解決死鎖的4種基本方法

一、解決死鎖的背景和引入

死鎖是多線程編程中常見的問題之一，它發生在兩個或多個線程互相等待對方釋放資源的情況下。這種情況下，所有的線程都被阻塞，並且沒有線程能夠繼續執行，導致程序無法正常運行。

解決死鎖問題是多線程編程中必不可少的一項技能。本文將介紹解決死鎖的四種基本方法和相關代碼示例，幫助讀者更好地理解和掌握該技能。

二、解決死鎖的基本方法

1. 避免死鎖

避免死鎖是最好的解決方案，它通過對資源的請求進行限制，使得死鎖不可能發生。Java中提供了一個專門的接口類java.util.concurrent.locks.Lock，可以使用它來避免死鎖。

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. 可以打破循環等待條件

循環等待是死鎖的核心之一。可以通過打破這個條件來解決死鎖問題。Java中提供了一個工具類java.util.concurrent.locks.ReentrantLock，通過使用該類的tryLock()方法可以嘗試獲取鎖資源，並在獲取失敗後立即釋放已經獲取的鎖資源。

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. 使用超時等待機制

使用超時等待機制可以避免線程因為無法獲取資源而一直等待的情況。Java中提供了一個接口類java.util.concurrent.locks.Condition，可以使用它來實現等待超時機制。

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. 按照順序獲取鎖

按照順序獲取鎖可以避免循環等待條件的發生。可以通過對資源的順序進行管理，在獲取鎖資源時按照固定的順序獲取，從而避免死鎖。

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、解決死鎖的最佳方法

解決死鎖最好的方法是通過分析和設計，確保在程序運行時不會出現死鎖的情況。其中，最簡單的方法就是在設計時盡量減少鎖的數量，避免出現循環等待等情況。