LinkedHashMap 閱讀筆記

以前看了 HashMap 的源碼，看 LinkedHashMap 要輕鬆很多。

 

先看數據的儲存形式：

    /**
     * LinkedEntry adds nxt/prv double-links to plain HashMapEntry.
     */
    static class LinkedEntry<K, V> extends HashMapEntry<K, V> {
        LinkedEntry<K, V> nxt;
        LinkedEntry<K, V> prv;

        /** Create the header entry */
        LinkedEntry() {
            super(null, null, 0, null);
            nxt = prv = this;
        }

        /** Create a normal entry */
        LinkedEntry(K key, V value, int hash, HashMapEntry<K, V> next,
                    LinkedEntry<K, V> nxt, LinkedEntry<K, V> prv) {
            super(key, value, hash, next);
            this.nxt = nxt;
            this.prv = prv;
        }
    }

看到 pre 和 nxt 就知道這是一個雙向鏈表結構。

構造函數裏面：nxt = prv = this; 說明這仍是一個環形雙向鏈表。

 

根據 HashMap 的源碼，咱們調用了 put 方法以後，put 方法會調用 addNewEntry 方法（若是 key 值不存在）。

    /**
     * Evicts eldest entry if instructed, creates a new entry and links it in
     * as head of linked list. This method should call constructorNewEntry
     * (instead of duplicating code) if the performance of your VM permits.
     *
     * <p>It may seem strange that this method is tasked with adding the entry
     * to the hash table (which is properly the province of our superclass).
     * The alternative of passing the "next" link in to this method and
     * returning the newly created element does not work! If we remove an
     * (eldest) entry that happens to be the first entry in the same bucket
     * as the newly created entry, the "next" link would become invalid, and
     * the resulting hash table corrupt.
     */
    @Override void addNewEntry(K key, V value, int hash, int index) {
        LinkedEntry<K, V> header = this.header;

        // Remove eldest entry if instructed to do so.
        LinkedEntry<K, V> eldest = header.nxt;
        if (eldest != header && removeEldestEntry(eldest)) {
            remove(eldest.key);
        }

        // Create new entry, link it on to list, and put it into table
        LinkedEntry<K, V> oldTail = header.prv;
        LinkedEntry<K, V> newTail = new LinkedEntry<K,V>(
                key, value, hash, table[index], header, oldTail);
        table[index] = oldTail.nxt = header.prv = newTail;
    }

每次 add 一個新的 Entry，都是插入到 Header 和 Tail 之間，即插入的 Entry 做爲新的 Tail。

removeEldestEntry() 一直返回 false，須要子類本身根據須要實現。

 

看 get 方法：

    /**
     * Returns the value of the mapping with the specified key.
     *
     * @param key
     *            the key.
     * @return the value of the mapping with the specified key, or {@code null}
     *         if no mapping for the specified key is found.
     */
    @Override public V get(Object key) {
        /*
         * This method is overridden to eliminate the need for a polymorphic
         * invocation in superclass at the expense of code duplication.
         */
        if (key == null) {
            HashMapEntry<K, V> e = entryForNullKey;
            if (e == null)
                return null;
            if (accessOrder)
                makeTail((LinkedEntry<K, V>) e);
            return e.value;
        }

        int hash = Collections.secondaryHash(key);
        HashMapEntry<K, V>[] tab = table;
        for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)];
                e != null; e = e.next) {
            K eKey = e.key;
            if (eKey == key || (e.hash == hash && key.equals(eKey))) {
                if (accessOrder)
                    makeTail((LinkedEntry<K, V>) e);
                return e.value;
            }
        }
        return null;
    }

與 HashMap 的 get() 差很少，不過多了

if (accessOrder)
    makeTail((LinkedEntry<K, V>) e);

這兩行代碼，看看 makeTail()：

    /**
     * Relinks the given entry to the tail of the list. Under access ordering,
     * this method is invoked whenever the value of a  pre-existing entry is
     * read by Map.get or modified by Map.put.
     */
    private void makeTail(LinkedEntry<K, V> e) {
        // Unlink e
        e.prv.nxt = e.nxt;
        e.nxt.prv = e.prv;

        // Relink e as tail
        LinkedEntry<K, V> header = this.header;
        LinkedEntry<K, V> oldTail = header.prv;
        e.nxt = header;
        e.prv = oldTail;
        oldTail.nxt = header.prv = e;
        modCount++;
    }

