I'm using the following 2 pieces of codes to read a large file.
This using a FileReader:
File file = new File("/Users/Desktop/shakes.txt");
FileReader reader = new FileReader(file);
int ch;
long start = System.currentTimeMillis();
while ((ch = reader.read()) != -1) {
System.out.print((char) ch);
}
long end = System.currentTimeMillis();
And the following using a BufferedReader:
File file = new File("/Users/Desktop/shakes.txt");
BufferedReader reader = new BufferedReader(new FileReader(file));
int ch;
long start = System.currentTimeMillis();
while ((ch = reader.read()) != -1) {
System.out.print((char) ch);
}
long end = System.currentTimeMillis();
Going by the documentation for BufferedReader:
It is therefore advisable to wrap a BufferedReader around any Reader whose read() operations may be costly, such as FileReaders and InputStreamReaders. Without buffering, each invocation of read() or readLine() could cause bytes to be read from the file, converted into characters, and then returned, which can be very inefficient.
Given this documentation and the default buffer size of 8192 of the BufferedReader class, shouldn't the overall time for reading the file with BufferedReader be quicker? Currently, both pieces of code run in ~3000ms on my machine. However, if I use 'readLine' in the BufferedReader, the performance substantially improves (~200ms).
Thoughts on something that I'm missing? Is it not expected that even with the 'read()' method, BufferedReader should give a better performance than reading from FileReader?
Using BufferedReader is indeed faster than using just FileReader.
I executed your code on my machine, with the following text file https://norvig.com/big.txt (6MB).
The initial result shows roughly the same time. About 17 seconds each.
However, this is because System.out.print() is a bottleneck (within the loop). Without print, the result is 4 times faster with BufferedReader. 200ms vs 50ms. (Compare it to 17s before)
In other words, don't use System.out.print() when benchmarking.
Example
An improved benchmark could look like this using StringBuilder.
File file = new File("/Users/Desktop/shakes.txt");
FileReader reader = new FileReader(file);
int ch;
StringBuilder sb = new StringBuilder();
long start = System.currentTimeMillis();
while ((ch = reader.read()) != -1) {
//System.out.print((char) ch);
sb.append((char) ch);
}
long end = System.currentTimeMillis();
System.out.println(sb);
The above code provides the same output but performs much faster. It will accurately show the difference in speed when using a BufferedReader.
Thoughts on something that I'm missing?
It should be faster to read a file a character at a time from a BufferedReader than a FileReader. (By orders of magnitude!) So I suspect that problem is in your benchmarks.
Your benchmark is measuring both reading the file, and writing it to standard output. So basically, your performance figures will be distorted by the overheads of writing the file. And if your output is being written to a "console", then those overheads include the overheads of painting characters to the screen ... and scrolling.
Your benchmark takes no account of vm startup overheads.
Your benchmark doesn't (obviously) take the effect of file caching. (The first time a file is read, it will be read from disc. If you read it again soon afterwards, you may be reading from a copy of the file cached in memory by the operating system. That will be faster.)
Related
I'm using the following method to read a file into a JTextArea:
public void readFile(File file) throws java.io.FileNotFoundException,
java.io.IOException {
if(file == null) return;
jTextArea1.setText("");
try(BufferedReader reader = new BufferedReader(new FileReader(file))){
String line = "";
while((line=reader.readLine())!=null){
jTextArea.append(line + "\n");
}
}
}
It works OK with a normal-sized file (a few hundred kilobytes), but when I tested a 30000-line file of 42 MB that Notepad can open in about 5 seconds, my file reader took forever. I couldn't wait for it to finish; I had waited for about 15-20 minutes and it was still working consuming 30% of my CPU usage.
Could you please give me a solution for this? I'm handling with text files only, not binary files, and all I know is using BufferedReader is the best.
The problem is likely not in the file reading but the processing. Repeated calls to append are likely to be very inefficient with large datasets.
Consider using a StringBuilder. This class is designed for quickly creating long strings from parts (on a single thread; see StringBuffer for a multi-threaded counterpart).
if(file == null) return;
StringBuilder sb = new StringBuilder();
jTextArea1.setText("");
try(BufferedReader reader = new BufferedReader(new FileReader(file))){
String line = "";
while((line==reader.readLine())!=null){
sb.append(line);
sb.append('\n');
}
jTextArea1.setText(sb.toString());
}
As suggested in the comments, you may wish to perform this action in a new thread so the user doesn't think your program has frozen.
Can I share an InputStream or OutputStream?
