Java单例模式利用HashMap实现缓存数据

public class People {

    private String name;

    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "People{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

import java.util.HashMap;
import java.util.Map;

public class CacheSingletonUtil {

    private static volatile CacheSingletonUtil cacheSingletonUtil;
    private static Map<String,Object> cacheSingletonMap;

    public static final String PEOPLE_LIST_KEY = "peopleList";

    private CacheSingletonUtil(){
        cacheSingletonMap = new HashMap<String, Object>();
    }

    /*
     * 单例模式有两种类型
     * 懒汉式：在真正需要使用对象时才去创建该单例类对象
     * 饿汉式：在类加载时已经创建好该单例对象，等待被程序使用
     */

    // 懒汉式单例模式
    public static CacheSingletonUtil getInstance(){
        if (cacheSingletonUtil == null){// 线程A和线程B同时看到cacheSingletonUtil = null，如果不为null，则直接返回cacheSingletonUtil
            synchronized (CacheSingletonUtil.class) {// 线程A或线程B获得该锁进行初始化
                if (cacheSingletonUtil == null) {// 其中一个线程进入该分支，另外一个线程则不会进入该分支
                    cacheSingletonUtil = new CacheSingletonUtil();
                }
            }
        }
        return cacheSingletonUtil;
    }

    /**
     * 添加到内存
     */
    public void addCacheData(String key,Object obj){
        cacheSingletonMap.put(key,obj);
    }

    /**
     * 从内存中取出
     */
    public Object getCacheData(String key){
        return cacheSingletonMap.get(key);
    }

    /**
     * 从内存中清除
     */
    public void removeCacheData(String key){
        cacheSingletonMap.remove(key);
    }

}

import org.apache.commons.collections.CollectionUtils;
import java.util.ArrayList;
import java.util.List;

public class CacheSingletonTest {

    public static void main(String[] args) {
        //测试查询
        testQuery();
    }

    private static void testQuery () {
        System.out.println("第一次查询开始");
        query();
        System.out.println("第一次查询结束");

        System.out.println("=============");

        System.out.println("第二次查询开始");
        query();
        System.out.println("第二次查询结束");
    }

    /*
     * 查询数据
     */
    private static List<People> query() {
        List<People> peopleList = null;

        List<People> cacheData = (List<People>) CacheSingletonUtil.getInstance().getCacheData(CacheSingletonUtil.PEOPLE_LIST_KEY);

        if (CollectionUtils.isNotEmpty(cacheData)) {
            System.out.println("从内存中读取");
            peopleList = cacheData;
        } else {
            System.out.println("从数据库中读取");
            peopleList = getData();
            // 添加到内存中
            CacheSingletonUtil.getInstance().addCacheData(CacheSingletonUtil.PEOPLE_LIST_KEY, peopleList);
        }

        for (People people : peopleList) {
            System.out.println("name : " + people.getName() + " age : " + people.getAge());
        }

        return peopleList;
    }

    /*
     * 删除数据
     */
    private void deleteCache () {
        CacheSingletonUtil.getInstance().removeCacheData(CacheSingletonUtil.PEOPLE_LIST_KEY);
    }

    private static List<People> getData() {
        People p1 = new People();
        p1.setName("Jack");
        p1.setAge(25);

        People p2 = new People();
        p2.setName("Brown");
        p2.setAge(28);

        List<People> peopleList = new ArrayList<>();
        peopleList.add(p1);
        peopleList.add(p2);

        return peopleList;
    }
}