I'm using Eclipse JDT to build a Java refactoring platform, for exploring different refactorings in memory before choosing one and saving it. I can create collections of working copies of the source files, edit them in memory, and commit the changes to disk using the JDT framework. However, I also want to generate new 'working copy' source files in memory as part of refactorings, and only create the corresponding real source file if I commit the working copy. I have seen various hints that this is possible, e.g. http://www.jarvana.com/jarvana/view/org/eclipse/jdt/doc/isv/3.3.0-v20070613/isv-3.3.0-v20070613.jar!/guide/jdt%5Fapi%5Fmanip.htm says "Note that the compilation unit does not need to exist in the Java model in order for a working copy to be created". So far I have only been able to create a new real file, i.e. ICompilationUnit newICompilationUnit = myPackage.createCompilationUnit(newName, "package piffle; public class Baz{private int i=0;}", false, null); This is not what I want. Does anyone know how to create a new 'working copy' source file, that does not appear in my file system until I commit it? Or any other mechanism to achieve the same thing?