Introduction to Aspect Oriented Software Development

Aspect Oriented Software Development Viviane Jonckers et al. 2007-2009 Many of the slides in this presentation are adapted from the AOSD-Europe white paper by J. Brichau and T. D’Hondt Others are adapted from material previously available on the AOSD.net web site by G. Kiczales and others.

Course Overview Separation Of Concerns and AOSD Aspect-Oriented Programming in AspectJ Framework-based AOP approaches Case study: Aspects in Enterprise Software Current research topics in AOSD p.

Course Organisation Theory + Practice All exercises for this course must be handed in by the given deadline Submit your solution through the website. A link will be posted on the course page: http://ssel.vub.ac.be/ssel/teaching:courses:aosd Package your solution in a ZIP-file with subfolders ex1, ex2, etc for each exercise. For questions you can contact Bruno De Fraine E-mail: [email_address] p.

Course Evaluation Presentation and participation in the discussions in the Exam Workshop (33%) Exercises assignments (33%) Project (33%) p.

Software Complexity p. Need for adequate software engineering techniques Functional Requirements Software Development Requirements Non-functional Requirements + + Complexity: =>

Evolution of Software Engineering p. Machine-level Programming Complex code Difficult to read & write Poor evolvability Poor maintainability Poor reusability Program Main program data

Evolution of Software Engineering p. Structured Programming + Language features for common program patterns Easier to read & write Poor evolvability Poor maintainability Poor reusability Program Main program data

Evolution of Software Engineering p. Procedural Programming + Procedural abstractions + Parameter passing + Recursion Easier to read & write Improved evolvability Improved maintainability Improved reusability Program Main program data Procedure 1 Procedure 2 Procedure n

Evolution of Software Engineering p. Modular Programming + Module abstractions Easier to read & write Better evolvability Better maintainability Better reusability Program Main program data module 1 data 1 Procedure 1 module 2 data 2 Procedure 2 Procedure n

Evolution of Software Engineering p. Object-oriented Programming + Encapsulation + Polymorphism + Inheritance Easier to read & write Better evolvability Better maintainability Better reusability Program Object 1 data Object 2 data Object 3 data Object 4 data

Evolution of Software Engineering p. Aspect-orientation Component orientation Generative Programming ....

Separation of Concerns p. [E.W. Dijkstra] Let me try to explain to you, what to my taste is characteristic for all intelligent thinking. 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 […] But nothing is gained – on the contrary – by tackling these various aspects simultaneously. It is what I sometimes have called “ the separation of concerns ” […]

Modular programming Organize code by grouping data/functionality Need for language mechanisms Drives evolution of languages & paradigms Separation of Concerns p. Concern : “ Something the developer needs to care about ” (e.g. functionality, QoS requirement,..) Separation of concerns : handle each concern separately

Separation of Concerns p. concern concern concern concern

Separation of Concerns SoC is one of the main goals of software engineering and allows to: reduce software complexity and improve comprehensibility; promote traceability within and across artifacts and throughout the lifecycle; limit the impact of change, facilitating evolution and non-invasive adaptation and customization; facilitate reuse; and simplify component integration. p. Tarr et al., ICSE 2000. Parnas, CACM 15(12), 1972.

Crosscutting Concern Example Implementation of Apache Tomcat webserver Analyzed implementation of 3 concerns: XML parsing concern URL pattern matching concern Logging concern p.

Crosscutting concerns Good modularity: handled by code in one class p. [Picture taken from the aspectj.org website] XML Parsing in Apache Tomcat

Crosscutting concerns Good modularity: handled by code in two classes related by inheritance p. [Picture taken from the aspectj.org website] URL handling in Apache Tomcat

BAD modularity: handled by code that is scattered over almost all classes Crosscutting concerns p. [Picture taken from the aspectj.org website] Logging in Apache Tomcat

