/*
* Copyright (c) 2019, 2019, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* 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
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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*/
package com.oracle.svm.core.util;
import java.util.function.Supplier;
import org.graalvm.compiler.serviceprovider.GraalUnsafeAccess;
// Checkstyle: stop
import sun.misc.Unsafe;
// Checkstyle: resume
An object reference that is set lazily to the reference returned by the provided Supplier
in a thread-safe manner: Supplier.get()
might be called more than once from different threads, but get()
will always return the same reference. /**
* An object reference that is set lazily to the reference returned by the provided {@link Supplier}
* in a thread-safe manner: {@link Supplier#get()} might be called more than once from different
* threads, but {@link #get()} will always return the same reference.
*/
public final class LazyFinalReference<T> {
private static final Object UNINITIALIZED = new Object();
private static final Unsafe UNSAFE = GraalUnsafeAccess.getUnsafe();
private static final long VALUE_OFFSET;
static {
try {
VALUE_OFFSET = UNSAFE.objectFieldOffset(LazyFinalReference.class.getDeclaredField("value"));
} catch (Throwable ex) {
throw VMError.shouldNotReachHere(ex);
}
}
private final Supplier<T> supplier;
Not required to be volatile because the value will be eventually consistent and inconsistency
is primitive to handle.
/**
* Not required to be volatile because the value will be eventually consistent and inconsistency
* is primitive to handle.
*/
@SuppressWarnings("unchecked") private T value = (T) UNINITIALIZED;
public LazyFinalReference(Supplier<T> supplier) {
this.supplier = supplier;
}
@SuppressWarnings("unchecked")
public T get() {
T v = value;
if (v == UNINITIALIZED) {
// Try volatile read first in case of memory inconsistency to avoid Supplier call
v = (T) UNSAFE.getObjectVolatile(this, VALUE_OFFSET);
if (v == UNINITIALIZED) {
T obj = supplier.get();
if (UNSAFE.compareAndSwapObject(this, VALUE_OFFSET, UNINITIALIZED, obj)) {
v = obj;
} else {
v = (T) UNSAFE.getObjectVolatile(this, VALUE_OFFSET);
}
assert v != UNINITIALIZED;
}
}
return v;
}
}