[英]Java - Write call to socket output stream blocking in full duplex
我正在编写一个客户端服务器应用程序,我想从两个不同的线程(一个线程用于读取,一个用于写入)在一个套接字上读取和写入。 我的系统几乎可以正常工作,但是有一个令人困惑的错误,我似乎无法解决。 读写工作完全相互独立,但是当我从一个线程中的Socket
的OutputStream
开始读取时,所有在不同线程中写入InputStream
调用都会无限期地阻塞。
我编写了一个小型测试程序,用于快速重现问题并尽可能多地消除外部变量。 我使用java.nio
的ServerSocketChannel
和SocketChannel
来建立连接,我使用java.io
的Socket
( SocketChannel
的底层套接字)是因为它易于与ObjectInputStream
和ObjectOutputStream
。 测试程序设计为运行两次; 第一次运行时,用户输入s
来启动服务器,第二次运行时,用户输入c
来运行客户端。
我的问题是:为什么在第二次调用objectOutput.writeObject( message );
执行以下程序块objectOutput.writeObject( message );
在server()
方法中? (该方法中的第四行至最后一行)
我在程序代码下方包含了预期输出和实际输出以及我认为它们的含义。
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Scanner;
public class Main {
private static final String IP_ADDRESS = "localhost";
private static final int WELL_KNOWN_PORT = 4000;
public static void main( String... args ) throws Exception {
Scanner scanner = new Scanner( System.in );
System.out.print( "choose (s)erver or (c)lient: " );
char choice = scanner.nextLine().charAt( 0 );
switch ( choice ) {
case 's':
server();
break;
case 'c':
client();
break;
default:
break;
}
scanner.close();
}
private static void server() throws Exception {
// initialize connection
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.bind( new InetSocketAddress( WELL_KNOWN_PORT ) );
System.out.println( "waiting for client to connect" );
SocketChannel socketChannel = serverSocketChannel.accept();
System.out.println( "client connected" );
socketChannel.configureBlocking( true );
while ( !socketChannel.finishConnect() )
Thread.sleep( 100 );
Socket socket = socketChannel.socket();
ObjectOutput objectOutput = new ObjectOutputStream( socket.getOutputStream() );
// write first object to stream
Message message = new Message( 1 );
System.out.println( "writing first object to object output stream: " + message );
objectOutput.writeObject( message );
System.out.println( "first object written to object output stream" );
objectOutput.flush();
System.out.println( "object output stream flushed" );
// start reading in a separate thread
new Thread( () -> {
ObjectInput objectInput = null;
try {
objectInput = new ObjectInputStream( socket.getInputStream() );
} catch ( IOException e ) {
e.printStackTrace();
}
Message messageIn = null;
try {
System.out.println( "reading on object input stream" );
messageIn = (Message) objectInput.readObject();
System.out.println( "read object on object input stream: " + messageIn );
} catch ( ClassNotFoundException | IOException e ) {
e.printStackTrace();
}
System.out.println( messageIn );
} ).start();
Thread.sleep( 100 ); // allow time for object listening to start
// write second object to stream
message = new Message( 2 );
System.out.println( "writing second object to object output stream: " + message );
objectOutput.writeObject( message ); // this call seems to block??
System.out.println( "second object written to object output stream" );
objectOutput.flush();
System.out.println( "object output stream flushed" );
}
private static void client() throws Exception {
// initialize connection
SocketChannel socketChannel = SocketChannel.open();
socketChannel.configureBlocking( true );
socketChannel.connect( new InetSocketAddress( IP_ADDRESS, WELL_KNOWN_PORT ) );
while ( !socketChannel.finishConnect() )
Thread.sleep( 100 );
Socket socket = socketChannel.socket();
ObjectOutput objectOutput = new ObjectOutputStream( socket.getOutputStream() );
ObjectInput objectInput = new ObjectInputStream( socket.getInputStream() );
// read first object
System.out.println( "reading first object on object input stream" );
Message message = (Message) objectInput.readObject();
System.out.println( "read first object on object input stream: " + message );
// read second object
System.out.println( "reading second object on object input stream" );
message = (Message) objectInput.readObject();
System.out.println( "read second object on object input stream: " + message );
// write confirmation message
message = new Message( 42 );
System.out.println( "writing confirmation message to object output stream: " + message );
objectOutput.writeObject( message );
System.out.println( "confirmation message written to object output stream" );
objectOutput.flush();
System.out.println( "object output stream flushed" );
}
private static class Message implements Serializable {
private static final long serialVersionUID = 5649798518404142034L;
private int data;
public Message( int data ) {
this.data = data;
}
@Override
public String toString() {
return "" + data;
}
}
}
服务器
预期输出:
choose (s)erver or (c)lient: s
waiting for client to connect
client connected
writing first object to object output stream: 1
first object written to object output stream
object output stream flushed
reading on object input stream
writing second object to object output stream: 2
second object written to object output stream
object output stream flushed
read object on object input stream: 42
