Question

I am working on an application which reads in a huge amount of data from a database into a Map<String,Map<String,Map<String,String>>>, processes it, and writes the processed reports to a spreadsheet using an in-house xml writer. The whole run can take about 12 hours.

I’m finding I’m getting

Exception in thread "CursorController-Thread-0" java.lang.OutOfMemoryError: Java heap space
    at java.lang.AbstractStringBuilder.<init>(AbstractStringBuilder.java:45)
    at java.lang.StringBuilder.<init>(StringBuilder.java:68)

When I attempt to write this jumbo file. For this reason I think it would be best to write each Map<String,Map<String,String>> (notice that’s a layer deeper)as it’s finished processing.

My question is, how can I make sure that the Map<String,Map<String,String>> is not retained in memory after I write it, since the Map>> will still contain it?

Answer 1

Once you’re done with the Map<String,Map<String,String>> mapped to by the key "key" you simply do

hugeMap.remove("key");

This will “null” out the entry in the hugeMap and make the Map<String,Map<String,String>> eligible for garbage collection (i.e., never be part of causing a heap space out of memory).