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How to create a Java String from the contents of a file?
o
I've been using this idiom for some time now.
o
And it seems to be the most wide-spread, at least in the sites I've visited.
p
Does anyone have a better/different way to read a file into a string in Java?
o
CODESNIPPET_JAVA1 .
o
Can anyone explain me in a very simple way what's with the NIO?
o
Each time I read about itI get lost in the nth mention of channel :(.
n
do remember that it's not guaranteed that the line separator in the file isn't necessary the same as the system's line separator.
o
Could you please insert a proper try finally that closes the reader?
o
Someone might actually use this example and introduce a bug into his code.
n
Code above has a bug of adding extra new line char at the last line.It should be something like followingif(line reader.readLine() ) != null){ stringBuilder.append( line );}while (line reader.readLine() ) != null) { stringBuilder.append( ls ); stringBuilder.append( line );}.
n
Java 7 introduces byte[] Files.readAllBytes(file); To those, who suggest the 'one-line' Scanner solution: Don't yo need to close it?
o
Read text from a file Here's a compact, robust idiom for Java 7, wrapped up in a utility method: CODESNIPPET_JAVA1 .
p
Memory utilization This method can temporarily require memory several times the size of the file, because for a short time the raw file contents (a byte array), the decoded characters (a character buffer), and a copy of the character data (in the new CODETERM1 instance) all reside in memory at once.
n
It is safest to apply to files that you know to be small relative to the available memory.
n
For reading large files, you need a different design for your program, one that reads a chunk of text from a stream, processes it, and then moves on to the next, reusing the same fixed-sized memory block.
n
Here, "large" depends on the computer specs.
o
Nowadays, this threshold might be many gigabytes of RAM.
o
Character encoding One thing that is missing from the sample in the original post is the character encoding.
o
There are some special cases where the platform default is what you want, but they are rare, and you should be able justify your choice.
p
The URL_http://docs.oracle.com/javase/7/docs/api/java/nio/charset/StandardChar sets.html [CODETERM2] class define some constants for the encodings required of all Java runtimes: CODESNIPPET_JAVA2 .
o
The platform default is available from URL_http://docs.oracle.com/javase/7/doc s/api/java/nio/charset/Charset.html#defaultCharset%28%29 [the-CODETERM3-class] itself: CODESNIPPET_JAVA3 .
o
Note: This answer largely replaces my Java 6 version.
o
The utility of Java 7 safely simplifies the code, and the old answer, which used a mapped byte buffer, prevented the file that was read from being deleted until the mapped buffer was garbage collected.
p
You can view the old version via the "edited" link on this answer.
o
Quite interesting.
p
What does the channel means.
o
I know that is to avoid block the "thread?
o
" They can be bidirectional ( or that's what I understood ) But, in more simple word, what are they?
o
Can you elaborate further?
o
In many ways, a ReadableByteChannel is like an InputStream, and WritableByteChannel is like an OutputStream.
o
Many concrete Channels implement both of these interfaces, so one object is bi-directional.
o
Some channels (SocketChannel) support non-blocking IO, but this isn't true of all channels.
o
Do you know the time- and memory-efficiencies of this idiom, or can at least estimate?
o
It's a beautiful idiom!.
p
Technically speaking, it's O(n) in time and space.
o
Qualitatively, due the immutability requirement of Strings, it's pretty hard on memory; temporarily there are two copies of the char data in memory, plus the room for the encoded bytes.
n
Assuming some single-byte encoding, it will (temporarily) require 5 bytes of memory for each character in the file.
n
Since the question asks specifically for a String, that's what I show, but if you can work with the CharBuffer returned by "decode", the memory requirement is much less.
n
Time- wise, I don't think you'll find anything faster in the core Java libs.
p
Possible typo?
o
NIO has a Charset (not CharSet) class called java.nio.charset.Charset.
o
Is this what CharSet should have been?
o
Note : after exercising a bit that code, I found out that you can't reliably delete the file right after reading it with this method, which may be a non issue in some case, but not mine.May it be in relation with this issue : URL_http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4715154 ?I finally went with the propos...
o
@Sbastien Nussbaumer: I also bumped on this problem.
o
Amazing that the bug has been marked "Will Not Fix".
n
This essentially means that FileChannel#map is, in general, unusable.
o
@Sbastien Nussbaumer: The bug has been deleted from the Oracle / Sun Bug Database: "This bug is not available.
n
" Google cached the site at URL_http://webcac he.googleusercontent.com/search?q=cache:bugs.sun.com/bugdatabase/view_bug.do%3 Fbug_id%3D4715154.
o
I tried reading an exe file using this method with default charset, append some data in it and again make it an exe, But it corrupted the exe, Any thoughts ?
o
@smilepleeeaz What you describe makes no sense.
