Java NIO 特性学习

Java NIO 包含几个核心的组件：

• Channels
• Buffer
• Selectors

Channels

可以理解为资源的一个流，通过这个流资源可以从Channel读取Data到一个Buffer中或者从一个Buffer中写入Data到Channel；

Channel Implementations

集中Jdk7常用的Channel上线

• FileChannel : 操作文件读取或者写入数据
• DatagramChannel : 从一个网络UDP连接中读取或写入数据
• SocketChannel : 从一个TCP网络连接中读取或写入数据
• ServerSocketChannel: 可以使用这个Channel来监听TCP连接，如同Web Server 当连接来时可以创建出一个SocketChannel

Base Channel Example

Read Data from Channel

1. RandomAccessFile aFile =newRandomAccessFile("data/nio-data.txt","rw");
2. FileChannel inChannel = aFile.getChannel();
4. ByteBuffer buf =ByteBuffer.allocate(48);
6. //write data to buffer
7. int bytesRead = inChannel.read(buf);
8. while(bytesRead!=-1){
10. System.out.println("Read " + bytesRead);
11. //swither buffer type to read data from buffer
12. buf.flip();
14. while(buf.hasRemaining()){
15. System.out.print((char) buf.get());
16. }
18. buf.clear();
19. bytesRead= inChannel.read(buf);
20. }
21. aFile.close();

Buffer

Java NIO Buffers 和Channels配合使用，Read from channels into buffer ,writtern from buffers into channels

Buffer Usage

1. 向buffer中写入数据
2. 调用buffer.flip()方法
3. 使用buffer.get()方法获取出数据
4. 调用buffer.clear() 或者 buffer.compact()清理已经读取的数据

    RandomAccessFile aFile = new RandomAccessFile("data/nio-data.txt" , "rw");
    FileChannel inChannel = aFile.getChannel();
    //Create Buffer with capacity of 48 byte
    ByteBuffer buf = ByteBuffer.allocate(48);
    int bytesRead = inChannel.read(buf);
    while(bytesRead!=-1){
      buf.flip();  //make buffer ready for read
          while(buf.hasRemaining()){
              System.out.print((char) buf.get()); // read 1 byte at a time
          }
      buf.clear(); //make buffer ready for writing
      bytesRead = inChannel.read(buf);
    }

Buffer Capacity , Position and Limit

Capacity

Buffer在创建的时候会分配规定大小的内存块，并且在buffer写入数据时只能写入这个固定大小的数据。如果Buffer已经放满数据，就需要先read 或者 clear 只能才能继续写入。

Position

Position 在读模式和模式下面含义不一样。Bufer init之后Position值为0，当数据写入one byte ，long etc Buffer Position相应的移动到下一个位置。并且Position<=Capacity-1

当调用filp()从Buffer读取数据时会set position = 0,position会随着读取过程向下移动；

Limit

在Write模式下，Limit等于Capacity的值，表示能写多少数据到Buffer中。

调用filp()进入Reade模式会set limit = position ,表示最多能读的数据量。

常用的Buffer 实现

• ByteBuffer
• MappedByteBuffer
• CharBuffer
• DoubleBuffer
• FloatBuffer
• IntBuffer
• LongBuffer
• ShortBuffer

SomeMethods of Buffer

Allocating a Buffer

通过使用allocate() 来分配Buffer大小

ByteBuffer buf = ByteBuffer.allocate(48);
CharBuffer buf = CharBuffer.allocate(1024);

Write Data to a Buffer

向Buffer中写入数据有两种方法：

• 从Channel中读取数据写入Buffer
• 调用Buffer put() 方法写入数据
  

int bytesRead = inChannel.read(buf); //read into buffer.
buf.put(127);

flip()

转换写状态到读状态，发生动作：set limit = position ; set position=0 ;

Reading Data from a Buffer

两种方法：

• 从Buffer中读取数据到Channel
• 读取Buffer中数据给自己,可以使用buffer自带方法 get() etc
  

//read from buffer into channel.
int bytesWritten = inChannel.write(buf);
byte aByte = buf.get();

rewind()

set position=0

mark() 和 reset()

mark用于打点记录position为止，之后使用reset方法可以将position重置到mark记录的位置。

Scatter / Gather /Transfers

Channel 可以执行读取操作将数据读取到多个Buffer中。
Channel 可以执行写操作将多个Buffer的数据写入Channle中。
Channel 可以在Channle之间做转换包括transferFrom() & transferTo

Scatter Reads :

例如我将html的header部分读取到第一个buffer中，将body部分读取到第二个Buffer中

ByteBuffer header = ByteBuffer.allocate(128);
ByteBuffer body   = ByteBuffer.allocate(1024);
ByteBuffer[] bufferArray = { header, body };
channel.read(buffers);

Gather Writes

ByteBuffer header = ByteBuffer.allocate(128);
ByteBuffer body   = ByteBuffer.allocate(1024);
//write data into buffers
ByteBuffer[] bufferArray = { header, body };
channel.write(buffers);

Transfers

TransferFrom()

RandomAccessFile fromFile = new RandomAccessFile("fromFile.txt", "rw");
FileChannel      fromChannel = fromFile.getChannel();
RandomAccessFile toFile = new RandomAccessFile("toFile.txt", "rw");
FileChannel      toChannel = toFile.getChannel();
long position = 0;
long count    = fromChannel.size();
toChannel.transferFrom(fromChannel, position, count);

TransferTo()

RandomAccessFile fromFile = new RandomAccessFile("fromFile.txt", "rw");
FileChannel      fromChannel = fromFile.getChannel();
RandomAccessFile toFile = new RandomAccessFile("toFile.txt", "rw");
FileChannel      toChannel = toFile.getChannel();
long position = 0;
long count    = fromChannel.size();
fromChannel.transferTo(position, count, toChannel);

Selector

Selector组件可以运行判断多个Channel，动态决定使用哪个Channel来执行Read 或者 Write操作。通过这个组件可以上线一个Thread 管理多个Channels 或者 多个网络连接Channel。

Create a Selector

通过使用Selector.open()方法来创建一个Selector

Selector selector = Selector.open();

注册Channels到Selector上

为了能让Selector管理Channels需要调用Selector.register()注册到Selector

channel.configureBlocking(false); //The Channel must be in non-blocking mode to be used with a Selector
SelectionKey key = channel.register(selector, SelectionKey.OP_READ);
注册Channel的时候需要制定Channel需要关心的事件，事件包括：

• Connect -> SelectionKey.OP_CONNECT
• Accept -> SelectionKey.OP_ACCEPT
• Read -> SelectionKey.OP_READ
• Write ->SelectionKey.OP_WRITE
  如果注册多个事件可以使用 int interestSet = SelectionKey.OP_READ | SelectionKey.OP_WRITE; 通过int interestSet = selectionKey.interestOps(); 可以得到所有Selector’s的事件。

Simple Demo：

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();
  }
}