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    <p>To keep things in perspective, consider running this code with <code>-Xmx64m</code>:</p> <pre><code>static long sum; public static void main(String[] args) { System.out.println("Warming up..."); for (int i = 0; i &lt; 100_000; i++) test(1); System.out.println("Main call"); test(5_500_000); System.out.println("Sum: " + sum); } static void test(int size) { // for (int i = 0; i &lt; 1; i++) { long[] a2 = new long[size]; sum += a2.length; } long[] a1 = new long[size]; sum += a1.length; } </code></pre> <p>Depending on whether you do the warmup or skip it, it will blow or not blow. This is because the JITted code properly <code>null</code>s out the var, whereas the interpreted code doesn't. Both behaviors are acceptable under the Java Language Specification, which means that you are at the mercy of the JVM with this.</p> <p><sup> Tested with <code>Java HotSpot(TM) 64-Bit Server VM (build 23.3-b01, mixed mode)</code> on OS X. </sup></p> <h2>Bytecode analysis</h2> <p>Look at the bytecode with the <code>for</code> loop (simple code, without the <code>sum</code> variable):</p> <pre><code>static void test(int); Code: 0: iconst_0 1: istore_1 2: goto 12 5: iload_0 6: newarray long 8: astore_2 9: iinc 1, 1 12: iload_1 13: iconst_1 14: if_icmplt 5 17: iload_0 18: newarray long 20: astore_1 21: return </code></pre> <p>and without:</p> <pre><code>static void test(int); Code: 0: iload_0 1: newarray long 3: astore_1 4: iload_0 5: newarray long 7: astore_1 8: return </code></pre> <p>No explicit <code>null</code>ing out in either case, but note that in the <em>no-for</em> example the same memory location is actually reused, in contrast with the <em>for</em> example. This would, if anything, lead to the expectation opposite to the observed behavior.</p> <h2>A twist...</h2> <p>Based on what we learned from the bytecode, try running this:</p> <pre><code>public static void main(String[] args) { { long[] a1 = new long[5_000_000]; } long[] a2 = new long[0]; long[] a3 = new long[5_000_000]; } </code></pre> <p><strong>No OOME thrown</strong>. Comment out the declaration of <code>a2</code>, and it is back. We <strong>allocate more</strong>, but <strong>occupy less</strong>? Look at the bytecode:</p> <pre><code>public static void main(java.lang.String[]); Code: 0: ldc #16 // int 5000000 2: istore_1 3: ldc #16 // int 5000000 5: newarray long 7: astore_2 8: iconst_0 9: newarray long 11: astore_2 12: ldc #16 // int 5000000 14: newarray long 16: astore_3 17: return </code></pre> <p>The location 2, used for <code>a1</code>, is reused for <code>a2</code>. The same is true for OP's code, but now we overwrite the location with a reference to an innocuous zero-length array, and use another location to store the reference to our huge array.</p> <h2>To sum it up...</h2> <p>Java Language Specification does not specify that any garbage object <em>must</em> be collected and the JVM spec only says that the "frame" with local variables is destroyed as a whole upon method completion. Therefore all behaviors that we have witnessed are by the book. The <em>invisible</em> state of an object (mentioned in the document linked to by <em>keppil</em>) is just a way to describe what happens to go on in some implementations and under some circumstances, but is in no way any kind of canonical behavior.</p>
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    1. CO... but the Java Virtual Machine Specification describes the internal data structures of the VM. In particular, it writes that [local variables](http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.6.1) are stored in a frame, and the [frame](http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.6) is destroyed when the method invocation completes. Similarly, the JVM spec [mandates](http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.5.3) the use of an "automatic storage management system (known as a garbage collector)".
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    2. COAdmittedly, "the Java virtual machine assumes no particular type of automatic storage management system".
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    3. CO@meriton So all the spec has to say is that the objects become unreachable when the method as a whole completes. Nothing is said of variables in or out of scope; also nothing specific is required to happen to an unreachable object at any particular time.
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