Java NIO 通道

一、通道（Channel）：用于源节点与目标节点的连接。在 Java NIO 中负责缓冲区中数据的传输。Channel 本身不存储数据，因此需要配合缓冲区进行传输。

二、通道的主要实现类
   java.nio.channels.Channel 接口：
   |--FileChannel
   |--SocketChannel
   |--ServerSocketChannel
   |--DatagramChannel

三、获取通道
1. Java 针对支持通道的类提供了 getChannel() 方法
   本地 IO：
   FileInputStream/FileOutputStream
   RandomAccessFile

   网络IO：
   Socket
   ServerSocket
   DatagramSocket

2. 在 JDK 1.7 中的 NIO.2 针对各个通道提供了静态方法 open()
3. 在 JDK 1.7 中的 NIO.2 的 Files 工具类的 newByteChannel()

四、通道之间的数据传输
transferFrom()
transferTo()

五、分散(Scatter)与聚集(Gather)
分散读取（Scattering Reads）：将通道中的数据分散到多个缓冲区中
聚集写入（Gathering Writes）：将多个缓冲区中的数据聚集到通道中

六、字符集：Charset
编码：字符串 -> 字节数组

解码：字节数组 -> 字符串

//利用通道完成文件的复制（非直接缓冲区）
public void test1(){
long start = System.currentTimeMillis();

FileInputStream fis = null;
FileOutputStream fos = null;
//①获取通道
FileChannel inChannel = null;
FileChannel outChannel = null;
try {
fis = new FileInputStream("d:/1.mkv");
fos = new FileOutputStream("d:/2.mkv");

inChannel = fis.getChannel();
outChannel = fos.getChannel();

//②分配指定大小的缓冲区
ByteBuffer buf = ByteBuffer.allocate(1024);

//③将通道中的数据存入缓冲区中
while(inChannel.read(buf) != -1){
buf.flip(); //切换读取数据的模式
//④将缓冲区中的数据写入通道中
outChannel.write(buf);
buf.clear(); //清空缓冲区
}
} catch (IOException e) {
e.printStackTrace();
} finally {
if(outChannel != null){
try {
outChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}

if(inChannel != null){
try {
inChannel.close();
} catch (IOException e) {
e.printStackTrace();
}
}

if(fos != null){
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}

if(fis != null){
try {
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}

long end = System.currentTimeMillis();
System.out.println("耗费时间为：" + (end - start));

}

//使用通道完成文件的复制(直接缓冲区内存映射文件)
public void test2() throws IOException{//2127-1902-1777
long start = System.currentTimeMillis();

FileChannel inChannel = FileChannel.open(Paths.get("d:/1.mkv"), StandardOpenOption.READ);
FileChannel outChannel = FileChannel.open(Paths.get("d:/2.mkv"), StandardOpenOption.WRITE, StandardOpenOption.READ, StandardOpenOption.CREATE);

//内存映射文件
MappedByteBuffer inMappedBuf = inChannel.map(MapMode.READ_ONLY, 0, inChannel.size());
MappedByteBuffer outMappedBuf = outChannel.map(MapMode.READ_WRITE, 0, inChannel.size());

//直接对缓冲区进行数据的读写操作
byte[] dst = new byte[inMappedBuf.limit()];
inMappedBuf.get(dst);
outMappedBuf.put(dst);

inChannel.close();
outChannel.close();

long end = System.currentTimeMillis();
System.out.println("耗费时间为：" + (end - start));
}

//通道之间的数据传输(直接缓冲区)

public void test3() throws IOException{
FileChannel inChannel = FileChannel.open(Paths.get("d:/1.mkv"), StandardOpenOption.READ);
FileChannel outChannel = FileChannel.open(Paths.get("d:/2.mkv"), StandardOpenOption.WRITE, StandardOpenOption.READ, StandardOpenOption.CREATE);

// inChannel.transferTo(0, inChannel.size(), outChannel);
outChannel.transferFrom(inChannel, 0, inChannel.size());

inChannel.close();
outChannel.close();
}

//分散和聚集
public void test4() throws IOException{
RandomAccessFile raf1 = new RandomAccessFile("1.txt", "rw");

//1. 获取通道
FileChannel channel1 = raf1.getChannel();

//2. 分配指定大小的缓冲区
ByteBuffer buf1 = ByteBuffer.allocate(100);
ByteBuffer buf2 = ByteBuffer.allocate(1024);

//3. 分散读取
ByteBuffer[] bufs = {buf1, buf2};
channel1.read(bufs);

for (ByteBuffer byteBuffer : bufs) {
byteBuffer.flip();
}

System.out.println(new String(bufs[0].array(), 0, bufs[0].limit()));
System.out.println("-----------------");
System.out.println(new String(bufs[1].array(), 0, bufs[1].limit()));

//4. 聚集写入
RandomAccessFile raf2 = new RandomAccessFile("2.txt", "rw");
FileChannel channel2 = raf2.getChannel();

channel2.write(bufs);
}

//获取所有字符集

public void test5(){
Map<String, Charset> map = Charset.availableCharsets();

Set<Entry<String, Charset>> set = map.entrySet();

for (Entry<String, Charset> entry : set) {
System.out.println(entry.getKey() + "=" + entry.getValue());
}
}

//字符集
public void test6() throws IOException{
Charset cs1 = Charset.forName("GBK");

//获取编码器
CharsetEncoder ce = cs1.newEncoder();

//获取解码器
CharsetDecoder cd = cs1.newDecoder();

CharBuffer cBuf = CharBuffer.allocate(1024);
cBuf.put("Java NIO学习！");
cBuf.flip();

//编码
ByteBuffer bBuf = ce.encode(cBuf);

for (int i = 0; i < 12; i++) {
System.out.println(bBuf.get());
}

//解码
bBuf.flip();
CharBuffer cBuf2 = cd.decode(bBuf);
System.out.println(cBuf2.toString());

System.out.println("------------------------------------------------------");

Charset cs2 = Charset.forName("GBK");
bBuf.flip();
CharBuffer cBuf3 = cs2.decode(bBuf);
System.out.println(cBuf3.toString());
}

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