Separation of Concerns. AspectJ. What if the concerns are Cross-Cutting? SoC: Programming Paradigms. Key theme: Modularity and Encapsulation

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1 Separation of Concerns and AspectJ EEC 625 Lecture #16 April 3, 2006 EEC 625: Software Design & Architecture Separation of Concerns Breaking a program into pieces that overlap in functionality as little as possible. It is, that one is willing to study in depth an aspect of one s subject matter in isolation for the sake of its own consistency, all the time knowing that one is occupying oneself only with one of the aspects. We know that a program must be correct and we can study it from that viewpoint only; we also know that it should be efficient and we can study its efficiency on another day, so to speak. In another mood we may ask ourselves whether, and if so: why, the program is desirable. Edsger Dijkstra EEC 625: Software Design & Architecture 1 SoC: Programming Paradigms Procedural Programming Procedures encapsulate different pieces of code. Each procedure is concerned only with a small piece of functionality. What if the concerns are Cross-Cutting? Can there be some concerns that cannot be cleanly modularized? What does one do with those? Object-Oriented Programming Classes and methods encapsulate different roles and associated operations. Key theme: Modularity and Encapsulation EEC 625: Software Design & Architecture 2 EEC 625: Software Design & Architecture 3 Example: A Simple Database A Sophisticated Database Consider a simple Database component on a server. Clients can query a Value corresponding to some Key in the database. Every query should be logged. Method logging on every method call Only authorized access is allowed. Client operation Search(k : Key) : V alue pre : k Database post : Search = Database[k] Search component Database Authenticate on every method call High availability is a key requirement. Fault-tolerance through replication; send every method call to three copies of object and vote on response EEC 625: Software Design & Architecture 4 EEC 625: Software Design & Architecture 5

2 Cross-cutting Concerns These concerns are all cross-cutting; they affect all methods; in the same way. Corollary: The implementation of such a cross-cutting concern should be modularized separately. Cross-cutting Concerns: Problems Code scattering Code tangling Several Solutions: Aspect-Oriented Programming, Software Containers, Subject-Oriented Programming,... EEC 625: Software Design & Architecture 6 EEC 625: Software Design & Architecture 7 Example: Logging in Tomcat Cost of Tangled Code redundant code same fragment of code in many places difficult to reason about non-explicit structure the big picture of the tangling isn t clear where is logging in org.apache.tomcat red shows lines of code that handle logging not in just one place not even in a small number of places difficult to change have to find all the code involved and be sure to change it consistently and be sure not to break it by accident EEC 625: Software Design & Architecture 8 EEC 625: Software Design & Architecture 9 Aspect-Oriented Programming crosscutting is inherent in complex systems The AOP Process crosscutting concerns have a clear purpose have a natural structure defined set of methods, module boundary crossings, lines of dataflow... capture crosscutting concerns explicitly... Component Aspect Aspect Weaver Final Code in a modular way with linguistic and tool support aspects are well-modularized crosscutting concerns EEC 625: Software Design & Architecture 10 EEC 625: Software Design & Architecture 11

3 What is an Aspect anyway? Aspect Weaving a property that cannot be cleanly encapsulated in a generalized procedure usually a non-functional element: logging, synchronization, caching developmental: used during the implementation phase by developer the process of combining the component and the aspect static: weaving occurs compile-time, source code transformation dynamic: run-time, aspect plug-in, performance consequences production: used for the final product as part of the design EEC 625: Software Design & Architecture 12 EEC 625: Software Design & Architecture 13 Anatomy of an Aspect Two main pieces: Where to add behavior: Join Points Pointcuts identify relevant join points What behavior is to be added: Advice Pointcuts Pointcuts pick out certain join points in the program flow. call(void Point.setXY(int, int)) A pointcut can be built out of other pointcuts with and, or, and not (spelled &&,, and!). pointcut move(): call(void Point.setXY(int, int)) call(void FigureElement.setP1(Point)) Pointcuts can be anonymous or named. EEC 625: Software Design & Architecture 14 EEC 625: Software Design & Architecture 15 Types of pointcuts Name-based crosscutting: Explicit naming of method call join point Property-based crosscutting: Specifying a pointcut in terms of properties of methods other than their exact name call(void Figure.make*(..)) call(public * FigureElement.*(..)) Types of pointcuts: Wildcards * denotes any number of characters expect period.. denotes any number of characters including any number of periods + denotes any subclass of a given type EEC 625: Software Design & Architecture 16 EEC 625: Software Design & Architecture 17

4 Signature Pattern * Type Signature Patterns Matched Types Type of name Types with name ending in Savings, Checking, etc. java.*.date Type Date in any direct subpackage of java java.util.date, java.sql.date java..* Any type in java package, and any subpackages javax..*model+ All types in javax package or its direct and indirect subpackages that have a name ending in Model and their subtypes. Operators and type signatures Signature Pattern!Vector Vector Hashtable javax..*model javax.swing.text.document Matched Types All types other than Vector Vector or Hashtable All types in the javax package that end in Model or javax.swing.text.document java.util.randomaccess+ All types that implement both && java.util.list+ the specified interfaces java.util.arraylist EEC 625: Software Design & Architecture 18 EEC 625: Software Design & Architecture 19 Join Point Kinded Pointcuts Pointcut Syntax Method execution execution(methodsignature) Method call call(methodsignature) Constructor exec execution(constructorsignature) Constructor call call(constructorsignature) Class initialization staticinitialization(typesignature) Field read access get(fieldsignature) Field write access set(fieldsignature) Exception handler handler(typesignature) Object init initialization(constructorsignature) Object pre-init preinitialization(constructorsignature) Advice execution adviceexecution() Control-Flow Based Pointcuts Capturing join points based on the control flow of join points captured by another pointcut. Pointcut cflow(call(*.debit(..)) cflowbelow(call(*.debit(..)) cflow(transactedops()) cflowbelow(execution(.new(..)) cflow(staticinitializer(bankingdatabase)) Description debit method in, including the call to debit() debit method in, excluding the call to debit() All join points in the control flow of the join points captured by the transactedops() pointcut constructor for excluding the call to the constructor itself All join points in the control flow during the class initialization of BankingDatabase EEC 625: Software Design & Architecture 20 EEC 625: Software Design & Architecture 21 Lexical-structure based pointcuts A lexical scope is a segment of source code, referring to the scope of the code as it was written, as opposed to the scope of the code when it is being executed (dynamic scope). Pointcut within() within(+) withincode(*.debit(..)) withincode(* *.getbalance(..)) Description and its subclasses any debit() method in any getbalance() method in any class that ends in Execution object pointcuts Match join points based on the types of the objects at execution time. this() All join points where this is of type target() All join points which are method calls, and the target of the method is of type EEC 625: Software Design & Architecture 22 EEC 625: Software Design & Architecture 23

5 Argument pointcuts Pointcuts based on types of arguments to a method, constructor, or exception handler. args(typepattern or ObjectIdentifier) Advice Next Time Programming using AspectJ EEC 625: Software Design & Architecture 24 EEC 625: Software Design & Architecture 25