For example, let's say I first have:
DataInputStream incoming = new DataInputStream(socket.getInputStream()));
...incoming being an object variable. Later on I temporarily do:
BufferedReader dataReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
I understand that the stream is concrete and reading from it will consume its input, no matter from where it's done... But after doing the above, can I still access both incoming and dataReader simultaneously or is the InputStream just connected to ONE object and therefore incoming loses its input once I declare dataReader? I understand that if I close the dataReader then I will close the socket as well and I will refrain from this but I'm wondering whether I need to "reclaim" the InputStream somehow to incoming after having "transferred" it to dataReader? Do I have to do:
incoming = new DataInputStream(socket.getInputStream());
again after this whole operation?
You are using a teaspoon and a shovel to move dirt from a hole.
I understand that the stream is concrete and reading from it will
consume its input, no matter from where it's done
Correct. The teaspoon and shovel both move dirt from the hole. If you are removing dirt asynchronously (i.e. concurrently) you could get into fights about who has what dirt - so use concurrent construct to provide mutually exclusive access. If access is not concurrent, in other words ...
1) move one or more teaspoons of dirt from the hole
2) move one or more shovels of dirt from the hole
3) move one or more teaspoons of dirt from the hole
...
No problem. Teaspoon and shovel both remove dirt. But once dirt gets removed, it's removed, they do not get the same dirt. Hope this helps. Let's start shovelling, I'll use the teaspoon. :)
As fast-reflexes found, be very careful about sharing streams, particularly buffered readers since they can gobble up a lot more bytes off the stream than they need, so when you go back to your other input stream (or reader) it may look like a whole bunch of bytes have been skipped.
Proof you can read from same input stream:
import java.io.*;
public class w {
public static void main(String[] args) throws Exception {
InputStream input = new FileInputStream("myfile.txt");
DataInputStream b = new DataInputStream(input);
int data, count = 0;
// read first 20 characters with DataInputStream
while ((data = b.read()) != -1 && ++count < 20) {
System.out.print((char) data);
}
// if prematurely interrupted because of count
// then spit out last char grabbed
if (data != -1)
System.out.print((char) data);
// read remainder of file with underlying InputStream
while ((data = input.read()) != -1) {
System.out.print((char) data);
}
b.close();
}
}
Input file:
hello OP
this is
a file
with some basic text
to see how this
works when moving dirt
from a hole with a teaspoon
and a shovel
Output:
hello OP
this is
a file
with some basic text
to see how this
works when moving dirt
from a hole with a teaspoon
and a shovel
Proof to show BufferedReader is NOT gauranteed to work as it gobbles up lots of chars from the stream:
import java.io.*;
public class w {
public static void main(String[] args) throws Exception {
InputStream input = new FileInputStream("myfile.txt");
BufferedReader b = new BufferedReader(new InputStreamReader(input));
// read three lines with BufferedReader
String line;
for (int i = 0; (line = b.readLine()) != null && i < 3; ++i) {
System.out.println(line);
}
// read remainder of file with underlying InputStream
int data;
while ((data = input.read()) != -1) {
System.out.print((char) data);
}
b.close();
}
}
Input file (same as above):
hello OP
this is
a file
with some basic text
to see how this
works when moving dirt
from a hole with a teaspoon
and a shovel
Output:
hello OP
this is
a file
This will be disastrous. Both streams will have corrupted data. How could Java possibly know which data to send to which Stream?
If you need to do two different things with the same data, you're better off storing it somewhere (possibly copying it into two Queue<String>), and then reading it that way.
Ok, I solved this myself.. interesting links:
http://www.coderanch.com/t/276168//java/InputStream-multiple-Readers
Multiple readers for InputStream in Java
Basically... the InputStream can be connected to multiple objects reading from it and consuming it. However, a BufferedReader reads ahead, so when involving one of those, it might be a good idea to implement some sort of signal when you're switching from for example a BufferedReader to a DataInputStream (that is you want to use the DataInputStream to process the InputStream all of a sudden instead of the BufferedReader). Therefore I stop sending data to the InputStream once I know that all data has been sent that is for the BufferedReader to handle. After this, I wait for the other part to process what it should with the BufferedReader. It then sends a signal to show that it's ready for new input. The sending part should be blocking until it receives the signal input and then it can start sending data again. If I don't use the BufferedReader after this point, it won't have a chance to buffer up all the input and "steal" it from the DataInputStream and everything works very well :) But be careful, one read operation from the BufferedReader and you will be back in the same situation... Good to know!
