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Swing Chapter 26. (Special topics) Swing and CORBA. Easy for reading, Click here!

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Swing Chapter 26. (Special topics) Swing and CORBA. Easy for reading, Click here!

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Subpages: 1. Java 2 and CORBA
2. Creating a CORBA interface
3. Creating a CORBA server
4. Creating a CORBA client

26.2  Creating a CORBA interface

To start the construction of a CORBA-enabled application we first need to define an IDL interface which will be used to retrieve data from a remote server. The example in this section presents two such interfaces and two user-defined exceptions in one module IDL file.

Note: IDL stands for Interface Definition Language. This language is specified by OMG and is used define the interfaces to CORBA objects. Java 2 uses a subset of IDL which is called "Java IDL" (see http://www.javasoft.com/products/jdk/idl/index.html for more details).

The Code: MarketData.idl

see \Chapter26\1

module MarketDataApp

{

  interface DataUnit {

    string getSymbol();

    string getName();

    double getLast();

    double getOpen();

    double getChange();

    double getChangePr();

    long getVolume();

  };

  exception CorbaSqlException {

    string reason;

  };

  exception LoginException {

  };

  interface MarketData {

    DataUnit getFirstData(in string name, in string password,

      in long year, in long month, in long day)

      raises(LoginException, CorbaSqlException);

    DataUnit getNextData()

      raises(CorbaSqlException);

  };

};

Java IDL is similar to Java and C++, but it does have some important distinctions. First, all parameters passed to the declared methods (see the MarketData interface) must have a direction attribute: in, out, or inout. Second, the raises keyword is used to declare exceptions which may be thrown by the associated method.

The MarketDataApp Java IDL file shown above declares the following:

The DataUnit interface holds data for a single row in our stocks table.

The MarketData interface delivers a sequence of DataUnit objects to a caller. The getFirstData() method returns the first DataUnit object available for a given year, month, and day, or null if no data is available. This method also takes a user's name and password as parameters, and is capable of throwing a LoginException or a CorbaSqlException. The getNextData() method returns the next available DataUnit object. By calling this method repeatedly we can retrieve all sequences of data objects (used to fill each row of our stocks table).

A LoginException exception will be thrown if the user's authentication fails.

The CorbaSqlException exception will be thrown to encapsulate an SQL exception thrown on the server side, and pass it to the caller.

To compile this IDL file you need to download the idltojava compiler. This tool is not included in the Java 2 release, but is freely available from Java Developers Connection web site at http://developer.java.sun.com/developer/earlyAccess/jdk12/idltojava.html. This tool should be placed in  the Java/bin directory. Once there, we can run the following command:

idltojava -fverbose -fno-cpp MarketData.idl

This command will generate a number of Java source files in a "MarketDataApp" sub-directory:

DataUnit.java: Java interface (listed below) corresponding to our DataUnit CORBA interface.

DataUnitHelper.java: helper class for the DataUnit interface. Helpers declare a set of useful static methods to help manage a given class, notably narrow(), which is the CORBA counterpart of a Java class cast.

DataUnitHolder.java: holder class for the DataUnit interface. Holders are required for CORBA operations that take out or inout arguments. Unlike CORBA in arguments, out and inout don't map directly to Java's pass-by-value semantics.

_DataUnitImplBase.java: this abstract class provides a base implementation of a CORBA object which all concrete implementations of the DataUnit interface must extend.

_DataUnitStub.java: this class represents the implementation of a client stub. It implements the DataUnit interface and provides the associated functionality for the client.

MarketData.java: Java interface (listed below) corresponding to our MarketData CORBA interface.

MarketDataHelper.java: helper class for MarketData interface.

MarketDataHolder.java: holder class for MarketData interface.

_MarketDataImplBase.java: this abstract class provides a base implementation of a CORBA object which all concrete implementations of the MarketData interface must extend.

_MarketDataStub.java: this class represents the implementation of client stub. It implements our MarketData interface and provides the associated functionality for the client.

CorbaSqlException.java: this class implements the SQL server-side exception.

CorbaSqlExceptionHelper.java: helper class for CorbaSqlException.

CorbaSqlExceptionHolder.java: holder class for CorbaSqlException.

LoginException.java: this class implements the authentication exception.

LoginExceptionHelper.java: helper class for LoginException class.

LoginExceptionHolder.java: holder class for LoginException class.



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