Scattering & Tangling p. scattering – code addressing one concern is spread around in the code tangling – code in one region addresses multiple concerns scattering and tangling tend to appear together; they describe different facets of the same problem /* * ==================================================================== * * The Apache Software License, Version 1.1 * * Copyright (c) 1999 The Apache Software Foundation. All rights * reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in * the documentation and/or other materials provided with the * distribution. * * 3. The end-user documentation included with the redistribution, if * any, must include the following acknowlegement: * "This product includes software developed by the * Apache Software Foundation (http://www.apache.org/)." * Alternately, this acknowlegement may appear in the software itself, * if and wherever such third-party acknowlegements normally appear. * * 4. The names "The Jakarta Project", "Tomcat", and "Apache Software * Foundation" must not be used to endorse or promote products derived * from this software without prior written permission. For written * permission, please contact apache@apache.org. * * 5. Products derived from this software may not be called "Apache" * nor may "Apache" appear in their names without prior written * permission of the Apache Group. * * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE * DISCLAIMED. IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. * ==================================================================== * * This software consists of voluntary contributions made by many * individuals on behalf of the Apache Software Foundation. For more * information on the Apache Software Foundation, please see * <http://www.apache.org/>. * * [Additional notices, if required by prior licensing conditions] * */ package org.apache.tomcat.session; import org.apache.tomcat.core.*; import org.apache.tomcat.util.StringManager; import java.io.*; import java.net.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; /** * Core implementation of an application level session * * @author James Duncan Davidson [duncan@eng.sun.com] * @author Jason Hunter [jch@eng.sun.com] * @author James Todd [gonzo@eng.sun.com] */ public class ApplicationSession implements HttpSession { private StringManager sm = StringManager.getManager("org.apache.tomcat.session"); private Hashtable values = new Hashtable(); private String id; private ServerSession serverSession; private Context context; private long creationTime = System.currentTimeMillis();; private long thisAccessTime = creationTime; private long lastAccessed = creationTime; private int inactiveInterval = -1; private boolean valid = true; ApplicationSession(String id, ServerSession serverSession, Context context) { this.serverSession = serverSession; this.context = context; this.id = id; this.inactiveInterval = context.getSessionTimeOut(); if (this.inactiveInterval != -1) { this.inactiveInterval *= 60; } } ServerSession getServerSession() { return serverSession; } /** * Called by context when request comes in so that accesses and * inactivities can be dealt with accordingly. */ void accessed() { // set last accessed to thisAccessTime as it will be left over // from the previous access lastAccessed = thisAccessTime; thisAccessTime = System.currentTimeMillis(); validate(); } void validate() { // if we have an inactive interval, check to see if we've exceeded it if (inactiveInterval != -1) { int thisInterval = (int)(System.currentTimeMillis() - lastAccessed) / 1000; if (thisInterval > inactiveInterval) { invalidate(); } } } // HTTP SESSION IMPLEMENTATION METHODS public String getId() { if (valid) { return id; } else { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } } public long getCreationTime() { if (valid) { return creationTime; } else { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } } /** * * @deprecated */ public HttpSessionContext getSessionContext() { return new SessionContextImpl(); } public long getLastAccessedTime() { if (valid) { return lastAccessed; } else { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } } public void invalidate() { serverSession.removeApplicationSession(context); // remove everything in the session Enumeration enum = values.keys(); while (enum.hasMoreElements()) { String name = (String)enum.nextElement(); removeValue(name); } valid = false; } public boolean isNew() { if (! valid) { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } if (thisAccessTime == creationTime) { return true; } else { return false; } } /** * @deprecated */ public void putValue(String name, Object value) { setAttribute(name, value); } public void setAttribute(String name, Object value) { if (! valid) { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } if (name == null) { String msg = sm.getString("applicationSession.value.iae"); throw new IllegalArgumentException(msg); } removeValue(name); // remove any existing binding if (value != null && value instanceof HttpSessionBindingListener) { HttpSessionBindingEvent e = new HttpSessionBindingEvent(this, name); ((HttpSessionBindingListener)value).valueBound(e); } values.put(name, value); } /** * @deprecated */ public Object getValue(String name) { return getAttribute(name); } public Object getAttribute(String name) { if (! valid) { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } if (name == null) { String msg = sm.getString("applicationSession.value.iae"); throw new IllegalArgumentException(msg); } return values.get(name); } /** * @deprecated */ public String[] getValueNames() { Enumeration e = getAttributeNames(); Vector names = new Vector(); while (e.hasMoreElements()) { names.addElement(e.nextElement()); } String[] valueNames = new String[names.size()]; names.copyInto(valueNames); return valueNames; } public Enumeration getAttributeNames() { if (! valid) { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } Hashtable valuesClone = (Hashtable)values.clone(); return (Enumeration)valuesClone.keys(); } /** * @deprecated */ public void removeValue(String name) { removeAttribute(name); } public void removeAttribute(String name) { if (! valid) { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } if (name == null) { String msg = sm.getString("applicationSession.value.iae"); throw new IllegalArgumentException(msg); } Object o = values.get(name); if (o instanceof HttpSessionBindingListener) { HttpSessionBindingEvent e = new HttpSessionBindingEvent(this,name); ((HttpSessionBindingListener)o).valueUnbound(e); } values.remove(name); } public void setMaxInactiveInterval(int interval) { if (! valid) { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } inactiveInterval = interval; } public int getMaxInactiveInterval() { if (! valid) { String msg = sm.getString("applicationSession.session.ise"); throw new IllegalStateException(msg); } return inactiveInterval; } } //----------------------------------------------------------------------- package org.apache.tomcat.session; import org.apache.tomcat.core.