Java NIO

ByteBuffer

ByteBuffer

使用byte[]数组存储

1. capacity
   数组长度

2. limit
   第一个不可读取或写入的index

3. mark
   标记当前position，通过reset返回到mark位置

4. position
   下一个要读取或写入的index

5. int remaining()
   获取剩余容量: limit-position

0<mark<position<limit<capacity

6. 判断缓冲区是否只读

ByteBuffer byteBuffer = ByteBuffer.wrap(new byte[] {1, 2, 3, 4, 5, 6});
System.out.println(byteBuffer.isReadOnly());  // false

7. 直接缓冲区
   

ByteBuffer byteBuffer = ByteBuffer.wrap(new byte[] {1, 2, 3, 4, 5, 6});
System.out.println(byteBuffer.isDirect());  // false

7. 还原缓冲区状态

public final Buffer clear() {
    position = 0;	
    limit = capacity;
    mark = -1;
    return this;
}

position=1; limit=capacity; mark=-1;

8. 对缓冲区进行反转 flip()
   将指针设置为0，mark设置为-1，当前位置设置为不可读写的位置
   将限制设置为当前位置

public final Buffer flip() {
    limit = position;
    position = 0;
    mark = -1;
    return this;
}

//  测试
ByteBuffer buffer = ByteBuffer.allocateDirect(10);
buffer.put(new byte[] {1, 2, 3, 4, 5, 6}, 2, 3);

//        buffer.rewind();
buffer.flip();
while (buffer.hasRemaining())
{
    System.out.println(buffer.get());
}
// 3  4  5

9. remind()
   将指针设置为0，mark设置为-1

public final Buffer rewind() {
    position = 0;
    mark = -1;
    return this;
}

//  测试
ByteBuffer buffer = ByteBuffer.allocateDirect(10);
buffer.put(new byte[] {1, 2, 3, 4, 5, 6}, 2, 3);

buffer.rewind();
while (buffer.hasRemaining())
{
    System.out.println(buffer.get());
}
// 3  4  5 0 0 0 0 0 0 0

10. putLong()
    将8个字节put到缓冲区中，如putLong(1) --> 1 2 3 4 5 6 7 8
    高位/低位 array() 字节顺序

11. asCharBuffer 视图转换
    两个字节转为一个字符 e.g

ByteBuffer buffer = ByteBuffer.allocateDirect(10);
buffer.put(new byte[] {0, 98, 0, 99, 0, 100, 0, 101, 0, 102});

buffer.rewind();

CharBuffer charBuffer = buffer.asCharBuffer();
while (charBuffer.hasRemaining())
{
    System.out.println(charBuffer.get());
}
// b c d e f

直接缓冲区和堆缓冲区

• 直接缓冲区的内存分配和释放的成本要高于非直接缓冲区。因此不要频繁的创建直接缓冲区

1. slice()
   内存共享缓冲区

2. 压缩缓冲区

 byte[] bytes = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
 ByteBuffer buffer = ByteBuffer.wrap(bytes);

 buffer.position(2);
 buffer.compact();
 System.out.println("current position:" + buffer.position());  // 8
 buffer.rewind();

 while (buffer.hasRemaining())
 {
     System.out.println(buffer.get());
 }

3
4
5
6
7
8
9
10
9
10

3. duplicate 复制缓冲区
   新缓冲区的内容一样，内存共享

通道和FileChannel的使用

通道概述

通道：数据传输的通路

NIO是将数据放在缓冲区进行管理，再利用通道进行数据传输到目的地

channel

jdk1.8 通道子接口

• AsynchronousChannel
• AsynchronousByteChannel
• ReadableByteChannel
• ScatteringByteChannel
• WriteableByteChannel
• GatheringByteChannel
• ByteChannel
• SeekableByteChannel
• NetworkChannel
• MulticastChannel
• InterruptipleChannel

1. AsynchronousChannel
   使通道支持异步IO操作
   （1）Future operation()
   Future用来判断IO操作是否完成，或者等待完成
   （2）回调的方式
   void operation(... A attachment, CompletionHandler<V, supper A> handler)

2. ReadableByteChannel
   只允许一个线程read，第二个会被阻塞
   1） 将通道当前位置读到ByteBuffer中
   2） read(ByteBuffer )是同步的

3. ScatteringByteChannel
   从通道中读取字节到多个缓冲区

public long read(ByteBuffer[] dsts) throws IOException;

4. GatheringByteChannel
   将多个缓冲区的内容写到通道中

5. ByteChannel
   整合ReadableByteChannel和WriteableChannel

6. SeekableByteChannel
   允许position改变

7. NetworkChannel
   与Socket进行关联
   bind(SocketAddress)

FilChannel

Charset charset = Charset.forName("GBK");//Java.nio.charset.Charset处理了字符转换问题。它通过构造CharsetEncoder和CharsetDecoder将字符序列转换成字节和逆转换。
CharsetDecoder decoder = charset.newDecoder();

System.out.println(System.getProperty("user.dir"));
FileInputStream fis = new FileInputStream(System.getProperty("user.dir")+"/src/main/resources/aa.txt");
FileChannel fileChannel = fis.getChannel();
ByteBuffer byteBuffer = ByteBuffer.allocate(5);
CharBuffer cb = CharBuffer.allocate(5);
while (fileChannel.isOpen() && fileChannel.read(byteBuffer) != -1)
{
    System.out.println("length:" + byteBuffer.position());
    byteBuffer.flip();
    decoder.decode(byteBuffer, cb, false);
    cb.flip();
    while (cb.hasRemaining())
    {
        System.out.println(cb.get());
    }
    cb.clear();
    byteBuffer.clear();
}