PDI plugin, getInputRowMeta() throw Non pointer exception - pentaho

Question

In my plugin, I try to get MetaData info from previous step, so that I can do some work on the row data based on it's MetaData info( column name).
It works fine as per the data, since r = getRow(), can get the data. but when calling getInputRowMeta() to get previous step's row meta, it throws Non pointer exception.
Code example as follow( mainly the processRow() function.)
public boolean processRow(StepMetaInterface smi, StepDataInterface sdi) throws KettleException {
meta = (NAAClientStepMeta) smi;
data = (NAAClientStepData) sdi;
option = meta.getOption();
message = option;
if (first) {
first = false;
if(option.equals("Publisher AccountIDs")){
getInputRowMeta().size(); // don't work here, throw exception.
RowMetaInterface rm = (RowMetaInterface) getInputRowMeta().clone();
data.outputRowMeta = rm;
meta.getFields(rm, getStepname(), null, null, this);
//String previousSteps[] = transMeta.getPrevStepNames("New Acquisio API Client");
//RowSet rowSet = this.findOutputRowSet(transMe);
//RowMetaInterface rowMeta = rowSet.getRowMeta();
r = getRow();
when calling getInputRowMeta(), it throws Exception as follows:
2015/08/05 14:11:00 - New Acquisio API Client 2.0 - ERROR (version 4.4.0 stable, build 17588 from 2012-11-21 16.02.21 by buildguy) : Unexpected error
2015/08/05 14:11:00 - New Acquisio API Client 2.0 - ERROR (version 4.4.0-stable, build 17588 from 2012-11-21 16.02.21 by buildguy) : java.lang.NullPointerException
2015/08/05 14:11:00 - New Acquisio API Client 2.0 - ERROR (version 4.4.0-stable, build 17588 from 2012-11-21 16.02.21 by buildguy) : at com.acquisio.kettle.plugins.NAAClientStep.processRow(NAAClientStep.java:183)
2015/08/05 14:11:00 - New Acquisio API Client 2.0 - ERROR (version 4.4.0-stable, build 17588 from 2012-11-21 16.02.21 by buildguy) : at org.pentaho.di.trans.step.RunThread.run(RunThread.java:50)
2015/08/05 14:11:00 - New Acquisio API Client 2.0 - ERROR (version 4.4.0-stable, build 17588 from 2012-11-21 16.02.21 by buildguy) : at java.lang.Thread.run(Thread.java:745)
Are there any place StepMeta class, StepData class I should set?

Answer 1

Thats absolutely right. The underlying PDI class doesn't know about the metadata of the incoming stream UNTIL it has read the first row. So your r=getRow() has to come earlier. A common occurance when building UDJC steps!