Question

I am getting this error in a program that creates several (hundreds of thousands) HashMap objects with a few (15-20) text entries each. These Strings have all to be collected (without breaking up into smaller amounts) before being submitted to a database.

According to Sun, the error happens “if too much time is being spent in garbage collection: if more than 98% of the total time is spent in garbage collection and less than 2% of the heap is recovered, an OutOfMemoryError will be thrown.”.

Apparently, one could use the command line to pass arguments to the JVM for

• Increasing the heap size, via “-Xmx1024m” (or more), or
• Disabling the error check altogether, via “-XX:-UseGCOverheadLimit”.

The first approach works fine, the second ends up in another java.lang.OutOfMemoryError, this time about the heap.

So, question: is there any programmatic alternative to this, for the particular use case (i.e., several small HashMap objects)? If I use the HashMap clear() method, for instance, the problem goes away, but so do the data stored in the HashMap! 🙂

The issue is also discussed in a related topic in StackOverflow.

Answer 1

You’re essentially running out of memory to run the process smoothly. Options that come to mind:

1. Specify more memory like you mentioned, try something in between like -Xmx512m first
2. Work with smaller batches of HashMap objects to process at once if possible
3. If you have a lot of duplicate strings, use String.intern() on them before putting them into the HashMap
4. Use the HashMap(int initialCapacity, float loadFactor) constructor to tune for your case