Read all lines with BufferedReader

One line of code using Java 8:

line =  buffer.lines().collect(Collectors.joining());

The idiomatic way to read all of the lines is while ((line = buffer.readLine()) != null). Also, I would suggest a try-with-resources statement. Something like

try (InputStreamReader instream = new InputStreamReader(System.in);
        BufferedReader buffer = new BufferedReader(instream)) {
    long length = 0;
    String line;
    while ((line = buffer.readLine()) != null) {
        length += line.length();
    }
    System.out.println("Read length: " + length);
} catch (Exception e) {
    e.printStackTrace();
}

If you want to end the loop when you receive an empty line, add a test for that in the while loop

while ((line = buffer.readLine()) != null) {
    if (line.isEmpty()) {
        break;
    }
    length += line.length();
}

JLS-14.15. The break Statement says

A break statement transfers control out of an enclosing statement.