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POwhy PhantomReference does not work?
 

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    <p>Demo code:</p> <pre><code>import java.lang.ref.PhantomReference; import java.lang.ref.ReferenceQueue; public class Main { public static void main(String[] args) throws InterruptedException { Object test = new Object(); ReferenceQueue&lt;Object&gt; q = new ReferenceQueue&lt;Object&gt;(); PhantomReference&lt;Object&gt; p = new PhantomReference&lt;Object&gt;(test, q); Object lock = new Object(); while (true) { synchronized (lock) { //q.poll() is null always,why? if (q.poll() != null) { break; } //System.gc(); lock.wait(); } } System.out.println(1111111); } } </code></pre> <p>I tested the code,but it always is dead loop. The code (System.out.println(1111111);) can not execute,q.poll() reurn null.</p> <p>I think if test object is removed by GC,q.poll() will return p object,then break loop,but invoke this demo code,it is not like my thought </p> <p>Edited: I modify the demo code,it can work now.</p> <pre><code>import java.lang.ref.PhantomReference; import java.lang.ref.ReferenceQueue; public class Main { public static void main(String[] args) throws InterruptedException { Object test = new Object(); ReferenceQueue&lt;Object&gt; q = new ReferenceQueue&lt;Object&gt;(); PhantomReference&lt;Object&gt; p = new PhantomReference&lt;Object&gt;(test, q); Object lock = new Object(); while (true) { synchronized (lock) { if (q.poll() != null) { break; } test = null; //it is important System.gc(); lock.wait(100);//should not lock.wait() } } System.out.println(1111111); } } </code></pre> <p>AS sb says,the statement( test=null) is key.GC collect test object that assign null.</p>
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