場景練習(xí)一：

轉(zhuǎn)賬功能

描述：
小新轉(zhuǎn)100給小明，同時小紅轉(zhuǎn)50給小新
原有賬戶余額

image.png

代碼初版設(shè)計(jì)
轉(zhuǎn)賬功能：

private static void transfer(Account from, Account to, BigDecimal amount) {
        if (null == from || null == to || null == amount) {
            log.info("【轉(zhuǎn)賬】失敗.參數(shù)不正確");
        }
        synchronized (from) {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                log.error("異常", e);
            }
            synchronized (to) {
                from.setBalance(from.getBalance().subtract(amount));

                to.setBalance(to.getBalance().add(amount));

            }
        }
    }

主函數(shù)調(diào)用

        Account xiaoming = new Account("1", BigDecimal.valueOf(100), "小明");
        Account xiaohong = new Account("2", BigDecimal.valueOf(200), "小紅");
        Account xiaoxin = new Account("3", BigDecimal.valueOf(300), "小新");

        System.out.println("小明轉(zhuǎn)賬前:" + xiaoming.toString());
        System.out.println("小紅轉(zhuǎn)賬前:" + xiaohong.toString());
        System.out.println("小新轉(zhuǎn)賬前:" + xiaoxin.toString());

        List<Future<Integer>> results = new ArrayList<>();
        results.add(executor.submit(new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                transfer(xiaoxin, xiaoming, BigDecimal.valueOf(100));
                return 1;
            }
        }));
        results.add(executor.submit(new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                transfer(xiaohong, xiaoxin, BigDecimal.valueOf(50));
                return 1;
            }
        }));

        for (Future<Integer> result : results) {
            try {
                result.get();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        System.out.println("------------------------------");
        System.out.println("小明轉(zhuǎn)賬后:" + xiaoming.toString());
        System.out.println("小紅轉(zhuǎn)賬后:" + xiaohong.toString());
        System.out.println("小新轉(zhuǎn)賬后:" + xiaoxin.toString());

結(jié)果：

小明轉(zhuǎn)賬前:Account(id=1, balance=100, name=小明)
小紅轉(zhuǎn)賬前:Account(id=2, balance=200, name=小紅)
小新轉(zhuǎn)賬前:Account(id=3, balance=300, name=小新)
------------------------------
小明轉(zhuǎn)賬后:Account(id=1, balance=200, name=小明)
小紅轉(zhuǎn)賬后:Account(id=2, balance=150, name=小紅)
小新轉(zhuǎn)賬后:Account(id=3, balance=250, name=小新)

看起來好像沒啥問題，可是會造成死鎖?。?！
設(shè)想，如果是兩個線程，線程A跑小新轉(zhuǎn)給小明100，線程B跑小明轉(zhuǎn)給小新50.這樣線程A先獲取小新這個賬戶鎖，同時線程B獲取小明這個賬戶鎖，雙方都拿了對方需要的鎖，這樣就相處等待??

所以呢，要改進(jìn)一下，改進(jìn)方法很多，可以采用lock獲取鎖失敗直接拒絕策略?；蛘咭部梢园凑找欢樞蚣渔i,這樣大家加鎖策略就都一致啦～～
優(yōu)化版：

private static void transferSuccess(Account from, Account to, BigDecimal amount) {
        if (null == from || null == to || null == amount) {
            log.info("【轉(zhuǎn)賬】失敗.參數(shù)不正確");
        }

        Account min = from.getId().compareTo(to.getId()) > 0 ? to : from;
        Account max = from.getId().compareTo(to.getId()) > 0 ? from : to;

        synchronized (min) {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (max) {
                from.setBalance(from.getBalance().subtract(amount));

                to.setBalance(to.getBalance().add(amount));
            }
        }
    }

??