基于读写锁实现一个内存缓存器

首先实现定时任务读取数据库信息到内存：

public class ScheduledTask {

    @Autowired
    private TestMapper testMapper;

    @Autowired
    private ScheduledTaskImpl scheduledTaskImpl;

    /**
     * Cron表达式由6~7项组成，中间用空格分开。从左到右依次是：秒、分、时、日、月、周几、年（可省略）。值可以是数字，也可以是以下符号：
     * ：所有值都匹配
     * ? ：无所谓，不关心，通常放在“周几”里
     * , ：或者
     * / ：增量值
     * - ：区间
     */

    private static ExecutorService executorService;

    public static ExecutorService getExecutorService() {
        if (null == executorService) {
            executorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
        }
        return executorService;

    }

    @Scheduled(cron = "0 5/5 * * * ?  ")
    public void executeCompanyCodeTask() {
        getExecutorService().execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Map idMap = new HashMap<>();
                    List list = testMapper.selectAll();
                    for(Test test: list){
                        idMap.put(test.getId(), object);
                    }
                scheduledTaskImpl.setIdMap(idMap);
                } catch (Exception e) {
                    e.printStackTrace();
                    log.error("更新信息出错", e);
                }
            }
        });
    }
}

接下来就是从内存中取数据：

public class ScheduledTaskImpl {

    @Autowired
    private TestfMapper testMapper;

    private Map idMap = new HashMap<>();

    private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    public void setIdMap(Map idMap){
        lock.writeLock().lock();//上写锁，不允许其他线程读也不允许写
        this.idMap = idMap;
        lock.writeLock().unlock();//释放写锁
    }

    public Test getById(Long id){
        Test test = null;
        lock.readLock().lock();//首先开启读锁，从缓存中去取
        try {
            test = idMap.get(id);
            if (test == null) {//如果缓存中没有释放读锁，上写锁
                lock.readLock().unlock();
                lock.writeLock().lock();
                try {
                    if(test == null){
                        test = testMapper.selectById(id);
                        if (test == null) {
                            throw new RuntimeException("数据不存在");
                        }
                        idMap.put(id, test);
                    }
                }finally {
                    lock.writeLock().unlock();//释放写锁
                }
                lock.readLock().lock();//然后再上读锁
            }
        }catch (Exception e) {
            log.error("根据id获取数据出错，" + e.getMessage());
        } finally {
            lock.readLock().unlock();//最后释放读锁
        }
        return test;
    }
}

参考博客：
1.https://www.jianshu.com/p/840b55ef4186
2.https://www.cnblogs.com/zzlp/p/5174745.html