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POWeird garbage production when instantiating Objects in Java
 

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  1. POWeird garbage production when instantiating Objects in Java
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    <p>I am profiling the garbage behavior of <code>java.lang.String</code> and it looks like each time you instantiate a string <strong>for the first time inside any class</strong> it always generate garbage. Would anyone know why?</p> <pre><code>public abstract class AbstractTest { protected static String SERGIO = "sergio"; private String someValue; public void init() { this.someValue = new String(SERGIO); } } public class Test extends AbstractTest { private static String JULIA = "julia"; private Runtime runtime = Runtime.getRuntime(); private String anotherValue; private String yetAnother; private void gc() throws InterruptedException { System.gc(); Thread.sleep(100); } private long usedMemory() { return runtime.maxMemory() - runtime.freeMemory(); } public void test() throws Exception { gc(); this.anotherValue = new String(SERGIO); // a bunch of garbage is created! long usedMemory = usedMemory(); gc(); long usedMemoryAfterGC = usedMemory(); System.out.println("Collected: " + (usedMemory - usedMemoryAfterGC)); gc(); this.yetAnother = new String(JULIA); // no more garbage usedMemory = usedMemory(); gc(); usedMemoryAfterGC = usedMemory(); System.out.println("Collected: " + (usedMemory - usedMemoryAfterGC)); } public static void main(String[] args) throws Exception { Test t = new Test(); t.test(); } </code></pre> <p><strong>Output:</strong></p> <p>Collected: 704336<br/> Collected: 0</p> <p>That 's fine. First time it creates garbage, then subsequent instantiations produce no garbage.</p> <p>What is weird is when you force a string creation in the superclass, it still creates garbage in the subclass the first time you instantiate a String there:</p> <pre><code>public void test() throws Exception { gc(); init(); // creates a String in the superclass gc(); this.yetAnother = new String(JULIA); long usedMemory = usedMemory(); gc(); long usedMemoryAfterGC = usedMemory(); System.out.println("Collected: " + (usedMemory - usedMemoryAfterGC)); } </code></pre> <p><strong>Output:</strong></p> <p>Collected: 348648</p> <p>Any idea why?</p> <p>(By the way I am running this on MAC and JDK 1.6.0_37)</p> <p><strong>EDIT1:</strong> I changed the code slightly to make it clear that string internalization is not the culprit here, at least it does not look like it is.</p> <p><strong>EDIT2:</strong> If you change String to Object throughout the code, you get the same garbage so I guess it has to do with how object allocation through <em>new</em> happens in Java. The first time you allocate an object in a class you get the garbage. The second time you don't. Weird it is <em>per class</em>.</p> <p><strong>EDIT3:</strong> I wrote a <a href="http://mentablog.soliveirajr.com/2012/11/real-time-java-programming-without-gc/" rel="nofollow">blog article</a> where I talk about how to force a GC to profile your applications for garbage creation, like I am doing in the code above.</p>
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    1. USTraderJoeChicago
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    1. COIt might be worth noting that calling System.gc(); does not guarantee that the garbage collector will run immediately. Its kind of like a "hey, when you get a chance, it would be nice if you'd run"
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    2. CODon't understand what you mean with "garbage" - what did you expect?
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    3. COgarbage = de-referenced objects in the heap that will be eventually collected by the gc. If you noticed, I am not the one producing the garbage but the JVM is doing that under the hood. The JDK classes often do that, but this time it looks kind of weird.
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