public  void execute() 
{
        if (file == null) {
            throw new BuildException("import requires file attribute");
        }
        if (getOwningTarget() == null
            || !"".equals(getOwningTarget().getName())) {
            throw new BuildException("import only allowed as a top-level task");
        }
        ProjectHelper helper =
                (ProjectHelper) getProject().
                    getReference(ProjectHelper.PROJECTHELPER_REFERENCE);
        if (helper == null) {
            // this happens if the projecthelper was not registered with the project.
            throw new BuildException("import requires support in ProjectHelper");
        }
        Vector importStack = helper.getImportStack();
        if (importStack.size() == 0) {
            // this happens if ant is used with a project
            // helper that doesn't set the import.
            throw new BuildException("import requires support in ProjectHelper");
        }
        if (getLocation() == null || getLocation().getFileName() == null) {
            throw new BuildException("Unable to get location of import task");
        }
        File buildFile = new File(getLocation().getFileName()).getAbsoluteFile();
        // Paths are relative to the build file they're imported from,
        // *not* the current directory (same as entity includes).
        File buildFileParent = new File(buildFile.getParent());
        File importedFile = FILE_UTILS.resolveFile(buildFileParent, file);
        getProject().log("Importing file " + importedFile + " from "
                         + buildFile.getAbsolutePath(), Project.MSG_VERBOSE);
        if (!importedFile.exists()) {
            String message =
                "Cannot find " + file + " imported from "
                + buildFile.getAbsolutePath();
            if (optional) {
                getProject().log(message, Project.MSG_VERBOSE);
                return;
            } else {
                throw new BuildException(message);
            }
        }
        if (importStack.contains(importedFile)) {
            getProject().log(
                "Skipped already imported file:\n   "
                + importedFile + "\n", Project.MSG_VERBOSE);
            return;
        }
        try {
            helper.parse(getProject(), importedFile);
        } catch (BuildException ex) {
            throw ProjectHelper.addLocationToBuildException(
                ex, getLocation());
        }
}

This relies on the task order model.

 public  void setFile(String file) 
{
        // I don't think we can use File - different rules
        // for relative paths.
        this.file = file;
}

the name of the file to import. How relative paths are resolved is still in flux: use absolute paths for safety.

 public  void setOptional(boolean optional) 
{
                                          
        this.optional = optional;
}

sets the optional attribute