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This short course will teach you how to use CORBA as found in the JavaTM 2 platform. Course OutlineDistributed ApplicationsCORBA products provide a framework for the development and execution of distributed applications. But why would one want to develop a distributed application in the first place? As you will see later, distribution introduces a whole new set of difficult issues. However, sometimes there is no choice; some applications by their very nature are distributed across multiple computers because of one or more of the following reasons:
Data are DistributedSome applications must execute on multiple computers because the data that the application must access exist on multiple computers for administrative and ownership reasons. The owner may permit the data to be accessed remotely but not stored locally. Or perhaps the data cannot be co-located and must exist on multiple heterogeneous systems for historical reasons. Computation is DistributedSome applications execute on multiple computers in order to take advantage of multiple processors computing in parallel to solve some problem. Other applications may execute on multiple computers in order to take advantage of some unique feature of a particular system. Distributed applications can take advantage of the scalability and heterogeneity of the distributed system. Users are DistributedSome applications execute on multiple computers because users of the application communicate and interact with each other via the application. Each user executes a piece of the distributed application on his or her computer, and shared objects, typically execute on one or more servers. A typical architecture for this kind of application is illustrated below. ![]() Prior to designing a distributed application, it is essential to understand some of the fundamental realities of the distributed system on which it will execute. Fundamental Realities of Distributed SystemsDistributed application developers must address a number of issues that can be taken for granted in a local program where all logic executes in the same operating system process. The following table summarizes some of the basic differences between objects that are co-located in the same process, and objects that interact across process or machine boundaries.
The communication between objects in the same process is orders of magnitude faster than communication between objects on different machines. The implication of this is that you should avoid designing distributed applications in which two or more distributed objects have very tight interactions. If they do have tight interactions, they should be co-located. When two objects are co-located, they fail together; if the process in which they execute fails, both objects fail. The designer of the objects need not be concerned with the behavior of the application if one of the objects is available and the other one is not. But if two objects are distributed across process boundaries, the objects can fail independently. In this case, the designer of the objects must be concerned with each of the object's behavior in the event the other object has failed. Similarly, in a distributed system the network can partition and both objects can execute independently assuming the other has failed. The default mode for most local programs is to operate with a single thread of control. Single threaded programming is easy. Objects are accessed in a well-defined sequential order according to the program's algorithms, and you need not be concerned with concurrent access. If you decide to introduce multiple threads of control within a local program, you must consider the possible orderings of access to objects and use synchronization mechanisms to control concurrent access to shared objects. But at least you have a choice of introducing multiple threads of control. In a distributed application, there are necessarily multiple threads of control. Each distributed object is operating in a different thread of control. A distributed object may have multiple concurrent clients. As the developer of the object and the developer of the clients, you must consider this concurrent access to objects and use the necessary synchronization mechanisms. When two objects are co-located in the same process, you need not be concerned about security. When the objects are on different machines, you need to use security mechanisms to authenticate the identity of the other object. Distributed Object SystemsDistributed object systems are distributed systems in which all entities are modeled as objects. Distributed object systems are a popular paradigm for object-oriented distributed applications. Since the application is modeled as a set of cooperating objects, it maps very naturally to the services of the distributed system. In spite of the natural mapping from object-oriented modeling to distributed object systems, do not forget the realities of distributed systems described above. Process boundaries really do matter and they will impact your design. That said, the next section of this course discusses the CORBA standard for distributed object systems. What is CORBA?CORBA, or Common Object Request Broker Architecture, is a standard architecture for distributed object systems. It allows a distributed, heterogeneous collection of objects to interoperate. The OMGThe Object Management Group (OMG) is responsible for defining CORBA. The OMG comprises over 700 companies and organizations, including almost all the major vendors and developers of distributed object technology, including platform, database, and application vendors as well as software tool and corporate developers. CORBA ArchitectureCORBA defines an architecture for distributed objects. The basic CORBA paradigm is that of a request for services of a distributed object. Everything else defined by the OMG is in terms of this basic paradigm. The services that an object provides are given by its interface. Interfaces are defined in OMG's Interface Definition Language (IDL). Distributed objects are identified by object references, which are typed by IDL interfaces. The figure below graphically
