面试官问你HashMap底层你用线程安全吊打他

梦里梦外; 2023-05-28 11:27 10阅读 0赞

面试中，HashMap可以说是必问的，既然这样，我们应该怎么准备怎么回答呢，看看这篇文章，估计你会懂点东西。  
**先看看这两张图，是其内部的存储结构**  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0hlWmhpWWluZ18_size_16_color_FFFFFF_t_70]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0hlWmhpWWluZ18_size_16_color_FFFFFF_t_70 1]  
说起HashMap，我们可以先从底层实现说起，HashMap是通过hash算法，基于数组、链表和红黑树实现的，hash算法是一种思想，只要符合该思想的算法都是hash算法，其核心就是给定一个key，通过hash可以对应一个h(key)，举个例子就是当我们要存储一个key为字符串的一组数据，那么我们就可以通过hash将key转化为数字，做为数组的索引，然后将value存入该索引所对应的空间内，如果我们的数组长度是16，那么当有20个key来进行hash的时候，肯定存在冲突，也就是h(key1) = h (key2)，此时解决的办法就是，让该索引指向一个链表，这样就可以让不同的key对应同一个h(key)，获取的时候通过该key确定到某一个索引，然后遍历链表，但是当链表数据量很大的时候，遍历是很耗时的，于是红黑树就出现了，当链表元素大于等于8个的时候，就采用红黑树去存储，时间复杂度直接从O(n)降低到了O(logn)。  
说到这，HashMap的核心思想也就差不多了，这些应该大部分人都知道，现在来点亮点，你可以说HashMap线程不安全，然后给他来这个例子进行演示，创建三个线程，同时去访问put方法

import java.util.HashMap;
    import java.util.Map;
    import java.util.concurrent.ConcurrentHashMap;
    
    public class TestHashMap implements Runnable {
        
        static Map<String,Object> map = new HashMap();
        //static Map<String,Object> map = new ConcurrentHashMap<>();
    
        @Override
        public void run() {
        
            for (int i = 0; i < 100; i ++) {
        
                map.put(i + "", "hello");
            }
        }
    
        public static void main(String[] args){
        
            Thread thread1 = new Thread(new TestHashMap());
            Thread thread2 = new Thread(new TestHashMap());
            Thread thread3 = new Thread(new TestHashMap());
    
    
            thread1.start();
            thread2.start();
            thread3.start();
            Thread currentThread = Thread.currentThread();
            try {
        
                currentThread.sleep(2000);
            } catch (Exception e) {
        
                System.out.println(e.getMessage());
            }
            System.out.println(map.size());
        }
    }

按正常来说应该输出100的，但是这里并没有，而是输出了204  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0hlWmhpWWluZ18_size_16_color_FFFFFF_t_70 2]  
你可以给put方法上锁，用synchronized

import java.util.HashMap;
    import java.util.Map;
    import java.util.concurrent.ConcurrentHashMap;
    
    public class TestHashMap implements Runnable {
        
        static Map<String,Object> map = new HashMap();
        //static Map<String,Object> map = new ConcurrentHashMap<>();
    
        @Override
        public void run() {
        
            for (int i = 0; i < 100; i ++) {
        
                synchronized (TestHashMap.class) {
         // 这里！！！
                    map.put(i + "", "hello");
                }
            }
        }
    
        public static void main(String[] args){
        
            Thread thread1 = new Thread(new TestHashMap());
            Thread thread2 = new Thread(new TestHashMap());
            Thread thread3 = new Thread(new TestHashMap());
    
    
            thread1.start();
            thread2.start();
            thread3.start();
            Thread currentThread = Thread.currentThread();
            try {
        
                currentThread.sleep(2000);
            } catch (Exception e) {
        
                System.out.println(e.getMessage());
            }
            System.out.println(map.size());
        }
    }

