<div dir="ltr">Stefan, <div><br></div><div> Thanks for your explanation.</div><div><br></div><div>Yumin</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Apr 29, 2016 at 1:15 AM, Stefan Karlsson <span dir="ltr"><<a href="mailto:stefan.karlsson@oracle.com" target="_blank">stefan.karlsson@oracle.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
Hi Yumin,<div><div class="h5"><br>
<br>
<div>On 2016-04-29 00:57, yumin qi wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr"><span style="font-size:12.8px">Hi, Runtime and GC
team</span>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"> My last email with a typo on
hotspot-gc-dev. So send again, sorry if you receive multiple
same content emails from me --- please add my email in your
reply since I could not see the email I sent out to open
alias. </div>
<div style="font-size:12.8px"> </div>
<div style="font-size:12.8px"> As stated in the subject, found
some weakly referenced object not GC'ed. </div>
<div style="font-size:12.8px"> </div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"> Code here is the test case we
used to check for this problem.</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"> There are two classes AAA and
AAB which are all loaded by the TestClassLoader but different
instances which are independent. TestClassLoader extends from
URLClassLoader.</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"> AAA has a field type of AAB
which will be set to null in the test.</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap">public class AAA {</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap"> private AAB aab;</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap"> public AAA() {</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap"> aab = new AAB();</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap"> }</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap"> public void clear() {</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap"> aab = null;</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap"> }</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap">}</span></font></div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px">public class AAB {</div>
<div style="font-size:12.8px"> }</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px">The two class have to be in
different jars(here AAA.jar contains AAA.class, and AAB.jar
contains AAB.jar), which are not in the class path so are
being loaded from different locations for the test.</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px">
<div>import java.net.URL;</div>
<div>import java.net.URLClassLoader;</div>
<div>import java.util.WeakHashMap;</div>
<div><br>
</div>
<div><br>
</div>
<div>public class TestLoader extends URLClassLoader {</div>
<div> public static
WeakHashMap<TestLoader,Object> map=new
WeakHashMap<TestLoader,Object>();</div>
<div> private static int count=0;</div>
<div> public TestLoader(URL[] urls){</div>
<div> super(urls);</div>
<div> map.put(this, new Object());</div>
<div> }</div>
<div> @SuppressWarnings("resource")</div>
<div> public Class<?> loadClass(String name)
throws ClassNotFoundException {</div>
<div> if (name.equals("AAB") &&
count==0) {</div>
<div> try {</div>
<div> count=1;</div>
<div> URL[] urls = new URL[1];</div>
<div> urls[0] = new
URL(<a>"file:///home/nijiaben/tmp/AAB.jar"</a>); // You need to use
your own location for AAB.jar here!!!</div>
<div> return new
TestLoader(urls).loadClass("AAB");</div>
<div> } catch (Exception e){</div>
<div> e.printStackTrace();</div>
<div> }</div>
<div> } else {</div>
<div> return super.loadClass(name);</div>
<div> }</div>
<div> return null;</div>
<div> }</div>
<div>}</div>
</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px">TestLoader puts itself in the
WeakHashMap --- upon "AAB" loading, it uses new instance of
TestLoader. the new Object is just a object as value which has
nothing to strongly or weakly to the key.</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"> TTest.java is the test program
to start with, basically self explained.</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px">
<div>import java.lang.reflect.Method;</div>
<div>import java.net.URL;</div>
<div><br>
</div>
<div>// Author: Jiapeng Li</div>
<div><br>
</div>
<div>public class TTest {<br>
</div>
<div> private Object aaa;</div>
<div> public static void main(String args[]){</div>
<div> try {</div>
<div> TTest tt = new TTest();</div>
<div> //Move tt to old gen and clear aab in aaa</div>
<div> test(tt);</div>
<div> // do a final full GC, the TestLoader for aab
should be purged.</div>
<div> System.gc();</div>
<div> System.out.println("finished"); // stop here
in debugger and dump heap</div>
<div> }catch (Exception e){</div>
<div> e.printStackTrace();</div>
<div> }</div>
<div> }</div>
<div><br>
</div>
<div> @SuppressWarnings("resource")</div>
<div> public static void test(TTest tt){</div>
<div> try {</div>
<div> // New instance of TestLoader which will load
AAA from AAA.jar</div>
<div> URL[] urls = new URL[1];</div>
<div> urls[0] = new
URL(<a>"file:///home/nijiaben/tmp/AAA.jar"</a>); // You have to
use your own location for AAA.jar here!!!</div>
<div> tt.aaa=new
TestLoader(urls).loadClass("AAA").newInstance();</div>
<div> // young GC will not purge the class loader,
after 10 times of full GC, it should move it to old gen</div>
<div> for (int i = 0; i < 10; i++) {</div>
<div> System.gc();</div>
<div> Thread.sleep(1000);</div>
<div> }</div>
<div> // set aaa.aab= null，so next full gc will
collect it</div>
<div> Method[]
methods=tt.aaa.getClass().getDeclaredMethods();</div>
<div> for (Method m : methods){</div>
<div> if (m.getName().equals("clear")) {</div>
<div> m.invoke(tt.aaa);</div>
<div> break;</div>
<div> }</div>
<div> }</div>
<div> } catch (Exception e) {</div>
<div> e.printStackTrace();</div>
<div> }</div>
<div> }</div>
<div>}</div>
</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"> After final full GC, there
should be no instance of AAB exists, but the instance of AAB's
class loader (TestLoader) still not cleaned by GC. See the
stop point above, we dumped heap after final full GC.
Following is the graph for reference which still holds for the
WeakHashMap.</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"><img src="cid:part1.01050001.06010204@oracle.com" alt="Inline
image 1" height="189" width="516"><br>
</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px"> Notice that the dependency
records the two instances of TestLoaders in order that they
are related, When AAA is loaded, it loads AAB and after AAB
loaded, dependency records the relationship in _dependencies. </div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px">
<div>but we don't have 'remove' method to disengage them for
GC. </div>
<div>When GC, _dependencies.oops_do, the is_alive Closure
still marks the TestLoader for AAB alive even though it has
nothing to refer to due to the dependency.</div>
<div><br>
</div>
<div>
<div>void ClassLoaderData::oops_do(OopClosure* f,
KlassClosure* klass_closure, bool must_claim) {</div>
<div> if (must_claim && !claim()) {</div>
<div> return;</div>
<div> }</div>
<div><br>
</div>
<div> f->do_oop(&_class_loader);</div>
<div> _dependencies.oops_do(f);</div>
<div> _handles->oops_do(f);</div>
<div> if (klass_closure != NULL) {</div>
<div> classes_do(klass_closure);</div>
<div> }</div>
<div>}</div>
</div>
<div><br>
</div>
</div>
<div style="font-size:12.8px">Is this a bug or a design
consideration? <br>
</div>
</div>
</blockquote>
<br></div></div>
It's by design. The AAA class depends on the AAB class:<span class=""><br>
<br>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap">public class AAA {</span></font></div>
<div style="font-size:12.8px"><font color="#f92672" face="Menlo,
Monaco, Consolas, Courier New, monospace"><span style="line-height:18.5714px;white-space:pre-wrap"> private AAB aab;</span></font></div>
<br></span>
so AAB can't be unloaded unless AAA is also unloaded.<br>
<br>
StefanK<span class=""><br>
<br>
<blockquote type="cite">
<div dir="ltr">
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px">Any comments are appreciated!</div>
<div style="font-size:12.8px"><br>
</div>
<div style="font-size:12.8px">Thanks</div>
</div>
</blockquote>
<br>
</span></div>
</blockquote></div><br></div>