depicts a request. A client holds
an object reference to a distributed object. The object reference
is typed by an interface. In the figure below the object reference
is typed by the
The ORBThe ORB is the distributed service that implements the request to the remote object. It locates the remote object on the network, communicates the request to the object, waits for the results and when available communicates those results back to the client. The ORB implements location transparency. Exactly the same request mechanism is used by the client and the CORBA object regardless of where the object is located. It might be in the same process with the client, down the hall or across the planet. The client cannot tell the difference. The ORB implements programming language independence for the request. The client issuing the request can be written in a different programming language from the implementation of the CORBA object. The ORB does the necessary translation between programming languages. Language bindings are defined for all popular programming languages. CORBA as a Standard for Distributed ObjectsOne of the goals of the CORBA specification is that clients and object implementations are portable. The CORBA specification defines an application programmer's interface (API) for clients of a distributed object as well as an API for the implementation of a CORBA object. This means that code written for one vendor's CORBA product could, with a minimum of effort, be rewritten to work with a different vendor's product. However, the reality of CORBA products on the market today is that CORBA clients are portable but object implementations need some rework to port from one CORBA product to another. CORBA 2.0 added interoperability as a goal in the specification. In particular, CORBA 2.0 defines a network protocol, called IIOP (Internet Inter-ORB Protocol), that allows clients using a CORBA product from any vendor to communicate with objects using a CORBA product from any other vendor. IIOP works across the Internet, or more precisely, across any TCP/IP implementation. Interoperability is more important in a distributed system than portability. IIOP is used in other systems that do not even attempt to provide the CORBA API. In particular, IIOP is used as the transport protocol for a version of JavaTM RMI (so called "RMI over IIOP"). Since EJB is defined in terms of RMI, it too can use IIOP. Various application servers available on the market use IIOP but do not expose the entire CORBA API. Because they all use IIOP, programs written to these different API's can interoperate with each other and with programs written to the CORBA API. CORBA ServicesAnother important part of the CORBA standard is the definition of a set of distributed services to support the integration and interoperation of distributed objects. As depicted in the graphic below, the services, known as CORBA Services or COS, are defined on top of the ORB. That is, they are defined as standard CORBA objects with IDL interfaces, sometimes referred to as "Object Services."
There are several CORBA services. The popular ones are described in detail in another module of this course. Below is a brief description of each:
CORBA ProductsCORBA is a specification; it is a guide for implementing products. Several vendors provide CORBA products for various programming languages. The CORBA products that support the Java programming language include:
Providing detailed information about all of these products is beyond the scope of this introductory course. This course will just use examples from both Sun's Java 2 ORB and Inprise's VisiBroker 3.x for Java products. The Stock ApplicationThe stock trading application is a distributed application that illustrates the Java programming language and CORBA. In this introductory module only a small simple subset of the application is used. Feel free to expand upon the application to enhance it once you are more comfortable with CORBA. The stock application allows multiple users to watch the activity of stocks. The user is presented with a list of available stocks identified by their stock symbols. The user can select a stock and then press the "view" button.
Selecting the "view" button results in a report about the stock, indicating the name of the company, the stock symbol, the current price, the last time it was updated, the trading volume, and a graph that shows the stock price over some interval. This report is automatically updated as new stock data becomes available.
The stock report also lets the user set an alarm by pressing the "Alarm" button. The alarm can be set to activate when the price of the stock falls below a certain price or when it exceeds a certain price.
When the price of the stock satisfies the alarm's condition, it activates and the user is notified.
Later the application could be extended to allow users to buy and sell stocks. Some Objects in the Stock ApplicationFrom the above description, you can easily identify the following distributed objects in the application.