I have a large text file but doesn't have any line break. It just contains a long String (1 huge line of String with all ASCII characters), but so far anything works just fine as I can be able to read the whole line into memory in Java, but i am wondering if there could be a memory leak issue as the file becomes so big like 5GB+ and the program can't read the whole file into memory at once, so in that case what will be the best way to read such file ? Can we break the huge line into 2 parts or even multiple chunks ?
Here's how I read the file
BufferedReader buf = new BufferedReader(new FileReader("input.txt"));
String line;
while((line = buf.readLine()) != null){
}
A single String can be only 2 billion characters long and will use 2 byte per character, so if you could read a 5 GB line it would use 10 GB of memory.
I suggest you read the text in blocks.
Reader reader = new FileReader("input.txt");
try {
char[] chars = new char[8192];
for(int len; (len = reader.read(chars)) > 0;) {
// process chars.
}
} finally {
reader.close();
}
This will use about 16 KB regardless of the size of the file.
There won't be any kind of memory-leak, as the JVM has its own garbage collector. However you will probably run out of heap space.
In cases like this, it is always best to import and process the stream in manageable pieces. Read in 64MB or so and repeat.
You also might find it useful to add the -Xmx parameter to your java call, in order to increase the maximum heap space available within the JVM.
its better to read the file in chunks and then concatenate the chunks or do whatever you want wit it, because if it is a big file you are reading you will get heap space issues
an easy way to do it like below
InputStream is;
OutputStream os;
byte buffer[] = new byte[1024];
int read;
while((read = is.read(buffer)) != -1)
{
// do whatever you need with the buffer
}
In addition to the idea of reading in chunks, you could also look at memory mapping areas of the file using java.nio.MappedByteBuffer. You would still be limited to a maximum buffer size of Integer.MAX_VALUE. This may be better than explicitly reading chunks if you will be making scattered accesses within a chunk.
To read chunks from file or write same to some file this could be used:
{
in = new FileReader("input.txt");
out = new FileWriter("output.txt");
char[] buffer = new char[1024];
int l = 0;
while ( (l = in.read(buffer)) > 0 ) {
out.write(buffer, 0, l);
}
You won't run into any memory leak issues, but possible heap space issues. To avoid heap issues, use a buffer.
It all depends on how you are currently reading the line. It is possible to avoid all heap issues by using a buffer.
public void readLongString(String superlongString, int size, BufferedReader in){
char[] buffer = new char[size];
for(int i=0;i<superlongString.length;i+=size;){
in.read(buffer, i, size)
//do stuff
}
}
I've never had close experiences with Java IO API before and I'm really frustrated now. I find it hard to believe how strange and complex it is and how hard it could be to do a simple task.
My task: I have 2 positions (starting byte, ending byte), pos1 and pos2. I need to read lines between these two bytes (including the starting one, not including the ending one) and use them as UTF8 String objects.
For example, in most script languages it would be a very simple 1-2-3-liner like that (in Ruby, but it will be essentially the same for Python, Perl, etc):
f = File.open("file.txt").seek(pos1)
while f.pos < pos2 {
s = f.readline
# do something with "s" here
}
It quickly comes hell with Java IO APIs ;) In fact, I see two ways to read lines (ending with \n) from regular local files:
RandomAccessFile has getFilePointer() and seek(long pos), but it's readLine() reads non-UTF8 strings (and even not byte arrays), but very strange strings with broken encoding, and it has no buffering (which probably means that every read*() call would be translated into single undelying OS read() => fairly slow).
BufferedReader has great readLine() method, and it can even do some seeking with skip(long n), but it has no way to determine even number of bytes that has been already read, not mentioning the current position in a file.
I've tried to use something like:
FileInputStream fis = new FileInputStream(fileName);
FileChannel fc = fis.getChannel();
BufferedReader br = new BufferedReader(
new InputStreamReader(
fis,
CHARSET_UTF8
)
);
... and then using fc.position() to get current file reading position and fc.position(newPosition) to set one, but it doesn't seem to work in my case: looks like it returns position of a buffer pre-filling done by BufferedReader, or something like that - these counters seem to be rounded up in 16K increments.
Do I really have to implement it all by myself, i.e. a file readering interface which would:
allow me to get/set position in a file
buffer file reading operations
allow reading UTF8 strings (or at least allow operations like "read everything till the next \n")
Is there a quicker way than implementing it all myself? Am I overseeing something?
import org.apache.commons.io.input.BoundedInputStream
FileInputStream file = new FileInputStream(filename);
file.skip(pos1);
BufferedReader br = new BufferedReader(
new InputStreamReader(new BoundedInputStream(file,pos2-pos1))
);
If you didn't care about pos2, then you woundn't need Apache Commons IO.
