Java死锁避免的五种方法举例总结

public void transfer(Account from, Account to, int amount) {
    // 规定：先锁 id 小的账户，再锁 id 大的账户
    Account first = from.getId() < to.getId() ? from : to;
    Account second = from.getId() < to.getId() ? to : from;

    synchronized (first) {
        synchronized (second) {
            if (from.getBalance() >= amount) {
                from.debit(amount);
                to.credit(amount);
            }
        }
    }
}

public boolean transfer(Account from, Account to, int amount, long timeout) throws InterruptedException {
    long startTime = System.currentTimeMillis();
    while (true) {
        if (from.lock.tryLock()) { // 尝试获取第一把锁
            try {
                if (to.lock.tryLock()) { // 尝试获取第二把锁
                    try {
                        if (from.getBalance() >= amount) {
                            from.debit(amount);
                            to.credit(amount);
                            return true;
                        }
                    } finally {
                        to.lock.unlock();
                    }
                }
            } finally {
                from.lock.unlock(); // 释放第一把锁
            }
        }
        // 超时检查
        if (System.currentTimeMillis() - startTime >= timeout) {
            return false;
        }
        Thread.sleep(100); // 避免 CPU 忙等待
    }
}

public class AccountManager {
    private static final Object globalLock = new Object();

    public void transfer(Account from, Account to, int amount) {
        synchronized (globalLock) { // 全局锁保护所有账户
            if (from.getBalance() >= amount) {
                from.debit(amount);
                to.credit(amount);
            }
        }
    }
}

Lock lock1 = new ReentrantLock();
Lock lock2 = new ReentrantLock();

public void methodA() {
    boolean gotLock1 = false;
    boolean gotLock2 = false;
    try {
        gotLock1 = lock1.tryLock(1, TimeUnit.SECONDS);
        gotLock2 = lock2.tryLock(1, TimeUnit.SECONDS);
        if (gotLock1 && gotLock2) {
            // 执行业务逻辑
        }
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    } finally {
        if (gotLock1) lock1.unlock();
        if (gotLock2) lock2.unlock();
    }
}

ThreadMXBean bean = ManagementFactory.getThreadMXBean();
long[] threadIds = bean.findDeadlockedThreads();
if (threadIds != null) {
    System.out.println("检测到死锁！");
}

public void transfer(Account from, Account to, int amount) {
    synchronized (from) {  // 线程1：锁 from → 等 to
        synchronized (to) { // 线程2：锁 to → 等 from
            if (from.getBalance() >= amount) {
                from.debit(amount);
                to.credit(amount);
            }
        }
    }
}