这样是可以解决线程问题，但是，你这有点暴力了吧，直接把整个put方法都给上锁了，你看看put方法中有多少代码，都给锁上，有点过分  
**Hash Map的put源码**

public V put(K key, V value) {
        
            return putVal(hash(key), key, value, false, true);
        }
    
        /**
         * Implements Map.put and related methods.
         *
         * @param hash hash for key
         * @param key the key
         * @param value the value to put
         * @param onlyIfAbsent if true, don't change existing value
         * @param 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;
            if ((tab = table) == null || (n = tab.length) == 0)
                n = (tab = resize()).length;
            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;
        }

好家伙，你这一锁，都给锁上了，不好，于是来了**ConcurrentHashMap**，线程安全的，先看看人家是怎么锁的

public V put(K key, V value) {
        
            return putVal(key, value, false);
        }
    
    /** Implementation for put and putIfAbsent */
        final V putVal(K key, V value, boolean onlyIfAbsent) {
        
            if (key == null || value == null) throw new NullPointerException();
            int hash = spread(key.hashCode());
            int binCount = 0;
            for (Node<K,V>[] tab = table;;) {
        
                Node<K,V> f; int n, i, fh;
                if (tab == null || (n = tab.length) == 0)
                    tab = initTable();
                else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
        
                    if (casTabAt(tab, i, null,
                                 new Node<K,V>(hash, key, value, null)))
                        break;                   // no lock when adding to empty bin
                }
                else if ((fh = f.hash) == MOVED)
                    tab = helpTransfer(tab, f);
                else {
        
                    V oldVal = null;
                    synchronized (f) {
         // 在这!!!!!!!!!!!!
                        if (tabAt(tab, i) == f) {
        
                            if (fh >= 0) {
        
                                binCount = 1;
                                for (Node<K,V> e = f;; ++binCount) {
        
                                    K ek;
                                    if (e.hash == hash &&
                                        ((ek = e.key) == key ||
                                         (ek != null && key.equals(ek)))) {
        
                                        oldVal = e.val;
                                        if (!onlyIfAbsent)
                                            e.val = value;
                                        break;
                                    }
                                    Node<K,V> pred = e;
                                    if ((e = e.next) == null) {
        
                                        pred.next = new Node<K,V>(hash, key,
                                                                  value, null);
                                        break;
                                    }
                                }
                            }
                            else if (f instanceof TreeBin) {
        
                                Node<K,V> p;
                                binCount = 2;
                                if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                               value)) != null) {
        
                                    oldVal = p.val;
                                    if (!onlyIfAbsent)
                                        p.val = value;
                                }
                            }
                        }
                    }
                    if (binCount != 0) {
        
                        if (binCount >= TREEIFY_THRESHOLD)
                            treeifyBin(tab, i);
                        if (oldVal != null)
                            return oldVal;
                        break;
                    }
                }
            }
            addCount(1L, binCount);
            return null;
        }

这样一来，锁的就少了，效率当然也就高了，说实话，用着也方便，下面来展示高端操作

import java.util.HashMap;
    import java.util.Map;
    import java.util.concurrent.ConcurrentHashMap;
    
    public class TestHashMap implements Runnable {
        
        //static Map<String,Object> map = new HashMap();
        static Map<String,Object> map = new ConcurrentHashMap<>();
    
        @Override
        public void run() {
        
            for (int i = 0; i < 100; i ++) {
        
                map.put(i + "", "hello");
            }
        }
    
        public static void main(String[] args){
        
            Thread thread1 = new Thread(new TestHashMap());
            Thread thread2 = new Thread(new TestHashMap());
            Thread thread3 = new Thread(new TestHashMap());
    
    
            thread1.start();
            thread2.start();
            thread3.start();
            Thread currentThread = Thread.currentThread();
            try {
        
                currentThread.sleep(2000);
            } catch (Exception e) {
        
                System.out.println(e.getMessage());
            }
            System.out.println(map.size());
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0hlWmhpWWluZ18_size_16_color_FFFFFF_t_70 3]  
最后，如果考虑线程安全，ConcurrentHashMap香呀。

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