The Stock object is now used to illustrate the CORBA distributed object model. Implementing a CORBA ClientThis section covers what you need to know to use CORBA objects from the Java programming language. It examines OMG IDL interfaces, the Java programming language binding for IDL interfaces, object references, and requests, how to obtain object references, and how, as a client, to create distributed objects. After reading this section and completing the exercises, you should be able to write a client using the Java programming language. Again, the stock example is used to illustrate the client's model of CORBA. CORBA Objects are Described by IDL InterfacesThe OMG Interface Definition Language IDL supports the specification of object interfaces. An object interface indicates the operations the object supports, but not how they are implemented. That is, in IDL there is no way to declare object state and algorithms. The implementation of a CORBA object is provided in a standard programming language, such as the Java programming language or C++. An interface specifies the contract between code using the object and the code implementing the object. Clients only depend on the interface. IDL interfaces are programming language neutral. IDL defines language bindings for many different programming languages. This allows an object implementor to choose the appropriate programming language for the object. Similarly, it allows the developer of the client to choose the appropriate and possibly different programming language for the client. Currently, the OMG has standardized on language bindings for the C, C++, Java, Ada, COBOL, Smalltalk, Objective C, and Lisp programming languages. So by using OMG IDL, the following can be described without regards to any particular programming language:
The IDL data types are:
Again, IDL says nothing about object implementations. Here's the IDL interface for the example stock objects: module StockObjects { struct Quote { string symbol; long at_time; double price; long volume; }; exception Unknown{}; interface Stock { // Returns the current stock quote. Quote get_quote() raises(Unknown); // Sets the current stock quote. void set_quote(in Quote stock_quote); // Provides the stock description, // e.g. company name. readonly attribute string description; }; interface StockFactory { Stock create_stock( in string symbol, in string description ); }; }; Note that the above example defines an IDL module named
The module defines a scope for these names. Within the module,
a data structure IDL declarations are compiled with an IDL compiler and converted to their associated representations in the target programming languages according to the standard language binding. (This course uses the Java language binding in all of the examples. Later you will see the Java binding in more depth.) Object References and RequestsClients issue a request on a CORBA object using an object
reference. An object reference identifies the distributed
object that will receive the request. Here's a Java programming language code
fragment that obtains a Stock theStock = ... try { Quote current_quote = theStock.get_quote(); } catch (Throwable e) { } Object references can be passed around the distributed object
system, i.e. as parameters to operations and returned as results
of requests. For example, notice that the StockFactory factory = ... Stock theStock = ... try { theStock = factory.create( "GII", "Global Industries Inc."); } catch (Throwable e) { } Note that issuing a request on a CORBA object is not all that different from issuing a request on a Java object in a local program. The main difference is that the CORBA objects can be anywhere. The CORBA system provides location transparency, which implies that the client cannot tell if the request is to an object in the same process, on the same machine, down the hall, or across the planet. Another difference from a local Java object is that the life time of the CORBA object is not tied to the process in which the client executes, nor to the process in which the CORBA object executes. Object references persist; they can be saved as a string and recreated from a string. The following Java code converts the String stockString = orb.object_to_string(theStock); The string can be stored or communicated outside of the distributed object system. Any client can convert the string back to an object reference and issue a request on the distributed object. This Java code converts the string back to a org.omg.CORBA.Object obj = orb.string_to_object(stockString); Stock theStock = StockHelper.narrow(obj); Note that the resulting type of the IDL Type SystemIDL interfaces can be defined in terms of other IDL interfaces.
You previously saw a Consider another IDL module: module ReportingObjects { exception EventChannelFailure{}; interface Reporting { // Receive events in push mode CosEventComm::PushSupplier push_events( in CosEventComm::PushConsumer consumer) raises(EventChannelFailure); // Receive events in pull mode CosEventComm::PullSupplier pull_events( in CosEventComm::PullConsumer consumer) raises(EventChannelFailure); }; }; The Given the definition of the interface ReportingStock: Reporting, Stock { }; A
All CORBA interfaces implicitly inherit the Object references are typed by IDL interfaces. In a Java program
you could type an object reference to be a ReportingStock theReportingStock; Clients can pass this object reference to an operation expecting
a supertype. For example assume there is an interface EventManager { : void register(in Reporting event_supplier); : }; The following is a legal request because a EventManager manager = ... ReportingStock theReportingStock = ... manager->register(theReportingStock); // ok However, the following is not a legal request because a EventManager manager = ... Stock theStock = ... manager->register(theStock); // type error IDL Type OperationsGiven that IDL interfaces can be arranged in a hierarchy, a
small number of operations are defined on that hierarchy. The
org.omg.CORBA.Object obj = ... Stock theStock = StockHelper.narrow(obj); The if (obj._is_a(StockHelper.id()) ... The IDL:StockObjects/Stock:1.0 Finally, it is possible to widen an object reference, that is cast it to a less specific interface: Stock theStock = theReportingStock; There are no special operations to widen an object reference. It is accomplished exactly as in the Java programming language. Request Type CheckingThe IDL compiler for Java programming language generates client-side stubs, which represent the CORBA object locally in the Java programming language. The generated code also represents in the Java programming language all of the IDL interfaces and data types used to issue requests. The client code thus depends on the generated Java code.
