如果您正在阅读固定长度的文件,Neil Coffey 的解决方案非常好。但是对于具有可变长度的文件(数据不断进入),通过 InputStreamReader 直接在 FileInputStream 或 FileChannel 输入流上使用 BufferedReader 会出现一些问题。对于前考虑的情况
1)您想从某个偏移量读取数据到当前文件长度。因此,您在 FileInputStream/FileChannel(通过 InputStreamReader)上使用 BR 并使用其 readLine 方法。但是,当您忙于读取数据时,假设添加了一些数据,这导致 BF 的 readLine 读取的数据比您预期的要多(之前的文件长度)
2)您完成了 readLine 的内容,但是当您尝试读取当前文件长度/通道位置时,突然添加了一些数据,这导致当前文件长度/通道位置增加,但您已经读取的数据少于此。
在上述两种情况下,很难知道您已读取的实际数据(您不能只使用使用 readLine 读取的数据长度,因为它会跳过一些字符,如回车)
所以最好以缓冲字节读取数据并使用 BufferedReader 包装器。我写了一些这样的方法
/** Read data from offset to length bytes in RandomAccessFile using BufferedReader
* @param offset
* @param length
* @param accessFile
* @throws IOException
*/
public static void readBufferedLines(long offset, long length, RandomAccessFile accessFile) throws IOException{
if(accessFile == null) return;
int bufferSize = BYTE_BUFFER_SIZE;// constant say 4096
if(offset < length && offset >= 0){
int index = 1;
long curPosition = offset;
/*
* iterate (length-from)/BYTE_BUFFER_SIZE times to read into buffer no matter where new line occurs
*/
while((curPosition + (index * BYTE_BUFFER_SIZE)) < length){
accessFile.seek(offset); // seek to last parsed data rather than last data read in to buffer
byte[] buf = new byte[bufferSize];
int read = accessFile.read(buf, 0, bufferSize);
index++;// Increment whether or not read successful
if(read > 0){
int lastnewLine = getLastLine(read,buf);
if(lastnewLine <= 0){ // no new line found in the buffer reset buffer size and continue
bufferSize = bufferSize+read;
continue;
}
else{
bufferSize = BYTE_BUFFER_SIZE;
}
readLine(buf, 0, lastnewLine); // read the lines from buffer and parse the line
offset = offset+lastnewLine; // update the last data read
}
}
// Read last chunk. The last chunk size in worst case is the total file when no newline occurs
if(offset < length){
accessFile.seek(offset);
byte[] buf = new byte[(int) (length-offset)];
int read = accessFile.read(buf, 0, buf.length);
if(read > 0){
readLine(buf, 0, read);
offset = offset+read; // update the last data read
}
}
}
}
private static void readLine(byte[] buf, int from , int lastnewLine) throws IOException{
String readLine = "";
BufferedReader reader = new BufferedReader(new InputStreamReader(new ByteArrayInputStream(buf,from,lastnewLine) ));
while( (readLine = reader.readLine()) != null){
//do something with readLine
System.out.println(readLine);
}
reader.close();
}
private static int getLastLine(int read, byte[] buf) {
if(buf == null ) return -1;
if(read > buf.length) read = buf.length;
while( read > 0 && !(buf[read-1] == '\n' || buf[read-1] == '\r')) read--;
return read;
}
public static void main(String[] args) throws IOException {
RandomAccessFile accessFile = new RandomAccessFile("C:/sri/test.log", "r");
readBufferedLines(0, accessFile.length(), accessFile);
accessFile.close();
}