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Asked: 2026-05-23T09:52:13+00:00

I’m running the following code more or less out of the box http://download.oracle.com/javase/1.4.2/docs/guide/nio/example/Grep.java I’m

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I’m running the following code more or less out of the box

http://download.oracle.com/javase/1.4.2/docs/guide/nio/example/Grep.java

I’m using the following VM arguments

-Xms756m -Xmx1024m

It crashes with OutOfMemory on a 400mb file. What am I doing wrong?

Stack trace:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.nio.HeapCharBuffer.<init>(Unknown Source)
    at java.nio.CharBuffer.allocate(Unknown Source)
    at java.nio.charset.CharsetDecoder.decode(Unknown Source)
    at com.alluvialtrading.tools.Importer.<init>(Importer.java:46)
    at com.alluvialtrading.tools.ReutersImporter.<init>(ReutersImporter.java:24)
    at com.alluvialtrading.tools.ReutersImporter.main(ReutersImporter.java:20)
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1 Answer

  1. Editorial Team

    You are not doing anything wrong.

    The problem is that the application maps the entire file into memory, and then creates a 2nd in-heap copy of the file. The mapped file is not consuming heap space, though it does use part of the JVM’s virtual address space.

    It is the 2nd copy, and the process of creating it that is actually filling the heap. The 2nd copy contains the file content expanded into 16-bit characters. A contiguous array of ~400 million characters (800 million bytes) is too big for a 1Gb heap, considering how the heap spaces are partitioned.

    In short, the application is simply using too much memory.

    You could try increasing the maximum heap size, but the real problem is that the application is too simple-minded in the way it manages memory.

    The other point to make is application you are running is an example designed to illustrate how to use NIO. It is not designed to be a general purpose, production quality utility. You need to adjust your expectations accordingly.

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