Java Try-With-Resources 未知资源计数

Question

我需要打开 N 个多播套接字（其中 N 来自参数列表的大小）。然后我将在一个循环中将相同的数据发送到 N 个套接字中的每一个，最后关闭每个套接字。我的问题是，如何使用 try-with-resources 块执行此操作？以下是我将如何使用单个资源执行此操作：

final int port = ...;
try (final MulticastSocket socket = new MulticastSocket(port)) {
    // Do a bunch of sends of small packet data over a long period of time
    ...
}

我能想到的使用多个端口执行此操作的唯一方法如下：

final List<Integer> ports = ...;
final List<MulticastSocket> sockets = new ArrayList<>(ports.size());
try {
    for (final Integer port : ports) {
        sockets.add(new MulticastSocket(port));
    }

    // Do a bunch of sends of small packet data over a long period of time
    ...
} finally {
    for (final MulticastSocket socket : sockets) {
        try {
            socket.close();
        } catch (final Throwable t) {
            // Eat the exception
        }
    }
}

是否有更简洁的方法来完成此任务，或者我提出的解决方案是否已达到最佳效果？

Answer 1

您所做的几乎是最好的。

您可以创建一个 AutoCloseable 通用多关闭器，其中包含一个 List<AutoCloseable> 并接受作为构造函数参数的 closeables 计数和要调用以创建每个 closeable 的工厂，然后当它的 close() 被调用时关闭它们，这样你就可以像这样使用它：

try( MultiCloser<MulticastSocket> multiCloser = 
         new MultiCloser<>( ports.size(), i -> new MulticastSocket( ports.get( i ) ) )
{
    for( MulticastSocket socket : multiCloser.getItems() )
    {
        do something with the socket
    }
}

...但这可能有点矫枉过正。

Answer 2

递归执行以保持 try-with-resources 的保证：

void foo(List<Integer> ports, List<Socket> sockets) {
  if (sockets.size() == ports.size()) {
    // Do something with your sockets.
  } else {
    try (Socket s = new MulticastSocket(ports.get(sockets.size())) {
      sockets.add(s);
      foo(ports, sockets);
      // You could call sockets.remove(sockets.size()-1) here.
      // Not convinced whether it's worth it.
    }
  }
}

Answer 3

使用 ArrayList 存储 MulticastSocket 实例有什么意义？

你是这么说的：

I will then send the same data to each of the N sockets within a loop, and finally, close each socket.

因此您可以在循环中创建它们并为每次迭代发送相同的处理。
为此，您应该稍微更改一下设计。
MulticastSocket 的处理任务可以由功能接口执行，该接口还允许指定要使用的端口。

例如：

@FunctionalInterface
public interface SocketProcessor {
    void process(MulticastSocket multicastSocket) ;
}

您可以有一个方法将此功能接口作为参数来应用处理：

public static void processSocket(SocketProcessor socketProcessor, Integer port) throws IOException {
  try (final MulticastSocket socket = new MulticastSocket(port)) {
    socketProcessor.process(socket);
  }
}

最后，您可以从客户端代码创建一个带有 lambda 的 socketProcessor 实例：

SocketProcessor socketProcessor = (MulticastSocket socket) -> {
    socket.send(...);
    socket.send(...);
};

然后您可以在端口上循环，以便使用合适的端口和刚刚创建的 SocketProcessor 实例调用 processSocket :

for (final Integer port : ports) {
    try {
      processSocket(socketProcessor, port);
    } catch (IOException e) {
      // do processing
    }
}

这个解决方案不一定要更短（实际上不会更长），但它确实更清晰。
两个主要问题是分开的：

processSocket(SocketProcessor) 执行样板代码
SocketProcessor 定义具体任务。

Answer 4

受到 Mike Nakis 提出的想法的启发，我想出了以下 class...

package myNamespace;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;

import myNamespace.ThrowingFunction;
import myNamespace.ThrowingSupplier;

/** Collection of AutoCloseable objects */
public class ResourceCollection<T extends AutoCloseable>
        implements Iterable<T>, AutoCloseable {

    /** Resources owned by this instance */
    private final List<T> myResources;

    /**
     * Constructor
     * @param allocator Function used to allocate each resource
     * @param count     Number of times to call the allocator
     * @throws E Thrown if any of the allocators throw
     */
    public <E extends Throwable> ResourceCollection(
            final ThrowingSupplier<T, E> allocator, final int count)
            throws E {
        myResources = new ArrayList<>(count);
        try {
            while (myResources.size() < count) {
                final T resource = allocator.getThrows();
                myResources.add(resource);
            }
        } catch (final Throwable e) {
            close();
            throw e;
        }
    }

    /**
     * Constructor
     * @param allocator Function used to allocate each resource
     * @param input     List of input parameters passed to the allocator
     * @throws E Thrown if any of the allocators throw
     */
    public <U, E extends Throwable> ResourceCollection(
            final ThrowingFunction<U, T, E> allocator, final Collection<U> input)
            throws E {
        myResources = new ArrayList<>(input.size());
        try {
            for (final U value : input) {
                final T resource = allocator.applyThrows(value);
                myResources.add(resource);
            }
        } catch (final Throwable e) {
            close();
            throw e;
        }
    }

    /**
     * Gets the number of resources in the collection
     * @return The number of resources in the collection
     */
    public int size() {
        return myResources.size();
    }

    /**
     * Gets whether the collection contains no resources
     * @return Whether the collection contains no resources
     */
    public boolean isEmpty() {
        return myResources.isEmpty();
    }

    /**
     * Gets the resource at index i
     * @param i The index of a resource, in the range [0, size())
     * @return The resource at index i
     */
    public T get(final int i) {
        return myResources.get(i);
    }

    @Override
    public Iterator<T> iterator() {
        return myResources.iterator();
    }

    @Override
    public void close() {
        final ListIterator<T> resourceIter =
                myResources.listIterator(myResources.size());
        while (resourceIter.hasPrevious()) {
            final T resource = resourceIter.previous();
            if (resource != null) {
                try {
                    resource    .close ();
                    resourceIter.remove();
                } catch (final Throwable t) {
                    // Eat the exception
                }
            }
        }
    }

}

Java Try-With-Resources 未知资源计数

Java Try-With-Resources Unknown Resource Count

java

try-with-resources

autocloseable