Would you dare to change HashMap implementation?

Nikita Salnikov TarnovskiNovember 12th, 2013Last Updated: November 12th, 2013

1 26 2 minutes read

There are bold engineers working for the Oracle nowadays. I came to this conclusion when trying to nail down a Heisenbug yesterday. Not too surprisingly, the bug seemed to disappear when I was trying to find the solution. Several hours later, the “Heisen”-part of the bug was removed, when the problem was traced down to minor differences between the JDK7 updates.

But back to the bravery claim. In order to understand the case I am describing I extracted it into a really simple test snippet for you to try out:

class OOM {
	public static void main(String[] args) {
		java.util.Map m = new java.util.HashMap(10_000_000);
	}
}

Now when I launch the class on my 64bit Mac OS X with the JDK7u40 or later:

my:tmp user$ /path-to/jdk1.7.0_40/bin/java -Xmx96m OOM
my:tmp user$

You see the command prompt returning and the JVM successfully completing its job. Now, launch the same class with JDKu25 or earlier:

my:tmp user$ /path-to/jdk1.7.0_25/bin/java -Xmx96m OOM
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.util.HashMap.(HashMap.java:283)
    at java.util.HashMap.(HashMap.java:297)
    at OOM.main(OOM.java:3)

And you see a different result. Initializing a HashMap with 100M entries fails to allocate enough resources in our ~100m heap and the JVM exits with an OutOfMemoryError being thrown.

Source of the HashMap is clearly the #1 suspect in this case. And indeed, when you compare the source code of the JDK 7u25 to the next release (named u40, kudos for naming!), you see a significant difference. The Hashmap(initialCapacity, loadFactor) constructor now ignores your will to construct a HashMap with the initial size being equal to initialCapacity. Instead, you see the underlying array being allocated lazily only when the first put() method is called on the map.

A seemingly very reasonable change – JVM is lazy by nature in different aspects, so why not postpone the allocation of large data structures until the need for such allocation becomes imminent. So in that sense a good call.

In the sense that a particular application was performing tricks via reflection and directly accessing the internal structures of the Map implementations – maybe not. But again, one should not bypass the API and start being clever, so maybe the particular developer is now a bit more convinced that each newly found concept is not applicable everywhere.

Would you have made the change yourself if you were the API developer? I am not convinced I would have had the guts, knowing that there has to be around bazillion apps out there depending on all kind of weird aspects of the implementation. But I do vote for reasonable changes within the JDK and can count this one definitely among the good ones.

Reference: Would you dare to change HashMap implementation? from our JCG partner Nikita Salnikov Tarnovski at the Plumbr Blog blog.