The FRACTAL component model and its support in Java

class M implements I { abstract void _super_m (); abstract void _this_n (); public int count; public void m () { ++count; ‡This is no longer true with version 1.1 of AspectJ, but this was the case in 2002 when JULIA was developed. Copyright c © 2006 John Wiley & Sons, Ltd. Softw. Pract. Exper. 2006; 36:1257–1284 DOI: 10.1002/spe THE FRACTAL COMPONENT MODEL AND ITS SUPPORT IN JAVA 1267 _this_n(); _super_m(); } } Mixin classes can be mixed, resulting in normal classes. More precisely, the result of mixing several mixin classes M1, . . . , Mn, in this order, is a normal class that is equivalent to a class Mn extending the Mn−1 class, itself extending the Mn−2 class, . . . itself extending the M1 class (constructors are ignored; an empty public constructor is generated for the mixed classes). Several mixin classes can be mixed only if each method and field required by a mixin class Mi is provided by a mixin class Mj , with j < i (each required method and field may be provided by a different mixin). For example, if N and O designate the following mixins: abstract class N implements I { abstract class O implements I { abstract void _super_m (); public void m () { System.out.println("m"); public void m () { } System.out.println("m called"); public void n () { _super_m(); System.out.println("n"); } } } }class N implements I { abstract class O implements I { abstract void _super_m (); public void m () { System.out.println("m"); public void m () { } System.out.println("m called"); public void n () { _super_m(); System.out.println("n"); } } } } then the mixed class O N M is equivalent to the following class (note that this class implements all of the interfaces implemented by the mixin classes): public class C55d992cb_0 implements I, Generated { // from O private void m$1 () { System.out.println("m"); } public void n () { System.out.println("n"); } // from N private void m$0 () { System.out.println("m called"); m$1(); } // from M public int count; public void m () { ++count; n(); m$0(); } } Copyright c © 2006 John Wiley & Sons, Ltd. Softw. Pract. Exper. 2006; 36:1257–1284 DOI: 10.1002/spe 1268 E. BRUNETON ET AL. The mixed classes are generated dynamically, directly in bytecode form with ASM [10], by the MixinClassGenerator class. In order to ease debugging, the class generator keeps the line numbers of the mixin classes in the mixed class. More precisely, a line number l of the mixin class at index i (in the list of mixin classes, and starting from 1) is transformed into 1000 ∗ i + l. For example, if a new Exception().printStackTrace() were added in the N.m method, the stack trace would contain a line at C55d992cb 0.m$0(ONM:2005), meaning that the exception was created in method m$0 of the C55d992cb 0 class, whose source is the ONM mixed class, at line 5 of mixin 2, i.e. at line 5 of the N mixin class.