Java NIO (1)
看了下java核心技术这本书 关于nio的部分介绍比较少,而且如果自己写服务器的话nio用的还是比较多,整理一下nio的资料
java中nio主要是三个组件
- Buffers
- Channels
- Selectors
Buffer可以理解为缓冲区,NIO中的Buffer实现很多,一般常用的就是ByteBuffer
ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
ByteBuffer mapedBuffer = ByteBuffer.allocateDirect(1024);
一般情况下我们不必直接操作byteBuffer,都是用一个Channel来操作
FileChannel fileChannel = FileChannel.open(path);
int hadread = fileChannel.read(byteBuffer);
调用flip切换为读写模式
int hadread;
while ((hadread=fileChannel.read(byteBuffer))>0){
byteBuffer.flip();
byte[] bytes = new byte[1024];
System.out.println(hadread);
byteBuffer.get(bytes,0,hadread);
for (int i = 0; i < hadread; i++) {
System.out.println(bytes[i]);
}
byteBuffer.flip();
}
如果遇到了网络编程,我们可能会需要类似于linux select epoll的功能,java nio已经为我们提供了
一般情况下,我们使用单个线程处理多个Channels,因为如果线程过多,可能造成频繁的上下文切换导致开销过高(跟太频繁系统调用一个道理)
selector就为我们提供了这样的功能
不过selector只能跟SocketChannel一起使用,因为Channel必须处于非阻塞模式下
FileChannel没有configureBlocking这个方法
Selector selector = Selector.open();
SocketChannel channel = server.accept();
// 有新的连接并不代表这个通道就有数据,
// 这里将这个新的 SocketChannel 注册到 Selector,监听 OP_READ 事件,等待数据
channel.configureBlocking(false);
//注册给某个selector
SelectionKey key = channel.register(selector,
Selectionkey.OP_READ);
SelectionKey key;
Channel channel = key.channel();
Selector selector = key.selector();
注册完之后,就可以像linux那样select
//调用select
// @return The number of keys, possibly zero,
// whose ready-operation sets were updated
int readys = selector.select();
//获取就绪事件的集合
Set selectedKeys = selector.selectedKeys();
//这里返回的是所有就绪事件,比如说可以读,可以写,可以连接
//完整实例:
Selector selector = Selector.open();
channel.configureBlocking(false);
SelectionKey key = channel.register(selector, SelectionKey.OP_READ);
while(true) {
int readyChannels = selector.select();
if(readyChannels == 0) continue;
Set selectedKeys = selector.selectedKeys();
Iterator keyIterator = selectedKeys.iterator();
while(keyIterator.hasNext()) {
SelectionKey key = keyIterator.next();
if(key.isAcceptable()) {
// a connection was accepted by a ServerSocketChannel.
} else if (key.isConnectable()) {
// a connection was established with a remote server.
} else if (key.isReadable()) {
// a channel is ready for reading
} else if (key.isWritable()) {
// a channel is ready for writing
}
keyIterator.remove();
}
}
这个网站里面nio写的很好
还有这个