Executing a compiled class' main from another class

[Meidan]

Hi all,

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

 

[Rhino]

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

 

[Gordon Beaton]

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

 

[Meidan]

Or even simpler:

Foo.main(args);

No need for an instance, constructors or reflection.

/gordon

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

 

[BartCr]

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

 

[Meidan]

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.

 

[Jeffrey Schwab]

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.");
}
}

 

[zero]

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

 

[zero]

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.

 

[Thomas Okken]

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();
}

 

[Meidan]

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();
}

I get a java.lang.ClassNotFoundException when Class.forName("Foo") is
executed.

I'm using Eclipse and I found in project properties the parameter "java
build path", I added to it the location of my .class file. But still no
luck.......

 

[Gordon Beaton]

I get a java.lang.ClassNotFoundException when Class.forName("Foo")
is executed.

So you've got classpath issues. Does "Foo" have a real name? Does it
belong to a package?

If so, the classpath should not point to the directory containing the
classfile. It should point to a directory containing a hierarchy of
directories that correspond to the component parts of the package name.

Learn more about the necessary relationship between the classpath and
the directory structure here:

http://www.yoda.arachsys.com/java/packages.html

Type "javap Foo" to see what the file contains if you don't already
know.

/gordon

 

[Roedy Green]

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

of course. Most of the time you don't get source from the authors.

 

[Meidan]

So you've got classpath issues. Does "Foo" have a real name? Does it
belong to a package?

If so, the classpath should not point to the directory containing the
classfile. It should point to a directory containing a hierarchy of
directories that correspond to the component parts of the package name.

Learn more about the necessary relationship between the classpath and
the directory structure here:

http://www.yoda.arachsys.com/java/packages.html

Type "javap Foo" to see what the file contains if you don't already
know.

/gordon

My file system structure is:
Exercise
FruitGamePlayer
.metadata(dir)
Player
.metadata
.classpath
.project
Tester.java
MyPlayer.java
(and some more .class files for internal use in MyPlayer)

FruitGamesServer
.metadata(dir)
GamesServer
.project
GameDefinition.class
GameServerMain.class
HandlingServers.class
PlayerServer.class
.classpath
.project

What do I have to do in order to call GameServerMain.main() from
Tester.java?
Note that I can call it from the command line by java -classpath .;..
GameServerMain from within the GameServer Directory.

 

[Meidan]

The contents of my main .classpath file:
<?xml version="1.0" encoding="UTF-8"?>
<classpath>
<classpathentry kind="src" path="FruitGamePlayer/Player"/>
<classpathentry kind="src" path="FruitGameServer/GameServer"/>
<classpathentry kind="con"
path="org.eclipse.jdt.launching.JRE_CONTAINER"/>
<classpathentry kind="con" path="FruitGameServer/GameServer"/>
<classpathentry kind="con" path="FruitGameServer/GameServer"/>
<classpathentry kind="output" path="FruitGamePlayer/Player"/>
<classpathentry kind="output" path="FruitGameServer/GameServer"/>
</classpath>