java中如何读取文件？

读取文件有多种方式，基于传统的输入流方式或基于nio的Buffer缓冲对象和管道读取方式甚至非常快速的内存映射读取文件。

java中四种读取文件方式：（推荐：java视频教程）

1、RandomAccessFile：随机读取，比较慢优点就是该类可读可写可操作文件指针

2、FileInputStream：io普通输入流方式，速度效率一般

3、Buffer缓冲读取：基于nio Buffer和FileChannel读取，速度较快

4、内存映射读取：基于MappedByteBuffer,速度最快

RandomAccessFile读取

//RandomAccessFile类的核心在于其既能读又能写

public void useRandomAccessFileTest() throws Exception {

    RandomAccessFile randomAccessFile = new RandomAccessFile(new File("e:/nio/test.txt"), "r");

    byte[] bytes = new byte[1024];
    int len = 0;
    while ((len = randomAccessFile.read(bytes)) != -1) {
        System.out.println(new String(bytes, 0, len, "gbk"));
    }

    randomAccessFile.close();
}

FielInputStream读取

//使用FileInputStream文件输入流,比较中规中矩的一种方式,传统阻塞IO操作。

public void testFielInputStreamTest() throws Exception {

    FileInputStream inputStream = new FileInputStream(new File("e:/nio/test.txt"));

    // 使用输入流读取文件,以下代码块几乎就是模板代码
    byte[] bytes = new byte[1024];
    int len = 0;
    while ((len = inputStream.read(bytes)) != -1) {// 如果有数据就一直读写,否则就退出循环体,关闭流资源。
        System.out.println(new String(bytes, 0, len, "gbk"));
    }
    inputStream.close();
}

Buffer缓冲对象读取

// nio 读取

public void testBufferChannel() throws Exception {

    FileInputStream inputStream = new FileInputStream(new File("e:/nio/test.txt"));

    FileChannel fileChannel = inputStream.getChannel();
    ByteBuffer buffer = ByteBuffer.allocate(1024);

    // 以下代码也几乎是Buffer和Channle的标准读写操作。
    while (true) {
        buffer.clear();
        int result = fileChannel.read(buffer);
        buffer.flip();
        if (result == -1) {
            break;
        }
        System.out.println(new String(buffer.array(), 0, result, "gbk"));
    }
    inputStream.close();
}

内存映射读取

public void testmappedByteBuffer() throws Exception {

    FileInputStream inputStream = new FileInputStream(new File("e:/nio/test.txt"));
    FileOutputStream outputStream = new FileOutputStream(new File("e:/nio/testcopy.txt"),true);

    FileChannel inChannel = inputStream.getChannel();
    FileChannel outChannel = outputStream.getChannel();

    System.out.println(inChannel.size());
    MappedByteBuffer mappedByteBuffer = inChannel.map(MapMode.READ_ONLY, 0, inChannel.size());

    System.out.println(mappedByteBuffer.limit());
    System.out.println(mappedByteBuffer.position());

    mappedByteBuffer.flip();
    outChannel.write(mappedByteBuffer);

    outChannel.close();
    inChannel.close();
    outputStream.close();
    inputStream.close();

}

//基于内存映射这种方式，这么写好像有问题。


MappedByteBuffer和RandomAcessFile这两个类要单独重点研究一下。

//TODO 大文件读取

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