Hi Jochen,
On 01/07/2016 03:05 PM, Jochen Theodorou wrote:
Hi Peter,
Am 06.01.2016 um 21:34 schrieb Peter Levart:
[...]
ClassValue API could use a different strategy and reference the cached
instance of type T from ClassValue<T> instance instead, but then you
would have a problem with ClassValue instances reachable from other
class loaders than the cached instances of type T.
yes, that is what I tried as well... I used my own map directly in
ClassValue and avoided the map ClassValue uses natively.
So the solution for this case is basically using a Soft- or
Weak-Reference in Dummy, right?
The solution to use any Class<?> key and not be dependent on it's origin
(i.e. the class loader) and therefore on it's lifetime, is to decouple
the cached value from the Class<?> key by an indirection through a
WeakReference, but then you should make sure the cached value doesn't go
away while it is still possible to retrieve it - the trick is to hook it
on a list that is reachable from the ClassValue instance that
constructed it:
import java.io.File;
import java.io.UncheckedIOException;
import java.lang.ref.WeakReference;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.nio.file.Paths;
import java.util.LinkedList;
import java.util.List;
import java.util.regex.Pattern;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class CVTest {
public static class MyClassValue extends
ClassValue<WeakReference<Dummy>> {
private final List<Dummy> dummies = new ArrayList<>();
@Override
protected WeakReference<Dummy> computeValue(Class type) {
Dummy ret = new Dummy();
// make it reachable from MyClassValue instance that
constructed it
synchronized(dummies) {
dummies.add(ret);
}
Dummy.o = this;
// decouple it from 'type' - make it only weakly reachable
from 'type'
return new WeakReference<>(ret);
}
}
public static class Dummy {
static Object o;
@Override
protected void finalize() throws Throwable {
System.out.println(this + " finalized");
}
}
public static void main(String[] args) throws Exception {
String[] cp = System.getProperty("java.class.path")
.split(Pattern.quote(File.pathSeparator));
URL[] urls = Stream.of(cp).map(f -> {
try {
return Paths.get(f).toUri().toURL();
} catch (MalformedURLException e) {
throw new UncheckedIOException(e);
}
}).collect(Collectors.toList()).toArray(new URL[0]);
for (long i = 0; i < 10; i++) {
URLClassLoader classLoader = new URLClassLoader(
urls, ClassLoader.getSystemClassLoader().getParent());
@SuppressWarnings("unchecked")
ClassValue<WeakReference<?>> cv =
(ClassValue<WeakReference<?>>)
classLoader.loadClass("CVTest$MyClassValue").newInstance();
WeakReference<?> valueRef = cv.get(Integer.TYPE);
// while 'cv' is alive, 'value' is alive too since 'cv' keeps a
// strong reference to 'value' in its 'dummies' list...
Object value = valueRef.get();
assert value != null;
assert value.getClass().getClassLoader() == classLoader;
System.out.println(i);
classLoader.close();
classLoader = null;
cv = null;
value = null;
System.gc();
Thread.sleep(200L);
}
}
}
...running this example produces:
0
CVTest$Dummy@6b027549 finalized
1
CVTest$Dummy@65c59659 finalized
2
CVTest$Dummy@61fa1e39 finalized
...
...
Regards, Peter
bye Jochen
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