我尝试将外部非线程安全库集成到我的Web项目中;我发现为每个客户端线程创建此对象的实例太昂贵了.

因此,我想创建一个具有以下属性的对象池.

>动态对象创建,动态创建池中的对象,而不是在构造函数中创建它们.池最初为空,当客户端线程获取资源对象时,池可以按需创建新资源.一旦创建的对象的数量达到了池的大小;然后将阻止新的客户端线程,并等待其他线程回收资源.
>池应该是公平的,公平性确保第一个要求的线程是获得的第一个线程;否则有些线程可能会永远等待.

我该怎么做？如果有一个有效的例子,我将不胜感激.

解决方法:

这个问题和解决方案从https://www.dbtsai.com/blog/2013/java-concurrent-dynamic-object-pool-for-non-thread-safe-objects-using-blocking-queue/总结

并发对象池可以由Java并发包中的阻塞队列构建,而ArrayBlockingQueue也支持我们需要的公平性.在此实现中,我使用ReentrantLock来控制是否可以在池中创建新对象.结果,在非动态创建模式中,即在构造函数中创建所有对象,该锁将始终被锁定;在动态创建模式中,每次只能创建一个对象,所以如果有另一个线程获取该对象,它将从pool.take()获取阻止删除的对象,并等待新的可用对象队列中的资源.

    public abstract class ResourcePool {
        private final BlockingQueue pool;
        private final ReentrantLock lock = new ReentrantLock();
        private int createdObjects = 0;
        private int size;
     
        protected ResourcePool(int size) {
            this(size, false);
        }
     
        protected ResourcePool(int size, Boolean dynamicCreation) {
            // Enable the fairness; otherwise, some threads
            // may wait forever.
            pool = new ArrayBlockingQueue<>(size, true);
            this.size = size;
            if (!dynamicCreation) {
                lock.lock();
            }
        }
     
        public Resource acquire() throws Exception {
            if (!lock.isLocked()) {
                if (lock.tryLock()) {
                    try {
                        ++createdObjects;
                        return createObject();
                    } finally {
                        if (createdObjects < size) lock.unlock();
                    }
                }
            }
            return pool.take();
        }
     
        public void recycle(Resource resource) throws Exception {
            // Will throws Exception when the queue is full,
            // but it should never happen.
            pool.add(resource);
        }
     
        public void createPool() {
            if (lock.isLocked()) {
                for (int i = 0; i < size; ++i) {
                    pool.add(createObject());
                    createdObjects++;
                }
            }
        }
     
        protected abstract Resource createObject();
    }

在以下示例中,有5个客户端线程同时在资源池中获取两个DataTimeFormat对象,这些客户端线程总共将执行30次计算.

    class DataTimeFormatResourcePool extends ResourcePool<SimpleDateFormat> {
 
    DataTimeFormatResourcePool(int size, Boolean dynamicCreation) {
        super(size, dynamicCreation);
        createPool();
    }
 
    @Override
    protected SimpleDateFormat createObject() {
        return new SimpleDateFormat("yyyyMMdd");
    }
 
    public Date convert(String input) throws Exception {
        SimpleDateFormat format = acquire();
        try {
            return format.parse(input);
        } finally {
            recycle(format);
        }
    }
}
 
public class ResourcePoolExample {
    public static void main(String args[]) {
        final DataTimeFormatResourcePool pool = new DataTimeFormatResourcePool(2, true);
 
        Callable<Date> task = new Callable<Date>() {
            @Override
            public Date call() throws Exception {
                return pool.convert("20130224");
            }
        };
 
        ExecutorService exec = Executors.newFixedThreadPool(5);
        List<Future<Date>> results = new ArrayList<>();
 
        for (int i = 0; i < 30; i++) {
            results.add(exec.submit(task));
        }
        exec.shutdown();
        try {
            for (Future<Date> result : results) {
                System.out.println(result.get());
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}