Software components• software re-use– libraries, etc.– inter-language linkage• the Microsoft way– COM: the Component Object Model– Visual Basic: scripting, embedding, viruses–.NET–C#• other approaches to components– CORBA, Java RMI, JavaBeans, ...Software re-use• how do we re-use code written by others?– "If I have seen further than others, it is because I have stood on the shoulders of giants."• source code– e.g., open source • libraries of compiled code– e.g., archives of object files on Unix, DLL's on Windows, Java packages, ...• classes– C++ Standard Template Library– Java Collection framework– ...• objects• components• mashups• application program interfaces (APIs)Libraries• linking to previously compiled code• static linking: all called routines are included in executable• dynamic linking– called routines located and linked in on demandshared libraries on Unix (.so == "shared object")dynamic link libraries (DLL's) on Windowsplug-ins in browsers• advantages of dynamic linking– no cost if a particular routine is not called– minor startup cost for initialization when called– minimal cost when running (extra indirection for call)– library code is shared among all simultaneous uses– can update libraries without updating entire program• some disadvantages– runs in same address space as rest of program, may lead to security issues– DLL hell on Windows: inconsistencies among versionsespecially after install then uninstallCOM: Microsoft's component object model• binary standard for creating & using components– components can be written in any languageIDL (interface definition language) to describe arguments and return values, generate necessary code– components can be in same process, separate process on same machine, or on some other machine (DCOM)DCOM transports include TCP/IP and HTTP– supporting libraries marshal arguments, call functions, retrieve resultsall happens transparently to process that uses it– integral part of Microsoft systemsavailable on non-MS operating systems (sort of?)• COM components are objects with interfaces– interface: functions that provides access to methodsbased on C++ virtual function calls, but implementable in any language– 128-bit GUID (globally unique identifiers)stored in Windows registry so others can find itActiveX• Microsoft's name for technologies and services based on COM• ActiveX components are COM objects– executable code that packages an object as.EXE (standalone executable).DLL (dynamic link library).OCX (visual interface control)• ActiveX controls– COM components with user-interface aspects– written in C++, Java, VB, …– can be used in web pages (analogous to applets, but less restricted)– can be controlled with VBScript, JScript and other scripting languages• ActiveX documents– lets users view and edit non-HTML documents through the browser– integrates existing documents into browser or any other application Calling a COM object• conceptually, what happens when a COM object is called from a program... • first time– find its codelook up in Windows registryregistered during install or when created or by explicit call– do any initializationWindows needs to keep track of what DLLs are in use– link it into current program (if a DLL)fill in calls with pointer to real code: vtbl• each subsequent method call– collect arguments into proper form ("marshalling")–call function– convert return value and output arguments into proper form• when done– do any finalization– release resourceslast user tells Windows that DLL is no longer in useAlternative approaches• CORBA (Common Object Request Broker Architecture)– industry consortium (OMG: Object Management Group)– client-server model, using objects– object-request broker (ORB)communicates client requests to target objects, finds object implementation, activates it if necessary, delivers request, and returns response– IDL (interface definition language) and compiler for specifying and implementing interfaces• Java RMI (Remote Method Invocation)– a remote procedure call mechanism– call objects located (usually) on other systems– very loosely equivalent to (D)COM– can pass objects, not just primitive types• Java Beans (marketing name for Java components)– an API for writing component software in Java– components expose features (methods & events)– visual application builder tools determine properties by "introspection" or "reflection": can query an object about its properties– loosely analogous to ActiveX components– attempting to solve same problems as COM and CORBA, but within JavaVisual Basic • a programming language– modern dialect of Basic (John Kemeny ('47, *49) and Tom Kurtz (*56), 1964)– reasonable control flow, data types, arrays, structures• a toolkit – standard library for math, file I/O, text manipulation– user interface components: buttons, text, menus, ...– extensible: easy access to entire Windows API and existing objectscan add own C/C++ code and create new controls– a "glue" language for assembling from pre-built pieces• an integrated development environment– interactive system for building and testing VB programsdraw interface by dragging and dropping componentsfill in behaviors in code templates, set properties like size, color, position, …manage/edit source code and other resourcesrun in controlled environment for test and debug, compile and export as .EXE file• an extension mechanism– embedded (as VBA) in many other programs, including Word, Excel,Powerpoint, Outlook; can easily extend their capabilities– a vehicle for distributing virusesComponent scripting• component exposes what it can do as an object interface: methods, properties, events– can control object from a programming language that can access objects• a large industry creates such components– written in VB, C++, etc.• VBScript is a scripting version of VB for controlling scriptableobjects– can use it to control scriptable programs– also CScript, WScript, PowerShell, ...• Visual Basic for Applications (VBA) is a version of VB that lives inside some programs– notably Word, Excel, other Office programs, Outlook, …– can use it to control them and other scriptable programs• in general, can do anything from a program that is possible fromkeyboard and mouse– macro recorder to create command sequences–