做用就是將 e 給插入到 header 和 tail 之間，和 addNewEntry() 的邏輯同樣。

 

accessOrder 的做用，爲 true 表示按照訪問順序排序，最近訪問的放到鏈表的最後（雙向鏈表也沒有先後之分）。

爲 false 表示按照插入順序排序，最後插入的方法鏈表的最後，從上面的方法裏面很容易看出來。

 

preModify 也調整了訪問順序：

    @Override void preModify(HashMapEntry<K, V> e) {
        if (accessOrder) {
            makeTail((LinkedEntry<K, V>) e);
        }
    }

 

其餘的方法暫時先放一邊，就這樣。

 

根據 LinkedHashMap 實現的 LruMemeryCache：

/**
 * A cache that holds strong references to a limited number of Bitmaps. Each time a Bitmap is accessed, it is moved to
 * the head of a queue. When a Bitmap is added to a full cache, the Bitmap at the end of that queue is evicted and may
 * become eligible for garbage collection.<br />
 * <br />
 * <b>NOTE:</b> This cache uses only strong references for stored Bitmaps.
 *
 * @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
 * @since 1.8.1
 */
public class LruMemoryCache implements MemoryCache {

	private final LinkedHashMap<String, Bitmap> map;

	private final int maxSize;
	/** Size of this cache in bytes */
	private int size;

	/** @param maxSize Maximum sum of the sizes of the Bitmaps in this cache */
	public LruMemoryCache(int maxSize) {
		if (maxSize <= 0) {
			throw new IllegalArgumentException("maxSize <= 0");
		}
		this.maxSize = maxSize;
		this.map = new LinkedHashMap<String, Bitmap>(0, 0.75f, true);
	}

	/**
	 * Returns the Bitmap for {@code key} if it exists in the cache. If a Bitmap was returned, it is moved to the head
	 * of the queue. This returns null if a Bitmap is not cached.
	 */
	@Override
	public final Bitmap get(String key) {
		if (key == null) {
			throw new NullPointerException("key == null");
		}

		synchronized (this) {
			return map.get(key);
		}
	}

	/** Caches {@code Bitmap} for {@code key}. The Bitmap is moved to the head of the queue. */
	@Override
	public final boolean put(String key, Bitmap value) {
		if (key == null || value == null) {
			throw new NullPointerException("key == null || value == null");
		}

		synchronized (this) {
			size += sizeOf(key, value);
			Bitmap previous = map.put(key, value);
			if (previous != null) {
				size -= sizeOf(key, previous);
			}
		}

		trimToSize(maxSize);
		return true;
	}

	/**
	 * Remove the eldest entries until the total of remaining entries is at or below the requested size.
	 *
	 * @param maxSize the maximum size of the cache before returning. May be -1 to evict even 0-sized elements.
	 */
	private void trimToSize(int maxSize) {
		while (true) {
			String key;
			Bitmap value;
			synchronized (this) {
				if (size < 0 || (map.isEmpty() && size != 0)) {
					throw new IllegalStateException(getClass().getName() + ".sizeOf() is reporting inconsistent results!");
				}

				if (size <= maxSize || map.isEmpty()) {
					break;
				}

				Map.Entry<String, Bitmap> toEvict = map.entrySet().iterator().next();
				if (toEvict == null) {
					break;
				}
				key = toEvict.getKey();
				value = toEvict.getValue();
				map.remove(key);
				size -= sizeOf(key, value);
			}
		}
	}

	/** Removes the entry for {@code key} if it exists. */
	@Override
	public final Bitmap remove(String key) {
		if (key == null) {
			throw new NullPointerException("key == null");
		}

		synchronized (this) {
			Bitmap previous = map.remove(key);
			if (previous != null) {
				size -= sizeOf(key, previous);
			}
			return previous;
		}
	}

	@Override
	public Collection<String> keys() {
		synchronized (this) {
			return new HashSet<String>(map.keySet());
		}
	}

	@Override
	public void clear() {
		trimToSize(-1); // -1 will evict 0-sized elements
	}

	/**
	 * Returns the size {@code Bitmap} in bytes.
	 * <p/>
	 * An entry's size must not change while it is in the cache.
	 */
	private int sizeOf(String key, Bitmap value) {
		return value.getRowBytes() * value.getHeight();
	}

	@Override
	public synchronized final String toString() {
		return String.format("LruCache[maxSize=%d]", maxSize);
	}
}