How to make Jasper Reports programmatically determine the Name of Columns within the report it self? - jasper-reports

Question

I am generating a report with that will have a 7 columns where the last 6 should have the the last 6 months listed. So as of the time of this writing it should be:
NAME -> September -> August -> July -> June -> May -> April
ss the column headers. I am trying to avoid having to pass them in as parameters, and am trying to get Jasper Reports to figure it out at runtime. I can get the first month pretty easily using a Text Field Expression. It looks like:
new java.text.SimpleDateFormat("MMMMM").format(new Date())
The issue comes in with the other months. I initially tried
new java.text.SimpleDateFormat("MMMMM").format(java.util.Calendar.getInstance().add(Calendar.MONTH, new Integer("-1)).getTime())
This does not work since Calendar.add does not return a Calendar instance. I then tried using a variable and then a combination of variables which also did not work.
How to make Jasper Reports programmatically determine the Name of Columns within the report it self?

Answer 1

I think the best approach to solving this problem is to use Commons Lang. That package provides utilities to make calculations like this very easy. By adding one extra jar you can then use expressions like this:
DateUtils.addMonths(new Date(),-1)
I find that easier to maintain than first creating a helper Calendar class and then using the ternary operator but ignoring its results.
$P{cal}.add(Calendar.MONTH, -1)
? null : $P{cal}.getTime()
If you ever need to generalize the solution then it's a lot easier to get a Date back from a SQL query than to get a Calendar. So you can quickly change to "DateUtils.addMonths($F{MyDate},-1)". Initializing a Calendar to match a date returned by a query isn't nearly as simple. But of course there's a certain benefit to not having to add one more .jar file. So sometimes that ternary operator technique is the quickest way to get things done.
I wrote about using the Commons Lang approach a couple of years ago here: http://mdahlman.wordpress.com/2009/09/09/jasperreports-first-dates/

Answer 2

I also needed a certain format for the previous month. I ended up combining the other two answers:
new java.text.SimpleDateFormat("yyyyMM").format(
org.apache.commons.lang3.time.DateUtils.addMonths(
new java.util.Date(),
-6
)
)
This way I don't need to add another parameter. Adding the Commons Lang jar is a non issue for me since JasperServer 5.5 comes with version 3.0 out of the box.
I hope this helps someone who stumples upon this page, just like I did.

Answer 3

I found a working solution that is pretty ingenious (no I did not come up with it). I found it here. The gist of it is create a parameter called call, with a default value of:
Calendar.getInstance()
and un-check the option 'Use as a prompt'. Then in your text field expression you would do:
new java.text.SimpleDateFormat("MMMMM").format(
(
$P{cal}.add(Calendar.MONTH, -1)
? null : $P{cal}.getTime()
)
)
What happens is it will set the default value for the calendar instance, then execute the add method, which will resolve to false, so then it will then return the result from getTime() method which gets formatted how I want.