CS 242 Topic Interoperability � Many systems built using multiple languages • Fortran calls to C code, vice versa • Microsoft: VBasic, C, C++ � Communication between modules John Mitchell • Function call with primitive-type arg and result? • Shared objects? • Error handling and exceptions? Topic Three basic approaches � Many systems built using multiple languages � Client-Implementation Intermediary • Fortran calls to C code, vice versa • Convert messages from one implementation to another • Microsoft: VBasic, C, C++ • Example: Corba � Binary compatibility � Communication between modules • Compatible values passed between implementations • Example: COM • Function call with primitive-type arg and result? Yes � Neutral platform • Shared objects? Focus of this lecture • Error handling and exceptions? Still evolving • Run on multiple languages on virtual machine • Example: .Net Background: some related issues Corba Concept � Dynamic linking � Insert “broker” between client and server • Update parts of a system independently – C++: Link compiled code to stub ORB – Stub contains code to interact with DLL request • Share components among different applications � Implementation compatibility SERVER/OBJECT CLIENT • Can C++ component compiled with one compiler be IMPLEMENTATION dynamically linked with component from another? � Inter-language interoperability ORB • Two languages ∼ two compilers Result/ error 1
Corba Architecture Functions of ORB � Communication between client and server Object Client • Insulates application system configuration details Implementation � Specific steps • Intercepts calls • Finds object • Invokes method IDL ORB IDL Object Stub Interface Skeleton Adapter • Passes parameters • Returns results or error messages ORB Core Interface description language CORBA application development Object � Write the IDL interface Client Implementation � Map .idl file to target language (C++, Java, …) • IDL Compiler IDL ORB IDL Object � Develop a Client application Stub Interface Skeleton Adapter � Develop the Server ORB Core � Compile and run the application Interface Description • IDL generates ‘stubs’ and ‘skeleton’ programs for each interface • ‘Stub’ acts like a local function call, providing interface to ORB • ‘Skeleton’ yields server side implementation of IDL interface IDL Example Interface translation Hello.idl � IDL Statement Java Statement module HelloApp package HelloApp; module HelloApp { interface Hello { interface Hello public interface Hello string sayHello(); string sayHello(); String sayHello(); }; }; /* Hello.java as generated by idltojava */ module HelloApp { package HelloApp; HelloHolder.java Hello.java public interface Hello interface Hello { (delegates r/w methods) (IDL interface in Java) string sayHello(); extends org.omg.CORBA.Object { HelloHelper.java String sayHello(); _HelloStub.java }; (auxiliary functionality) (client stub) }; } _HelloImplBase.java (server skeleton) 2
Just to give you some idea of what this looks like … IDL Compiler Output Implementing the Client import HelloApp.*; � _HelloImplBase.java public class HelloClient { import org.omg.CosNaming.*; static Hello helloImpl; • Abstract class provides the server skeleton, basic CORBA functionality import org.omg.CosNaming.NamingContextPackage.*; public static void main(String args[]) { • Implements Hello.java interface import org.omg.CORBA.*; try{ // create and initialize the ORB • server class HelloServant extends _HelloImplBase ORB orb = ORB.init(args, null); � _HelloStub.java // get the root naming context • Client stub, with CORBA functionality for the client org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService"); • Implements Hello.java interface. // Use NamingContextExt instead of NamingContext; part of Interoperable naming Service � Hello.java NamingContextExt ncRef = NamingContextExtHelper.narrow(objRef); // resolve the Object Reference in Naming • Interface containing Java version of IDL interface, extends String name = "Hello"; org.omg.CORBA.Object helloImpl = HelloHelper.narrow(ncRef.resolve_str(name)); � HelloHelper.java System.out.println("Obtained a handle on server object: " + helloImpl); • Final class provides auxiliary functionality, including a narrow method System.out.println(helloImpl.sayHello()); to cast CORBA object references to proper type helloImpl.shutdown(); � HelloHolder.java } • Final class has public instance member of type Hello catch (Exception e) { System.out.println("ERROR : " + e) ; e.printStackTrace(System.out); • Provides operations for out and in/out arguments, which CORBA } } } assumes but do not map immediately to Java Hello World Architecture Inter-ORB Communication Client HelloServer.java Implementation Code Interface Application (Hello Description IDL ORB (Hello Servant.java in IDL Stub Interface Client.java) (Hello.idl) Server Skeleton Client Stubs (_HelloImplBase.java) ORB Core (_HelloStub.java) (Hello.java) Implementation ORB (LINUX) ORB (WNT) TCP/IP IDL IDL TCP/IP Internet Compiler Compiler LINUX Windows NT ORB IDL Object Inter-ORB Protocol Interface Skeleton Adapter Network Hardware Hardware (IIOP) ORB Core See http://java.sun.com/j2se/1.4.2/docs/guide/idl/jidlExample.html Corba Summary Corba ORB Architecture: more details � Interface definition language (IDL) • Define interface in “neutral language” • Compiler generates several related files automatically � Object request broker (ORB) • System intermediary handles requests, response 3
COM: Component Object Model Central ideas in COM � Clients program using interfaces, not classes � Purpose (marketing page) � Implementation code is dynamically linked • “COM is used by developers to create re-usable � Manage requirements at run time software components, link components together to • Object implementors declare runtime requirements build applications, and take advantage of Windows services. …” • System ensures that these requirements are met � Current incarnations � Evolution • COM+, Distributed COM (DCOM) , ActiveX Controls • Interfaces and dynamic linking are classic COM � References • Runtime requirements handled using Microsoft • Don Box, Essential COM Transaction Server (MTS) and COM+ • MS site: http://www.microsoft.com/com/ Motivation Evolution in appreciation for abstraction � 1980s: classes and objects � Build dynamically composable systems • Classes used for object implementation • Not all parts of application are statically linked • Classes also used for consumer/client type hierarchy � Independence of components � Dependencies between client and object • One change should not propagate to all source code • Client assumes complete knowledge of public interface • New components usable in place of old • Client may know even more under certain languages � Compatibility of components (C++) • Use components with different runtime requirements � 1990s: separate interface from implementation • Mix heterogeneous objects in a single process • Client to program in terms of abstract types • Completely hides implementation class from client Interface-Based Programming Interoperability � Define interfaces for classes � Use of interfaces separates implementation • C++ : use abstract base classes as interfaces • Different implementations can coexist � Associate implementations with interfaces • C++: inheritance • These can be built in different languages – Class FastString : public IFastString {...}; � Create implementation objects without exposing layout • Usually a creator or factory function • Manipulate object indirectly through intermediate structure • Class unsuitable for declaring variables – Want to avoid dependence on class � Client must be able delete object • Since new operator not used by the client, cannot call delete • Reference counting can be used 4
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