JAVA IO - 高效讀取大文件的後幾行

File file = new File("DemoRandomAccessFile.out");
RandomAccessFile raf = new RandomAccessFile(file, "r");

// Seek to the end of file
raf.seek(file.length() - n);
// Read it out.
raf.read(yourbyteArray, 0, n);

Below are two functions, one that returns the last non-blank line of a file without loading or stepping through the entire file, and the other that returns the last N lines of the file without stepping through the entire file:

What tail does is zoom straight to the last character of the file, then steps backward, character by character, recording what it sees until it finds a line break. Once it finds a line break, it breaks out of the loop. Reverses what was recorded and throws it into a string and returns. 0xA is the new line and 0xD is the carriage return.

public String tail( File file ) {
    RandomAccessFile fileHandler = null;
    try {
        fileHandler = new RandomAccessFile( file, "r" );
        long fileLength = fileHandler.length() - 1;
        StringBuilder sb = new StringBuilder();

        for(long filePointer = fileLength; filePointer != -1; filePointer--){
            fileHandler.seek( filePointer );
            int readByte = fileHandler.readByte();

            if( readByte == 0xA ) {
                if( filePointer == fileLength ) {
                    continue;
                } else {
                    break;
                }
            } else if( readByte == 0xD ) {
                if( filePointer == fileLength - 1 ) {
                    continue;
                } else {
                    break;
                }
            }

            sb.append( ( char ) readByte );
        }

        String lastLine = sb.reverse().toString();
        return lastLine;
    } catch( java.io.FileNotFoundException e ) {
        e.printStackTrace();
        return null;
    } catch( java.io.IOException e ) {
        e.printStackTrace();
        return null;
    } finally {
        if (fileHandler != null )
            try {
                fileHandler.close();
            } catch (IOException e) {
                /* ignore */
            }
    }
}

But you probably don't want the last line, you want the last N lines, so use this instead:

public String tail2( File file, int lines) {
    java.io.RandomAccessFile fileHandler = null;
    try {
        fileHandler = 
            new java.io.RandomAccessFile( file, "r" );
        long fileLength = fileHandler.length() - 1;
        StringBuilder sb = new StringBuilder();
        int line = 0;

        for(long filePointer = fileLength; filePointer != -1; filePointer--){
            fileHandler.seek( filePointer );
            int readByte = fileHandler.readByte();

            if( readByte == 0xA ) {
                if (line == lines) {
                    if (filePointer == fileLength) {
                        continue;
                    } else {
                        break;
                    }
                }
            } else if( readByte == 0xD ) {
                line = line + 1;
                if (line == lines) {
                    if (filePointer == fileLength - 1) {
                        continue;
                    } else {
                        break;
                    }
                }
            }
            sb.append( ( char ) readByte );
        }

        sb.deleteCharAt(sb.length()-1);
        String lastLine = sb.reverse().toString();
        return lastLine;
    } catch( java.io.FileNotFoundException e ) {
        e.printStackTrace();
        return null;
    } catch( java.io.IOException e ) {
        e.printStackTrace();
        return null;
    }
    finally {
        if (fileHandler != null )
            try {
                fileHandler.close();
            } catch (IOException e) {
                /* ignore */
            }
    }
}

Another Example

public static void main(String[] args) throws Exception {
    int n = 3;
    List<String> lines = new ArrayList<>();
    try (RandomAccessFile f = new RandomAccessFile("test", "r")) {
        ByteArrayOutputStream bout = new ByteArrayOutputStream();
        for (long length = f.length(), p = length - 1; p > 0 && lines.size() < n; p--) {
            f.seek(p);
            int b = f.read();
            if (b == 10) {
                if (p < length - 1) {
                    lines.add(0, getLine(bout));
                    bout.reset();
                }
            } else if (b != 13) {
                bout.write(b);
            }
        }
    }
    System.out.println(lines);
}

static String getLine(ByteArrayOutputStream bout) {
    byte[] a = bout.toByteArray();
    // reverse bytes
    for (int i = 0, j = a.length - 1; j > i; i++, j--) {
        byte tmp = a[j];
        a[j] = a[i];
        a[i] = tmp;
    }
    return new String(a);
}