java中讀取文件的方式有哪幾種

讀取文件有多種方式，基于傳統(tǒng)的輸入流方式或基于nio的Buffer緩沖對(duì)象和管道讀取方式甚至非常快速的內(nèi)存映射讀取文件。

java中四種讀取文件方式：

1、RandomAccessFile：隨機(jī)讀取，比較慢優(yōu)點(diǎn)就是該類(lèi)可讀可寫(xiě)可操作文件指針

2、FileInputStream：io普通輸入流方式，速度效率一般

3、Buffer緩沖讀?。夯趎io Buffer和FileChannel讀取，速度較快

4、內(nèi)存映射讀取：基于MappedByteBuffer,速度最快

RandomAccessFile讀取

//RandomAccessFile類(lèi)的核心在于其既能讀又能寫(xiě)

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讀取

//使用FileInputStream文件輸入流,比較中規(guī)中矩的一種方式,傳統(tǒng)阻塞IO操作。

public void testFielInputStreamTest() throws Exception {

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

    // 使用輸入流讀取文件,以下代碼塊幾乎就是模板代碼
    byte[] bytes = new byte[1024];
    int len = 0;
    while ((len = inputStream.read(bytes)) != -1) {// 如果有數(shù)據(jù)就一直讀寫(xiě),否則就退出循環(huán)體,關(guān)閉流資源。
        System.out.println(new String(bytes, 0, len, "gbk"));
    }
    inputStream.close();
}

Buffer緩沖對(duì)象讀取

// nio 讀取

public void testBufferChannel() throws Exception {

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

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

    // 以下代碼也幾乎是Buffer和Channle的標(biāo)準(zhǔn)讀寫(xiě)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();
}

內(nèi)存映射讀取

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();

}

//基于內(nèi)存映射這種方式，這么寫(xiě)好像有問(wèn)題。


MappedByteBuffer和RandomAcessFile這兩個(gè)類(lèi)要單獨(dú)重點(diǎn)研究一下。

//TODO 大文件讀取

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