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Tuesday, April 12, 2011

Incoming Connections in Talend Open Studio Components

To work with data in your custom Talend Open Studio component, retrieve an IConnection.  IConnection provides the variable name for a row of data; "row1" is common in smaller jobs.

The XML descriptor for a Talend Open Studio component defines the incoming connections for the component, accessed using the IConnection Java interface.  Most components define a single incoming connection.  Exceptions include one-shot calls like tPreJob or tJava.  Also, tUnite uses more than one connection expressed as a special MERGE parameter.

This line from an XML descriptor defines a single input connection.


From the following Design Workspace Perspective screenshot, the input connection is "row1" setting tRules' input with a tRowGenerator output.  Had the connection been configured later in the job creation, it might have had a different identifier (say "row5").  Which ever identifier is displayed in the map, that's the variable name that is used in a main, as set in a JET section.

Incoming Connection "row1" in tRules
In a main, "row1" can be used as a variable for things like assignments.  "row2.field1=row1.field1" will log (using tLogRow)  field1's data from row1 to the equivalent field in row2.  To avoid a hardcoded value, set up a variable in a JET block like

CodeGeneratorArgument codeGenArgument = (CodeGeneratorArgument) argument;
INode node = (INode)codeGenArgument.getArgument();
String cid = node.getUniqueName();

String incomingConnName = null;

List< ? extends IConnection> conns = node.getIncomingConnections();
if(conns!=null && conns.size()>0){
IConnection conn = conns.get(0);
incomingConnName = conn.getName();


In this particular configuration, the number of incoming connections will be at most one.  Other requirements may need a loop on the structure 'conns'.

Later, outside of the JET block, "incomingConnName" can be used as follows

jc = new org.apache.commons.jexl2.MapContext();
jc.set("<%= incomingConnName %>", <%= incomingConnName %>);

o = filter.evaluate(jc);

The jc.set() puts a variable (evaluated as "row1") into a context using the key determined by "incomingConnName" which is also "row1", but a String value.

When building a Talend Open Studio component that needs to work with input data, retrieve an IConnection object using the getIncomingConnections() call on the node.  The returned list can be pared down to a single item in most cases ("get(0)") that can be interrogated.


  1. Tnx for the great post!

    one question: what's the purpose of those jexl expressions and how to use them in generated java code?

  2. Hi Gabriele,

    I'm using JEXL for two things in tScriptRules, a Talend component I wrote a year ago and put on the Exchange.

    1. A more concise syntax: 'row1.name = "Carl"' versus 'row1 != null && row1.name.equals("Carl")'

    2. Transport. A JEXL expression is a string that can be stored in a file, a String variable, or a data structure

    This link explains more about using JEXL


    Good luck

    1. Tnx allot, Carl! It's definitively clear now. I can see JEXL expression can be really powerful, in TOS jobs.