conditional import

Question

I am thinking of adding dbus function to a java program that uses swing, so scripts can be used to perform some functions. This thing must run on windows too, where dbus is not available.

So i was thinking of doing the following:

dbus.java:

import dbus; //Whatever the module is called
class dbus implements some_interface {
    //blah blah
}

dbus_fake.java

class dbus_fake implements some_interface {
    //full of empty methods
}

dbus_manager.java

class dbus_manager {
    static some_interface get_dbus() {
        try {
            return new dbus(); //This should fail when loading the class, because the imports are not satisfied.
        } except {
            return new fake_dbus();
        }
    }
}

Do you think it's a good idea? Would it work? Is there a better way of doing it?

It will certainly work if you keep this into separate jars. One for your application, containing the interface. And a jar for each different platform. Simply put the jar of the platform into the classpath and voila. — Jan Goyvaerts, Feb 08 '13 at 15:17
If your project is going to be very big, you might want to look at Dependency Injection frameworks like Spring. They use properties files to managed which specific interface implementations actually get injected into your code. You could then have separate properties files for each operating system. — David Lavender, Feb 08 '13 at 15:20

gustafc · Accepted Answer · 2013-02-08T15:35:47.777

10

It is somewhat unreliable to rely on the constructor to throw ClassNotFoundExceptions, unfortunately (although it may work for your use case, right now).

I'd load the class using Class.forName for more reliable behavior (and in the same time use more Java-ish names ;):

class DBusManager {
    static SomeInterface getDBus() {
        try {
            return Class.forName("your.pkg.RealDBus")
                        .getConstructor()
                        .newInstance();
        } catch(ClassNotFoundException e) {
            return new FakeDBus();
        }
    }
}

edited Feb 08 '13 at 15:35

answered Feb 08 '13 at 15:16

gustafc

28,465
7
73
99

I was too lazy to think of good names for pseudo-code. Maybe the reflection is a good idea. – LtWorf Feb 08 '13 at 15:22

score 1 · Answer 2 · answered Feb 08 '13 at 15:30

1

Why so difficult ? Simply maintain a separate jar file for each platform. It makes the dependencies a lot better, easy to maintain it separately, extend with a new implementation, etc...

windows-dbus.jar
 |
 +- class DBus implements SomeInterface { fake code }


linux-dbus.jar
 |
 +- class DBus implements SomeInterface { real dbus code }

Depending on the platform you're on, include the appropriate jar in the classpath. And in the program request a new instance of DBus.

final SomeInterface bus = new DBus();

If your program is distributed with WebStart you can even mention what jar is destined to what platform.

answered Feb 08 '13 at 15:30

Jan Goyvaerts

2,913
4
35
48

Deploying several jar files is an hell, besides the documented way to run the thing is with ant. – LtWorf Feb 08 '13 at 16:00
That is matter of personal taste of course. :-) But you can easily build the OS-dependend classpath with ant: http://stackoverflow.com/questions/453170/using-ant-to-detect-os-and-set-property – Jan Goyvaerts Feb 08 '13 at 16:09

conditional import

2 Answers2

Related