Aug 29, 2016

The Internal Cache of Integers

There is an IntegerCache in java.lang.Integer which stores instances for values -128 though 127. This means that Integer.valueOf(17) always will return the very same instance, while Integer.of(200) will not. While this clearly has the advantage of reuse of commonly used Integer values, thus relieving the GC from some work, it also has implications for autoboxing and identity comparisons.

The Cache

An easy way to see the cache in action is to investigate identity for two small integers compared to numbers outside the scope of the cache. The following two statements both hold true.
Integer.valueOf(17) == Integer.valueOf(17)
and
Integer.valueOf(200) != Integer.valueOf(200)

The java.lang.Integer.IntegerCache and Autoboxing

The cache ensures that there is exactly one Integer instance for each of the 256 integer numbers closest to 0. While this may have impact for GC, a more fundamental effect is how this affects autoboxing as can be seen in the following example.


public class IntegerCacheTest {
  private static void test(Integer i, Integer i2) {
    System.out.println(i);
    if (i == i2) System.out.println("  the same");
    if (i != i2) System.out.println("  different");
    if (i.equals(i2)) System.out.println("  equal");
  }

  public static void main(String[] args) {
    test(17, 17);
    test(200, 200);
  }
}
Knowing about the caching of instances for integer 17 but not for 200 the reader may already have expected to get differing results for the two comparisons and indeed, the output is as follows.
17
  the same
  equal
200
  different
  equal
What happens is that 17 and 200 are autoboxed when sent as parameters to a method accepting Integers, but for the lower number the cache will ensure we get the identically same instance while the higher number is outside the size limit of the cache and therefore we get two different but equal instances.

Tweaking the Cache Size

The size of the cache can be tweaked with the -XX:AutoBoxCacheMax= option. Thus, if we run the above program setting the cache size to 1000 by adding -XX:AutoBoxCacheMax=1000 to the VM parameters, we get the following result.
17
  the same
  equal
200
  the same
  equal

The Cache Internals

The cache is used as follows, as seen in the source code of java.lang.Integer.

  public static Integer valueOf(int i) {
    if (i >= IntegerCache.low && i <= IntegerCache.high)
      return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
  }
The actual cache can be found in java.lang.Integer and looks as follows.


...
  private static class IntegerCache {
    static final int low = -128;
    static final int high;
    static final Integer cache[];

    static {
      // high value may be configured by property
      int h = 127;
      String integerCacheHighPropValue =
        sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
      if (integerCacheHighPropValue != null) {
        try {
          int i = parseInt(integerCacheHighPropValue);
          i = Math.max(i, 127);
          // Maximum array size is Integer.MAX_VALUE
          h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
        } catch( NumberFormatException nfe) {
          // If the property cannot be parsed into an int, ignore it.
        }
      }
      high = h;

      cache = new Integer[(high - low) + 1];
      int j = low;
      for(int k = 0; k < cache.length; k++)
        cache[k] = new Integer(j++);

      // range [-128, 127] must be interned (JLS7 5.1.7)
      assert IntegerCache.high >= 127;
    }

    private IntegerCache() {}
  }
...
Note how the cache is pre-filled on startup rather than on demand, meaning that the memory footprint of the cache is constant no matter which Integers are actually used in the VM. Tweaking the cache to a very large size thus comes with a steep prize in terms of memory consumption.

Interestingly enough, the parameter to set the size of the cache only affects the upper limit and never the lower, meaning that there is no simple way to get Integers smaller than -128 to be interned in the cache.

Edit: There is a reddit thread about this blog post with valuable feedback. For instance, it is pointed out that new instances of small Integers can still be created with the new operator, a feature that will be deprecated in Java 9.

Also published at DZone: The Internal Cache of Integers.


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