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8198540: Dynalink leaks memory when generating type converters
Reviewed-by: plevart, hannesw
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szegedi committed Feb 9, 2021
1 parent edd5fc8 commit 8f4c15f6417e471b372f74460fa94b9d84c4811d
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* This code is free software; you can redistribute it and/or modify it
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* published by the Free Software Foundation. Oracle designates this
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* by Oracle in the LICENSE file that accompanied this code.
* This code is distributed in the hope that it will be useful, but WITHOUT
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
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package jdk.dynalink;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.Map;
import java.util.Objects;
import java.util.function.BiFunction;
import java.util.function.Function;
import jdk.dynalink.internal.AccessControlContextFactory;

import static jdk.dynalink.internal.InternalTypeUtilities.canReferenceDirectly;

* Similar to ClassValue, but lazily associates a computed value with
* (potentially) every pair of types.
* @param <T> the value to associate with pairs of types.
final class BiClassValue<T> {
* Creates a new BiClassValue that uses the specified binary function to
* lazily compute the values.
* @param compute the binary function to compute the values. Ordinarily, it
* is invoked at most once for any pair of values. However,
* it is possible for it to be invoked concurrently. It can
* even be invoked concurrently multiple times for the same
* arguments under contention. In that case, it is undefined
* which of the computed values will be retained therefore
* returning semantically equivalent values is strongly
* recommended; the function should ideally be pure.
* Additionally, if the pair of types passed as parameters
* are from unrelated class loaders, the computed value is
* not cached at all and the function might be reinvoked
* with the same parameters in the future. Finally, a null
* return value is allowed, but not cached.
* @param <T> the type of the values
* @return a new BiClassValue that computes the values using the passed
* function.
static <T> BiClassValue<T> computing(final BiFunction<Class<?>, Class<?>, T> compute) {
return new BiClassValue<>(compute);

* A type-specific map that stores the values specific to pairs of types
* which include its class in one of the positions of the pair. Internally,
* it uses at most two maps named "forward" and "reverse". A BiClassValues
* for class C1 can store values for (C1, Cy) in its forward map as well
* as values for (Cx, C1) in its reverse map. The reason for this scheme
* is to avoid creating unwanted strong references from a parent class
* loader to a child class loader. If for a pair of classes (C1, C2)
* either C1 and C2 are in the same class loader, or C2 is in parent of C1,
* or C2 is a system class, forward map of C1's BiClassValues is used for
* storing the computed value. If C1 is in parent of C2, or C1 is a system
* class, reverse map of C2's BiClassValues is used for storing. If the
* class loaders are unrelated, the computed value is not cached and will
* be recomputed on every evaluation.
* NOTE that while every instance of this class is type-specific, it does
* not store a reference to the type Class object itself. BiClassValuesRoot
* creates the association from a type Class object to its BiClassValues'.
* @param <T> the type of the values
private final static class BiClassValues<T> {
// These will be used for compareAndExchange on forward and reverse fields.
private static final VarHandle FORWARD;
private static final VarHandle REVERSE;
static {
final MethodHandles.Lookup lookup = MethodHandles.lookup();
try {
FORWARD = lookup.findVarHandle(BiClassValues.class, "forward", Map.class);
REVERSE = lookup.findVarHandle(BiClassValues.class, "reverse", Map.class);
} catch (NoSuchFieldException | IllegalAccessException e) {
throw new AssertionError(e);

private Map<Class<?>, T> forward = Map.of();
private Map<Class<?>, T> reverse = Map.of();

T getForwardValue(final Class<?> c) {
return forward.get(c);

T getReverseValue(final Class<?> c) {
return reverse.get(c);

private T compute(final VarHandle mapHandle, final Class<?> c, final Function<Class<?>, T> compute) {
Map<Class<?>, T> map = (Map<Class<?>, T>) mapHandle.getVolatile(this);
T value;
T newValue = null;
while ((value = map.get(c)) == null) {
if (newValue == null) {
newValue = compute.apply(c);
if (newValue == null) {
@SuppressWarnings({"unchecked", "rawtypes"})
final Map.Entry<Class<?>, T>[] entries = map.entrySet().toArray(new Map.Entry[map.size() + 1]);
entries[map.size()] = Map.entry(c, newValue);
final var newMap = Map.ofEntries(entries);
final var witness = (Map<Class<?>, T>) mapHandle.compareAndExchange(this, map, newMap);
if (witness == map) {
value = newValue;
map = witness;
return value;

T computeForward(final Class<?> c, Function<Class<?>, T> compute) {
return compute(FORWARD, c, compute);

T computeReverse(final Class<?> c, Function<Class<?>, T> compute) {
return compute(REVERSE, c, compute);

// A named class used for "root" field so it can be static so it doesn't
// gain a synthetic this$0 reference as that'd cause a memory leak through
// unwanted anchoring to a GC root when used with system classes.
private static final class BiClassValuesRoot<T> extends ClassValue<BiClassValues<T>> {
@Override protected BiClassValues<T> computeValue(Class<?> type) {
return new BiClassValues<>();

private enum RetentionDirection {

private final BiClassValuesRoot<T> root = new BiClassValuesRoot<>();
private final BiFunction<Class<?>, Class<?>, T> compute;

private BiClassValue(final BiFunction<Class<?>, Class<?>, T> compute) {
this.compute = Objects.requireNonNull(compute);

final T get(final Class<?> c1, final Class<?> c2) {
// Most likely case: it is in the forward map of c1's BiClassValues
final BiClassValues<T> cv1 = root.get(c1);
final T v1 = cv1.getForwardValue(c2);
if (v1 != null) {
return v1;

// Next likely case: it is in the reverse map of c2's BiClassValues
final BiClassValues<T> cv2 = root.get(c2);
final T v2 = cv2.getReverseValue(c1);
if (v2 != null) {
return v2;

// Value is uncached, compute it and cache if possible.
switch (getRetentionDirection(c1, c2)) {
// loader of c1 can see loader of c2, store value for (c1, c2) in cv1's forward map
return cv1.computeForward(c2, cy -> compute.apply(c1, cy));
// loader of c2 can see loader of c1, store value for (c1, c2) in cv2's reverse map
return cv2.computeReverse(c1, cx -> compute.apply(cx, c2));
// Class loaders are unrelated; compute and return uncached.
return compute.apply(c1, c2);
throw new AssertionError(); // enum values exhausted

private static final AccessControlContext GET_CLASS_LOADER_CONTEXT =

private static RetentionDirection getRetentionDirection(Class<?> from, Class<?> to) {
return AccessController.doPrivileged((PrivilegedAction<RetentionDirection>) () -> {
final ClassLoader cl1 = from.getClassLoader();
final ClassLoader cl2 = to.getClassLoader();
if (canReferenceDirectly(cl1, cl2)) {
return RetentionDirection.FORWARD;
} else if (canReferenceDirectly(cl2, cl1)) {
return RetentionDirection.REVERSE;
return RetentionDirection.NEITHER;

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