java 爪哇蔚来。SocketChannel.read 方法一直返回 0。为什么？
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Java NIO. SocketChannel.read method all time return 0. Why?
提问by user471011
I try understand how works java NIO. In particular, how works SocketChannel.
我尝试了解 java NIO 的工作原理。特别是 SocketChannel 是如何工作的。
I wrote code below:
我在下面写了代码：
import java.io.*;
import java.net.*;
import java.nio.*;
import java.nio.channels.*;
public class Test {
public static void main(String[] args) throws IOException {
SocketChannel socketChannel = SocketChannel.open();
socketChannel.configureBlocking(false);
socketChannel.connect(new InetSocketAddress("google.com", 80));
while (!socketChannel.finishConnect()) {
// wait, or do something else...
}
String newData = "Some String...";
ByteBuffer buf = ByteBuffer.allocate(48);
buf.clear();
buf.put(newData.getBytes());
buf.flip();
while (buf.hasRemaining()) {
System.out.println(socketChannel.write(buf));
}
buf.clear().flip();
int bytesRead;
while ((bytesRead = socketChannel.read(buf)) != -1) {
System.out.println(bytesRead);
}
}
}
- I try connect to google server.
- Send request to the server;
- Read answer from the server.
- 我尝试连接到谷歌服务器。
- 向服务器发送请求；
- 从服务器读取答案。
but, method socketChannel.read(buf) all time return 0 and performs infinitely.
但是，方法 socketChannel.read(buf) 始终返回 0 并无限执行。
Where I made mistake??
我哪里出错了？？
回答by Ramesh PVK
Because NIO SocektChannelwill not block until the data is available for read. i.e, Non Blocking Channel can return 0 on read() operation.
因为NIO SocektChannel在数据可供读取之前不会阻塞。即，非阻塞通道可以return 0 on read() operation。
That is why while using NIO you should be using java.nio.channels.Selectorwhich gives you read notification on channel if the data is available.
这就是为什么在使用 NIO 时你应该使用java.nio.channels.Selector如果数据可用，它会给你在通道上读取通知。
On the otherhand, blocking channel will wait till the data is available and return how much data is available. i.e, Blocking channel will never return 0 on read() operation.
另一方面，阻塞通道将等待数据可用并返回可用数据量。即，阻塞通道永远不会return 0 on read() operation。
You can read more about NIO here:
您可以在此处阅读有关 NIO 的更多信息：
回答by Peter Lawrey
You have specifically configured
你专门配置了
socketChannel.configureBlocking(false);
If you left the default which is blockingthen you would never get a length of 0 returned.
如果您保留阻塞的默认值，那么您将永远不会得到长度为 0 的返回值。
回答by bestsss
While reading the title I was about the say: b/c there is no room in the buffer:
在阅读标题时，我想说：b/c 缓冲区中没有空间：
Unlike the answers so far the real culprit is buf.clear().flip();this effectively sets the buffer like that position=limit=0- hence no data read - change to buf.clear()and you are good to go.
与迄今为止的答案不同，真正的罪魁祸首是 buf.clear().flip();这有效地设置了这样的缓冲区position=limit=0- 因此没有读取数据 - 更改为buf.clear()，你很高兴。
Have fun w/ the NIO and use debugger before posting a question.
在发布问题之前与 NIO 一起玩得开心并使用调试器。
回答by Alexei Kaigorodov
If you really want to use non-blocking I/O, then either use selector, or use nio2 (AsynchronousSocketChannel et al.) from Java 7. Using selector is tricky, better find ready working examples. Using nio2 is relatively easy.
如果你真的想使用非阻塞 I/O，那么要么使用选择器，要么使用 Java 7 中的 nio2（AsynchronousSocketChannel 等）。使用选择器很棘手，最好找到现成的工作示例。使用 nio2 相对容易。
回答by user1187372
The technical explanation is that you didn't send a complete HTTP request to the google webserver, therefore it won't send you a reply until it receives a complete request.
技术解释是您没有向谷歌网络服务器发送完整的 HTTP 请求，因此它在收到完整请求之前不会向您发送回复。
This explains why you are reading 0 bytes which means that no data is available for reading.
这解释了为什么您正在读取 0 字节，这意味着没有数据可供读取。
