之前一直没有用过多线程，这次，写了一个多线程查询数据库的案列，并且比较一下多线程和单线程直接执行的效率，体会一下多线程同步。数据库中大概有1 W条数据。下面是核心代码：

单线程一次查询数据库

TestServiceImpl类

    @Override
    public List<Store> queryStoresInfo() {
        return testDao.queryStoresInfo();
    }

单线程For循环查询数据库

TestServiceImpl类

    @Override
    public List<Store> queryStoresInfoByFor() {
        List<Store> lists = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            List<Store> list = testDao.queryStoresInfo();
            lists.addAll(list);
        }
        return lists;
    }

多线程查询数据库(使用join()进行同步)

TestServiceImpl类（下面这个是我的一个错误的做法）

将join写在循环内，实际本身还是一个串行的运行过程。

    @Override
    public List<Store> queryStoresInfoByThread() throws InterruptedException {
        List<Store> lists = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(new TestThread(lists));
            thread.start();
            thread.join();
        }
        return lists;
    }

TestServiceImpl类（正确的做法）

    @Override
    public List<Store> queryStoresInfoByThread() throws InterruptedException {
        List<Store> lists = new ArrayList<>();
        List<Thread> threads = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(new TestThread(lists));
            thread.start();
            threads.add(thread);
        }
        for (Thread thread: threads){
            thread.join();
        }
        return lists;
    }

TestThread类

public class TestThread implements Runnable {

    TestDao testDao = SpringBeanUtils.getBean(TestDao.class);

    private List<Store> stores;

    public TestThread(List<Store> stores) {
        this.stores = stores;
    }

    @Override
    public void run() {
        System.err.println(Thread.currentThread().getName() +" query data start...");
        List<Store> store = testDao.queryStoresInfo();
        synchronized (stores) {
            stores.addAll(store);
        }
        System.err.println(Thread.currentThread().getName() +" query data end...");
    }

}

多线程查询数据库(使用CountDownLatch进行同步)

TestServiceImpl类

    @Override
    public List<Store> queryStoresInfoByThread() throws InterruptedException {
        List<Store> lists = new ArrayList<>();
        CountDownLatch latch = new CountDownLatch(10);

        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(new TestThread(lists, latch));
            thread.start();
        }
        latch.await();
        return lists;
    }

TestThread类

public class TestThread implements Runnable {

    TestDao testDao = SpringBeanUtils.getBean(TestDao.class);

    private List<Store> stores;

    private CountDownLatch latch;

    public TestThread(List<Store> stores,CountDownLatch latch) {
        this.stores = stores;
        this.latch = latch;
    }

    @Override
    public void run() {
        System.err.println(Thread.currentThread().getName() +" query data start...");
        List<Store> store = testDao.queryStoresInfo();
        synchronized (stores) {
            stores.addAll(store);
        }
        latch.countDown();
        System.err.println(Thread.currentThread().getName() +" query data end...");
    }

}

多线程查询数据库(使用线程池->CountDownLatch进行同步)

TestServiceImpl类

    @Override
    public List<Store> queryStoresInfoByThreadPool() {
        ExecutorService executorService = new ThreadPoolExecutor(5, 10, 1, TimeUnit.MINUTES,new ArrayBlockingQueue<Runnable>(10));

        List<Store> resultList = new ArrayList<>();
        CountDownLatch latch = new CountDownLatch(10);

        for (int i = 0; i < 10; i++) {
            executorService.execute(new TestThread(resultList,latch));
        }
        executorService.shutdown();
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        return resultList;
    }

TestThread类

public class TestThread implements Runnable {

    TestDao testDao = SpringBeanUtils.getBean(TestDao.class);

    private List<Store> stores;

    private CountDownLatch latch;

    public TestThread(List<Store> stores,CountDownLatch latch) {
        this.stores = stores;
        this.latch = latch;
    }

    @Override
    public void run() {
        System.err.println(Thread.currentThread().getName() +" query data start...");
        List<Store> store = testDao.queryStoresInfo();
        synchronized (stores) {
            stores.addAll(store);
        }
        latch.countDown();
        System.err.println(Thread.currentThread().getName() +" query data end...");
    }

}

运行效率比较