I wrote this code to read utf-8 using randomaccessfiles
//File: CyclicBuffer.java
public class CyclicBuffer {
private static final int size = 3;
private FileChannel channel;
private ByteBuffer buffer = ByteBuffer.allocate(size);
public CyclicBuffer(FileChannel channel) {
this.channel = channel;
}
private int read() throws IOException {
return channel.read(buffer);
}
/**
* Returns the byte read
*
* #return byte read -1 - end of file reached
* #throws IOException
*/
public byte get() throws IOException {
if (buffer.hasRemaining()) {
return buffer.get();
} else {
buffer.clear();
int eof = read();
if (eof == -1) {
return (byte) eof;
}
buffer.flip();
return buffer.get();
}
}
}
//File: UTFRandomFileLineReader.java
public class UTFRandomFileLineReader {
private final Charset charset = Charset.forName("utf-8");
private CyclicBuffer buffer;
private ByteBuffer temp = ByteBuffer.allocate(4096);
private boolean eof = false;
public UTFRandomFileLineReader(FileChannel channel) {
this.buffer = new CyclicBuffer(channel);
}
public String readLine() throws IOException {
if (eof) {
return null;
}
byte x = 0;
temp.clear();
while ((byte) -1 != (x = (buffer.get())) && x != '\n') {
if (temp.position() == temp.capacity()) {
temp = addCapacity(temp);
}
temp.put(x);
}
if (x == -1) {
eof = true;
}
temp.flip();
if (temp.hasRemaining()) {
return charset.decode(temp).toString();
} else {
return null;
}
}
private ByteBuffer addCapacity(ByteBuffer temp) {
ByteBuffer t = ByteBuffer.allocate(temp.capacity() + 1024);
temp.flip();
t.put(temp);
return t;
}
public static void main(String[] args) throws IOException {
RandomAccessFile file = new RandomAccessFile("/Users/sachins/utf8.txt",
"r");
UTFRandomFileLineReader reader = new UTFRandomFileLineReader(file
.getChannel());
int i = 1;
while (true) {
String s = reader.readLine();
if (s == null)
break;
System.out.println("\n line " + i++);
s = s + "\n";
for (byte b : s.getBytes(Charset.forName("utf-8"))) {
System.out.printf("%x", b);
}
System.out.printf("\n");
}
}
}
For #Ken Bloom A very quick go at a Java 7 version. Note: I don't think this is the most efficient way, I'm still getting my head around NIO.2, Oracle has started their tutorial here
Also note that this isn't using Java 7's new ARM syntax (which takes care of the Exception handling for file based resources), it wasn't working in the latest openJDK build that I have. But if people want to see the syntax, let me know.
/*
* Paths uses the default file system, note no exception thrown at this stage if
* file is missing
*/
Path file = Paths.get("C:/Projects/timesheet.txt");
ByteBuffer readBuffer = ByteBuffer.allocate(readBufferSize);
FileChannel fc = null;
try
{
/*
* newByteChannel is a SeekableByteChannel - this is the fun new construct that
* supports asynch file based I/O, e.g. If you declared an AsynchronousFileChannel
* you could read and write to that channel simultaneously with multiple threads.
*/
fc = (FileChannel)file.newByteChannel(StandardOpenOption.READ);
fc.position(startPosition);
while (fc.read(readBuffer) != -1)
{
readBuffer.rewind();
System.out.println(Charset.forName(encoding).decode(readBuffer));
readBuffer.flip();
}
}
Start with a RandomAccessFile and use read or readFully to get a byte array between pos1 and pos2. Let's say that we've stored the data read in a variable named rawBytes.
Then create your BufferedReader using
new BufferedReader(new InputStreamReader(new ByteArrayInputStream(rawBytes)))
Then you can call readLine on the BufferedReader.
Caveat: this probably uses more memory than if you could make the BufferedReader seek to the right location itself, because it preloads everything into memory.
I think the confusion is caused by the UTF-8 encoding and the possibility of double byte characters.
UTF8 doesn't specify how many bytes are in a single character. I'm assuming from your post that you are using single byte characters. For example, 412 bytes would mean 411 characters. But if the string were using double byte characters, you would get the 206 character.
