Java-NIO（五）：通道（Channel）的数据传输与内存映射文件

• 通道（Channel）的数据传输(采用非直接缓冲区)

     @Test
     public void testChannel() throws IOException {
         FileInputStream fileInputStream = new FileInputStream("Java NIO.pdf");
         FileOutputStream fileOutputStream = new FileOutputStream("2.pdf");

         // 1、获取通道
         FileChannel inChannel = fileInputStream.getChannel();
         FileChannel outChannel = fileOutputStream.getChannel();

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

         // 3、将通道的数据读入缓冲区
         while (inChannel.read(byteBuffer) != -1) {
             byteBuffer.flip();// 切换缓冲区为读模式
             // 4、把缓冲区的数据写入通道
             outChannel.write(byteBuffer);
             byteBuffer.clear();// 因为需要循环多次读，需要清空缓冲区。
         }

         byteBuffer.clear();
         inChannel.close();
         outChannel.close();
         fileInputStream.close();
         fileOutputStream.close();
     }

•  内存映射文件（采用直接缓冲区）

     /**
      * 内存映射文件
      *
      * @throws IOException
      */
     @Test
     public void testMemoryMappingFile() throws IOException {
         long start = System.currentTimeMillis();

         FileChannel inChannel = FileChannel.open(Paths.get("D:\\nio.zip"), StandardOpenOption.READ);
         // 注意：StandardOpenOption.CREATE
         // 如果文件已经存在，直接覆盖，StandardOpenOption.CREATE_NEW,如果文件已经存在，就抛出异常。
         FileChannel outChannel = FileChannel.open(Paths.get("E:\\nio.zip"), StandardOpenOption.READ,
                 StandardOpenOption.WRITE, StandardOpenOption.CREATE);

         // 获取内存映射文件
         MappedByteBuffer inMappedByteBuffer = inChannel.map(MapMode.READ_ONLY, 0, inChannel.size());
         MappedByteBuffer outMappedByteBuffer = outChannel.map(MapMode.READ_WRITE, 0, inChannel.size());

         // 直接对数据进行读写
         byte[] bytes = new byte[inMappedByteBuffer.limit()];
           // 此时，如果数据读超出了一定返回会抛出异常。如果内存不足时，会抛出java.lang.OutOfMemoryError: Java heap space
         inMappedByteBuffer.get(bytes);
         outMappedByteBuffer.put(bytes);

         inChannel.close();
         outChannel.close();
         long end = System.currentTimeMillis();

         System.out.println((end - start));
     }

• transferTo&transferFrom将数据从源通道传输到其他 Channel 中(采用直接缓存区)

     public void testTransfer() throws IOException {
         FileChannel inChannel = FileChannel.open(Paths.get("D:\\nio.zip"), StandardOpenOption.READ);
         // 注意：StandardOpenOption.CREATE
         // 如果文件已经存在，直接覆盖，StandardOpenOption.CREATE_NEW,如果文件已经存在，就抛出异常。
         FileChannel outChannel = FileChannel.open(Paths.get("E:\\nio.zip"), StandardOpenOption.READ,
                 StandardOpenOption.WRITE, StandardOpenOption.CREATE);

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