LRUキャッシュの実装

LinkedHashMapは、既存のLRU実装です.しかし,コンカレントメカニズムが欠けており,フルロード削除条件(removeEldestEntry)が実現されていないため,LinkedHashMapは拡張を予約したLRUクラスであると考えられる.
この文書では、クラスを継承してLRUキャッシュをさらに実装します.
まず、再入可能ロックを併用します.これは、読み取りロック後に書き込み可能かどうか、書き込みロック後に読み取り可能かどうかの問題です.一般的に、読んだ後、書くと汚いデータが発生します.書いた後、読むと問題ありません.
 
再ロック可能な読み書きロック、ロック順序テスト:

package lock;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class LockTest1{
	final static ReadWriteLock rwlock = new ReentrantReadWriteLock();
	final static Lock readLock = rwlock.readLock();
	final static Lock writeLock = rwlock.writeLock();

	public static void main(String[] args) throws InterruptedException {
		System.out.println("test readlock-writelock");
		lockR2W();
		System.out.println("test writelock-readlock");
		lockW2R();
		System.out.println("test readlock-readlock");
		lockRR();
		System.out.println("test writelock-writelock");
		lockWW();
	}

	private static void lockWW() {
		writeLock.lock();
		System.out.println("w lock.");
		try {
			if (writeLock.tryLock(1, TimeUnit.SECONDS))
				System.out.println("w lock.");
			else
				System.out.println("when w lock, cannot w lock again.");
		} catch (InterruptedException e0) {
			e0.printStackTrace();
		}
		try {
			writeLock.unlock();
			System.out.println("w unlock.");
			writeLock.unlock();
			System.out.println("w unlock.");
		} catch (Exception e) {
			//ignore;
		}
	}

	private static void lockRR() {
		readLock.lock();
		System.out.println("r lock.");
		readLock.lock();
		System.out.println("r lock.");
		readLock.unlock();
		System.out.println("r unlock.");
		readLock.unlock();
		System.out.println("r unlock.");
	}

	private static void lockW2R() throws InterruptedException {
		writeLock.lock();
		System.out.println("w lock.");
		if (readLock.tryLock(1, TimeUnit.SECONDS))
			System.out.println("r lock.");
		else
			System.out.println("when w lock, cannot r lock.");
		writeLock.unlock();
		System.out.println("w unlock.");
		readLock.unlock();
		System.out.println("r unlock.");
	}

	private static void lockR2W() {
		readLock.lock();
		System.out.println("r lock.");
		try {
			if (writeLock.tryLock(1, TimeUnit.SECONDS))
				System.out.println("w lock.");
			else
				System.out.println("when r lock, cannot w lock.");
		} catch (InterruptedException e0) {
			e0.printStackTrace();
		}
		try {
			readLock.unlock();
			System.out.println("r unlock.");
			writeLock.unlock();
			System.out.println("w unlock.");
		} catch (Exception e) {
			//ignore;
		}
	}
}

テスト結果:

test readlock-writelock

r lock.
when r lock, cannot w lock.
r unlock.
test writelock-readlock

w lock.
r lock.
w unlock.
r unlock.
test readlock-readlock

r lock.
r lock.
r unlock.
r unlock.
test writelock-writelock

w lock.
w lock.
w unlock.
w unlock.

 
LRUは、最近アクセスされたデータが常にキャッシュから最も速く読み取れることを意味する.そこで、次に、最新データの位置とキャッシュがいっぱいになったときのデータの処理をテストします.

package lru;

import java.util.Iterator;
import java.util.Map.Entry;

public class LRUTest{
	public static void main(String[] args) {
		int maxCapacity = 5;
		OjadbMap<Integer, String> lruMap = new OjadbMap<Integer, String>(maxCapacity);
		for (int i = 0; i < maxCapacity; i++) {
			lruMap.put(i, "data[" + i + "]");
		}
		lruMap.get(3);
		Iterator<Entry<Integer, String>> iter = lruMap.entrySet().iterator();
		while (iter.hasNext()) {
			System.out.println(iter.next().getValue());
		}
		System.out.println();
		lruMap.put(9, "data[ 9 ]");
		Iterator<Entry<Integer, String>> iter1 = lruMap.entrySet().iterator();
		while (iter1.hasNext()) {
			System.out.println(iter1.next().getValue());
		}
		
	}
}

テスト結果:

data[0]
data[1]
data[2]
data[4]
data[3] // 。

data[1]
data[2]
data[4]
data[3]
data[ 9 ] // ( ， )。

 
本明細書で使用するLRUキャッシュクラス:
(参考:このクラスは私のオープンソースデータベースプロジェクトで、キャッシュ部分の実装クラスです.
http://code.google.com/p/ojadbにアクセスしてプロジェクト全体を表示できます.

/*******************************************************************************************
 * oJadb is free software: you can redistribute it and/or modify it under 
 * the terms of the GNU General Public License as published by the Free Software Foundation, 
 * either version 3 of the License, or (at your option) any later version.
 * 
 * oJadb 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 for more details.
 * 
 * You should have received a copy of the GNU General Public License along with this library. 
 * If not, see <http://www.gnu.org/licenses/>.
 * 
 * Author:EricHan
 * Time:2008-8-28
 ******************************************************************************************/
package lru;

import java.util.ArrayList;
import java.util.Collection;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class OjadbMap<K, V> extends LinkedHashMap<K, V>{
	private static final long serialVersionUID = -6970580442519842034L;
	private final int maxCapacity;
	private static final float DEFAULT_LOAD_FACTOR = 0.75f;
	//private final Lock lock = new ReentrantLock();
	private final ReadWriteLock rwlock = new ReentrantReadWriteLock();
	private final Lock readLock = rwlock.readLock();
	private final Lock writeLock = rwlock.writeLock();

	public OjadbMap(int maxCapacity) {
		super(maxCapacity, DEFAULT_LOAD_FACTOR, true);
		this.maxCapacity = maxCapacity;
	}

	@Override
	protected boolean removeEldestEntry(java.util.Map.Entry<K, V> eldest) {
		return size() > maxCapacity;
	}

	@Override
	public boolean containsKey(Object key) {
		try {
			readLock.lock();
			return super.containsKey(key);
		} finally {
			readLock.unlock();
		}
	}

	@Override
	public V get(Object key) {
		try {
			readLock.lock();
			return super.get(key);
		} finally {
			readLock.unlock();
		}
	}

	@Override
	public V put(K key, V value) {
		try {
			writeLock.lock();
			return super.put(key, value);
		} finally {
			writeLock.unlock();
		}
	}

	public int size() {
		try {
			readLock.lock();
			return super.size();
		} finally {
			readLock.unlock();
		}
	}

	public void clear() {
		try {
			writeLock.lock();
			super.clear();
		} finally {
			writeLock.unlock();
		}
	}

	public Collection<Map.Entry<K, V>> getAll() {
		try {
			readLock.lock();
			return new ArrayList<Map.Entry<K, V>>(super.entrySet());
		} finally {
			readLock.unlock();
		}
	}
}