Java NIO Study (2)

Java NIO Buffer的三个重要属性

Buffer中3个重要状态属性的含义：position（位置），limit（限制）与capacity（容量）。

• example:

    public static void main(String[] args) {
        IntBuffer buffer = IntBuffer.allocate(10);
        System.out.println("capacity:" + buffer.capacity());

        for (int i = 0; i < 5; ++i) {
            int randomNumber = new SecureRandom().nextInt(20);
            buffer.put(randomNumber);
        }
        System.out.println("before flip limit:" + buffer.limit());

        buffer.flip();

        System.out.println("after flip limit:" + buffer.limit());
        System.out.println("enter while loop");
        while (buffer.hasRemaining()) {
            System.out.println("position:" + buffer.position());
            System.out.println("limit:" + buffer.limit());
            System.out.println("capacity:" + buffer.capacity());
            System.out.println("元素:" + buffer.get());
        }
        System.out.println("position:" + buffer.position());
        System.out.println("limit:" + buffer.limit());
        System.out.println("capacity:" + buffer.capacity());
    }

buffer-flip方法以及3个属性.jpg

文件通道用法

通过NIO读取文件涉及到3个步骤：

1 从FileInputStrem获取FileChannel对象

2 创建Buffer

3 将数据从Channel读取到Buffer

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绝对方法和相对方法的含义：

1 相对方法：limit值与position值会在操作时被考虑到。

2 绝对方法：完全忽略掉limit值与position值。

• example:

    public static void main(String[] args) throws Exception {
        FileInputStream inputStream = new FileInputStream("test.png");
        FileOutputStream outputStream = new FileOutputStream("output.png");

        FileChannel inputStreamChannel = inputStream.getChannel();
        FileChannel outputStreamChannel = outputStream.getChannel();
        
        ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
        
        while (true) {
            byteBuffer.clear();
            int read = inputStreamChannel.read(byteBuffer);
            System.out.println(read);
            
            if (-1 == read) {
                break;
            }
            
            byteBuffer.flip();
            outputStreamChannel.write(byteBuffer);
        }

        inputStream.close();
        outputStream.close();
    }