Add Xtext nature to Eclipse Project

addNature.java

	public static void addNature(IProject project) throws CoreException {
		if (!project.hasNature("org.eclipse.xtext.ui.shared.xtextNature")) {
			IProjectDescription description = project.getDescription();
			String[] prevNatures = description.getNatureIds();
			String[] newNatures = new String[prevNatures.length + 1];
			System.arraycopy(prevNatures, 0, newNatures, 0, prevNatures.length);
			newNatures[prevNatures.length] = "org.eclipse.xtext.ui.shared.xtextNature";
			description.setNatureIds(newNatures);

			project.setDescription(description, null);
		}

usage.java

 public IProject getProjectHandle() {
        return ResourcesPlugin.getWorkspace().getRoot().getProject(
                getProjectName());
    }

/* 
....
*/

IProject project = firstPage.getProjectHandle();
try {
			project.create(null);
			project.open(null);
			SlrProjectSupport.addNature(project);
		} catch (CoreException e) {
			e.printStackTrace();
		}

Another option is to @Inject ToggleXtextNatureAction helper with helper.toggleNature(project)