重写hashcode()和equals()->解决HashMap内存溢出问题
Object.class
//此方法是操作系统提供的本地方法,java不做实现 https://blog.csdn.net/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 } }