«

Hi all,

Can I execute programmaticlly the main of a class which I only the
..class file of?

"Meidan" <> wrote in message
news: oups.com...
> Hi all,
>
> Can I execute programmaticlly the main of a class which I only the
> .class file of?
>
You can but do you really want to?

If you want to execute a file called Foo.class directly from the command
line, you automatically execute main() first (assuming Foo is an
application, not an applet). For example:

java Foo

will always execute the main() method first.

If you invoke Foo from a class called Bar, it will look like this:

=======================
public class Bar {

Foo myFoo = new Foo();
}
========================

but the method called will be the appropriate Foo constructor (the one that
takes no parameters in this example) and not main().

It would be rather unusual to want to invoke the main() method of Foo from
Bar. Invoking some other method of Foo wouldn't be that unusual but main()
is typically only invoked by running Foo from the command line.

However, you could probably invoke main() via reflection if you really
wanted to. I believe it is somewhat expensive but it should be possible. I
don't have a link to a tutorial on Reflection handy but you should be able
to find one via Google if you really want to execute Foo's main from another
class.

Rhino

On Wed, 14 Dec 2005 13:25:21 -0500, Rhino wrote:
> If you invoke Foo from a class called Bar, it will look like this:

Or even simpler:

Foo.main(args);

No need for an instance, constructors or reflection.

/gordon

--
[ do not email me copies of your followups ]
g o r d o n + n e w s @ b a l d e r 1 3 . s e

Gordon Beaton wrote:
> On Wed, 14 Dec 2005 13:25:21 -0500, Rhino wrote:
> > If you invoke Foo from a class called Bar, it will look like this:
>
> Or even simpler:
>
> Foo.main(args);
>
> No need for an instance, constructors or reflection.
>
> /gordon
>
> --
> [ do not email me copies of your followups ]
> g o r d o n + n e w s @ b a l d e r 1 3 . s e

I already tried that.
But it wont compile - it says Foo cannot be resolved.

Well, you'll need to make sure Foo is in the correct place in your
classpath when compiling

BartCr wrote:
> Well, you'll need to make sure Foo is in the correct place in your
> classpath when compiling

But I didn't compile Foo, I only have the .class.

Meidan wrote:
> Hi all,
>
> Can I execute programmaticlly the main of a class which I only the
> ..class file of?

// Yes.

import java.io.PrintWriter;
import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;

public class ClassMain {

static PrintWriter err = new PrintWriter(System.err, true);

public static void main(String[] args) {
callMain(Helper.class, args);
}

static void callMain(Class c, Object args) {
try {
Method m = c.getDeclaredMethod(
"main", String[].class);

m.invoke(null, args);

} catch(NoSuchMethodException x) {
err.println(x);
} catch(IllegalAccessException x) {
err.println(x);
} catch(InvocationTargetException x) {
err.println(x);
}
}
}

class Helper {

static PrintWriter out = new PrintWriter(System.out, true);

public static void main(String[] args) {
out.println("Hello from Helper.");
}
}

"Meidan" <> wrote in news:1134587714.717782.182840
@g14g2000cwa.googlegroups.com:

>
> BartCr wrote:
>> Well, you'll need to make sure Foo is in the correct place in your
>> classpath when compiling
>
> But I didn't compile Foo, I only have the .class.
>

Well, you'll need to make sure Foo is in the correct place in your
classpath when compiling Bar.

ie: javac -classpath path/to/Foo/ Bar.java

--
Beware the False Authority Syndrome

"BartCr" <> wrote in news:1134587444.240877.166390
@g14g2000cwa.googlegroups.com:

> Well, you'll need to make sure Foo is in the correct place in your
> classpath when compiling
>

Also make sure you import it if it's defined in a package.

--
Beware the False Authority Syndrome

> But I didn't compile Foo, I only have the .class.

That's not a problem, all you need is the .class. But, it needs to be
in your classpath, so that the compiler can find it; the compiler needs
to see it so that it can check your code (e.g. to see whether or not
Foo contains a main() method, or else it won't be able to generate code
for the Foo.main() call).

Alternatively, use the reflection approach mentioned earlier:

try {
Class c = Class.forName("Foo");
String[] args = new String[] { "Hello", "world" };
Method m = c.getMethod("main", new Class[] { args.getClass() });
m.invoke(null, args);
} catch (Exception e) {
// Possible exceptions include ClassNotFoundException,
// NoSuchMethodException, InvocationTargetException
e.printStackTrace();
}