Loading Classes

Question

Hi,
Here is my problem. I have a directory with several classes (foo_1, foo_2, ...) which all extend a base class (foo); I want to create a vector with an instance of each class in it. The problem is I dont know what the classes will be called or even how many classes will be in the directory when I generate my vector, so I dont know to do this. Does anyone have a suggestion? =)

Answer 1

File f = new File("/path");
File[] files = f.listFiles();
List l = new ArrayList(files.length());
for (int cntr = 0, len = files.length(); cntr < len; cntr++)
l.add(files[cntr]);
That will create a List of the actual File objects. Now if you want to load the class, you would need a little more work. The path you pass uses / (or \ on windows), to separate it, so you would need something to replace / or \ with . to get the fully qualified package name, OR if you already know the fully qualified package name, do this:
String[] fileNames = f.list(new FileNameFilter()
public boolean accept(File dir, String name)
// do code here to determine if file is acceptable:
// possibly checking if file is not dir and only file...
if (dir.isFile())
return true;
Now you have a list of filenames and if you use the filename filter above to block out dirs, you should only have file names (although on linux/unix you may get symbolic link names as well, not sure).
From here you would do something like:
List l = new ArrayList(fileNames.length());
for (int cntr = 0, len = fileNames.length(); cntr < len; cntr++)
l.add(Class.forName(fullyQualifiedPackageName + "." + fileNames[cntr], true, getClass().getClassLoader()).newInstance());
I have not tested this, but it should work. Basically, the first example gets you the File objects, in case that is what you wanted. The above example gets you the list of file names as String objects, you then append that name (in the loop to do the whole dir) to the fully qualified package (how you get that is up to you, perhaps some code to get the dir name, or some other way), and you are dynamically loading it AND creating an instance of it via the Class.forName() call.
Hope that helps.