Java 新手 求解 死锁要怎么处理

题是 10 个线程 对应 20 个账号 每个线程执行 100 次 任意 2 个账号之间的金额交易

代码实现中遇到了死锁

假设线程 A 对 账户 A 账户 B 执行交易 （ A 转钱到 B ） 线程 B 对账户 B 和账户 A 执行交易（ B 转钱到 A ）

我代码写的是先锁一个账号 判断账号余额是否足够，然后锁另一个账户 最后执行转账

但是同时有 2 个线程执行了 2 个相同账户的反向操作时，就会死锁了。

package cn.bobmao.logic.service; import java.util.ArrayList; import java.util.List; import java.util.Random; import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.atomic.AtomicInteger; public class Test { class Account { int index; int balance = 100; public Account(int index) { this.index = index; } } public static void main(String[] args) throws InterruptedException { new Test().run(); } public void run() throws InterruptedException { AtomicInteger sum = new AtomicInteger(0); List<Account> accounts = new ArrayList<>(); for (int i = 0; i < 20; i++) { accounts.add(new Account(i)); } ExecutorService executorService = Executors.newFixedThreadPool(10); CountDownLatch latch = new CountDownLatch(1000); for (int i = 0; i < 10; i++) { executorService.submit(new Pay(accounts, latch, sum)); } latch.await(); int count = 0; for (Account account : accounts) { count += account.balance; } System.out.println(count); } class Pay implements Runnable { List<Account> accounts; CountDownLatch latch; AtomicInteger sum; public Pay(List<Account> accounts, CountDownLatch latch, AtomicInteger sum) { this.accounts = accounts; this.latch = latch; this.sum = sum; } @Override public void run() { for (int i = 0; i < 100; i++) { int mOney= new Random().nextInt(100); // a->b int a = new Random().nextInt(20); int b = new Random().nextInt(20); latch.countDown(); if (a == b) { System.out.println("当前执行次数" + sum.getAndIncrement()); continue; } Account ac = accounts.get(a); synchronized (ac) { System.out.println("锁 ac " + ac.index + " " + Thread.currentThread().getName()); if (ac.balance >= money) { Account bc = accounts.get(b); synchronized (bc) { System.out.println("锁 bc " + ac.index + " " + Thread.currentThread().getName()); ac.balance = ac.balance - money; bc.balance = bc.balance + money; //latch.countDown(); } System.out.println("解锁 bc " + ac.index + " " + Thread.currentThread().getName()); } } System.out.println("解锁 ac " + ac.index + " " + Thread.currentThread().getName()); System.out.println("当前执行次数" + sum.getAndIncrement()); } } } }