重写hashcode()和equals()-＞解决HashMap内存溢出问题

最新推荐文章于 2022-10-08 15:46:36 发布

氯雷他定.

最新推荐文章于 2022-10-08 15:46:36 发布

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分类专栏： JAVA基础文章标签： java hashmap redis hashcode vue

本文链接：https://blog.youkuaiyun.com/menjiyufei/article/details/111028246

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本文深入探讨了Java中HashMap和ConcurrentHashMap的工作原理，包括hashCode和equals方法的重要性，以及它们如何影响内存管理和性能。通过具体代码示例，展示了重写hashCode和equals方法对于避免内存溢出和提高效率的关键作用。

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Object.class

//此方法是操作系统提供的本地方法，java不做实现 https://blog.youkuaiyun.com/cjf1002361126/article/details/52750528
public native int hashCode();

//默认比较内存地址
public boolean equals(Object obj) {
   return (this == obj);
}

HashMap 1.8源码

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}
static final int hash(Object key) {
      int h;　　　// hashCode()方法决定了向数组哪个位置的桶内放数据，重写它让具有相同url的对象对应相同的数组位置
      return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
public boolean containsKey(Object key) {　　// 根据hash()找到桶下标，循环遍历桶比较Node的key与指定的key是否相同
     return getNode(hash(key), key) != null;
}

final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}

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;// equals()方法比较相同则覆盖旧值,所以重写它让具有相同url的对象达到比较结果相同的目的
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        ......
}

未重写hashCode()和equals()导致的内存溢出BUG

hashCode():保证获取正确的数组下标

equals():保证Node的key能正确匹配，保证不重复创建节点、更新节点的值

https://zhuanlan.zhihu.com/p/172388495

bug在于没有红色部分的代码

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;

public class Monitor {
    public static class MonitorKey {
        String url;
        String desc;

        public MonitorKey(String url, String desc) {
            super();
            this.url = url;
            this.desc = desc;
        }

        public String getUrl() {
            return url;
        }

        @Override
        public int hashCode() {
            return url.hashCode();
        }

        @Override
        public boolean equals(Object obj) {
            if (obj instanceof MonitorKey) {
                MonitorKey key = (MonitorKey)obj;
                return url.equals(key.getUrl());
            }
            return false;
        }
    }

    public static class MonitorValue {
        AtomicInteger count = new AtomicInteger();
        float avgTime;
        AtomicLong totalTime = new AtomicLong();
    }

    private Map<MonitorKey, MonitorValue> monitors = new ConcurrentHashMap<>();

    public void visit(String url, String desc, long timeCost) {
        MonitorKey key = new MonitorKey(url, desc);
        /* BUG：此处具有相同url的MonitorKey计算出的hashCode值不同，导致ConcurrentHashMap内存溢出 */
        // putIfAbsent：key不存在则上锁(CAS+Synchronized)添加节点
        MonitorValue oldValue = monitors.putIfAbsent(key, new MonitorValue());
        MonitorValue newValue = monitors.get(key);
        newValue.count.getAndIncrement();
        newValue.totalTime.getAndAdd(timeCost);
        newValue.avgTime = newValue.totalTime.get() / newValue.count.get();
    }

    public static void main(String[] args) {
        Monitor monitor = new Monitor();
        monitor.visit("key1", "key1desc", 1);
        monitor.visit("key2", "key2desc", 1);
        monitor.visit("key1", "key1desc", 1);
        monitor.visit("key2", "key2desc", 1);
        System.out.println(monitor.monitors.size());// 无红色部分代码：4.有红色部分代码：2
    }
}