实际输出:
choose (s)erver or (c)lient: s
waiting for client to connect
client connected
writing first object to object output stream: 1
first object written to object output stream
object output stream flushed
reading on object input stream
writing second object to object output stream: 2
应用程序成功发送第一个对象,但在第二个对象上无限期地阻塞。 我能看到的唯一区别是第二次写调用发生在一个单独的线程上进行读操作时。 我的第一直觉是,也许Socket
不支持从不同线程同时读取和写入,但是我对 Stack Overflow 的搜索表明它们确实支持这种同时操作(全双工)。 这是我对上面代码的操作感到困惑的主要原因。
客户
预期输出:
choose (s)erver or (c)lient: c
reading on object input stream
read first object on object input stream: 1
reading second object on object input stream
read second object on object input stream: 2
writing confirmation message to object output stream: 42
confirmation message written to object output stream
object output stream flushed
实际输出:
choose (s)erver or (c)lient: c
reading first object on object input stream
read first object on object input stream: 1
reading second object on object input stream
这确认第一个对象被客户端成功发送和接收。 由于服务器中的这种奇怪的阻塞行为,客户端似乎正在等待服务器从未发送过的第二个对象。
非常感谢任何人可以提供的任何建议。 如果可以通过另一种方式轻松实现全双工,我愿意重写我的代码,但是如果有使用上述结构的解决方案,我更愿意坚持使用它,以便不必重构大部分代码。
这段代码有很多问题,我必须一行一行地理解:
private static void server() throws Exception {
// initialize connection
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.bind( new InetSocketAddress( WELL_KNOWN_PORT ) );
System.out.println( "waiting for client to connect" );
SocketChannel socketChannel = serverSocketChannel.accept();
上面没有“初始化连接”。 客户端初始化连接。 此代码接受它。
System.out.println( "client connected" );
socketChannel.configureBlocking( true );
这是默认设置。 您不需要断言默认值。
while ( !socketChannel.finishConnect() )
Thread.sleep( 100 );
你不应该这样称呼。 finishConnect()
用于在非阻塞模式下调用connect()
客户端。 你是一个服务器,你没有调用connect()
,你没有处于非阻塞模式。 如果您是处于非阻塞模式的客户端,则不应在睡眠循环中调用它:您应该将Selector.select()
与OP_CONNECT
。
Socket socket = socketChannel.socket();
ObjectOutput objectOutput = new ObjectOutputStream( socket.getOutputStream() );
当您使用阻塞模式和输出流时,根本无法理解为什么要使用ServerSocketChannel
和SocketChannel
,事实上,这至少是问题的一部分。 一个鲜为人知的事实是,来自 NIO 通道的流使用通道上的同步进行读取和写入,因此它们根本不是全双工的,即使底层 TCP 连接是。 删除所有这些并使用ServerSocket
和Socket
重写。
// write first object to stream
Message message = new Message( 1 );
System.out.println( "writing first object to object output stream: " + message );
objectOutput.writeObject( message );
System.out.println( "first object written to object output stream" );
objectOutput.flush();
System.out.println( "object output stream flushed" );
// start reading in a separate thread
new Thread( () -> {
ObjectInput objectInput = null;
try {
objectInput = new ObjectInputStream( socket.getInputStream() );
} catch ( IOException e ) {
e.printStackTrace();
}
不要写这样的代码。 像下面这样依赖于前面的try
块是否成功的代码必须在该try
块内。 否则,例如以下代码可能会得到NullPointerExceptions
。
Message messageIn = null;
try {
System.out.println( "reading on object input stream" );
messageIn = (Message) objectInput.readObject();
System.out.println( "read object on object input stream: " + messageIn );
} catch ( ClassNotFoundException | IOException e ) {
e.printStackTrace();
}
同上。
System.out.println( messageIn );
} ).start();
Thread.sleep( 100 ); // allow time for object listening to start
// write second object to stream
message = new Message( 2 );
System.out.println( "writing second object to object output stream: " + message );
objectOutput.writeObject( message ); // this call seems to block??
System.out.println( "second object written to object output stream" );
objectOutput.flush();
System.out.println( "object output stream flushed" );
}
请参阅上文,了解为什么在从 NIO 通道派生的流的情况下,在单独的线程中执行此操作无法正常工作。
private static void client() throws Exception {
// initialize connection
SocketChannel socketChannel = SocketChannel.open();
socketChannel.configureBlocking( true );
socketChannel.connect( new InetSocketAddress( IP_ADDRESS, WELL_KNOWN_PORT ) );
while ( !socketChannel.finishConnect() )
Thread.sleep( 100 );
上面的最后两行毫无意义,因为连接已经完成,因为您处于阻塞模式。
Socket socket = socketChannel.socket();
ObjectOutput objectOutput = new ObjectOutputStream( socket.getOutputStream() );
ObjectInput objectInput = new ObjectInputStream( socket.getInputStream() );
// read first object
System.out.println( "reading first object on object input stream" );
Message message = (Message) objectInput.readObject();
System.out.println( "read first object on object input stream: " + message );
// read second object
System.out.println( "reading second object on object input stream" );
message = (Message) objectInput.readObject();
System.out.println( "read second object on object input stream: " + message );
// write confirmation message
message = new Message( 42 );
System.out.println( "writing confirmation message to object output stream: " + message );
objectOutput.writeObject( message );
System.out.println( "confirmation message written to object output stream" );
objectOutput.flush();
System.out.println( "object output stream flushed" );
}
您可以按原样使用其余部分,但这里的 NIO 通道再次毫无意义。 您也可以使用Socket
。
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