a class. Check their web site.

If you really have a class file, then you need for the directory
containing the class file to be in the classpath (set by the CLASSPATH
variable or by the -classpath option to the java executable) assuming
the class is in the default package. If the class is in a package, e.g.
com.xxx.MyClass , and the class file is in
C:\myJava\com\x xx\MyClass.clas s, then the directory C:\myJava should be
added to the classpath.

If you do not have the option of modifying the classpath, you can load
the file into an array of bytes and the Java API provides a means of
loading the bytes as a class. See the java.lang.Class and
java.lang.Class Loader APIs.

HTH,
Ray

[Fu Bo Xia]

i've tried the following for creating a java.lang.Class object for
c:\myJava\myApp .class

//set system property java.class.path
String currentCP = System.getPrope rty("java.class .path").toStrin g();
System.setPrope rty("java.class .path", (currentCP + ";" +
"C:\\myJava "));
//set system property java.library.pa th
String currentLP =
System.getPrope rty("java.libra ry.path").toStr ing();
System.setPrope rty("java.libra ry.path", (currentLP + ";" +
"C:\\myJava "));
//set system property java.ext.dirs
String currentED = System.getPrope rty("java.ext.d irs").toString( );
System.setPrope rty("java.ext.d irs", (currentED + ";" +
"C:\\myJava "));

//create Class object
Class myClass = Class.forName(" myApp");
but the Class.forName method is still throwing ClassNotFoundEx ception
any suggestions?

fu bo

"Neomorph" <ne******@nospa m.demon.co.uk> wrote in message
news:j4******** *************** *********@4ax.c om...
On Mon, 1 Sep 2003 12:07:09 +1000, "Fu Bo Xia" <fu****@optusho me.com.au>
two-finger typed:

Reflection is the only way, since you have the class name only in a string.

Since the class file needs to be found through the classpath, you could try
setting the java.class.path system property, and then use reflection to
load the class, the byte loading method explained below.

Cheers.

[Neomorph]

On Tue, 2 Sep 2003 15:40:45 +1000, "Fu Bo Xia" <fu****@optusho me.com.au>
two-finger typed:

i've tried the following for creating a java.lang.Class object for
c:\myJava\myAp p.class

//set system property java.class.path
String currentCP = System.getPrope rty("java.class .path").toStrin g();
System.setPrope rty("java.class .path", (currentCP + ";" +
"C:\\myJava")) ;
Change the ";" into File.pathSepara tor to keep it cross-platform.
//set system property java.library.pa th
String currentLP =
System.getProp erty("java.libr ary.path").toSt ring();
System.setPrope rty("java.libra ry.path", (currentLP + ";" +
"C:\\myJava")) ;
Only neccessary when the class you want to load is using a native library.
//set system property java.ext.dirs
String currentED = System.getPrope rty("java.ext.d irs").toString( );
System.setPrope rty("java.ext.d irs", (currentED + ";" +
"C:\\myJava")) ;
When there's a JAR file in the directory, but you could add that into the
classpath directory as well.

I wonder if there is a way to reinitialize the classloader after these
changes ? And if it's neccessary to do that for what you're trying to do
here...

//create Class object
Class myClass = Class.forName(" myApp");
but the Class.forName method is still throwing ClassNotFoundEx ception
any suggestions?
Raymond mentioned loading the bytes as a class: Raymond DeCampo <rd******@twcny .rr.com> wrote:

If you do not have the option of modifying the classpath, you can load
the file into an array of bytes and the Java API provides a means of
loading the bytes as a class. See the java.lang.Class and
java.lang.Class Loader APIs.

You probably need to write your own classloader.

Do you actually have the API Documentation ?
It's usually a seperate download from Sun's website, but there is also a
Window Helpfile implementation of several JDK versions API's:
http://www.confluent.fr/javadoc/javadoce.html

fu bo

"Neomorph" <ne******@nospa m.demon.co.uk> wrote in message
news:j4******* *************** **********@4ax. com...
On Mon, 1 Sep 2003 12:07:09 +1000, "Fu Bo Xia" <fu****@optusho me.com.au>
two-finger typed:

Reflection is the only way, since you have the class name only in a string.

Since the class file needs to be found through the classpath, you couldtry
setting the java.class.path system property, and then use reflection to
load the class, the byte loading method explained below.

Cheers.

[Raymond DeCampo]

Fu Bo,

OK, I broke down and did it, just to make sure it could be done. The
code is really raw and needs to be cleaned up greatly, but I'll leave
that to you.

Here's the output (you'll have to adjust to Windows, I'm on Linux):

$ java -cp . LoadClassFromFi le tmp/HelloWorld.clas s
Hello World!
$

Real exciting huh? I'll leave the HelloWorld class as an exercise, but
here's the LoadClassFromFi le class:

