hadoop WeakReferenceThreadMap 源码
haddop WeakReferenceThreadMap 代码
文件路径：/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/impl/WeakReferenceThreadMap.java
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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 org.apache.hadoop.fs.impl;
import java.lang.ref.WeakReference;
import java.util.function.Consumer;
import java.util.function.Function;
import javax.annotation.Nullable;
import org.apache.hadoop.util.WeakReferenceMap;
import static java.util.Objects.requireNonNull;
/**
* A WeakReferenceMap for threads.
* @param <V> value type of the map
*/
public class WeakReferenceThreadMap<V> extends WeakReferenceMap<Long, V> {
public WeakReferenceThreadMap(final Function<? super Long, ? extends V> factory,
@Nullable final Consumer<? super Long> referenceLost) {
super(factory, referenceLost);
}
/**
* Get the value for the current thread, creating if needed.
* @return an instance.
*/
public V getForCurrentThread() {
return get(currentThreadId());
}
/**
* Remove the reference for the current thread.
* @return any reference value which existed.
*/
public V removeForCurrentThread() {
return remove(currentThreadId());
}
/**
* Get the current thread ID.
* @return thread ID.
*/
public long currentThreadId() {
return Thread.currentThread().getId();
}
/**
* Set the new value for the current thread.
* @param newVal new reference to set for the active thread.
* @return the previously set value, possibly null
*/
public V setForCurrentThread(V newVal) {
requireNonNull(newVal);
long id = currentThreadId();
// if the same object is already in the map, just return it.
WeakReference<V> existingWeakRef = lookup(id);
// The looked up reference could be one of
// 1. null: nothing there
// 2. valid but get() == null : reference lost by GC.
// 3. different from the new value
// 4. the same as the old value
if (resolve(existingWeakRef) == newVal) {
// case 4: do nothing, return the new value
return newVal;
} else {
// cases 1, 2, 3: update the map and return the old value
return put(id, newVal);
}
}
}
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