*; import org.apache.tomcat.util.StringManager; import java.io.*; import java.net.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; /** * Core implementation of a server session * * @author James Duncan Davidson [duncan@eng.sun.com] * @author James Todd [gonzo@eng.sun.com] */ public class ServerSession { private StringManager sm = StringManager.getManager("org.apache.tomcat.session"); private Hashtable values = new Hashtable(); private Hashtable appSessions = new Hashtable(); private String id; private long creationTime = System.currentTimeMillis();; private long thisAccessTime = creationTime; private long lastAccessed = creationTime; private int inactiveInterval = -1; ServerSession(String id) { this.id = id; } public String getId() { return id; } public long getCreationTime() { return creationTime; } public long getLastAccessedTime() { return lastAccessed; } public ApplicationSession getApplicationSession(Context context, boolean create) { ApplicationSession appSession = (ApplicationSession)appSessions.get(context); if (appSession == null && create) { // XXX // sync to ensure valid? appSession = new ApplicationSession(id, this, context); appSessions.put(context, appSession); } // XXX // make sure that we haven't gone over the end of our // inactive interval -- if so, invalidate and create // a new appSession return appSession; } void removeApplicationSession(Context context) { appSessions.remove(context); } /** * Called by context when request comes in so that accesses and * inactivities can be dealt with accordingly. */ void accessed() { // set last accessed to thisAccessTime as it will be left over // from the previous access lastAccessed = thisAccessTime; thisAccessTime = System.currentTimeMillis(); } void validate() void validate() { // if we have an inactive interval, check to see if // we've exceeded it if (inactiveInterval != -1) { int thisInterval = (int)(System.currentTimeMillis() - lastAccessed) / 1000; if (thisInterval > inactiveInterval) { invalidate(); ServerSessionManager ssm = ServerSessionManager.getManager(); ssm.removeSession(this); } } } synchronized void invalidate() { Enumeration enum = appSessions.keys(); while (enum.hasMoreElements()) { Object key = enum.nextElement(); ApplicationSession appSession = (ApplicationSession)appSessions.get(key); appSession.invalidate(); } } public void putValue(String name, Object value) { if (name == null) { String msg = sm.getString("serverSession.value.iae"); throw new IllegalArgumentException(msg); } removeValue(name); // remove any existing binding values.put(name, value); } public Object getValue(String name) { if (name == null) { String msg = sm.getString("serverSession.value.iae"); throw new IllegalArgumentException(msg); } return values.get(name); } public Enumeration getValueNames() { return values.keys(); } public void removeValue(String name) { values.remove(name); } public void setMaxInactiveInterval(int interval) { inactiveInterval = interval; } public int getMaxInactiveInterval() { return inactiveInterval; } // XXX // sync'd for safty -- no other thread should be getting something // from this while we are reaping. This isn't the most optimal // solution for this, but we'll determine something else later. synchronized void reap() { Enumeration enum = appSessions.keys(); while (enum.hasMoreElements()) { Object key = enum.nextElement(); ApplicationSession appSession = (ApplicationSession)appSessions.get(key); appSession.validate(); } } } ; /** * Standard implementation of the Session interface. This object is * serializable, so that it can be stored in persistent storage or transferred * to a different JVM for distributable session support. * * IMPLEMENTATION NOTE: An instance of this class represents both the * internal (Session) and application level (HttpSession) view of the session. * However, because the class itself is not declared public, Java logic outside * of the <code>org.apache.tomcat.session</code> package cannot cast an * HttpSession view of this instance back to a Session view. * * @author Craig R. McClanahan * @version $Revision: 1.2 $ $Date: 2000/05/15 17:54:10 $ */ final class StandardSession implements HttpSession, Session { // ----------------------------------------------------------- Constructors /** * Construct a new Session associated with the specified Manager. * * @param manager The manager with which this Session is associated */ public StandardSession(Manager manager) { super(); this.manager = manager; } /** * The last accessed time for this Session. */ private long lastAccessedTime = creationTime; /** * The Manager with which this Session is associated. */ private Manager manager = null; /** * The maximum time interval, in seconds, between client requests before * the servlet container may invalidate this session. A negative time * indicates that the session should never time out. */ private int maxInactiveInterval = -1; /** * Flag indicating whether this session is new or not. */ private boolean isNew = true; /** * Flag indicating whether this session is valid or not. */ private boolean isValid = false; /** * The string manager for this package. */ private StringManager sm = StringManager.getManager("org.apache.tomcat.session"); /** * The HTTP session context associated with this session. */ private static HttpSessionContext sessionContext = null; /** * The current accessed time for this session. */ private long thisAccessedTime = creationTime; // ----------------------------------------------------- Session Properties /** * Set the creation time for this session. This method is called by the * Manager when an existing Session instance is reused. * * @param time The new creation time */ public void setCreationTime(long time) { this.creationTime = time; this.lastAccessedTime = time; this.thisAccessedTime = time; } /** * Return the session identifier for this session. */ public String getId() { return (this.id); } /** * Set the session identifier for this session. * * @param id The new session identifier */ public void setId(String id) { if ((this.id != null) && (manager != null) && (manager instanceof ManagerBase)) ((ManagerBase) manager).remove(this); this.id = id; if ((manager != null) && (manager instanceof ManagerBase)) ((ManagerBase) manager).add(this); } /** * Return descriptive information about this Session implementation and * the corresponding version number, in the format * <code><description>/<version></code>. */ public String getInfo() { return (this.info); } /** * Return the last time the client sent a request associated with this * session, as the number of milliseconds since midnight, January 1, 1970 * GMT. Actions that your application takes, such as getting or setting * a value associated with the session, do not affect the access time. */ public long getLastAccessedTime() { return (this.lastAccessedTime); } /** * Return the Manager within which this Session is valid. */ public Manager getManager() { return (this.manager); } /** * Set the Manager within which this Session is valid. * * @param manager The new Manager */ public void setManager(Manager manager) { this.manager = manager; } /** * Return the maximum time interval, in seconds, between client requests * before the servlet container will invalidate the session. A negative * time indicates that the session should never time out. * * @exception IllegalStateException if this method is called on * an invalidated session */ public int getMaxInactiveInterval() { return (this.maxInactiveInterval); /** * Update the accessed time information for this session. This method * should be called by the context when a request comes in for a particular * session, even if the application does not reference it. */ public void access() { this.lastAccessedTime = this.thisAccessedTime; this.thisAccessedTime = System.currentTimeMillis(); this.isNew=false; } /** * Perform the internal processing required to invalidate this session, * without triggering an exception if the session has already expired. */ public void expire() { // Remove this session from our manager's active sessions if ((manager != null) && (manager instanceof ManagerBase)) ((ManagerBase) manager).remove(this); // Unbind any objects associated with this session Vector results = new Vector(); Enumeration attrs = getAttributeNames(); while (attrs.hasMoreElements()) { String attr = (String) attrs.nextElement(); results.addElement(attr); } Enumeration names = results.elements(); while (names.hasMoreElements()) { String name = (String) names.nextElement(); removeAttribute(name); } // Mark this session as invalid setValid(false); } /** } /** * Set the <code>isNew</code> flag for this session. * * @param isNew The new value for the <code>isNew</code> flag */ void setNew(boolean isNew) { this.isNew = isNew; } /** * Set the <code>isValid</code> flag for this session. * * @param isValid The new value for the <code>isValid</code> flag */ void setValid(boolean isValid) { this.isValid = isValid; } // ------------------------------------------------- HttpSession Properties /** * Return the time when this session was created, in milliseconds since * midnight, January 1, 1970 GMT. * * @exception IllegalStateException if this method is called on an * invalidated session */ public long getCreationTime() { return (this.creationTime); } /** * Return the session context with which this session is associated. * * @deprecated As of Version 2.1, this method is deprecated and has no * replacement. It will be removed in a future version of the * Java Servlet API. */ public HttpSessionContext getSessionContext() { if (sessionContext == null) sessionContext = new StandardSessionContext(); return (sessionContext); } // ----------------------------------------------HttpSession Public Methods /** * Return the object bound with the specified name in this session, or * <code>null</code> if no object is bound with that name. * * @param name Name of the attribute to be returned * * @exception IllegalStateException if this method is called on an * invalidated session */ public Object getAttribute(String name) { return (attributes.get(name)); } /** * Return an <code>Enumeration</code> of <code>String</code> objects * containing the names of the objects bound to this session. * * @exception IllegalStateException if this method is called on an * invalidated session */ public Enumeration getAttributeNames() { return (attributes.keys()); } /** * Return the object bound with the specified name in this session, or * <code>null</code> if no object is bound with that name. * * @param name Name of the value to be returned * * @exception IllegalStateException if this method is called on an * invalidated session * * @deprecated As of Version 2.2, this method is replaced by * <code>getAttribute()</code> */ public Object getValue(String name) { return (getAttribute(name)); } /** * Return the set of names of objects bound to this session. If there * are no such objects, a zero-length array is returned. * * @exception IllegalStateException if this method is called on an * invalidated session * * @deprecated As of Version 2.2, this method is replaced by * <code>getAttributeNames()</code> */ public String[] getValueNames() { Vector results = new Vector(); Enumeration attrs = getAttributeNames(); while (attrs.hasMoreElements()) { String attr = (String) attrs.nextElement(); results.addElement(attr); } String names[] = new String[results.size()]; for (int i = 0; i < names.length; i++) names[i] = (String) results.elementAt(i); return (names); } /** * Invalidates this session and unbinds any objects bound to it. * * @exception IllegalStateException if this method is called on * an invalidated session */ public void invalidate() { // Cause this session to expire expire(); } /** * Return <code>true</code> if the client does not yet know about the * session, or if the client chooses not to join the session. For * example, if the server used only cookie-based sessions, and the client * has disabled the use of cookies, then a session would be new on each * request. * * @exception IllegalStateException if this method is called on an * invalidated session */ public boolean isNew() { return (this.isNew); } * Remove the object bound with the specified name from this session. If * the session does not have an object bound with this name, this method * does nothing. * * After this method executes, and if the object implements * <code>HttpSessionBindingListener</code>, the container calls * <code>valueUnbound()</code> on the object. * * @param name Name of the object to remove from this session. * * @exception IllegalStateException if this method is called on an * invalidated session */ public void removeAttribute(String name) { synchronized (attributes) { Object object = attributes.get(name); if (object == null) return; attributes.remove(name); // System.out.println( "Removing attribute " + name ); if (object instanceof HttpSessionBindingListener) { ((HttpSessionBindingListener) object).valueUnbound (new HttpSessionBindingEvent((HttpSession) this, name)); } } } * Bind an object to this session, using the specified name. If an object * of the same name is already bound to this session, the object is * replaced. * * After this method executes, and if the object implements * <code>HttpSessionBindingListener</code>, the container calls * <code>valueBound()</code> on the object. * * @param name Name to which the object is bound, cannot be null * @param value Object to be bound, cannot be null * * @exception IllegalArgumentException if an attempt is made to add a * non-serializable object in an environment marked distributable. * @exception IllegalStateException if this method is called on an * invalidated session */ public void setAttribute(String name, Object value) { if ((manager != null) && manager.getDistributable() && !(value instanceof Serializable)) throw new IllegalArgumentException (sm.getString("standardSession.setAttribute.iae")); synchronized (attributes) { removeAttribute(name); attributes.put(name, value); if (value instanceof HttpSessionBindingListener) ((HttpSessionBindingListener) value).valueBound (new HttpSessionBindingEvent((HttpSession) this, name)); } } // -------------------------------------------- HttpSession Private Methods /** * Read a serialized version of this session object from the specified * object input stream. * * IMPLEMENTATION NOTE: The reference to the owning Manager * is not restored by this method, and must be set explicitly. * * @param stream The input stream to read from * * @exception ClassNotFoundException if an unknown class is specified * @exception IOException if an input/output error occurs */ private void readObject(ObjectInputStream stream) throws ClassNotFoundException, IOException { // Deserialize the scalar instance variables (except Manager) creationTime = ((Long) stream.readObject()).longValue(); id = (String) stream.readObject(); lastAccessedTime = ((Long) stream.readObject()).longValue(); maxInactiveInterval = ((Integer) stream.readObject()).intValue(); isNew = ((Boolean) stream.readObject()).booleanValue(); isValid = ((Boolean) stream.readObject()).booleanValue(); // Deserialize the attribute count and attribute values int n = ((Integer) stream.readObject()).intValue(); for (int i = 0; i < n; i++) { String name = (String) stream.readObject(); Object value = (Object) stream.readObject(); attributes.put(name, value); } } /** * Write a serialized version of this session object to the specified * object output stream. * * IMPLEMENTATION NOTE: The owning Manager will not be stored * in the serialized representation of this Session. After calling * <code>readObject()</code>, you must set the associated Manager * explicitly. * * IMPLEMENTATION NOTE: Any attribute that is not Serializable * will be silently ignored. If you do not want any such attributes, * be sure the <code>distributable</code> property of our associated * Manager is set to <code>true</code>. * * @param stream The output stream to write to * * @exception IOException if an input/output error occurs */ private void writeObject(ObjectOutputStream stream) throws IOException { // Write the scalar instance variables (except Manager) stream.writeObject(new Long(creationTime)); stream.writeObject(id); stream.writeObject(new Long(lastAccessedTime)); stream.writeObject(new Integer(maxInactiveInterval)); stream.writeObject(new Boolean(isNew)); stream.writeObject(new Boolean(isValid)); // Accumulate the names of serializable attributes Vector results = new Vector(); Enumeration attrs = getAttributeNames(); while (attrs.hasMoreElements()) { String attr = (String) attrs.nextElement(); Object value = attributes.get(attr); if (value instanceof Serializable) results.addElement(attr); } // Serialize the attribute count and the attribute values stream.writeObject(new Integer(results.size())); Enumeration names = results.elements(); while (names.hasMoreElements()) { String name = (String) names.nextElement(); stream.writeObject(name); stream.writeObject(attributes.get(name)); } } crosscut invalidate(StandardSession s): s & (int getMaxInactiveInterval() | long getCreationTime() | Object getAttribute(String) | Enumeration getAttributeNames() | String[] getValueNames() | void invalidate() | boolean isNew() | void removeAttribute(String) | void setAttribute(String, Object)); static advice(StandardSession s): invalidate(s) { before { if (!s.isValid()) throw new IllegalStateException (s.sm.getString("standardSession." + thisJoinPoint.methodName + ".ise")); } } } // -------------------------------------------------------------- Private Class /** * This class is a dummy implementation of the <code>HttpSessionContext</code> * interface, to conform to the requirement that such an object be returned * when <code>HttpSession.getSessionContext()</code> is called. * * @author Craig R. McClanahan * * @deprecated As of Java Servlet API 2.1 with no replacement. The * interface will be removed in a future version of this API. */ final class StandardSessionContext implements HttpSessionContext { private Vector dummy = new Vector(); /** * Return the session identifiers of all sessions defined * within this context. * * @deprecated As of Java Servlet API 2.1 with no replacement. * This method must return an empty <code>Enumeration</code> * and will be removed in a future version of the API. */ public Enumeration getIds() { return (dummy.elements()); } /** * Return the <code>HttpSession</code> associated with the * specified session identifier. * * @param id Session identifier for which to look up a session * * @deprecated As of Java Servlet API 2.1 with no replacement. * This method must return null and will be removed in a * future version of the API. */ public HttpSession getSession(String id) { return (null); } }

