Using AWT and Swing with OPJ

By default, if an application has active windows when a checkpoint occurs, the windows are treated as roots of persistence, and will be recreated when the application resumes.

This is one case where the absence of persistence threads can cause a problem if, as is typical, the main application class creates the windows. This will result in duplicate windows appearing on subsequent runs. There are two ways to work around this problem. The first is to use a boolean flag to indicate whether this is the first run, e.g.

public class Main {
  private static boolean firstRun = true;
  public static void main(String[] args) {
    if (firstRun) {
      // create windows
      firstRun = false;
    }
    // use windows
  }
}

The second solution, which is convenient if the source code cannot be modified, is to exploit the opj command line option "-mainonce". This option encapsulates the notion of "firstRun" in the system itself, avoiding the execution of "main" completely on subsequent runs. Effectively it is as if the entire body of the main method was guarded by the test of "firstRun". Of course, this only works if the code in main conforms to the model, and doesn't execute any other code that needs to be run on every resumption..

As an example, the SwingSet demo program can be executed successfully this way, and provides a good example of the effect of persistent user interfaces. To run the SwingSet demo, execute the following, where $OPJ is the path to your installation:

% setenv PATH $OPJ/bin:$PATH
% cd $OPJ/demo/jfc/SwingSet
% opj -config $OPJ/sample_store_configs/tmp.pjc -jar SwingSet.jar -mainonce -mainroot SwingSet

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