LinkedHashMap和LruCache源码分析

/**
    * LinkedHashMap entry.
    */
   private static class LinkedHashMapEntry<K,V> extends HashMapEntry<K,V> {
       // These fields comprise the doubly linked list used for iteration.
       LinkedHashMapEntry<K,V> before, after;

       LinkedHashMapEntry(int hash, K key, V value, HashMapEntry<K,V> next) {
           super(hash, key, value, next);
       }

       /**
        * Removes this entry from the linked list.
        */
       // 对自身在链表中进行移除
       private void remove() {
           before.after = after;
           after.before = before;
       }

       /**
        * Inserts this entry before the specified existing entry in the list.
        */
         //插入到LinkedHashMapEntry existingEntry之前
       private void addBefore(LinkedHashMapEntry<K,V> existingEntry) {
           after  = existingEntry;
           before = existingEntry.before;
           before.after = this;
           after.before = this;
       }

       /**
        * This method is invoked by the superclass whenever the value
        * of a pre-existing entry is read by Map.get or modified by Map.set.
        * If the enclosing Map is access-ordered, it moves the entry
        * to the end of the list; otherwise, it does nothing.
        */
       void recordAccess(HashMap<K,V> m) {
           LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
           if (lm.accessOrder) {
               lm.modCount++;
               remove();
               addBefore(lm.header);
           }
       }

       void recordRemoval(HashMap<K,V> m) {
           remove();
       }
   }

 /**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        if (key == null)
            return putForNullKey(value);
        int hash = sun.misc.Hashing.singleWordWangJenkinsHash(key);
        int i = indexFor(hash, table.length);
        for (HashMapEntry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                //调用子类LinkedHashMapEntry的recordAccess方法
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        //调用LinkedHashMap.addEntry方法
        addEntry(hash, key, value, i);
        return null;
    }
    
/**
     * Adds a new entry with the specified key, value and hash code to
     * the specified bucket.  It is the responsibility of this
     * method to resize the table if appropriate.
     *
     * Subclass overrides this to alter the behavior of put method.
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {
        //扩容
        if ((size >= threshold) && (null != table[bucketIndex])) {
            resize(2 * table.length);
            hash = (null != key) ? sun.misc.Hashing.singleWordWangJenkinsHash(key) : 0;
            bucketIndex = indexFor(hash, table.length);
        }
        //增加新的key  value  调用LinkedHashMap.createEntry
        createEntry(hash, key, value, bucketIndex);
    }

 /**
     * This override alters behavior of superclass put method. It causes newly
     * allocated entry to get inserted at the end of the linked list and
     * removes the eldest entry if appropriate.
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {
        // Previous Android releases called removeEldestEntry() before actually
        // inserting a value but after increasing the size.
        // The RI is documented to call it afterwards.
        // **** THIS CHANGE WILL BE REVERTED IN A FUTURE ANDROID RELEASE ****
        
        //得到需要移除的对象
        // Remove eldest entry if instructed
        LinkedHashMapEntry<K,V> eldest = header.after;
        //需要移除的对象是否是自身
        if (eldest != header) {
            boolean removeEldest;
            size++;
            try {
                  //得到是否移除的对象的标识
                removeEldest = removeEldestEntry(eldest);
            } finally {
                size--;
            }
            if (removeEldest) {
                //调用HashMap的removeEntryForKey进行移除
                removeEntryForKey(eldest.key);
            }
        }
        //调用HashMap的addEntry
        super.addEntry(hash, key, value, bucketIndex);
    }
    
protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
        return false;
    }

    /**
     * Like addEntry except that this version is used when creating entries
     * as part of Map construction or "pseudo-construction" (cloning,
     * deserialization).  This version needn't worry about resizing the table.
     *
     * Subclass overrides this to alter the behavior of HashMap(Map),
     * clone, and readObject.
     */
       //增加新的key  value
    void createEntry(int hash, K key, V value, int bucketIndex) {
        HashMapEntry<K,V> e = table[bucketIndex];
        table[bucketIndex] = new HashMapEntry<>(hash, key, value, e);
        size++;
    }

/**
     * Removes the mapping for the specified key from this map if present.
     *
     * @param  key key whose mapping is to be removed from the map
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
    public V remove(Object key) {
        Entry<K,V> e = removeEntryForKey(key);
        return (e == null ? null : e.getValue());
    }