Cost of scattered and tangled code redundant code same or similar fragment of code in many places difficult to reason about the big picture isn’t clear difficult to change have to find all the code involved and be sure to change it consistently get no help from OO tools p.

Examples of CCC Logging Caching Security Access Control Confidentiality Transaction Management Persistance Error recovery … p. public void boe(String s, Key k) { log(“entering method boe with arguments … “); … … log(“exiting method boe“); } public String compute(Object input) { Object[] args = new Object[] {input}; Object res = cache.fetch(“myclz.compute”,args); if(res!=null) //cache contains value return (String) res; … //store result into computedResult cache.store(“myclz.compute”,args,computedResult); return computedResult; } public void sensitive(Object input) { if(!checkPermissions(currentUser,”sensitive”,this)) throw new AccessException(“access denied “); … } public void transactional(Object input) { Transaction t = transactionManager.startTransaction(); try{ … }catch(Throwable t) { transactionManager.rollback(t); } transactionManager.commit(t); } public void setProperty(String value) { … . getPersistanceManager.saveObject(this); }

BAD modularity: handled by code that is scattered over almost all classes Aspect-Oriented Programming p. [Picture taken from the aspectj.org website] Logging in Apache Tomcat

Aspect-Oriented Programming p. Logging Aspect Logging in Apache Tomcat … pointcut joinpoint