o
EXE files are not text.
o
And they have a well-defined structure.
o
First you are destroying the content of the file by decoding it as text, then you are destroying the structure by appending text to it.
o
How could this fail to corrupt the EXE?
o
I don't really care about encoding, could I just use Charset.defaultCharset() instead of having an argument accepting it?
o
@yannbane If you are sure that the file will always be encoded with the platform's default character set, yes, you could do that.
o
If there's a mismatch, then you'll care about encoding.
o
Commons URL_http://commons.apache.org/proper/commons-io/apidocs/org/apache/com mons/io/FileUtils.html#readFileToString%28java.io.File%29 [CODETERM1] : CODESNIPPET_JAVA1 .
o
Reads the contents of a file into a String using the default encoding for the VM.
o
The file is always closed.
o
Parameters: CODETERM2 \- the file to read, must not be null Returns: the file contents, never null Throws: - CODETERM3 \- in case of an I/O error Since: Commons IO 1.3.1 by Oscar Reyes** I've found the code used ( indirectly ) by that class: URL_http://www.docjar.com/html/api/org/apache/commons/io/IOUtils.java.h...
o
Very similar to the one use by Ritche_W .
o
I don't find that method in the URL you provide.
o
It's in the class org.apache.commons.io.FileUtils.
o
I'm using FileUtils too, but I'm wondering what is better betwwen using FileUtils or the accepted nio answer?
o
@Guillaume: The biggest question is whether you're comfortable having a dependency on a 3rd party library.
o
If you do have Commons IO or [Guava]( URL_http://stackoverflow.com/a/2224519/56285 ) in your project, then use that (just for code simplicity; otherwise there likely won't be a noticeable difference).
o
From URL_http://weblogs.java.net/blog/pat/archive/2004/10/stupid_scanner.html [this-page] the one-line solution: CODESNIPPET_JAVA1 .
o
or CODESNIPPET_JAVA2 .
o
If you want to set the charset .
o
+1 I thought the selimeter should be \\\Z.
o
\\\A works because there is no "other beginning of file", so you are in fact read the last token...which is also the first.
p
Never tried with \\\Z.
o
Also note you can read anything that is Readable , like Files, InputStreams, channels...I sometimes use this code to read from the display window of eclipse, when I'm not sure if I'm reading one file or another...yes, classpath confuses me.
n
As the poster, I can say I really don't know if and when the file is properly close...I never write this one in production code, I use it only for tests or debug.
o
It has a limit of 1024 chars I think.
o
Scanner implements Closeable (it invokes close on the source) - so while elegant it shouldn't really be a one-liner.
p
The default size of the buffer is 1024, but Scanner will increase the size as necessary (see Scanner#makeSpace()).
o
This one fails for empty files with a java.util.NoSuchElementException.
o
If you're looking for an alternative that doesn't involve a 3rd party library (e.g.
o
commons IO), you can use the URL_http://java.sun.com/javase/6/docs/api/java/util/Scanner.html [Scanner] class CODESNIPPET_JAVA1 .
o
I think this is the best way.
p
Check out URL_http://java.sun.com/docs/books/tutorial/essential/io/scanning.html .
o
The Scanner constructor that accepts a String doesn't treat the string as the name of a file to read, but as the text to be scanned.
o
I make that mistake all the time.
n
:-/.
o
@Alan, good catch.
p
I edited Don's answer slightly to fix that (I hope).
o
fileContents.append(scanner.nextLine()).append(lineSeparator);.
o
Change the initialization statement to Scanner scanner new Scanner((Readable) new BufferedReader(new FileReader(file)));.
o
Otherwise you may only capture part of the file.
o
URL_http://code.google.com/p/guava-libraries/ [Guava] has a method similar to the one from Commons IOUtils that Willi aus Rohr mentioned: CODESNIPPET_JAVA1 .
o
by Oscar Reyes** This is the (simplified) underlying code on the cited library: CODESNIPPET_JAVA2 .
o
(by Jonik): The above doesn't match the source code of recent Guava versions.
o
For the current source, see the classes URL_http://code.google.com/p/guava-libraries/source/browse/guava/src/com/google/common/io/Files.java [Files] , URL_http://code.google.com/p/guava-libraries/source/browse/guava/src/com/google/common/io/CharStreams.java [CharStreams] , URL_http://code.google.com/p/guava-libraries/s...
o
This code has casting from long to int which could pop up some crazy behaviour with big files.
n
Has extra spaces and where do you close the inputstream?
o
@M-T-A: The stream closed, note the use of Closer in [CharSource]( URL_http://code.google.com/p/guava - libraries/source/browse/guava/src/com/google/common/io/CharSource.java).
o
The code in the answer isn't the actual, current Guava source.
o