防止重复提交

LRUMap

LRU 是 Least Recently Used 的缩写，即最近最少使用，是一种常用的数据淘汰算法，选择最近最久未使用的数据予以淘汰。

 <!-- 集合工具类 apache commons collections -->
<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-collections4 -->
<dependency>
  <groupId>org.apache.commons</groupId>
  <artifactId>commons-collections4</artifactId>
  <version>4.4</version>
</dependency>

import org.apache.commons.collections4.map.LRUMap;

/**
 * 幂等性判断
 */
public class IdempotentUtils {

    // 根据 LRU(Least Recently Used，最近最少使用)算法淘汰数据的 Map 集合，最大容量 100 个
    private static LRUMap<String, Integer> reqCache = new LRUMap<>(100);

    /**
     * 幂等性判断
     * @return
     */
    public static boolean judge(String id, Object lockClass) {
        synchronized (lockClass) {
            // 重复请求判断
            if (reqCache.containsKey(id)) {
                // 重复请求
                System.out.println("请勿重复提交！！！" + id);
                return false;
            }
            // 非重复请求，存储请求 ID
            reqCache.put(id, 1);
        }
        return true;
    }
}

扩展知识——LRUMap 实现原理分析

LRUMap 的本质是持有头结点的环回双链表结构，它的存储结构如下：

AbstractLinkedMap.LinkEntry entry;

当调用查询方法时，会将使用的元素放在双链表 header 的前一个位置，源码如下：

public V get(Object key, boolean updateToMRU) {
    LinkEntry<K, V> entry = this.getEntry(key);
    if (entry == null) {
        return null;
    } else {
        if (updateToMRU) {
            this.moveToMRU(entry);
        }

        return entry.getValue();
    }
}
protected void moveToMRU(LinkEntry<K, V> entry) {
    if (entry.after != this.header) {
        ++this.modCount;
        if (entry.before == null) {
            throw new IllegalStateException("Entry.before is null. This should not occur if your keys are immutable, and you have used synchronization properly.");
        }

        entry.before.after = entry.after;
        entry.after.before = entry.before;
        entry.after = this.header;
        entry.before = this.header.before;
        this.header.before.after = entry;
        this.header.before = entry;
    } else if (entry == this.header) {
        throw new IllegalStateException("Can't move header to MRU This should not occur if your keys are immutable, and you have used synchronization properly.");
    }

}

如果新增元素时，容量满了就会移除 header 的后一个元素，添加源码如下：

 protected void addMapping(int hashIndex, int hashCode, K key, V value) {
     // 判断容器是否已满    
     if (this.isFull()) {
         LinkEntry<K, V> reuse = this.header.after;
         boolean removeLRUEntry = false;
         if (!this.scanUntilRemovable) {
             removeLRUEntry = this.removeLRU(reuse);
         } else {
             while(reuse != this.header && reuse != null) {
                 if (this.removeLRU(reuse)) {
                     removeLRUEntry = true;
                     break;
                 }
                 reuse = reuse.after;
             }
             if (reuse == null) {
                 throw new IllegalStateException("Entry.after=null, header.after=" + this.header.after + " header.before=" + this.header.before + " key=" + key + " value=" + value + " size=" + this.size + " maxSize=" + this.maxSize + " This should not occur if your keys are immutable, and you have used synchronization properly.");
             }
         }
         if (removeLRUEntry) {
             if (reuse == null) {
                 throw new IllegalStateException("reuse=null, header.after=" + this.header.after + " header.before=" + this.header.before + " key=" + key + " value=" + value + " size=" + this.size + " maxSize=" + this.maxSize + " This should not occur if your keys are immutable, and you have used synchronization properly.");
             }
             this.reuseMapping(reuse, hashIndex, hashCode, key, value);
         } else {
             super.addMapping(hashIndex, hashCode, key, value);
         }
     } else {
         super.addMapping(hashIndex, hashCode, key, value);
     }
 }

判断容量的源码：

public boolean isFull() {
  return size >= maxSize;
}

** 容量未满就直接添加数据：

super.addMapping(hashIndex, hashCode, key, value);

如果容量满了，就调用 reuseMapping 方法使用 LRU 算法对数据进行清除。

 综合来说：LRUMap 的本质是持有头结点的环回双链表结构，当使用元素时，就将该元素放在双链表 header 的前一个位置，在新增元素时，如果容量满了就会移除 header 的后一个元素。