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Here’s the Java code I used to connect to a ROB server:{String sValue;// Connect to servertry{Peer = new Socket( sServer, nPort.intValue() );c_In = new DataInputStream( Peer.getInputStream() );c_Out = new DataOutputStream( Peer.getOutputStream() );}catch ( Exception e ){System.out.println( "Error: " + e );return false;}After the connection is established, you can simplyread and write from the two streams. Of course, you stillhave to implement your own protocol. In this case I hadto use the same protocol used for the ROB library frommy last article. Nothing fancy; I only send the totalmessage size in the first four bytes so that the receivingapplication knows how many bytes to wait for. Javadoesn’t even need to convert this four bytes (longnumber) from host order to network order, as the Centuralibrary must, since Java uses the network order itself.Applets and securityIf you just use the virtual machine in a Java application,then you don’t have any security limitation; you canconnect wherever you like. However, if you’re using theROB client in an applet, you can connect only to the serverfrom which you downloaded the applet.Here’s an example. Say your HTML looks like this:This applet is able to connect only to a ROB serverrunning on www.propublishing.com.How to use the ROB_ClientIt’s really simple. Include the “rob” directory with thethree classes into your Java CLASSPATH setting. Thenimport the “rob” package in your Java application orapplet. If you want to distribute your applet over theInternet, you must copy the “rob” directory (package) intothe same directory where you have the applet.The following sample code shows how to access, inJava, a simple Centura ROB that connects to the database,loads a result set, and returns the content of each row.This code works using jview.exe. The applet version is alittle bit different because you have to deal with differentevents: loading of the applet, unloading, painting, etc.import rob.*;public class Test{static ROB_Client rob;public static void main( String args[] )}}// create the ROBrob = new ROB_Client( “localhost/10100”,“TableLoader” );// test validityif ( !rob.isAllocated() )return;// initialize ROB’s attributesrob.setm( “sDatabase”, “demo” );rob.setm( “sUser”, “sysadm” );rob.setm( “sPassword”, “sysadm” );rob.setm( “sTable” ,”systables” );rob.setm( “sColumn”, “name” );rob.setm( “sLike”, “sys%” );// prepare the result setrob.exec( “load”, “” );// print the rows countSystem.out.println( “number of rows=” +rob.exec( “getCount”, “” ) );// print each valuesValue = rob.exec( “getFirst”, “” );while ( sValue.length()>0 ){System.out.println(sValue );sValue = rob.exec( “getNext”, “” );}// free the object and disconnect.// the disconnection by itself would be enough// because the server automatically frees// all the objects associated with the connection.rob.free();rob.disconnect();First, you create an object from the class ROB_Client,then initialize the object with the “new” operator. SinceJava has constructors (SAL doesn’t), you can immediatelyspecify the server to connect to and the remote object toallocate. In theSAL version youhad to useclient.connect( )andclient.allocate( ).In the exampleabove I initializethe object’smember toconnect to the“Demo”Database and toread the “name”column, only forvalues that beginwith “sys%”,from the“systables”table. Then Iprint out all theretrieved valuesusing getFirst( )and getNext( ).Obviously, in theserver code, the Figure 2. The Java client result screen.4 Centura Pro May 1998 http://www.ProPublishing.com
getFirst( ) method should reset an internal index andreturn the first row of the result set; the getNext( ) methodshould increase the index and return the next row in theresult set. The Remote Object “TableLoader” is includedin the server test code. See Figure 2 for the test client resultwith the jvm, Figure 3 for the same code in an applet, andFigure 4 for the server transactions log.The test applet (included with the sample code)connects and tells the server to load the result set onlyonce, when the browser loads it, and retrieves all the rowseach time it needs to repaint its window. This approachcauses the applet to generate network traffic each time thewindow needs to be repainted. It would be better toconnect, load all the rows, and disconnect when theapplet gets loaded the first time and then use the localstorage to refresh the screen. Changing the code could bean interesting first test of the library.Error handlingThere are two kinds of errors: local errors, generated bythe application (mostly because of connection problems),and remote errors, generated by the remote object. In bothcases the function being executed fails. The get* functionsreturn false when they fail; the set* and exec functionsreturn a null string; and in both cases you can use thegetLastError( ) method to retrieve the actual error number.Another approach would be to derive your own clientclass from ROB_Client and implement the iError( ) virtualmethod, which is called by ROB_Client every time anerror occurs. This is probably not the best way to handleerrors since Java has the capability to use exceptions—butI’m not very familiar with it yet; I’m sure that any Javaprogrammer can do better than that.Fixes to the Centura ROB class libraryTesting the Java client gave me a chance to fix some errorsin the first version of the ROB library. The first problemwas the Winsock version clash. I was requesting version2.0, which caused Centura’s TCP/IP router to fail if calledafter the ROB library had already allocated the Winsock.The reason is that the Winsock can be allocated multipletimes in the same process, but each subsequent allocationmust request the same version requested by the firstallocation. Centura’s routers request 1.1 and the ROBlibrary was requesting 2.0.The second problem was the way I handled blockingoperations. I was waiting in a loop, with yielding enabled,waiting for the entire packet to be received. This approachcause the server to go into a recursion if the messageswere too close to each other. Now I detect a blockingoperation right in the Winsock library, and I don’t get outof there until the whole packet is received or until atimeout error occurs. CPDownload the software discussed in this article from ProPublishing’s Web site or obtain it from this month’sCompanion Disk.Figure 3. The Java applet-clientresult screen.Figure 4. The server transactions log.Gianluca Pivatodesigns LANs, WANs,databases, software,and anything thatinteracts with thesystem. Hegraduated in Italywith a degree indigital electronics,and he’s been in thecomputer businesssince 1982. Currently,he’s working on aproject for an Italianpatent andtrademark firm. Hiscompany is PivatoConsulting, inc. andhe can be reached atgianlucapivato@pipeline.com. He’salso selling Centuratools through hisnew online company,XSal,athttp://xsal.hexagon.net.http://www.ProPublishing.comCentura Pro May 1998 5
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