Here is an example of how to use the TrueFileFilter class from the Apache Commons IO library to return all files in a directory:

import org.apache.commons.io.filefilter.TrueFileFilter;

import java.io.File;
import java.io.FileFilter;

public class Main {
    public static void main(String[] args) {
        File dir = new File("/path/to/directory");

        // Creating a FileFilter
        FileFilter allFilesFilter = TrueFileFilter.INSTANCE;

        // Retrieving all files in the directory
        File[] files = dir.listFiles(allFilesFilter);

        for (File file : files) {
            System.out.println(file.getName());
        }
    }
}

In this example, the TrueFileFilter.INSTANCE is used to filter all files in a directory. The listFiles method is then used to retrieve all files in the directory that match the filter, in this case all files in the directory. The names of the files are then printed to the console.

It's worth mentioning that TrueFileFilter.INSTANCE is a singleton instance of TrueFileFilter class, which always returns true for any file that passed to its accept method, that means it accepts all files.