How do I create a Java string from the contents of a file?
How do I create a Java string from the contents of a file?
Question
I've been using the idiom below for some time now. And it seems to be the most wide-spread, at least on the sites I've visited.
Is there a better/different way to read a file into a string in Java?
private String readFile(String file) throws IOException {
BufferedReader reader = new BufferedReader(new FileReader (file));
String line = null;
StringBuilder stringBuilder = new StringBuilder();
String ls = System.getProperty("line.separator");
try {
while((line = reader.readLine()) != null) {
stringBuilder.append(line);
stringBuilder.append(ls);
}
return stringBuilder.toString();
} finally {
reader.close();
}
}
Popular Answer
Read all text from a file
Java 11 added the readString() method to read small files as a String, preserving line terminators:
String content = Files.readString(path, StandardCharsets.US_ASCII);
For versions between Java 7 and 11, here's a compact, robust idiom, wrapped up in a utility method:
static String readFile(String path, Charset encoding)
throws IOException
{
byte[] encoded = Files.readAllBytes(Paths.get(path));
return new String(encoded, encoding);
}
Read lines of text from a file
Java 7 added a convenience method to read a file as lines of text, represented as a List<String>. This approach is "lossy" because the line separators are stripped from the end of each line.
List<String> lines = Files.readAllLines(Paths.get(path), encoding);
Java 8 added the Files.lines() method to produce a Stream<String>. Again, this method is lossy because line separators are stripped. If an IOException is encountered while reading the file, it is wrapped in an UncheckedIOException, since Stream doesn't accept lambdas that throw checked exceptions.
try (Stream<String> lines = Files.lines(path, encoding)) {
lines.forEach(System.out::println);
}
This Stream does need a close() call; this is poorly documented on the API, and I suspect many people don't even notice Stream has a close() method. Be sure to use an ARM-block as shown.
If you are working with a source other than a file, you can use the lines() method in BufferedReader instead.
Memory utilization
The first method, that preserves line breaks, can temporarily require memory several times the size of the file, because for a short time the raw file contents (a byte array), and the decoded characters (each of which is 16 bits even if encoded as 8 bits in the file) reside in memory at once. It is safest to apply to files that you know to be small relative to the available memory.
The second method, reading lines, is usually more memory efficient, because the input byte buffer for decoding doesn't need to contain the entire file. However, it's still not suitable for files that are very large relative to available memory.
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. Here, "large" depends on the computer specs. Nowadays, this threshold might be many gigabytes of RAM. The third method, using a Stream<String> is one way to do this, if your input "records" happen to be individual lines. (Using the readLine() method of BufferedReader is the procedural equivalent to this approach.)
Character encoding
One thing that is missing from the sample in the original post is the character encoding. 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.
The StandardCharsets class defines some constants for the encodings required of all Java runtimes:
String content = readFile("test.txt", StandardCharsets.UTF_8);
The platform default is available from the Charset class itself:
String content = readFile("test.txt", Charset.defaultCharset());
Note: This answer largely replaces my Java 6 version. 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. You can view the old version via the "edited" link on this answer.
Read more… Read less…
If you're willing to use an external library, check out Apache Commons IO (200KB JAR). It contains an org.apache.commons.io.FileUtils.readFileToString() method that allows you to read an entire File into a String with one line of code.
Example:
import java.io.*;
import java.nio.charset.*;
import org.apache.commons.io.*;
public String readFile() throws IOException {
File file = new File("data.txt");
return FileUtils.readFileToString(file, StandardCharsets.UTF_8);
}
A very lean solution based on Scanner:
Scanner scanner = new Scanner( new File("poem.txt") );
String text = scanner.useDelimiter("\\A").next();
scanner.close(); // Put this call in a finally block
Or, if you want to set the charset:
Scanner scanner = new Scanner( new File("poem.txt"), "UTF-8" );
String text = scanner.useDelimiter("\\A").next();
scanner.close(); // Put this call in a finally block
Or, with a try-with-resources block, which will call scanner.close() for you:
try (Scanner scanner = new Scanner( new File("poem.txt"), "UTF-8" )) {
String text = scanner.useDelimiter("\\A").next();
}
Remember that the Scanner constructor can throw an IOException. And don't forget to import java.io and java.util.
Source: Pat Niemeyer's blog
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
String content = new String(Files.readAllBytes(Paths.get("readMe.txt")), StandardCharsets.UTF_8);
since java 7 you can do it this way.
If you're looking for an alternative that doesn't involve a third-party library (e.g. Commons I/O), you can use the Scanner class:
private String readFile(String pathname) throws IOException {
File file = new File(pathname);
StringBuilder fileContents = new StringBuilder((int)file.length());
try (Scanner scanner = new Scanner(file)) {
while(scanner.hasNextLine()) {
fileContents.append(scanner.nextLine() + System.lineSeparator());
}
return fileContents.toString();
}
}
Guava has a method similar to the one from Commons IOUtils that Willi aus Rohr mentioned:
import com.google.common.base.Charsets;
import com.google.common.io.Files;
// ...
String text = Files.toString(new File(path), Charsets.UTF_8);
EDIT by PiggyPiglet
Files#toString is deprecated, and due for removal Octobor 2019. Instead use
Files.asCharSource(new File(path), StandardCharsets.UTF_8).read();
EDIT by Oscar Reyes
This is the (simplified) underlying code on the cited library:
InputStream in = new FileInputStream(file);
byte[] b = new byte[file.length()];
int len = b.length;
int total = 0;
while (total < len) {
int result = in.read(b, total, len - total);
if (result == -1) {
break;
}
total += result;
}
return new String( b , Charsets.UTF_8 );
Edit (by Jonik): The above doesn't match the source code of recent Guava versions. For the current source, see the classes Files, CharStreams, ByteSource and CharSource in com.google.common.io package.