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    <p>I have finally got round to checking the Hotspot JVM source code and found the following code.</p> <p>In referenceProcessor.cpp:</p> <pre><code>void ReferenceProcessor::process_discovered_references( BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc, AbstractRefProcTaskExecutor* task_executor) { NOT_PRODUCT(verify_ok_to_handle_reflists()); assert(!enqueuing_is_done(), "If here enqueuing should not be complete"); // Stop treating discovered references specially. disable_discovery(); bool trace_time = PrintGCDetails &amp;&amp; PrintReferenceGC; // Soft references { TraceTime tt("SoftReference", trace_time, false, gclog_or_tty); process_discovered_reflist(_discoveredSoftRefs, _current_soft_ref_policy, true, is_alive, keep_alive, complete_gc, task_executor); } update_soft_ref_master_clock(); // Weak references { TraceTime tt("WeakReference", trace_time, false, gclog_or_tty); process_discovered_reflist(_discoveredWeakRefs, NULL, true, is_alive, keep_alive, complete_gc, task_executor); } </code></pre> <p>The function process_discovered_reflist has the following signature:</p> <pre><code>void ReferenceProcessor::process_discovered_reflist( DiscoveredList refs_lists[], ReferencePolicy* policy, bool clear_referent, BoolObjectClosure* is_alive, OopClosure* keep_alive, VoidClosure* complete_gc, AbstractRefProcTaskExecutor* task_executor) </code></pre> <p>This shows that WeakRefs are being unconditionally cleared by ReferenceProcessor::process_discovered_references.</p> <p>Searching the Hotspot code for process_discovered_reference shows that the CMS collector (which is what I am using) calls this method from the following call stack.</p> <pre><code>CMSCollector::refProcessingWork CMSCollector::checkpointRootsFinalWork CMSCollector::checkpointRootsFinal </code></pre> <p>This call stack looks like it is invoked every time a CMS collection is run.</p> <p>Assuming this is true, the only explanation for a long lived weakly referenced object would be either a subtle JVM bug or if the GC had not been run.</p>
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