The AOSD idea crosscutting is inherent in complex systems “ tyranny of the dominant decomposition” crosscutting concerns have a clear purpose What have some regular interaction points Where/When AOP proposes to capture crosscutting concerns explicitly... in a modular way with programming language support and with tool support p.

Tyranny of the Dominant Decomposition Not only for a given decomposition But for all possible decompositions Not only in object-orientation! Also in other paradigms Not only in implementation! Also in analysis & design stages p. Given one out of many possible decompositions of the problem... (mostly core functionality concerns) ...then some subproblems cannot be modularized! (non-functional, functional, added after the facts,...)

Aspectual Decomposition p. [Gregor Kiczales] Many existing programming languages, including object-oriented languages, procedural languages and functional languages, can be seen as having a common root in that their key abstraction and composition mechanisms are all rooted in some form of generalized procedure .

Explicit Invocation p. Program Main program data Procedure 1 Procedure 2 Procedure n Program Main program data module 1 data 1 Procedure 1 module 2 data 2 Procedure 2 Procedure n Program Object 1 data Object 2 data Object 3 data Object 4 data

Explicit Invocation p. Program Object 1 data Object 2 data Object 3 data Object 4 data Aspect Program Object 1 data Object 2 data Object 3 data Object 4 data

Implicit Invocation p. Program Object 1 data Object 2 data Object 3 data Object 4 data Aspect Program Object 1 data Object 2 data Object 3 data Object 4 data

Implicit Invocation is Captured by an aspect p. Aspect captures its own invocation that crosscuts other modules Objects are invoked by other objects through message sends

AOSD Terminology joinpoint: point in the program’s execution e.g. Executing method setX on type Y pointcut: A set of joinpoints e.g. The execution of all methods which name start with “set” on type Y advice: What to do at a certain pointcut: e.g. Print a logging string before the program continues. weaving: applying the advice to all joinpoints defined in the pointcut declaration p.

