最简洁的方法来读取Java中的文件/输入流的内容？

Question

What ist most concise way to read the contents of a file or input stream in Java? Do I always have to create a buffer, read (at most) line by line and so on or is there a more concise way? I wish I could do just

String content = new File("test.txt").readFully();

Answer 1

Use the Apache Commons IOUtils package. In particular the IOUtils class provides a set of methods to read from streams, readers etc. and handle all the exceptions etc.

eg

InputStream is = ...
String contents = IOUtils.toString(is);
// or
List lines = IOUtils.readLines(is)

Answer 2

I think using a Scanner is quite OK with regards to conciseness of Java on-board tools:

Scanner s = new Scanner(new File("file"));
StringBuilder builder = new StringBuilder();
while(s.hasNextLine()) builder.append(s.nextLine());

Also, it's quite flexible, too (eg regular expressions support, number parsing).

Answer 3

Helper functions. I basically use a few of them, depending on the situation

• cat method that pipes an InputStream to an OutputStream
• method that calls cat to a ByteArrayOutputStream and extracts the byte array, enabling quick read of an entire file to a byte array
• Implementation of Iterator<String> that is constructed using a Reader; it wraps it in a BufferedReader and readLine's on next()
• ...

Either roll your own or use something out of commons-io or your preferred utility library.

Answer 4

String content = (new RandomAccessFile(new File("test.txt"))).readUTF();

不幸的是，Java对源文件是有效的UTF8非常挑剔，否则你会得到一个EOFException或UTFDataFormatException。

Answer 5

To give an example of such an helper function:

String[] lines = NioUtils.readInFile(componentxml);

The key is to try to close the BufferedReader even if an IOException is thrown.

/**
 * Read lines in a file. <br />
 * File must exist
 * @param f file to be read
 * @return array of lines, empty if file empty
 * @throws IOException if prb during access or closing of the file
 */
public static String[] readInFile(final File f) throws IOException
{
    final ArrayList lines = new ArrayList();
    IOException anioe = null;
    BufferedReader br = null; 
    try 
    {
        br = new BufferedReader(new FileReader(f));
        String line;
        line = br.readLine();
        while(line != null)
        {
            lines.add(line);
            line = br.readLine();
        }
        br.close();
        br = null;
    } 
    catch (final IOException e) 
    {
        anioe = e;
    }
    finally
    {
        if(br != null)
        {
            try {
                br.close();
            } catch (final IOException e) {
                anioe = e;
            }
        }
        if(anioe != null)
        {
            throw anioe;
        }
    }
    final String[] myStrings = new String[lines.size()];
    //myStrings = lines.toArray(myStrings);
    System.arraycopy(lines.toArray(), 0, myStrings, 0, lines.size());
    return myStrings;
}

(if you just want a String, change the function to append each lines to a StringBuffer (or StringBuilder in java5 or 6)

Answer 6

You have to create your own function, I suppose. The problem is that Java's read routines (those I know, at least) usually take a buffer argument with a given length.

A solution I saw is to get the size of the file, create a buffer of this size and read the file at once. Hoping the file isn't a gigabyte log or XML file...

The usual way is to have a fixed size buffer or to use readLine and concatenate the results in a StringBuffer/StringBuilder.

Answer 7

Or the Java 8 way:

try {
    String str = new String(Files.readAllBytes(Paths.get("myfile.txt")));
    ...
} catch (IOException ex) {
    Logger.getLogger(getClass().getName()).log(Level.SEVERE, null, ex);
}

One may pass an appropriate Charset to the String constructor.

Answer 8

I don't think reading using BufferedReader is a good idea because BufferedReader will return just the content of line without the delimeter. When the line contains nothing but newline character, BR will return a null although it still doesn't reach the end of the stream.

Answer 9

String org.apache.commons.io.FileUtils.readFileToString（文件文件）

Answer 10

Pick one from here.

How do I create a Java string from the contents of a file?

The favorite was:

private static String readFile(String path) throws IOException {
  FileInputStream stream = new FileInputStream(new File(path));
  try {
    FileChannel fc = stream.getChannel();
    MappedByteBuffer bb = fc.map(FileChannel.MapMode.READ_ONLY, 0, fc.size());
    /* Instead of using default, pass in a decoder. */
    return CharSet.defaultCharset().decode(bb).toString();
  }
  finally {
    stream.close();
  }
}

Posted by erickson