Java 並發協作：死鎖形成與避免策略

死鎖核心成因

死鎖發生於多線程互相持有對方需要的鎖，並等待對方釋放，形成循環等待。四個必要條件（Coffman 條件）：

1. 互斥條件：資源獨佔
2. 請求與保持：持有資源又請求新資源
3. 不剝奪條件：資源只能主動釋放
4. 循環等待：線程鏈首尾相連

完整死鎖代碼示例

/** 
 * Java線程：並發協作-死鎖
 * @author Administrator
 */
public class Test {
    public static void main(String[] args) {
        DeadlockRisk dead = new DeadlockRisk();
        new MyThread(dead, 1, 2).start();
        new MyThread(dead, 3, 4).start();
        new MyThread(dead, 5, 6).start();
        new MyThread(dead, 7, 8).start();
    }
}

class MyThread extends Thread {
    private DeadlockRisk dead; private int a, b;
    MyThread(DeadlockRisk dead, int a, int b) { this.dead = dead; this.a = a; this.b = b; }
    @Override public void run() { dead.read(); dead.write(a, b); }
}

class DeadlockRisk {
    private static class Resource { public int value; }
    private Resource resourceA = new Resource();
    private Resource resourceB = new Resource();
    
    public int read() {
        synchronized (resourceA) {
            System.out.println("read():" + Thread.currentThread().getName() + "獲取了resourceA的鎖！");
            synchronized (resourceB) {
                System.out.println("read():" + Thread.currentThread().getName() + "獲取了resourceB的鎖！");
                return resourceB.value + resourceA.value;
            }
        }
    }
    
    public void write(int a, int b) {
        synchronized (resourceB) {  // 反序獲取鎖！
            System.out.println("write():" + Thread.currentThread().getName() + "獲取了resourceB的鎖！");
            synchronized (resourceA) {
                System.out.println("write():" + Thread.currentThread().getName() + "獲取了resourceA的鎖！");
                resourceA.value = a; resourceB.value = b;
            }
        }
    }
}

死鎖運行現象

程序卡住，無法繼續執行，控制台無新輸出。典型線程狀態：

• 線程1：持有 resourceA，等待 resourceB
• 線程2：持有 resourceB，等待 resourceA

死鎖避免與排查策略

• 固定鎖獲取順序：所有線程按相同順序獲取多把鎖（如按 hashCode 排序）
• 使用 tryLock 超時機制：ReentrantLock.tryLock(timeout) 避免無限等待
• 縮小鎖範圍：減少持有鎖時間，降低衝突概率
• 避免嵌套鎖：重構代碼，單一鎖完成操作
• 排查工具：jstack、VisualVM、Arthas 分析線程堆棧定位死鎖

代碼修正示例：統一鎖順序

// 修正：read 和 write 都按 resourceA → resourceB 順序獲取鎖
public int read() {
    synchronized (resourceA) {
        synchronized (resourceB) { ... }
    }
}
public void write(int a, int b) {
    synchronized (resourceA) {  // 改為先鎖 A
        synchronized (resourceB) { ... }
    }
}

出處：https://lavasoft.blog.51cto.com/62575/222074