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建议大家从【Java技术专题-JVM研究系列（39）Java各种类型对象占用内存情况分析（上）】开始学习比较好，这样子会有一个承接和过度。根据前面的学习的内存占用计算规则，可以计算出一个对象在内存中的占用空间大小情况，下面举例分析下Java中的Enum, ArrayList及HashMap的内存占用情况，读者可以仿照分析计算过程来计算其他数据结构的内存占用情况。

注： 下面的分析计算基于HotSpot Jvm, JDK1.8, 64位机器，开启指针压缩。。

对象头

这里只关注其内存占用大小。在64位机器上，默认不开启指针压缩（-XX:-UseCompressedOops）的情况下，对象头占用12bytes，开启指针压缩（-XX:+UseCompressedOops）则占用16bytes。

实例数据：

对象引用（reference）类型在64位机器上，关闭指针压缩时占用8bytes， 开启时占用4bytes，一般指的是局部变量表或者操作数栈中的reference类型或者针对于成员变量情况下的地址引用（shallow size）。

注： 下面的分析计算基于HotSpot Jvm, JDK1.8, 64位机器，开启指针压缩。

枚举类

创建enum时，编译器会生成一个相关的类，这个类继承自java.lang.Enum。Enum类拥有两个属性变量，分别为int的ordinal和String的name, 相关源码如下：

public abstract class Enum<E extends Enum<E>>
        implements Comparable<E>, Serializable {
    /**
     * The name of this enum constant, as declared in the enum declaration.
     * Most programmers should use the {@link #toString} method rather than
     * accessing this field.
     */
    private final String name;

    /**
     * The ordinal of this enumeration constant (its position
     * in the enum declaration, where the initial constant is assigned
     * an ordinal of zero).
     *
     * Most programmers will have no use for this field.  It is designed
     * for use by sophisticated enum-based data structures, such as
     * {@link java.util.EnumSet} and {@link java.util.EnumMap}.
     */
    private final int ordinal;
}

以下面的TestEnum为例进行枚举类的内存占用分析

public enum TestEnum {
        ONE(1, "one"),
        TWO(2, "two");

        private int code;
        private String desc;

        TestEnum(int code, String desc) {
            this.code = code;
            this.desc = desc;
        }

        public int getCode() {
            return code;
        }

        public String getDesc() {
            return desc;
        }
}

这里TestEnum的每个实例除了父类的两个属性外，还拥有一个int的code及String的desc属性，所以一个TestEnum的实例本身所占用的内存大小为：

12(header) + 4(ordinal) + 4(name reference) + 4(code) + 4(desc reference) = 28(padding) -> 32 bytes.

总共占用的内存大小为：

按照上面对字符串类型的分析，desc和name都占用：48bytes。 所以TestEnum.ONE占用总内存大小为：

12(header) + 4(ordinal) + 4(code) + 48 * 2(desc, name) + 4(desc reference) + 4(name reference) = 128 (bytes)

ArrayList

ArrayList实现List接口，底层使用数组保存所有元素。其操作基本上是对数组的操作。下面分析下源代码：

底层使用数组保存数据：

	/**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

构造方法

ArrayList提供了三种方式的构造器，可以构造一个默认的空列表、构造一个指定初始容量的空列表及构造一个包含指定collection元素的列表，这些元素按照该collection的迭代器返回它们的顺序排列。

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @ c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

存储

ArrayList提供了set(int index, E element)、add(E e)、add(int index, E element)、addAll(Collection<? extends E> c)等，这里着重介绍一下add(E e)方法。

    /**
     * Appends the specified element to the end of this list.
     *
     * @ e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

add方法将指定的元素添加到此列表的尾部。这里注意下ensureCapacityInternal方法，这个方法会检查添加后元素的个数是否会超过当前数组的长度，如果超出，数组将会进行扩容。

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Increases the capacity of this <tt>ArrayList</tt> instance, if
     * necessary, to ensure that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     *
     * @   minCapacity   the desired minimum capacity
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table
            ? 0
            // larger than default for default empty table. It's already
            // supposed to be at default size.
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

如果初始时没有指定ArrayList大小，在第一次调用add方法时，会初始化数组默认最小容量为10。看下grow方法的源码：

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @ minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

从上述代码可以看出，数组进行扩容时，会将老数组中的元素重新拷贝一份到新的数组中，每次数组扩容的增长是原容量的1.5倍。这种操作的代价是很高的，因此在实际使用时，应该尽量避免数组容量的扩张。当可预知要保存的元素的数量时，要在构造ArrayList实例时，就指定其容量，以避免数组扩容的发生。或者根据实际需求，通过调用ensureCapacity方法来手动增加ArrayList实例的容量。

内存占用

下面开始分析ArrayList的内存占用情况。ArrayList继承AbstractList类，AbstractList拥有一个int类型的modCount属性，ArrayList本身拥有一个int类型的size属性和一个数组属性。 所以一个ArrayList实例本身的的大小为：

12(header) + 4(modCount) + 4(size) + 4(elementData reference) = 24 (bytes)

下面分析一个只有一个Integer(1)元素的ArrayList<Integer>实例占用的内存大小。

   ArrayList<Integer> testList = Lists.newArrayList();
   testList.add(1);

根据上面对ArrayList原理的介绍，当调用add方法时，ArrayList会初始化一个默认大小为10的数组，而数组中保存的Integer(1)实例大小为16 bytes。则testList占用的内存大小为：

24(ArrayList itselft) + 16(elementData array header) + 10 * 4(elemetData reference) + 16(Integer) = 96 (bytes)