As you previously saw, passing an object reference typed by
the IDL to Java BindingThe Java binding for IDL maps the various IDL constructs to corresponding Java constructs. The following table shows how the IDL constructs are represented in the Java programming language. For comparison, the C++ binding is also shown.
Each of the IDL data types are represented in the Java programming language as follows:
When discussing data type mapping, one term you run across frequently is marshaling. Marshaling is the conversion of a language-specific data structure into the CORBA IIOP streaming format. IIOP data can then be transmitted over a network to its destination, where it is then unmarshaled from IIOP back into a language-dependent data structure. IDL to Java CompilerCORBA products provide an IDL compiler that converts IDL into
the Java programming language. The IDL compiler available for the Java 2 SDK is
called For the stock example, the command "
The developer compiles the IDL using the IDL compiler and then compiles the generated code using the Java compiler. The compiled code must be on the classpath of the running Java program. Obtaining Object ReferencesYou may have noticed that there are three fundamental mechanisms in which a piece of code can obtain an object reference:
These fundamental mechanisms are supported by the ORB. Using these mechanisms, it is possible to define higher level services for locating objects in the distributed object system. The Client's Model of Object CreationYou may decide to export the ability to create an object to the distributed object system. You can accomplish this by defining a factory for the object. Factories are simply distributed objects that create other distributed objects. There is nothing special about a factory. It is just another distributed object: It has an IDL interface, it is implemented in some programming language, and clients issue standard CORBA requests on factory objects. There is no standard interface for a factory. Recall in the
interface StockFactory { Stock create_stock( in string stock_symbol, in string stock_description); }; To create a stock object, a client simply issues a request on the factory. Another object implementor could define an object factory differently. ExceptionsAs you have seen in the stock example, CORBA has a concept
of exceptions that is very similar to that of the Java programming language;
naturally, CORBA exceptions are mapped to Java exceptions. When
you issue a CORBA request, you must use the Java programming language's There are two types of CORBA exceptions, System Exceptions
and User Exceptions. System Exceptions are thrown
when something goes wrong with the system--for instance, if
you request a method that doesn't exist on the server, if there's
a communication problem, or if the ORB hasn't been initialized
correctly. The Java class CORBA System Exceptions can contain "minor codes" which may provide additional information about what went wrong. Unfortunately, these are vendor-specific, so you need to tailor your error recovery routines to the ORB you're using. User Exceptions are generated if something goes wrong
inside the execution of the remote method itself. These are declared
inside the IDL definition for the object, and are automatically
generated by the Since User Exceptions are subclasses of Object ImplementationsNOTE: the previous section discussed the client's view of CORBA, that is, how a JavaTM client issues a request on a CORBA object. The client's view is standard across most CORBA products. Basically, the standard worked and there are only minor differences. Unfortunately, the same is not the case for the implementation view of CORBA. As such, some of the details given here might not match a particular CORBA product. Notes on different CORBA products appear as appendices. This section describes what you need to know to implement a simple CORBA object in the Java programming language. It examines the Java server-side language binding for IDL, implementing objects and servers, implementation packaging issues, and CORBA object adaptors. After completing this section, you should be able to write a simple CORBA object and server in the Java programming language. Again, the stock example is used to illustrate the implementation model of CORBA. CORBA object implementations are completely invisible to their clients. A client can only depend on the IDL interface. In the Java programming language, or C++, this is not the case. The user of an object declares variables by a class name; doing so makes the code depend on much more than just the interface. The client depends on the object implementation programming language, the name of the class, the implementation class hierarchy, and, in C++, even the object layout. The complete encapsulation for CORBA objects means the object implementor has much more freedom. Object implementations can be provided in a number of supported programming languages. This is not necessarily the same one the clients are written in. (Of course, here everything is in the Java programming language, but CORBA does notrequire this.) The same interface can be implemented in multiple ways. There
is no limit. In the stock example, the following are possible
implementations of the
Providing an ImplementationRecall that given an IDL file, the IDL compiler generates various files for a CORBA client. In addition to the files generated for a client, it also generates a skeleton class for the object implementation. A skeleton is the entry point into the distributed object. It unmarshals the incoming data, calls the method implementing the operation being requested, and returns the marshaled results. The object developer need only compile the skeleton and not be concerned with the insides of it. The object developer can focus on providing the implementation of the IDL interface. To implement a CORBA object in the Java programming language, the developer
simply implements a Java class that extends the generated skeleton
class and provides a method for each operation in the interface.