////// BEGIN LoadClassFromFi le.java
import java.io.*;
import java.lang.refle ct.*;

public class LoadClassFromFi le
{
public static void main(String[] args) throws Exception
{
for (int i = 0; i < args.length; i++)
{
Class klass = loadClass(args[i]);
Method main = klass.getMethod ("main", new Class[]
{String[].class});
main.invoke(nul l, new Object[] {new String[0]});
}
}

private static Class loadClass(final String filename) throws
ClassNotFoundEx ception
{
ClassLoader loader = new ClassLoader()
{
protected Class findClass(Strin g name) throws
ClassNotFoundEx ception
{
// Always load the file
InputStream in = null;
try
{
in = new BufferedInputSt ream(new
FileInputStream (filename));
ByteArrayOutput Stream byteStr = new
ByteArrayOutput Stream();
int byt = -1;
while ((byt = in.read()) != -1)
{
byteStr.write(b yt);
}
byte byteArr[] = byteStr.toByteA rray();
return defineClass(nul l, byteArr, 0, byteArr.length) ;
}
catch (final RuntimeExceptio n rex)
{
throw rex;
}
catch (final Exception ex)
{
ex.printStackTr ace();
throw new ClassNotFoundEx ception(name);
}
finally
{
if (in != null)
{
try
{
in.close();
}
catch (final IOException ioe)
{
ioe.printStackT race();
}
}
}
}
};
return loader.loadClas s("garbage");
}
}
///// END LoadClassFromFi le.java

Again, this code needs a lot of work, but it should give you the basic idea.

Ray

[Fu Bo Xia]

thanks ray, this baby works like a charm : )
i used your codes to write a customer implementation of ClassLoader:

//start of ClassFileLoader .java
import java.io.*;
import java.lang.refle ct.*;

/**
* A custom implementation of ClassLoader that loads class by it's file
name.
*
* use the loadClass(Strin g classFileName) method to return a
java.lang.Class object
* for the class identified by classFileName, the classFileName can be
relative or
* absolute file name eg. C:\myJava\myApp .class
*/
public class ClassFileLoader extends ClassLoader{

public static void main(String[] args) throws Exception {

final String Usage = "USAGE: java ClassFileLoader [class files...]";

if (args.length > 0) {

for (int i = 0; i < args.length; i++)
{
Class klass = new ClassFileLoader ().loadClass((a rgs[i]));
System.out.prin t(klass.getName ());
}
} else {
System.out.prin tln(Usage);
}
}

/**
* original code by Raymond DeCampo <rd******@twcny .rr.com>
*/
protected Class findClass(Strin g fileName) throws ClassNotFoundEx ception {

// Always load the file
InputStream in = null;
try {
in = new BufferedInputSt ream(new FileInputStream (fileName));
ByteArrayOutput Stream byteStr = new ByteArrayOutput Stream();
int byt = -1;
while ((byt = in.read()) != -1) {
byteStr.write(b yt);
}
byte[] byteArr = byteStr.toByteA rray();
return defineClass(nul l, byteArr, 0, byteArr.length) ;

} catch (final RuntimeExceptio n rex) {
throw rex;
} catch (final Exception ex) {
ex.printStackTr ace();
throw new ClassNotFoundEx ception(fileNam e);
} finally {
if (in != null) {
try {
in.close();
} catch (final IOException ioe) {
ioe.printStackT race();
}
}
}
}
}
//end of ClassFileLoader .java
thanks again,
fu bo
"Raymond DeCampo" <rd******@twcny .rr.com> wrote in message
news:gP******** **********@twis ter.nyroc.rr.co m...

Fu Bo,

OK, I broke down and did it, just to make sure it could be done. The
code is really raw and needs to be cleaned up greatly, but I'll leave
that to you.

Here's the output (you'll have to adjust to Windows, I'm on Linux):

$ java -cp . LoadClassFromFi le tmp/HelloWorld.clas s
Hello World!
$

Real exciting huh? I'll leave the HelloWorld class as an exercise, but
here's the LoadClassFromFi le class:

////// BEGIN LoadClassFromFi le.java
import java.io.*;
import java.lang.refle ct.*;

public class LoadClassFromFi le
{
public static void main(String[] args) throws Exception
{
for (int i = 0; i < args.length; i++)
{
Class klass = loadClass(args[i]);
Method main = klass.getMethod ("main", new Class[]
{String[].class});
main.invoke(nul l, new Object[] {new String[0]});
}
}

private static Class loadClass(final String filename) throws
ClassNotFoundEx ception
{
ClassLoader loader = new ClassLoader()
{
protected Class findClass(Strin g name) throws
ClassNotFoundEx ception
{
// Always load the file
InputStream in = null;
try
{
in = new BufferedInputSt ream(new
FileInputStream (filename));
ByteArrayOutput Stream byteStr = new
ByteArrayOutput Stream();
int byt = -1;
while ((byt = in.read()) != -1)
{
byteStr.write(b yt);
}
byte byteArr[] = byteStr.toByteA rray();
return defineClass(nul l, byteArr, 0, byteArr.length) ;
}
catch (final RuntimeExceptio n rex)
{
throw rex;
}
catch (final Exception ex)
{
ex.printStackTr ace();
throw new ClassNotFoundEx ception(name);
}
finally
{
if (in != null)
{
try
{
in.close();
}
catch (final IOException ioe)
{
ioe.printStackT race();
}
}
}
}
};
return loader.loadClas s("garbage");
}
}
///// END LoadClassFromFi le.java

Again, this code needs a lot of work, but it should give you the basic idea.
Ray

[Raymond DeCampo]

Fu Bo Xia wrote:

thanks ray, this baby works like a charm : )
i used your codes to write a customer implementation of ClassLoader:

No problem, I'll be on contact concerning the licensing.



Just kidding :)

Ray