The original java.io package didn't deal well with this multi-byte confusion. So, they added more classes to deal specifically with strings. The package mixes two different types of file handlers (and they can be confusing until the nomenclature is sorted out). The stream classes provide for direct data I/O without any conversion. The reader classes convert files to strings with full support for multi-byte characters. That might help clarify part of the problem.
Since you state you are using UTF-8 characters, you want the reader classes. In this case, I suggest FileReader. The skip() method in FileReader allows you to pass by X characters and then start reading text. Alternatively, I prefer the overloaded read() method since it allows you to grab all the text at one time.
If you assume your "bytes" are individual characters, try something like this:
FileReader fr = new FileReader( new File("x.txt") );
char[] buffer = new char[ pos2 - pos ];
fr.read( buffer, pos, buffer.length );
...
I'm late to the party here, but I ran across this problem in my own project.
After much traversal of Javadocs and Stack Overflow, I think I found a simple solution.
After seeking to the appropriate place in your RandomAccessFile, which I am here calling raFile, do the following:
FileDescriptor fd = raFile.getFD();
FileReader fr = new FileReader(fd);
BufferedReader br = new BufferedReader(fr);
Then you should be able to call br.readLine() to your heart's content, which will be much faster than calling raFile.readLine().
The one thing I'm not sure about is whether UTF8 strings are handled correctly.
The java IO API is very flexible. Unfortunately sometimes the flexibility makes it verbose. The main idea here is that there are many streams, writers and readers that implement wrapper patter. For example BufferedInputStream wraps any other InputStream. The same is about output streams.
The difference between streams and readers/writers is that streams work with bytes while readers/writers work with characters.
Fortunately some streams, writers and readers have convenient constructors that simplify coding. If you want to read file you just have to say
InputStream in = new FileInputStream("/usr/home/me/myfile.txt");
if (in.markSupported()) {
in.skip(1024);
in.read();
}
It is not so complicated as you afraid.
Channels is something different. It is a part of so called "new IO" or nio. New IO is not blocked - it is its main advantage. You can search in internet for any "nio java tutorial" and read about it. But it is more complicated than regular IO and is not needed for most applications.
I currently use the following function to do a simple HTTP GET.
public static String download(String url) throws java.io.IOException {
java.io.InputStream s = null;
java.io.InputStreamReader r = null;
//java.io.BufferedReader b = null;
StringBuilder content = new StringBuilder();
try {
s = (java.io.InputStream)new URL(url).getContent();
r = new java.io.InputStreamReader(s);
//b = new java.io.BufferedReader(r);
char[] buffer = new char[4*1024];
int n = 0;
while (n >= 0) {
n = r.read(buffer, 0, buffer.length);
if (n > 0) {
content.append(buffer, 0, n);
}
}
}
finally {
//if (b != null) b.close();
if (r != null) r.close();
if (s != null) s.close();
}
return content.toString();
}
I see no reason to use the BufferedReader since I am just going to download everything in sequence. Am I right in thinking there is no use for the BufferedReader in this case?
In this case, I would do as you are doing (use a byte array for buffering and not one of the stream buffers).
There are exceptions, though. One place you see buffers (output this time) is in the servlet API. Data isn't written to the underlying stream until flush() is called, allowing you to buffer output but then dump the buffer if an error occurs and write an error page instead. You might buffer input if you needed to reset the stream for rereading using mark(int) and reset(). For example, maybe you'd inspect the file header before deciding on which content handler to pass the stream to.
Unrelated, but I think you should rewrite your stream handling. This pattern works best to avoid resource leaks:
InputStream stream = new FileInputStream("in");
try { //no operations between open stream and try block
//work
} finally { //do nothing but close this one stream in the finally
stream.close();
}
If you are opening multiple streams, nest try/finally blocks.
Another thing your code is doing is making the assumption that the returned content is encoded in your VM's default character set (though that might be adequate, depending on the use case).
You are correct, if you use BufferedReader for reading HTTP content and headers you will want InputStreamReader so you can read byte for byte.
BufferedReader in this scenario sometimes does weird things...escpecially when it comes to reading HTTP POST headers, sometimes you will be unable to read the POST data, if you use the InputStreamReader you can read the content length and read that many bytes...
Each invocation of one of an InputStreamReader's read() methods may cause one or more bytes to be read from the underlying byte-input stream. To enable the efficient conversion of bytes to characters, more bytes may be read ahead from the underlying stream than are necessary to satisfy the current read operation.
My gut tells me that since you're already performing buffering by using the byte array, it's redundant to use the BufferedReader.