In the example, the IDL compiler generates the skeleton class
public class StockImpl extends StockObjects._StockImplBase { private Quote _quote=null; private String _description=null; public StockImpl( String name, String description) { super(); _description = description; } public Quote get_quote() throws Unknown { if (_quote==null) throw new Unknown(); return _quote; } public void set_quote(Quote quote) { _quote = quote; } public String description() { return _description; } } Interface versus Implementation HierarchiesNotice that there are two separate hierarchies: an interface
hierarchy and an implementation hierarchy. Recall that the interface
hierarchy for the example of a
In IDL this is represented as: interface ReportingStock: Reporting, Stock { }; Now suppose there is an implementation of a
In the Java programming language, this class hierarchy is represented as: class ReportingStockImpl implements ReportingStock extends _ReportingStockImplBase { ... } Since the Java programming language only supports single inheritance of
implementation classes, implementations often create an instance
of another class and delegate to it. In the above example,
the Other class hierarchies implementing the same interface hierarchy are possible. Furthermore, if you need to change the class hierarchy of the implementation in some way, the clients are not affected. Implementation Type CheckingJust as type checking is done at the client for the request to a distributed object, type checking is also done for the object implementation. The IDL compiler for the Java programming language generates object skeletons and Java code to represent all of the IDL interfaces and data types used in the interface definition. The implementation code thus depends on the generated Java code.
If there are any type errors in the object implementation,
the Java compiler, not the IDL compiler, catches the errors at
compile time. Thus, in the example, suppose the developer erroneously
implemented the Quote StockImpl.get_quote() { double price = ...; return price; } The Java compiler would detect this error at compile time. Implementing a Server Using the Java 2 ORBYou previously saw how to provide an implementation of a CORBA object in the Java programming language. The remaining task is to define a server that when run makes the services of its objects available to clients. A server that will run with the Java 2 ORB needs to do the following:
The server must instantiate at least one object since objects are the only way to offer services in CORBA systems. Here's an implementation of the stock objects server. This code depends on the Java 2 ORB. public class theServer { public static void main(String[] args) { try { // Initialize the ORB. org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(args,null); // Create a stock object. StockImpl theStock = new StockImpl("GII", "Global Industries Inc."); // Let the ORB know about the object orb.connect(theStock); // Write stringified object //reference to a file PrintWriter out = new PrintWriter(new BufferedWriter( new FileWriter(args[0]))); out.println( orb.object_to_string(theStock) ); out.close(); // wait for invocations from clients java.lang.Object sync = new java.lang.Object(); synchronized (sync) { sync.wait(); } } catch (Exception e) { System.err.println( "Stock server error: " + e); e.printStackTrace(System.out); } } } Notice that the server does a Implementing a Server Using VisiBroker 3.xYou previously saw how to provide a server using the Java 2 ORB. If you are using Inprise's VisiBroker 3.x for Java ORB you need to do the following:
The server must instantiate at least one object since objects are the only way to offer services in CORBA systems. Here's an implementation of the stock objects server. This code depends on VisiBroker 3.x:. public class theServer { public static void main(String[] args) { try { // Initialize the ORB. org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(args,null); // Initialize the BOA. org.omg.CORBA.BOA boa = ((com.visigenic.vbroker.orb.ORB)orb) .BOA_init(); // Create a stock object. StockImpl theStock = new StockImpl(" GII","Global Industries Inc."); // Write stringified object //reference to a file PrintWriter out = new PrintWriter(new BufferedWriter( new FileWriter(args[0]))); out.println( orb.object_to_string(theStock) ); out.close(); // Tell the BOA that the object //is ready to // receive requests. boa.obj_is_ready(theStock); // Tell the boa that the //server is ready. This // call blocks. boa.impl_is_ready(); } catch (Exception e) { System.err.println(" Stock server error: " + e); e.printStackTrace(System.out); } } } Notice that the server does a Differences Between Server ImplementationsThe following summarizes the differences between implementing a transient CORBA server using the Java 2 ORB and implementing a transient server using Inprise's VisiBroker 3.x:
