How to read a text-file resource into Java unit test?

Essential InputStream to String Conversion

When we need a String out of our resource, we can convert an InputStream into a String like this:

String result = new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8))
                  .lines().collect(Collectors.joining("\n"));
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This uses Java 8's streams. Make sure to specify character encoding (UTF-8 is typically a safe bet).

Libraries That Do the Heavy Lifting

Thank goodness for libraries like Apache Commons IO and Google Guava. They can make file operations a breeze.

String content = IOUtils.toString(inputStream, StandardCharsets.UTF_8);
// Courtesy of Apache Commons IO. Thanks, Apache!
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When Size Does Matter: Dealing with Large Files

Working with large files? Let's avoid a game of OutOfMemoryError whack-a-mole. You can read the file line by line or with streams. Be ready to handle cases when the file can't be found or is inaccessible.

The Future is Now: Java NIO

For Java 7 and onwards, the java.nio.file.Files class offers methods for reading a file:

List<String> lines = Files.readAllLines(Paths.get(resource.toURI()), StandardCharsets.UTF_8);
String content = String.join("\n", lines);
// Wall of text, meet your nemesis: line breaks!
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Architect the Robustness

Don't forget to handle potential errors and exceptions that might occur during file reading.

// Nobody's perfect; we need to plan for exceptions too
catch(IOException ex) {
    throw new UncheckedIOException("Resource reading went wrong", ex);
}
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Clean Kitchen Philosophy: Close Your Resources

Prevent resource leakage by closing your resources. InputStream is auto-closable, so it makes it easier. Here's how:

try(InputStream is = this.getClass().getResourceAsStream("/example.txt")) {
  // Resource handling code here...
}
// Look, ma! No leaks!
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