在 Java 中实现非阻塞套接字的最佳方法是什么？
或者有这样的事情吗？我有一个通过套接字与服务器通信的程序，但如果数据/连接出现问题，我不希望套接字调用阻塞/导致延迟。
最佳答案
Java 非阻塞套接字 ，在 Java 2 Standard Edition 1.4 中引入，允许应用程序之间进行网络通信，而不会阻塞使用套接字的进程。但是什么是非阻塞套接字，它在哪些上下文中有用，以及它是如何工作的？
什么是非阻塞套接字？
非阻塞套接字允许在 channel 上进行 I/O 操作而不阻塞使用它的进程。这意味着，我们可以使用单个线程来处理多个并发连接并获得“异步高性能”读/写操作(有些人可能不同意)
好的，它在哪些情况下有用？
假设您想实现一个接受不同客户端连接的服务器。还假设您希望服务器能够同时处理多个请求。使用传统方式，您有两种选择来开发这样的服务器:
两种解决方案都有效，但采用第一种方法您必须开发整个线程管理解决方案，并存在相关的并发和冲突问题。第二种解决方案使应用程序依赖于非 JDK 外部模块，并且您可能必须使库适应您的需要。通过非阻塞套接字，您可以实现非阻塞服务器，而无需直接管理线程或求助于外部模块。
这个怎么运作？
在详细介绍之前，您需要了解几个术语:
Java NIO 有一个名为
Selector 的类它允许单个线程检查多个 channel 上的 I/O 事件。这怎么可能？那么，selector可以检查 channel 的“就绪”事件，例如客户端尝试连接或读/写操作。这是 Selector 的每个实例可以监控更多 socket channel 从而有更多的联系。现在，当 channel 上发生某些事情(事件发生)时，selector通知申请处理请求 . selector通过创建 来做到这一点事件键 (或选择键)，它们是 SelectionKey 的实例类(class)。每个key持有有关的信息谁在提出请求 和 请求的类型是什么 ，如图 1 所示。
图1:结构图一个基本的实现
服务器实现由一个无限循环组成，其中
selector等待事件并创建事件键。 key 有四种可能的类型:通常
acceptable key 是在服务器端创建的。实际上，这种键只是简单地通知服务器客户端需要连接，然后服务器将套接字 channel 个性化并将其关联到选择器以进行读/写操作。此后，当接受的客户端读取或写入某些内容时，选择器将创建 readable或 writeable该客户的 key ..现在您已准备好按照建议的算法用 Java 编写服务器。套接字 channel 的创建，
selector ，并且可以通过这种方式进行套接字选择器注册:final String HOSTNAME = "127.0.0.1";
final int PORT = 8511;
// This is how you open a ServerSocketChannel
serverChannel = ServerSocketChannel.open();
// You MUST configure as non-blocking or else you cannot register the serverChannel to the Selector.
serverChannel.configureBlocking(false);
// bind to the address that you will use to Serve.
serverChannel.socket().bind(new InetSocketAddress(HOSTNAME, PORT));
// This is how you open a Selector
selector = Selector.open();
/*
* Here you are registering the serverSocketChannel to accept connection, thus the OP_ACCEPT.
* This means that you just told your selector that this channel will be used to accept connections.
* We can change this operation later to read/write, more on this later.
*/
serverChannel.register(selector, SelectionKey.OP_ACCEPT);
首先我们创建一个
SocketChannel 的实例与 ServerSocketChannel.open()方法。接下来，configureBlocking(false)调用设置此 channel 作为非阻塞 .到服务器的连接是由 serverChannel.socket().bind() 建立的方法。 HOSTNAME代表服务器的IP地址，PORT是通讯端口。最后，调用 Selector.open()创建 selector 的方法实例并将其注册到 channel和注册类型。本例中，注册类型为OP_ACCEPT ，这意味着选择器仅报告客户端尝试连接到服务器。其他可能的选项是:OP_CONNECT , 将由客户端使用； OP_READ ;和 OP_WRITE .现在我们需要使用无限循环来处理这个请求。一个简单的方法如下:
// Run the server as long as the thread is not interrupted.
while (!Thread.currentThread().isInterrupted()) {
/*
* selector.select(TIMEOUT) is waiting for an OPERATION to be ready and is a blocking call.
* For example, if a client connects right this second, then it will break from the select()
* call and run the code below it. The TIMEOUT is not needed, but its just so it doesn't
* block undefinable.
*/
selector.select(TIMEOUT);
/*
* If we are here, it is because an operation happened (or the TIMEOUT expired).
* We need to get the SelectionKeys from the selector to see what operations are available.
* We use an iterator for this.
