为什么多线程共享变量不能用HashMap、ArrayList、TreeSet等类

并发产生的原因浅析

　　　　变量的值是放在堆中，堆又是所有线程共享的，每个线程都有自己的工作内存，每次修改内存的值会从主内存中变量的值加载到自己的工作内存中，修改完把工作内存的值写回主内存。这个机制就导致程序无法按人的逻辑执行得出正确结果，因为你无法得知内存的值到底是几。

以HashMap为例:

      全局变量

      

  /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * An empty table instance to share when the table is not inflated.
     */
    static final Entry<?,?>[] EMPTY_TABLE = {};

    /**
     * The table, resized as necessary. Length MUST Always be a power of two.
     */
    transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

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

    /**
     * The next size value at which to resize (capacity * load factor).
     * @serial
     */
    // If table == EMPTY_TABLE then this is the initial capacity at which the
    // table will be created when inflated.
    int threshold;

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

    /**
     * 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 default threshold of map capacity above which alternative hashing is
     * used for String keys. Alternative hashing reduces the incidence of
     * collisions due to weak hash code calculation for String keys.
     * <p/>
     * This value may be overridden by defining the system property
     * {@code jdk.map.althashing.threshold}. A property value of {@code 1}
     * forces alternative hashing to be used at all times whereas
     * {@code -1} value ensures that alternative hashing is never used.
     */
    static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;

看过源码的都知道，每次put新值的时候size都会加一。

我们获取HashMap的大小都会调用size()方法,而size方法返回的是这个全局变量size的值。

 public int size() {
            return size;
        }

假设有两个线程，线程A在写值，线程B在读取HashMap大小，哪个线程先执行是由CPU决定的，所以size的值会使1或者0.假设这个程序的逻辑是先写值，然后根据这个HashMap的size判断后续执行什么操作，那么多线程的情况下，这个后续操作就会有可能执行或者有可能不执行，我们就无法保证程序得出正确的结果即我们想要的结果。这就是多线程引发的问题。

/**
 * The default initial capacity - MUST be a power of two.
 */
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

/**
 * The maximum capacity, used if a higher value is implicitly specified
 * by either of the constructors with arguments.
 * MUST be a power of two <= 1<<30.
 */
static final int MAXIMUM_CAPACITY = 1 << 30;

/**
 * The load factor used when none specified in constructor.
 */
static final float DEFAULT_LOAD_FACTOR = 0.75f;

/**
 * An empty table instance to share when the table is not inflated.
 */
static final Entry<?,?>[] EMPTY_TABLE = {};

/**
 * The table, resized as necessary. Length MUST Always be a power of two.
 */
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

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

/**
 * The next size value at which to resize (capacity * load factor).
 * @serial
*/
// If table == EMPTY_TABLE then this is the initial capacity at which the
// table will be created when inflated.
int threshold;

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

/**
 * 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 default threshold of map capacity above which alternative hashing is
 * used for String keys. Alternative hashing reduces the incidence of
 * collisions due to weak hash code calculation for String keys.
 * <p/>
* This value may be overridden by defining the system property
 * {@code jdk.map.althashing.threshold}. A property value of {@code 1}
 * forces alternative hashing to be used at all times whereas
 * {@code -1} value ensures that alternative hashing is never used.
 */
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;