These are the only differences for transient object servers. There are further API differences of CORBA products due to persistence and automatic activation of servers. Packaging Object ImplementationsAs illustrated above, you should separate the implementations of your objects from the implementation of the server. This allows you to mix and match object implementations in a server. The object implementation does not depend on the server. The server, of course depends on the object implementations that it contains. Another advantage of carefully isolating object implementation code from server code is portability. Most of the product-specific code exists in the server, not in the object implementation. A good strategy is to package an object implementation with its generated stubs and skeletons as a JavaBean component. This allows the implementation to be manipulated by JavaBean design tools. Object AdaptersThe CORBA specification defines the concept of an object adapter. An object adapter is a framework for implementing CORBA objects. It provides an API that object implementations use for various low level services. According to the CORBA specification, an object adapter is responsible for the following functions:
The architecture supports the definition of many kinds of object adapters. The specification includes the definition of the basic object adapter (BOA). In the previous section, you saw some server code that uses the services of VisiBroker's implementation of the BOA. The BOA has been implemented in various CORBA products. Unfortunately, since the specification of the BOA was not complete, the various BOA implementations differ in some significant ways. This has compromised server portability. To address this shortcoming, an entirely new object adapter was added, the portable object adapter (POA). Unfortunately, the POA is not yet supported in many products. In any event, the BOA and the POA are described here. Activation on Demand by the Basic Object Adapter (BOA)One of the main tasks of the BOA is to support on-demand object activation. When a client issues a request, the BOA determines if the object is currently running and if so, it delivers the request to the object. If the object is not running, the BOA activates the object and then delivers the request. The BOA defines four different models for object activation:
Portable Object Adapter (POA)According to the specification, "The intent of the POA, as its name suggests, is to provide an object adapter that can be used with multiple ORB implementations with a minimum of rewriting needed to deal with different vendors' implementations." However, most CORBA products do not yet support the POA. The POA is also intended to allow persistent objects -- at least, from the client's perspective. That is, as far as the client is concerned, these objects are always alive, and maintain data values stored in them, even though physically, the server may have been restarted many times, or the implementation may be provided by many different object implementations. The POA allows the object implementor a lot more control. Previously, the implementation of the object was responsible only for the code that is executed in response to method requests. Now, additionally, the implementor has more control over the object's identity, state, storage, and lifecycle. The POA has support for many other features, including the following:
For more detail on the POA, please see the specification. A word on multithreading. Each POA has a threading policy that determines how that particular POA instance will deal with multiple simultaneous requests. In the single thread model, all requests are processed one at a time. The underlying object implementations can therefore be lazy and thread-unsafe. Of course, this can lead to performance problems. In the alternate ORB-controlled model, the ORB is responsible for creating and allocating threads and sending requests in to the object implementations efficiently. The programmer doesn't need to worry about thread management issues; however, the programmer definitely has to make sure the objects are all thread-safe. ResourcesWeb Sites
Documentation and Specs
Books
MiscellaneousIf you don't have the RMI over IIOP compiler is available at http://java.sun.com/products/rmi-iiop/. JacORB - Free Java ORB, including POA, DII, DSI, CosNaming OrbixWeb from Iona WebSphere Application Server from IBM A nice list of ORBs on a nice CORBA infomation site About The Java 2 ORBThe Java IDL ORB that ships with the JavaTM 2 platform allows applications to run either as stand-alone Java applications or as applets within Java-enabled browsers. It uses IIOP as its native protocol. The Sun Java ORB is fairly generic. This is good, because there
are few surprises; however, there are many advanced features of
CORBA that are missing. There is no Interface Repository (though
Java IDL clients can access an Interface Repository provided by
another Java or C++ ORB), Transaction Service, or POA, for example.
