CountDownLatch降低数据库查询压力（缺点：增大内存消耗）

//线程池初始化
int thread = 3;
ExecutorService executorService = Executors.newFixedThreadPool(thread);
final CountDownLatch countDownLatch = new CountDownLatch(thread);

//线程执行

executorService.submit(new Callable<Object>() {
    @Override
    public Object call() throws Exception {
       　AMapper.select();
        countDownLatch.countDown();
        return null;
    }
});

executorService.submit(new Callable<Object>() {
    @Override
    public Object call() throws Exception {
       　BMapper.select();
        countDownLatch.countDown();
        return null;
    }
});

executorService.submit(new Callable<Object>() {
    @Override
    public Object call() throws Exception {
       　CMapper.select();
        countDownLatch.countDown();
        return null;
    }
});

try {
    countDownLatch.await();
　　 //组装数据
} catch (InterruptedException e) {
    e.printStackTrace();
}