Today, we will continue the series of articles about Java as programming language and we will try to present a case when strange things happens under the hood. As a result, we can end up with different production issues.
Let's suppose we have the following code in production:
package ro.mihaisurdeanu;
public class Main {
public static void main(String[] args) {
Integer a = 100;
Integer b = 100;
System.out.println(a == b);
}
}Can you guess the output printed? Correct answer: true.
For sure this is not the expected answer. Why? You probably remember the purpose of "==" operator. This operator is used to compare the equality between two object references. If the answer is true, for sure the reference is the same. In other words, a and b points to the same object in memory. Really nice!
To understand what happens under the hood, an idea will be to actually see the implementation for static method Integer.valueOf – one pattern for creating a new integer: (OpenJDK 11)
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}Behind the scene there is a nice caching system to avoid creating new instances and to optimize memory usage.
/**
* Cache to support the object identity semantics of autoboxing for values between
* -128 and 127 (inclusive) as required by JLS.
*
* The cache is initialized on first usage. The size of the cache
* may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
* During VM initialization, java.lang.Integer.IntegerCache.high property
* may be set and saved in the private system properties in the
* jdk.internal.misc.VM class.
*/
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer[] cache;
static Integer[] archivedCache;
static {
// high value may be configured by property
int h = 127;
String integerCacheHighPropValue =
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;
// Load IntegerCache.archivedCache from archive, if possible
VM.initializeFromArchive(IntegerCache.class);
int size = (high - low) + 1;
// Use the archived cache if it exists and is large enough
if (archivedCache == null || size > archivedCache.length) {
Integer[] c = new Integer;
int j = low;
for(int k = 0; k < c.length; k++)
c[k] = new Integer(j++);
archivedCache = c;
}
cache = archivedCache;
// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}Please also note that Integer and Long are immutable. Everything between [-128, 127] is cached by default. As you can see, you can also play with some VM properties to extend or reduce the default range.
In fact, this an example of a flyweight pattern. ;)
