Java - Read file and split into multiple files
I have a file which I would like to read in Java and split this file into n (user input) output files. Here is how I read the file:
int n = 4;
BufferedReader br = new BufferedReader(new FileReader("file.csv"));
try {
String line = br.readLine();
while (line != null) {
line = br.readLine();
}
} finally {
br.close();
}
How do I split the file - file.csv into n files?
Note - Since the number of entries in the file are of the order of 100k, I can't store the file content into an array and then split it and save into multiple files.
Since one file can be very large, each split file could be large as well.
Example:
Source File Size: 5GB
Num Splits: 5: Destination
File Size: 1GB each (5 files)
There is no way to read this large split chunk in one go, even if we have such a memory. Basically for each split we can read a fix size byte-array which we know should be feasible in terms of performance as well memory.
NumSplits: 10 MaxReadBytes: 8KB
public static void main(String[] args) throws Exception
{
RandomAccessFile raf = new RandomAccessFile("test.csv", "r");
long numSplits = 10; //from user input, extract it from args
long sourceSize = raf.length();
long bytesPerSplit = sourceSize/numSplits ;
long remainingBytes = sourceSize % numSplits;
int maxReadBufferSize = 8 * 1024; //8KB
for(int destIx=1; destIx <= numSplits; destIx++) {
BufferedOutputStream bw = new BufferedOutputStream(new FileOutputStream("split."+destIx));
if(bytesPerSplit > maxReadBufferSize) {
long numReads = bytesPerSplit/maxReadBufferSize;
long numRemainingRead = bytesPerSplit % maxReadBufferSize;
for(int i=0; i<numReads; i++) {
readWrite(raf, bw, maxReadBufferSize);
}
if(numRemainingRead > 0) {
readWrite(raf, bw, numRemainingRead);
}
}else {
readWrite(raf, bw, bytesPerSplit);
}
bw.close();
}
if(remainingBytes > 0) {
BufferedOutputStream bw = new BufferedOutputStream(new FileOutputStream("split."+(numSplits+1)));
readWrite(raf, bw, remainingBytes);
bw.close();
}
raf.close();
}
static void readWrite(RandomAccessFile raf, BufferedOutputStream bw, long numBytes) throws IOException {
byte[] buf = new byte[(int) numBytes];
int val = raf.read(buf);
if(val != -1) {
bw.write(buf);
}
}
import java.io.*;
import java.util.Scanner;
public class split {
public static void main(String args[])
{
try{
// Reading file and getting no. of files to be generated
String inputfile = "C:/test.txt"; // Source File Name.
double nol = 2000.0; // No. of lines to be split and saved in each output file.
File file = new File(inputfile);
Scanner scanner = new Scanner(file);
int count = 0;
while (scanner.hasNextLine())
{
scanner.nextLine();
count++;
}
System.out.println("Lines in the file: " + count); // Displays no. of lines in the input file.
double temp = (count/nol);
int temp1=(int)temp;
int nof=0;
if(temp1==temp)
{
nof=temp1;
}
else
{
nof=temp1+1;
}
System.out.println("No. of files to be generated :"+nof); // Displays no. of files to be generated.
//---------------------------------------------------------------------------------------------------------
// Actual splitting of file into smaller files
FileInputStream fstream = new FileInputStream(inputfile); DataInputStream in = new DataInputStream(fstream);
BufferedReader br = new BufferedReader(new InputStreamReader(in)); String strLine;
for (int j=1;j<=nof;j++)
{
FileWriter fstream1 = new FileWriter("C:/New Folder/File"+j+".txt"); // Destination File Location
BufferedWriter out = new BufferedWriter(fstream1);
for (int i=1;i<=nol;i++)
{
strLine = br.readLine();
if (strLine!= null)
{
out.write(strLine);
if(i!=nol)
{
out.newLine();
}
}
}
out.close();
}
in.close();
}catch (Exception e)
{
System.err.println("Error: " + e.getMessage());
}
}
}
Though its a old question but for reference I am listing out the code which I used to split large files to any sizes and it works with any Java versions above 1.4 .
Sample Split and Join blocks were like below:
public void join(String FilePath) {
long leninfile = 0, leng = 0;
int count = 1, data = 0;
try {
File filename = new File(FilePath);
//RandomAccessFile outfile = new RandomAccessFile(filename,"rw");
OutputStream outfile = new BufferedOutputStream(new FileOutputStream(filename));
while (true) {
filename = new File(FilePath + count + ".sp");
if (filename.exists()) {
//RandomAccessFile infile = new RandomAccessFile(filename,"r");
InputStream infile = new BufferedInputStream(new FileInputStream(filename));
data = infile.read();
while (data != -1) {
outfile.write(data);
data = infile.read();
}
leng++;
infile.close();
count++;
} else {
break;
}
}
outfile.close();
} catch (Exception e) {
e.printStackTrace();
}
}
public void split(String FilePath, long splitlen) {
long leninfile = 0, leng = 0;
int count = 1, data;
try {
File filename = new File(FilePath);
//RandomAccessFile infile = new RandomAccessFile(filename, "r");
InputStream infile = new BufferedInputStream(new FileInputStream(filename));
data = infile.read();
while (data != -1) {
filename = new File(FilePath + count + ".sp");
//RandomAccessFile outfile = new RandomAccessFile(filename, "rw");
OutputStream outfile = new BufferedOutputStream(new FileOutputStream(filename));
while (data != -1 && leng < splitlen) {
outfile.write(data);
leng++;
data = infile.read();
}
leninfile += leng;
leng = 0;
outfile.close();
count++;
}
} catch (Exception e) {
e.printStackTrace();
}
}
Complete java code available here in File Split in Java Program link.
a clean solution to edit.
this solution involves loading the entire file into memory.
set all line of a file in List<String> rowsOfFile;
edit maxSizeFile to choice max size of a single file splitted
public void splitFile(File fileToSplit) throws IOException {
long maxSizeFile = 10000000 // 10mb
StringBuilder buffer = new StringBuilder((int) maxSizeFile);
int sizeOfRows = 0;
int recurrence = 0;
String fileName;
List<String> rowsOfFile;
rowsOfFile = Files.readAllLines(fileToSplit.toPath(), Charset.defaultCharset());
for (String row : rowsOfFile) {
buffer.append(row);
numOfRow++;
sizeOfRows += row.getBytes(StandardCharsets.UTF_8).length;
if (sizeOfRows >= maxSizeFile) {
fileName = generateFileName(recurrence);
File newFile = new File(fileName);
try (PrintWriter writer = new PrintWriter(newFile)) {
writer.println(buffer.toString());
}
recurrence++;
sizeOfRows = 0;
buffer = new StringBuilder();
}
}
// last rows
if (sizeOfRows > 0) {
fileName = generateFileName(recurrence);
File newFile = createFile(fileName);
try (PrintWriter writer = new PrintWriter(newFile)) {
writer.println(buffer.toString());
}
}
Files.delete(fileToSplit.toPath());
}
method to generate Name of file:
public String generateFileName(int numFile) {
String extension = ".txt";
return "myFile" + numFile + extension;
}