newbie java design question

B

bob_roberts

- new to java (from C++ world)
Setup:
1. Abstract class A
2. Child C inherits from A.
3. Child D inherits from A.

In a client, I want to determine which child implementation to use
(via a config file, for instance). The main catch is that I do not
want any dependencies on either C or D for the client. That is, if
C is an implementation that invokes third-party code, the client which
decided (via config) to invoke C should make no reference to jar files
for child D (or any of its third-party jar files).

I've been looking into the URLClassLoader option for this, since I could
dynamically determine which implementation class to load and have at it.

In C++, I could just use dlopen() on a shared object, and load a
library(ies)
which included references to third-party libraries. Then I would
reference the loaded child class through a base class pointer and I don't
have any dependencies on shared objects which may not exist
on the system in which I'm currently running the client. I'm kind of
looking for something along those lines.

Is there an accepted "pattern" or general rule of thumb that I should look
into?

Or have I completely missed some key point in Java which would
allow me to do this more easily?

Thanks in advance...

bobr
 
M

Matt Rose

bob_roberts said:
- new to java (from C++ world)
Setup:
1. Abstract class A
2. Child C inherits from A.
3. Child D inherits from A.

In a client, I want to determine which child implementation to use
(via a config file, for instance). The main catch is that I do not
want any dependencies on either C or D for the client. That is, if
C is an implementation that invokes third-party code, the client which
decided (via config) to invoke C should make no reference to jar files
for child D (or any of its third-party jar files).

I've been looking into the URLClassLoader option for this, since I could
dynamically determine which implementation class to load and have at it.

In C++, I could just use dlopen() on a shared object, and load a
library(ies)
which included references to third-party libraries. Then I would
reference the loaded child class through a base class pointer and I don't
have any dependencies on shared objects which may not exist
on the system in which I'm currently running the client. I'm kind of
looking for something along those lines.

Is there an accepted "pattern" or general rule of thumb that I should look
into?

Or have I completely missed some key point in Java which would
allow me to do this more easily?

If you're not using something like Spring, (or even if you are!) you
could reflect the class.

Something along the lines of:

//read the fully qualified classname from your config file into
AImplClassName
Class aImpl = Class.forName(AImplClassName);
Constructor[] constructors = aImpl.getConstructors();
//Choose which constructor you want to invoke, create an Object[] with
the arguments for it called args
A a = (A) chosenConstructor.newInstance(args);
return a;

Is that what you're after?

Matt
 
O

Oliver Wong

bob_roberts said:
- new to java (from C++ world)
Setup:
1. Abstract class A
2. Child C inherits from A.
3. Child D inherits from A.

In a client, I want to determine which child implementation to use
(via a config file, for instance). The main catch is that I do not
want any dependencies on either C or D for the client. That is, if
C is an implementation that invokes third-party code, the client which
decided (via config) to invoke C should make no reference to jar files
for child D (or any of its third-party jar files).

I've been looking into the URLClassLoader option for this, since I could
dynamically determine which implementation class to load and have at it.

In C++, I could just use dlopen() on a shared object, and load a
library(ies)
which included references to third-party libraries. Then I would
reference the loaded child class through a base class pointer and I don't
have any dependencies on shared objects which may not exist
on the system in which I'm currently running the client. I'm kind of
looking for something along those lines.

Is there an accepted "pattern" or general rule of thumb that I should look
into?

Or have I completely missed some key point in Java which would
allow me to do this more easily?

For a "pattern", look at Factory or Abstract Factory.

For Java support, look at
http://java.sun.com/j2se/1.5.0/docs/api/javax/imageio/spi/ServiceRegistry.html

So your client code would see A and the factory. The factory would use the
ServiceRegistry to see what classes are available that implements A, and
return an appropriate instance of that class to the client.

- Oliver
 
B

bob_roberts

Matt Rose said:
bob_roberts said:
- new to java (from C++ world)
Setup:
1. Abstract class A
2. Child C inherits from A.
3. Child D inherits from A.

In a client, I want to determine which child implementation to use
(via a config file, for instance). The main catch is that I do not
want any dependencies on either C or D for the client. That is, if
C is an implementation that invokes third-party code, the client which
decided (via config) to invoke C should make no reference to jar files
for child D (or any of its third-party jar files).

I've been looking into the URLClassLoader option for this, since I could
dynamically determine which implementation class to load and have at it.

In C++, I could just use dlopen() on a shared object, and load a
library(ies)
which included references to third-party libraries. Then I would
reference the loaded child class through a base class pointer and I don't
have any dependencies on shared objects which may not exist
on the system in which I'm currently running the client. I'm kind of
looking for something along those lines.

Is there an accepted "pattern" or general rule of thumb that I should
look
into?

Or have I completely missed some key point in Java which would
allow me to do this more easily?

If you're not using something like Spring, (or even if you are!) you
could reflect the class.

Something along the lines of:

//read the fully qualified classname from your config file into
AImplClassName
Class aImpl = Class.forName(AImplClassName);
Constructor[] constructors = aImpl.getConstructors();
//Choose which constructor you want to invoke, create an Object[] with
the arguments for it called args
A a = (A) chosenConstructor.newInstance(args);
return a;

Is that what you're after?

Matt


Thanks for the reply. That sounds like a good idea. I wasn't aware that
the constructors could be invoked like this, hence, my newbie state.

Oliver Wong also replied with an idea about using ServiceRegistry which I
will
look into as well.

Thanks to both of you for taking the time to answer this!

bobr
 
T

Thomas Fritsch

Matt Rose said:
If you're not using something like Spring, (or even if you are!) you
could reflect the class.

Something along the lines of:

//read the fully qualified classname from your config file into
AImplClassName
Class aImpl = Class.forName(AImplClassName);
Constructor[] constructors = aImpl.getConstructors();
//Choose which constructor you want to invoke, create an Object[] with
the arguments for it called args
A a = (A) chosenConstructor.newInstance(args);
return a;

If the implementation classes have a no-arguments constructor, this can be
simplified even further:
A a = (A) Class.forName(AImplClassName).newInstance();
return a;
 

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Members online

Forum statistics

Threads
473,744
Messages
2,569,484
Members
44,905
Latest member
Kristy_Poole

Latest Threads

Top