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// Copyright 2014 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package com.google.devtools.build.skyframe;
import com.google.common.base.Function;
import com.google.common.base.Preconditions;
import java.io.Serializable;
/**
* A {@link SkyKey} is effectively a pair (type, name) that identifies a Skyframe value.
*/
public final class SkyKey implements Serializable {
private final SkyFunctionName functionName;
/**
* The name of the value.
*
* <p>This is deliberately an untyped Object so that we can use arbitrary value types (e.g.,
* Labels, PathFragments, BuildConfigurations, etc.) as value names without incurring
* serialization costs in the in-memory implementation of the graph.
*/
private final Object argument;
/**
* Cache the hash code for this object. It might be expensive to compute. It is transient because
* argument's hash code might not be stable across JVM instances.
*/
private transient int hashCode;
public SkyKey(SkyFunctionName functionName, Object valueName) {
this.functionName = Preconditions.checkNotNull(functionName);
this.argument = Preconditions.checkNotNull(valueName);
// 'hashCode' is non-volatile and non-final, so this write may in fact *not* be visible to other
// threads. But this isn't a concern from a correctness perspective. See the comments in
// #hashCode for more details.
this.hashCode = computeHashCode();
}
public SkyFunctionName functionName() {
return functionName;
}
public Object argument() {
return argument;
}
@Override
public String toString() {
return functionName + ":" + argument;
}
@Override
public int hashCode() {
// We use the hash code caching strategy employed by java.lang.String. There are three subtle
// things going on here:
//
// (1) We use a value of 0 to indicate that the hash code hasn't been computed and cached yet.
// Yes, this means that if the hash code is really 0 then we will "recompute" it each time. But
// this isn't a problem in practice since a hash code of 0 should be rare.
//
// (2) Since we have no synchronization, multiple threads can race here thinking there are the
// first one to compute and cache the hash code.
//
// (3) Moreover, since 'hashCode' is non-volatile, the cached hash code value written from one
// thread may not be visible by another.
//
// All three of these issues are benign from a correctness perspective; in the end we have no
// overhead from synchronization, at the cost of potentially computing the hash code more than
// once.
int h = hashCode;
if (h == 0) {
h = computeHashCode();
hashCode = h;
}
return h;
}
private int computeHashCode() {
return 31 * functionName.hashCode() + argument.hashCode();
}
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null) {
return false;
}
if (getClass() != obj.getClass()) {
return false;
}
SkyKey other = (SkyKey) obj;
return argument.equals(other.argument) && functionName.equals(other.functionName);
}
public static final Function<SkyKey, Object> NODE_NAME = new Function<SkyKey, Object>() {
@Override
public Object apply(SkyKey input) {
return input.argument();
}
};
}
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