Java Thread.join怎样避免线程饥饿现象

为了避免Java中的线程饥饿现象，您可以采取以下措施：

1. 公平锁：使用java.util.concurrent.locks.ReentrantLock的公平锁模式。在创建锁的时候，通过传入true参数来声明这是一个公平锁。公平锁会按照线程请求锁的顺序来分配，从而避免了线程饥饿。

import java.util.concurrent.locks.ReentrantLock;

public class FairLockExample {
    private final ReentrantLock fairLock = new ReentrantLock(true);

    public void doSomething() {
        fairLock.lock();
        try {
            // Do some work here
        } finally {
            fairLock.unlock();
        }
    }
}

2. 使用java.util.concurrent.Semaphore信号量：信号量可以用来限制对共享资源的访问。通过设置合适的许可数量，可以确保线程按照预期的顺序执行，从而避免线程饥饿。

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

public class SemaphoreExample {
    private final Semaphore semaphore = new Semaphore(3); // Allow 3 threads to access the resource concurrently

    public void doSomething() {
        try {
            semaphore.acquire();
            // Do some work here
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        } finally {
            semaphore.release();
        }
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        SemaphoreExample example = new SemaphoreExample();

        for (int i = 0; i < 10; i++) {
            executorService.submit(() -> example.doSomething());
        }

        executorService.shutdown();
    }
}

3. 使用java.util.concurrent.PriorityBlockingQueue优先级队列：优先级队列可以根据元素的自然顺序或者自定义的比较器来对元素进行排序。这样，高优先级的线程将优先执行，从而避免了线程饥饿。

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.TimeUnit;

class Task implements Runnable, Comparable {
    @Override
    public int compareTo(Task other) {
        // Define the priority order here (higher priority first)
        return Integer.compare(other.priority, this.priority);
    }

    @Override
    public void run() {
        // Do some work here
    }

    public int priority;
}

public class PriorityQueueExample {
    private final PriorityBlockingQueue taskQueue = new PriorityBlockingQueue<>();

    public void addTask(Task task) {
        taskQueue.add(task);
    }

    public void startProcessing() {
        ExecutorService executorService = Executors.newSingleThreadExecutor();

        executorService.submit(() -> {
            while (!taskQueue.isEmpty()) {
                try {
                    Task task = taskQueue.take();
                    task.run();
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                }
            }
        });

        executorService.shutdown();
    }

    public static void main(String[] args) throws InterruptedException {
        PriorityQueueExample example = new PriorityQueueExample();

        // Add tasks with different priorities
        example.addTask(new Task(3));
        example.addTask(new Task(1));
        example.addTask(new Task(2));

        example.startProcessing();
    }
}

通过使用这些方法，您可以有效地避免线程饥饿现象，确保所有线程都能公平地访问共享资源。