*/
Iterator<SelectionKey> keys = selector.selectedKeys().iterator();
while (keys.hasNext()) {
SelectionKey key = keys.next();
// remove the key so that we don't process this OPERATION again.
keys.remove();
// key could be invalid if for example, the client closed the connection.
if (!key.isValid()) {
continue;
}
/*
* In the server, we start by listening to the OP_ACCEPT when we register with the Selector.
* If the key from the keyset is Acceptable, then we must get ready to accept the client
* connection and do something with it. Go read the comments in the accept method.
*/
if (key.isAcceptable()) {
System.out.println("Accepting connection");
accept(key);
}
/*
* If you already read the comments in the accept() method, then you know we changed
* the OPERATION to OP_WRITE. This means that one of these keys in the iterator will return
* a channel that is writable (key.isWritable()). The write() method will explain further.
*/
if (key.isWritable()) {
System.out.println("Writing...");
write(key);
}
/*
* If you already read the comments in the write method then you understand that we registered
* the OPERATION OP_READ. That means that on the next Selector.select(), there is probably a key
* that is ready to read (key.isReadable()). The read() method will explain further.
*/
if (key.isReadable()) {
System.out.println("Reading connection");
read(key);
}
}
}
您可以找到 the implementation source here
注意:异步服务器
我们可以部署一个异步服务器来替代非阻塞实现。例如，您可以使用
AsynchronousServerSocketChannel类，它为面向流的监听套接字提供异步 channel 。要使用它，首先执行它的静态
open()方法，然后 bind()它到一个特定的 端口 .接下来，您将执行其 accept()方法，传递给它一个实现 CompletionHandler 的类界面。大多数情况下，您会发现该处理程序被创建为匿名内部类。从这里
AsynchronousServerSocketChannel对象，你调用 accept()告诉它开始监听连接，传递给它一个自定义 CompletionHandler实例。当我们调用 accept() ，它立即返回。请注意，这与传统的阻塞方法不同；而accept()方法 阻塞，直到客户端连接到它 , AsynchronousServerSocketChannel accept()方法为您处理。这里有一个例子:
public class NioSocketServer
{
public NioSocketServer()
{
try {
// Create an AsynchronousServerSocketChannel that will listen on port 5000
final AsynchronousServerSocketChannel listener = AsynchronousServerSocketChannel
.open()
.bind(new InetSocketAddress(5000));
// Listen for a new request
listener.accept(null, new CompletionHandler<AsynchronousSocketChannel, Void>()
{
@Override
public void completed(AsynchronousSocketChannel ch, Void att)
{
// Accept the next connection
listener.accept(null, this);
// Greet the client
ch.write(ByteBuffer.wrap("Hello, I am Echo Server 2020, let's have an engaging conversation!\n".getBytes()));
// Allocate a byte buffer (4K) to read from the client
ByteBuffer byteBuffer = ByteBuffer.allocate(4096);
try {
// Read the first line
int bytesRead = ch.read(byteBuffer).get(20, TimeUnit.SECONDS);
boolean running = true;
while (bytesRead != -1 && running) {
System.out.println("bytes read: " + bytesRead);
// Make sure that we have data to read
if (byteBuffer.position() > 2) {
// Make the buffer ready to read
byteBuffer.flip();
// Convert the buffer into a line
byte[] lineBytes = new byte[bytesRead];
byteBuffer.get(lineBytes, 0, bytesRead);
String line = new String(lineBytes);
// Debug
System.out.println("Message: " + line);
// Echo back to the caller
ch.write(ByteBuffer.wrap(line.getBytes()));
// Make the buffer ready to write
byteBuffer.clear();
// Read the next line
bytesRead = ch.read(byteBuffer).get(20, TimeUnit.SECONDS);
} else {
// An empty line signifies the end of the conversation in our protocol
running = false;
}
}
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
} catch (TimeoutException e) {
// The user exceeded the 20 second timeout, so close the connection
ch.write(ByteBuffer.wrap("Good Bye\n".getBytes()));
System.out.println("Connection timed out, closing connection");
}
System.out.println("End of conversation");
try {
// Close the connection if we need to
if (ch.isOpen()) {
ch.close();
}
} catch (I/OException e1)
{
e1.printStackTrace();
}
}
@Override
public void failed(Throwable exc, Void att)
{
///...
}
});
} catch (I/OException e) {
e.printStackTrace();
}
}
public static void main(String[] args)
{
NioSocketServer server = new NioSocketServer();
try {
Thread.sleep(60000);
} catch (Exception e) {
e.printStackTrace();
}
}
}
您可以找到 the full code here
关于java - 非阻塞套接字，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/3895461/