HashMap

HashMap的数据结构

HashMap是一个“链表散列”的数据结构，即数组和链表的结合体。

从图上可以看出，HashMap底层是一个数组结构，数组中的每一项又是一个链表。当新建一个HashMap的时候，初始化一个数组，源码如下：

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

Node是链表中一个结点，一个Node对象保存了一对HashMap的Key,Value以及指向下一个节点的指针，源码如下：

     /**
     * Basic hash bin node, used for most entries.  (See below for
     * TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
     */
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;

        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }
     }

构造方法

HashMap提供了四种方式的构造器，分别为指定初始容量及负载因子构造器，指定初始容量构造器，不指定初始容量及负载因子构造器，以及根据已有Map生成新Map的构造器。

    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and load factor.
     *
     * @  initialCapacity the initial capacity
     * @  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        this.loadFactor = loadFactor;
        this.threshold = tableSizeFor(initialCapacity);
    }

    /**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and the default load factor (0.75).
     *
     * @  initialCapacity the initial capacity.
     * @throws IllegalArgumentException if the initial capacity is negative.
     */
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

    /**
     * Constructs a new <tt>HashMap</tt> with the same mappings as the
     * specified <tt>Map</tt>.  The <tt>HashMap</tt> is created with
     * default load factor (0.75) and an initial capacity sufficient to
     * hold the mappings in the specified <tt>Map</tt>.
     *
     * @   m the map whose mappings are to be placed in this map
     * @throws  NullPointerException if the specified map is null
     */
    public HashMap(Map<? extends K, ? extends V> m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

如果不指定初始容量及负载因子，默认的初始容量为16， 负载因子为0.75。

负载因子衡量的是一个散列表的空间的使用程度，负载因子越大表示散列表的装填程度越高，反之愈小。对于使用链表法的散列表来说，查找一个元素的平均时间是O(1+a)，因此如果负载因子越大，对空间的利用更充分，然而后果是查找效率的降低；如果负载因子太小，那么散列表的数据将过于稀疏，对空间造成严重浪费。

HashMap有一个容量阈值属性threshold，是根据初始容量和负载因子计算得出threshold=capacity*loadfactor， 如果HashMap中数组元素的个数超过这个阈值，则HashMap会进行扩容。HashMap底层的数组长度总是2的n次方，每次扩容容量为原来的2倍。 扩容的目的是为了减少hash冲突，提高查询效率。而在HashMap数组扩容之后，最消耗性能的点就出现了：原数组中的数据必须重新计算其在新数组中的位置，并放进去，这就是resize。

数据的存储

    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    /**
     * Implements Map.put and related methods
     * @ hash hash for key
     * @ key the key
     * @ value the value to put
     * @ onlyIfAbsent if true, don't change existing value
     * @ evict if false, the table is in creation mode.
     * @return previous value, or null if none
     */
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //初始化数组的大小为16，容量阈值为16*0.75=12
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //如果key的hash值对应的数组位置没有元素，则新建Node放入此位置
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

从上面的源代码中可以看出：当我们往HashMap中put元素的时候，先根据key的hashCode重新计算hash值，根据hash值得到这个元素在数组中的位置（即下标）。

如果数组该位置上已经存放有其他元素了，那么在这个位置上的元素将以链表的形式存放，新加入的放在链头，最先加入的放在链尾。如果数组该位置上没有元素，就直接将该元素放到此数组中的该位置上。

HashMap内存占用

这里分析一个只有一组键值对的HashMap, 结构如下：

Map<Integer, Integer> testMap = Maps.newHashMap();
testMap.put(1, 2);

首先分析HashMap本身的大小。HashMap对象拥有的属性包括：

    /**
     * The table, initialized on first use, and resized as
     * necessary. When allocated, length is always a power of two.
     * (We also tolerate length zero in some operations to allow
     * bootstrapping mechanics that are currently not needed.)
     */
    transient Node<K,V>[] table;

    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient int modCount;

    /**
     * The next size value at which to resize (capacity * load factor).
     *
     * @serial
     */
    // (The javadoc description is true upon serialization.
    // Additionally, if the table array has not been allocated, this
    // field holds the initial array capacity, or zero signifying
    // DEFAULT_INITIAL_CAPACITY.)
    int threshold;

    /**
     * The load factor for the hash table.
     *
     * @serial
     */
    final float loadFactor;

HashMap继承了AbstractMap<K,V>, AbstractMap有两个属性：

transient Set<K> keySet;
transient Collection<V> values;

所以一个HashMap对象本身的大小为：

12(header) + 4(table reference) + 4(entrySet reference) + 4(size) + 4(modCount) + 4(threshold) + 8(loadFactor) + 4(keySet reference) + 4(values reference) = 48(bytes)

接着分析testMap实例在总共占用的内存大小。

根据上面对HashMap原理的介绍，可知每对键值对对应一个Node对象。根据上面的Node的数据结构，一个Node对象的大小为：

12(header) + 4(hash reference) + 4(key reference) + 4(value reference) + 4(next pointer reference) = 28 (padding) -> 32(bytes)

加上Key和Value两个Integer对象，一个Node占用内存总大小为：32 + 2 * 16 = 64(bytes)

下面分析HashMap的Node数组的大小。

根据上面HashMap的原理可知，在不指定容量大小的情况下，HashMap初始容量为16，所以testMap的Node[]占用的内存大小为：

16(header) + 16 * 4(Node reference) + 64(Node) = 144(bytes)

所以，testMap占用的内存总大小为：

48(map itself) + 144(Node[]) = 192(bytes)

这里只用一个例子说明如何对HashMap进行占用内存大小的计算，根据HashMap初始化容量的大小，以及扩容的影响，HashMap占用内存大小要进行具体分析，