Java NIO
IO v.s. NIO
功能
- IO 面向流 Stream ；NIO 面向缓冲区 Buffer
- IO 是阻塞式的；NIO 是非阻塞的
使用场景
- IO 适用，连接数目比较小，并且一次发送大量数据的场景
- NIO 适用，服务器需要支持大量长连接。比如10000个连接以上，并且每个客户端并不会频繁地发送太多数据
NIO 原理
实现 NIO 的主要原理是轮询。即，一个线程，同时监听多个接口，如果这个接口有数据就读取，没有就检查下一个。
但是，轮询的缺点是浪费CPU资源，因为大部分时间可能都是无数据可读的。 所以，一种常用的改进方案是I/O多路复用（IOmultiplexing），即监听时，由事件驱动，当某个接口的数据好了，再过去读。
NIO 的三种模型
- Reactor 单线程模型
- Reactor 多线程模型
- 主从 Reactor 多线程模型
参考 -> https://blog.csdn.net/jiyiqinlovexx/article/details/51204726
NIO API
Core components
- Channels
- Channels read data into Buffers
- Buffers
- Buffers write data into Channels
- Selectors
- Selector allows a single thread to handle multiple Channels
Multiple sources read/write from multiple channels into buffer(s), thus non-blocking.
Channel
Channel is similar to Stream, it is the gate for read/write.
Channels implementation
- FileChannel (File NIO)
- DatagramChannel (UDP NIO)
- SocketChannel (TCP NIO)
- ServerSocketChannel (TCP NIO)
//create channel
RandomAccessFile aFile = new RandomAccessFile("data/nio-data.txt", "rw");
FileChannel inChannel = aFile.getChannel();
//create buffer with capacity of 48 bytes
ByteBuffer buf = ByteBuffer.allocate(48);
//read from channel into buffer
int bytesRead = inChannel.read(buf);
while (bytesRead != -1) {
buf.flip(); //filp the buffer mode from write to read
while(buf.hasRemaining()){
System.out.print((char) buf.get()); // read 1 byte at a time
}
buf.clear(); //filp the buffer mode from read to write
bytesRead = inChannel.read(buf);
}
aFile.close();
Buffer
A buffer is essentially a block of memory into which you can write data, which you can then later read again.
Usage
- Write data into the Buffer
- Call buffer.flip()
- Read data out of the Buffer
- Call buffer.clear() or buffer.compact()
Buffer implementation
- ByteBuffer
- CharBuffer
- DoubleBuffer
- FloatBuffer
- IntBuffer
- LongBuffer
- ShortBuffer
Selector
- It can examine one or more NIO Channel’s, and determine which channels are ready for reading/writing.
- This way a single thread can manage multiple channels, and thus multiple network connections.
Usage
- open a Selector
- register Channels for this Selector
- select ready Channels from Selector by calling select()
- select() will block until there is an event ready for one of the registered Channels
- process the return results
- get a set of SelectionKey, this key contains
- the interest set
- the ready set
- the Channel
- the Selector
- an attached object (optional)
- iterate the set to do operations
- get a set of SelectionKey, this key contains
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<SelectionKey> selectedKeys = selector.selectedKeys();
Iterator<SelectionKey> 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();
}
}
