解决死锁的4种基本方法

一、解决死锁的背景和引入

死锁是多线程编程中常见的问题之一，它发生在两个或多个线程互相等待对方释放资源的情况下。这种情况下，所有的线程都被阻塞，并且没有线程能够继续执行，导致程序无法正常运行。

解决死锁问题是多线程编程中必不可少的一项技能。本文将介绍解决死锁的四种基本方法和相关代码示例，帮助读者更好地理解和掌握该技能。

二、解决死锁的基本方法

1. 避免死锁

避免死锁是最好的解决方案，它通过对资源的请求进行限制，使得死锁不可能发生。Java中提供了一个专门的接口类java.util.concurrent.locks.Lock，可以使用它来避免死锁。

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LockAvoidDeadlock {
    private Lock lock1 = new ReentrantLock();
    private Lock lock2 = new ReentrantLock();

    public void resource1() throws InterruptedException {
        lock1.lock();
        Thread.sleep(100);
        lock2.lock();
        System.out.println(Thread.currentThread().getName() + " has acquired lock1 and lock2.");
        lock2.unlock();
        lock1.unlock();
    }

    public void resource2() throws InterruptedException {
        lock2.lock();
        Thread.sleep(100);
        lock1.lock();
        System.out.println(Thread.currentThread().getName() + " has acquired lock2 and lock1.");
        lock1.unlock();
        lock2.unlock();
    }
}

2. 可以打破循环等待条件

循环等待是死锁的核心之一。可以通过打破这个条件来解决死锁问题。Java中提供了一个工具类java.util.concurrent.locks.ReentrantLock，通过使用该类的tryLock()方法可以尝试获取锁资源，并在获取失败后立即释放已经获取的锁资源。

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class BreakCycleWait {
    private Lock lock1 = new ReentrantLock();
    private Lock lock2 = new ReentrantLock();

    public void resource1() throws InterruptedException {
        boolean lock1Acquired = lock1.tryLock();
        Thread.sleep(100);
        boolean lock2Acquired = lock2.tryLock();
        if (lock1Acquired && lock2Acquired) {
            System.out.println(Thread.currentThread().getName() + " has acquired lock1 and lock2.");
        }
        if (lock1Acquired) {
            lock1.unlock();
        }
        if (lock2Acquired) {
            lock2.unlock();
        }
    }

    public void resource2() throws InterruptedException {
        boolean lock2Acquired = lock2.tryLock();
        Thread.sleep(100);
        boolean lock1Acquired = lock1.tryLock();
        if (lock1Acquired && lock2Acquired) {
            System.out.println(Thread.currentThread().getName() + " has acquired lock2 and lock1.");
        }
        if (lock2Acquired) {
            lock2.unlock();
        }
        if (lock1Acquired) {
            lock1.unlock();
        }
    }
}

3. 使用超时等待机制

使用超时等待机制可以避免线程因为无法获取资源而一直等待的情况。Java中提供了一个接口类java.util.concurrent.locks.Condition，可以使用它来实现等待超时机制。

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class TimeoutWait {
    private Lock lock1 = new ReentrantLock();
    private Lock lock2 = new ReentrantLock();
    private Condition condition1 = lock1.newCondition();
    private Condition condition2 = lock2.newCondition();

    public void resource1() throws InterruptedException {
        lock1.lock();
        Thread.sleep(100);
        if (!lock2.tryLock()) {
            condition1.await();
        }
        System.out.println(Thread.currentThread().getName() + " has acquired lock1 and lock2.");
        lock2.unlock();
        lock1.unlock();
    }

    public void resource2() throws InterruptedException {
        lock2.lock();
        Thread.sleep(100);
        if (!lock1.tryLock()) {
            condition2.await();
        }
        System.out.println(Thread.currentThread().getName() + " has acquired lock2 and lock1.");
        lock1.unlock();
        lock2.unlock();
    }
}

4. 按照顺序获取锁

按照顺序获取锁可以避免循环等待条件的发生。可以通过对资源的顺序进行管理，在获取锁资源时按照固定的顺序获取，从而避免死锁。

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class OrderlyLock {
    private Lock lock1 = new ReentrantLock();
    private Lock lock2 = new ReentrantLock();

    public void resource1() throws InterruptedException {
        lock1.lock();
        Thread.sleep(100);
        lock2.lock();
        System.out.println(Thread.currentThread().getName() + " has acquired lock1 and lock2.");
        lock2.unlock();
        lock1.unlock();
    }

    public void resource2() throws InterruptedException {
        lock1.lock();
        Thread.sleep(100);
        lock2.lock();
        System.out.println(Thread.currentThread().getName() + " has acquired lock2 and lock1.");
        lock2.unlock();
        lock1.unlock();
    }
}

三、解决死锁的最佳方法

解决死锁最好的方法是通过分析和设计，确保在程序运行时不会出现死锁的情况。其中，最简单的方法就是在设计时尽量减少锁的数量，避免出现循环等待等情况。