Java NIO相关操作

1,buffer相关操作：

private static void bufferTest() {
IntBuffer buffer = IntBuffer.allocate(100);

System.out.println("buffer position:" + buffer.position() + ",buffer limit:" + buffer.limit()
+ ",buffer capacity:" + buffer.capacity());

buffer.put(10);
int[] data = { 10, 2, 3, 4, 5, 6, 5, 6, 7, 8 };
buffer.put(data);

System.out.println("buffer position:" + buffer.position() + ",buffer limit:" + buffer.limit()
+ ",buffer capacity:" + buffer.capacity());

buffer.flip();

System.out.println("buffer position:" + buffer.position() + ",buffer limit:" + buffer.limit()
+ ",buffer capacity:" + buffer.capacity());

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

2，channel写数据相关操作：

private static void channelWriteTest() throws Exception {

String[] data = { “hadoop”, “spark”, “scala”, “bigdata”, “大数据” };

File file = new File(“d://nio_channel.txt”);

FileOutputStream out = new FileOutputStream(file);

FileChannel channel = out.getChannel();

ByteBuffer buffer = ByteBuffer.allocate(1024);

for (String str : data) {

buffer.put((str + “\t”).getBytes(“UTF-8”));

}

buffer.flip();

channel.write(buffer);

channel.close();

out.close();

}

3，channel 读数据相关操作：

private static void chanelReadTest() throws Exception {
File file = new File("D://settings.xml");
FileInputStream inputStream = new FileInputStream(file);
FileChannel channel = inputStream.getChannel();
ByteBuffer buffer = ByteBuffer.allocate(1024 * 1024);
int tmp = 0;
while ((tmp = channel.read(buffer)) != -1) {
buffer.flip();
channel.read(buffer);
Charset charset = null;
CharsetDecoder decoder = null;
CharBuffer charBuffer = null;
charset = Charset.forName("UTF-8");
decoder = charset.newDecoder();
// charBuffer = decoder.decode(buffer);//用这个的话，只能输出来一次结果，第二次显示为空
charBuffer = decoder.decode(buffer.asReadOnlyBuffer());
System.out.println(charBuffer.toString());
buffer.clear();
}
inputStream.close();
}

4，内存映射的方式读取文件：

private static void chanelMapTest() throws Exception {
File file = new File("D://settings.xml");
FileInputStream inputStream = new FileInputStream(file);
FileChannel fileChannel  = inputStream.getChannel();

MappedByteBuffer buffer = fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, file.length());//将文件映射到内存

byte[] data = new byte[(int)file.length()];
int index = 0;
while(buffer.hasRemaining()){
data[index++] = buffer.get();
}
System.out.println(new String(data,"UTF-8"));
inputStream.close();
}

总结：

java nio 读写数据基于buffer,channel,来实现，channel是双向的，既可以把数据读入缓存区，也可以把缓存区的数据写入channel。

java读取文件的几种方式比较：

1，RundomAccessFile,读取文件较慢，能随机访问文件内容

2，传统java io操作，inputstream 读取文件较慢，效率不高

3，BufferderReader,包装读取文件较快

4，MappedByteBuffer 通过内存映射的方式读取文件最快，最为方便，如上第4中方式。