Problems implementing plug-ins....!

[Parth Bhatt]

Hello,

I'm on a project that involves the use of Plug-ins. Each plug-in is implemented as a class.

As per the advice given to me in my earlier post, I've created an interface called Plugin.
And each class that wants to act as a plug-in, implements this interface.

Furthermore, I Have a String array that holds class names of all available plug-ins.
Now, my problem is to initialize the object of type Plugin with any of the strings in the string array.

The following program illustrates this problem.

    
class Test{

Plugin plugin;
 
public Test(){
 
String pluginList={"MyPlugin1","MyPlugin2","MyPlugin3"};
 
/*
  I wish to initialize plugin with one of MyPlugin1, MyPlugin2 or MyPlugin3
  e.g. plugin=new MyPlugin2();
*/
 
plugin.readMessage();
plugin.sendMessage();
}
 
public static void main(String[] args){
 
Test t=new Test();
}
}

Please advise...!

[Stan James]

Look at Class in the JavaDoc. The bits you'll need to figure out are forName and newInstance:

Class.forName(someString).newInstance();

To have a variable called "plugin" that can point to an instance of any of your various plugin classes, they'll all need something in common. Are you comfortable with extending an abstract class or implementing an interface?

Then you'll extend your recipe to:

AbstractPlugin plugin = Class.forName...

How will you choose which plugin to use? Maybe an index into your array of classnames?

Show us what you come up with!

[Parth Bhatt]

hi,
i think i've got my solution... Thanks a ton, Stan...

Actually, what i plan to do is : have a plugins directory inside my application directory. My application will discover the plugins at startup.

The following apply to each plugin :

1) Each plugin comes as a jar file.
2) A class in each plugin(i.e. jar file) implements an interface called Plugin, which forces the class to have a certain set of methods.
3) Each jar file has a "plugin.properties" file which has the name of the class that implements plugin

for example : This is what my interface Plugin would look like :

    
interface Plugin{
 
   void sendMessage(String);
   void readMessage();
}

now each class that wants to act as a plugin(this will be inside the jar file) implements this interface.

for example : take into consideration the class MyPlugin :

  
class MyPlugin implements Plugin{
 
   public void sendMessage(String msg){
 
          System.out.println("Sending Message...:"+msg);
 
     }
 
   public void readMessage(){
 
          System.out.println("Receiving Message...!");
     }
 
}

Inside My main class, i'll have the following lines of code :

    
   Plugin plugin=Class.forName("MyPlugin").newInstance();/*Thanks to you for this line  :) */
 
   .....
 
  /*and then call the methods*/
   
  plugin.sendMessage("My Message");
  plugin.readMessage();

Which plugin to load depends upon the option that the user chooses from the GUI...

Loading a perticular Plugin involves,
1) reading the jarfile for the class that implements Plugin in the file plugin.properties.
2) creating an instance of that class.

The second step is where i was stuck...
What i had posted as a question was a simplified version of this problem so thst it would be easy for people to answet it...!

Thanks again Stan, Do commment upon this mechenism of implementing plugins.

[Ulf Dittmer]

The mechanism seems OK, but this:

Class.forName("MyPlugin").newInstance()

will only work if the jar files are in your classpath. If they're not, you will need to create your own classloader which knows how to load classes/jar files from a directory named "plugins".

[Parth Bhatt]

hi Ulf,

Yup !! i'm aware of the fact that my jar file should be in the CLASSPATH... i plan to create an Environment Variable on the target machine for this pourpose...!

Thanks for validating my idea...