For a complete list of these unimplemented features, see the Java IDL is structured with a "pluggable ORB" architecture,
which allows you to instantiate ORBs from other vendors from within
the Java Virtual Machine. This is accomplished through setting
environment variables, or system
properties, or at run time through the use of a idltojava NotesIf you don't have the By default, Bad command or file name Couldn't open temporary file idltojava: fatal error: cannot preprocess input; No such file or directory If you get this message, it means you must invoke idltojava -fno-cpp foo.idl System PropertiesThe Quoted verbatim from the Java IDL guide: Currently, the following configuration properties are defined for all ORB implementations:
The name of a Java class that implements the
The name of a Java class that implements the In addition to the standard properties listed above, Java IDL also supports the following properties:
The host name of a machine running a server or daemon that
provides initial bootstrap services, such as a name service.
The default value for this property is
The port the initial naming service listens to. The default
value is VisiBroker 3.xHere are some additional details for the VisiBroker 3.x implementation of CORBA. See the product documentation for more details. VisiBroker ToolsVisiBroker for Java ships with a number of tools. Some are replacements or wrappers for the standard JavaTM compiler and interpreter. Others are specific to the VisiBroker product. The important ones are:
Using VisiBroker with Java 2To make VisiBroker for Java 3.4 work with the Java 2 platform, a number of changes are necessary, involving both code and configuration. The Java 2 platform ships with a standard implementation of
CORBA classes in the org.omg.CORBA.ORB orb = org.omg.CORBA.ORB.init(args, null); org.omg.CORBA.BOA boa = ((com.visigenic.vbroker.orb.ORB)orb).BOA_init( ); This example applies to the server code. For this cast to work, you must also guarantee that the ORB
returned by the
Portable Stubs and SkeletonsBy default, VisiBroker for Java creates stub and skeleton code that is interoperable but not portable. This makes sense in the VisiBroker world, since the non-portable code is more efficient and slightly smaller. However, if your code needs to run on several different ORBs, you can use the command idl2java -portable -no_bind Foo.idl and the stubs and skeletons will be portable. There are a few reasons for this. One is if you're writing an applet that will run inside a remote web browser environment, such as Netscape Communicator or the Java Plug-in. The latter uses JavaIDL; the former may be running an older version of VisiBroker. What's the difference between portable and proprietary versions? A portable stub uses DII (Dynamic Invocation Interface) to marshal the object request; a portable skeleton uses DSI (Dynamic Skeleton Interface). The proprietary versions make direct calls (to the ORB or the implementation), and hence do not have to go through the overhead of creating and parsing the various DII and DSI objects. Note that your code doesn't need to change--this all happens
behind the scenes with Using the BOA with VisiBrokerThe VisiBroker BOA uses a slightly modified version of the standard BOA initialization sequence. For VisiBroker, follow the following boilerplate code. // create and initialize the ORB ORB orb = ORB.init(args, null); The VisiBroker BOA is a customized, proprietary implementation of the CORBA.BOA interface. It has several methods that are not part of the standard interface. In order to use these proprietary methods, you must cast the ORB to a VisiBroker class, as follows. // Initialize the BOA. // Must cast to VBJ ORB for //Java 2 compatibility org.omg.CORBA.BOA boa = ((com.inprise.vbroker.CORBA.ORB)orb).BOA_init(); VisiBroker objects are usually persistent. In VisiBroker terms, this means that they are initialized with a name. This is not needed with a portable, non-VisiBroker object implementation. // create object and register it //with the ORB Stock theStock = new StockImpl(name); The VisiBroker BOA skips the // Export the newly created object. boa.obj_is_ready(theStock); The // Wait for incoming requests boa.impl_is_ready(); Using the VisiBroker Smart AgentVisiBroker for Java ships with its own location service, called the smart agent. The smart agent is a distributed location service. It collaborates with other smart agents running on the network to locate a suitable implementation of an object. If there is more than one implementation available; the smart agent selects one. This provides a degree of fault tolerance and load balancing. If a machine goes down, the smart agent will automatically find another implementation on another machine to service the request. The client is unaware of this. If you create a persistent object, by passing in a name when you call its constructor, then the BOA will automatically inform the smart agent. The name you passed in the constructor will become the name it is known by on the smart agent network. On the client side, the
proprietary Helper object defines a
method // "bind" (actually lookup) the //object reference Stock theStock = StockHelper.bind(orb, "GII"); Return to Top of PageCopyright © 1998-1999 MageLang Institute. All Rights Reserved. |