    /**
     * Removes and returns the entry associated with the specified key
     * in the HashMap.  Returns null if the HashMap contains no mapping
     * for this key.
     */
    final Entry<K,V> removeEntryForKey(Object key) {
        if (size == 0) {
            return null;
        }
        int hash = (key == null) ? 0 : sun.misc.Hashing.singleWordWangJenkinsHash(key);
        int i = indexFor(hash, table.length);
        HashMapEntry<K,V> prev = table[i];
        HashMapEntry<K,V> e = prev;

        while (e != null) {
            HashMapEntry<K,V> next = e.next;
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k)))) {
                modCount++;
                size--;
                if (prev == e)
                    table[i] = next;
                else
                    prev.next = next;
                //调用LinkedHashMapEntry.recordRemoval
                e.recordRemoval(this);
                return e;
            }
            prev = e;
            e = next;
        }

        return e;
    }

/**
         * Removes this entry from the linked list.
         */
        private void remove() {
            before.after = after;
            after.before = before;
        }

void recordRemoval(HashMap<K,V> m) {
            remove();
        }

/**
    * Returns the value to which the specified key is mapped,
    * or {@code null} if this map contains no mapping for the key.
    *
    * <p>More formally, if this map contains a mapping from a key
    * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
    * key.equals(k))}, then this method returns {@code v}; otherwise
    * it returns {@code null}.  (There can be at most one such mapping.)
    *
    * <p>A return value of {@code null} does not <i>necessarily</i>
    * indicate that the map contains no mapping for the key; it's also
    * possible that the map explicitly maps the key to {@code null}.
    * The {@link #containsKey containsKey} operation may be used to
    * distinguish these two cases.
    */
   public V get(Object key) {
       //调用HashMap的getEntry
       LinkedHashMapEntry<K,V> e = (LinkedHashMapEntry<K,V>)getEntry(key);
       if (e == null)
           return null;
       e.recordAccess(this);
       return e.value;
   }

/**
    * Caches {@code value} for {@code key}. The value is moved to the head of
    * the queue.
    *
    * @return the previous value mapped by {@code key}.
    */
   public final V put(K key, V value) {
       if (key == null || value == null) {
           throw new NullPointerException("key == null || value == null");
       }

       V previous;
       synchronized (this) {
           putCount++;
           //根据key, value获取大小，我们需要重写sizeOf自行赋值
           size += safeSizeOf(key, value);
           //加入到LinkedHashMap
           previous = map.put(key, value);
           //如果key已经存在了previous 就！=null  就移除刚加的大小
           if (previous != null) {
               size -= safeSizeOf(key, previous);
           }
       }

       if (previous != null) {
           //重写移除的逻辑 
           entryRemoved(false, key, previous, value);
       }
       //根据maxSize去除多余的元素
       trimToSize(maxSize);
       return previous;
   }

/**
    * Removes the entry for {@code key} if it exists.
    *
    * @return the previous value mapped by {@code key}.
    */
   public final V remove(K key) {
       if (key == null) {
           throw new NullPointerException("key == null");
       }

       V previous;
       synchronized (this) {
           //从LinkedHashMap中移除
           previous = map.remove(key);
           if (previous != null) {
               size -= safeSizeOf(key, previous);
           }
       }

       if (previous != null) {
           entryRemoved(false, key, previous, null);
       }

       return previous;
   }

/**
     * Returns the value for {@code key} if it exists in the cache or can be
     * created by {@code #create}. If a value was returned, it is moved to the
     * head of the queue. This returns null if a value is not cached and cannot
     * be created.
     */
    public final V get(K key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V mapValue;
        synchronized (this) {
            //从LinkedHashMap中取出元素
            mapValue = map.get(key);
            if (mapValue != null) {
                hitCount++;
                return mapValue;
            }
            missCount++;
        }

        /*
         * Attempt to create a value. This may take a long time, and the map
         * may be different when create() returns. If a conflicting value was
         * added to the map while create() was working, we leave that value in
         * the map and release the created value.
         */
        //如果没有这个key，那么你可以重写create来创建一个新的value 
        V createdValue = create(key);
        if (createdValue == null) {
            return null;
        }
      
      //新的创建的value 添加到LinkedHashMap
        synchronized (this) {
            createCount++;
            mapValue = map.put(key, createdValue);

            if (mapValue != null) {
                // There was a conflict so undo that last put
                map.put(key, mapValue);
            } else {
                size += safeSizeOf(key, createdValue);
            }
        }

        if (mapValue != null) {
            entryRemoved(false, key, createdValue, mapValue);
            return mapValue;
        } else {
            trimToSize(maxSize);
            return createdValue;
        }
    }