Aspects p. Aspect Aspect applicability code Aspect functionality code Where / when What Pointcut Advice

Joinpoints p. Application Object 1 data Object 2 data Object 3 data Object 4 data Aspect joinpoint: ● A join point is a point of interest in some artefact in the software lifecycle through which two or more concerns may be composed. Examples in implementation artefact: - message sends - method executions - error throwing - variable assignments - ...

Join point Model Specific to each aspect-oriented programming language E.g. AspectJ join point model: key points in dynamic call graph p. A join point model defines the kinds of join points available and how they are accessed and used.

Pointcuts p. A pointcut is a predicate that matches join points. A pointcut is a relationship ‘join point -> boolean’, where the domain of the relationship is all possible join points. Aspect Aspect applicability code Aspect functionality code Pointcut Advice

Advice p. Aspect Aspect applicability code Aspect functionality code Pointcut Advice

Example: Synchronised buffer p. Tangling! Crosscutting concerns! class Buffer { char[] data; int nrOfElements; Semaphore sema; bool isEmpty() { bool returnVal; sema.writeLock(); returnVal := nrOfElements == 0; sema.unlock(); return returnVal; } } Synchronisation concern Buffer functionality concern

Synchronisation as an Aspect p. When a Buffer object receives the message isEmpty, first make sure the object is not being accessed by another thread through the get or put methods When to execute the aspect (pointcut) Composition of when and what (kind of advice) What to do at the join point (advice) When a Buffer object receives the message isEmpty, first make sure the object is not being accessed by another thread through the get or put methods

Synchronisation as an Aspect p. class Buffer { char[] data; int nrOfElements; bool isEmpty() { bool returnVal; returnVal := nrOfElements == 0; return returnVal; } } aspect Syncronizer Semaphore sem; before: reception(Buffer.isEmpty) { sema.writeLock();} after: reception(Buffer.isEmpty) { sema.unlock(); } Aspect Pointcut Advice

Example Domain-specific Aspect Language p. class Stack { push(...) { ... } pop() { ... } ... } coordinator BoundedStackCoord { selfExclusive {pop,push}; mutExclusive {pop,push}; } Synchronisation concern is separated from other concerns using an aspect language for synchronisation

Advice code Domain-specific Aspect Languages Language targeted towards one kind of aspect COOL & RIDL (Synchronisation) RG (Loop fusion optimization) Describe a concern But Limited in expressiveness New Language for each kind of aspect?? General-purpose Aspect Languages More-general aspects possible Describe crosscutting p.

Asymmetric vs. Symmetric Decompositions p. Described until now Asymmetric Symmetric Crosscutting concerns modularized in special module (aspect). All concerns modularized in same kind of module

The Asymmetric AOSD Model (AspectJ style) describe crosscutting concerns as separate, independent entities and weave them with the base program p. Aspects Base program Weaving

The Symmetric AOSD Model (HyperJ style) describe different concerns as separate program fragments and weave them p. Program Fragments Weaving

Symmetric Approaches p. Multidimensional Separation of Concerns

Symmetric Approaches p. composition specification ~ in terms of join points

The Synchronised buffer, revisited p. class Buffer { char[] data; int nrOfElements; bool isEmpty() { bool returnVal; returnVal := nrOfElements == 0; return returnVal; } } class Synchronizer { Semaphore sema; bool lock() { return sema.writeLock; } bool unlock() { return sema.unlock(); } }

Towards AOSD Aspects in the whole lifecycle Requirements Analysis&design Architecure Implementation Verification Testing … Aspect aware technology Aspect aware tools p.

AOSD Timeline p. 1997 Introduction of AOP 2001 2002 2004 { 2000 Advanced Separation of Concerns First AOSD Conference AOP Workshops Advanced Separation of Concerns Workshops AOSD EU Aspect-Oriented Software Development

Conclusions Separation of Concerns Crosscutting concerns phenomenon Aspect-orientation Modularisation of crosscutting concerns Aspects: implicit invocation Join points, pointcuts, advices Aspect-oriented Software Development AO in the entire software development lifecycle p.

Introduction to Aspect Oriented Software Development

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