Java多线程编程：解决活跃等待与优雅终止问题

// PrinterThread.java - 负责打印字符的线程
public class PrinterThread extends Thread {

    private String letter;
    private int internal; // 每次打印后的休眠时间
    private int amount;   // 打印次数

    public PrinterThread(String letter, int internal, int amount){
        this.letter = letter;
        this.internal = internal;
        this.amount = amount;
    }

    @Override
    public void run(){
        for (int i = 1; i <= amount; i++) {
            System.out.println(letter + " - " + i);
            try {
                Thread.sleep(internal);
            } catch (InterruptedException e) {
                // 处理中断异常，通常是重新设置中断状态并退出
                Thread.currentThread().interrupt(); 
                System.out.println(letter + " 线程被中断，提前退出。");
                break; 
            }
        }
        System.out.println(letter + " 线程完成。");
    }
}

// Main.java - 主程序入口
public class Main {
    public static void main(String[] args) {
        PrinterThread printerThread = new PrinterThread("A", 1, 1000);
        PrinterThread printerThread1 = new PrinterThread("B", 1, 1000);
        PrinterThread printerThread2 = new PrinterThread("C", 1, 1); // C线程很快完成
        PrinterThread printerThread3 = new PrinterThread("D", 5, 50);

        printerThread.start();
        printerThread1.start();
        printerThread2.start();

        System.out.println("前三个线程A、B、C已启动。");

        // 问题代码段：活跃等待
        while(!printerThread3.isAlive()){ // 循环条件：D线程尚未启动
            // 检查是否有任一前置线程完成
            if (!printerThread.isAlive() || !printerThread1.isAlive() || !printerThread2.isAlive()) {
                printerThread3.start(); // 启动D线程
            }
        }
        // 程序可能在此处终止，但其他线程可能仍在运行，或主线程陷入无限忙等
        System.out.println("主线程完成其逻辑，但可能未等待所有子线程。");
    }
}

public class Main {
    public static void main(String[] args) {
        // ... (PrinterThread initialization and start of first three threads as before) ...

        System.out.println("前三个线程A、B、C已启动。");

        // 修正后的活跃等待逻辑：在D线程启动后立即跳出循环
        while(!printerThread3.isAlive()){ // 循环条件：D线程尚未启动
            if (!printerThread.isAlive() || !printerThread1.isAlive() || !printerThread2.isAlive()) {
                printerThread3.start(); // 启动D线程
                System.out.println("检测到A/B/C中至少一个已完成，D线程已启动。");
                break; // 关键：D线程启动后，立即跳出等待循环
            }

            // 为了避免在D线程启动前持续忙等，可以加入短暂的睡眠
            try {
                Thread.sleep(10); // 短暂等待10毫秒，减少CPU空转
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
                System.err.println("主线程在等待D线程启动时被中断。");
                break; // 中断时也退出循环
            }