zoukankan      html  css  js  c++  java
  • Java HashMap、HashTable、ConcurrentHashMap区别

    0. Java 容器

    1. HashTable、HashMap、ConcurrentHashMap 线程安全对比

      HashMap:异步的,线程不安全;

      HashTable:同步的,synchronized针对的是整个hash表,是独占锁,同一时刻只能有一个线程访问操作,线程安全;

      ConcurrentHashMap:异步的,使用分段锁机制,当某线程访问操作某段数据,其它段数据依然可以让其它线程访问操作,线程安全;

    2. 分析

      HashTable

      • HashTable底层是通过数组+链表实现的,KV都不能为NULL,线程安全的;
      • 初始size为11,扩容:newsize = oldsize * 2 + 1;
      • 计算index的方法:index = (hash & 0x7FFFFFFF) % tab.length;
        /**
         * Constructs a new, empty hashtable with a default initial capacity (11)
         * and load factor (0.75).
         */
        public Hashtable() {
            this(11, 0.75f);
        }

      HashMap

      • HashMap底层是通过数组+链表实现的,KV都可以为NULL,线程不安全;
      • 初始size为16,扩容:newsize = oldsize * 2,size一般为2的n次幂;
      • HashMap扩容针对整个Map,每次扩容后,需要对数组中的元素重新计算位置,再重新插入;
      • HashMap都是先扩容再判断是否需要扩容,所以会有无效扩容(无效扩容就是扩容后再不插入数据的扩容就叫无效扩容);
      • HashMap容量大于75%时就会触发扩容;
      • 计算index的方法:index = (hash & 0x7FFFFFFF) % tab.length;

      ConcurrentHashMap

      • ConcurrentHashMap底层是通过数组+链表实现的,是线程安全的;
      • 通过将Map分为N段Segment,提供相同线程安全(就是分段加锁)。默认分为16段;
      • ConcurrentHashMap当数组容量超过75%会触发扩容,但是不会对整个Map扩容,在插入前判断是否需要扩容,有效避免无效扩容;
        /**
         * The default initial table capacity.  Must be a power of 2
         * (i.e., at least 1) and at most MAXIMUM_CAPACITY.
         */
        private static final int DEFAULT_CAPACITY = 16;
    
        /**
         * The default concurrency level for this table. Unused but
         * defined for compatibility with previous versions of this class.
         */
        private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
    
        /**
         * The load factor for this table. Overrides of this value in
         * constructors affect only the initial table capacity.  The
         * actual floating point value isn't normally used -- it is
         * simpler to use expressions such as {@code n - (n >>> 2)} for
         * the associated resizing threshold.
         */
        private static final float LOAD_FACTOR = 0.75f;
  • 相关阅读:
    Compiler OverView
    二叉查找树
    [C++] manage background threads with boost::thread
    模拟类式继承
    JavaScript Curry
    Get GetProcAddress Function Address
    ASP.MVC Session为null
    ASP.NET MVC项目Forms身份验证HttpContext.Current.User为空
    Java Bean Validation学习笔记
    java开发中sql注入正则表达式检测
  • 原文地址:https://www.cnblogs.com/naray/p/13157785.html
Copyright © 2011-2022 走看看