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? 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. 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[size]; 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. 🙂
Adaugă comentariu