Java BufferedReader.readLine() 不等待 EOL？

Question

Sorry if I'm missing something obvious here...but please take a look at this code snippet:

String readString;
String writeString = "O hai world.";
BufferedReader br = new BufferedReader(
    new InputStreamReader( 
        new ByteArrayInputStream(writeString.getBytes()),
        "UTF-8"),
    1024);
readString = br.readLine();
System.out.println("readString: " + readString);

I'd expect this to print "readString: null" since I thought the BufferedReader would encounter an EOF before detecting a valid EOL, but instead this prints "readString: O hai world". This seems contrary to what the Javadocs for BufferedReader say readLine() will do:

Reads a line of text. A line is considered to be terminated by any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.

Returns: A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached

I can't see any reason why my string would be re-interpreted to terminate with '\n' and/or '\r'...can someone please illuminate me? Thanks!

EDIT : To provide some context, I'm trying to write JUnit tests to validate a Reader class that I wrote that's designed to read on System.in. Using ByteArrayInputStreams seemed like a reasonable way to simulate System.in (see this relevant SO post ).

When my Reader captures a line, it currently relies on BufferedReader.readLine(). For my purposes, my Reader's lines MUST all have been terminated with '\n' or '\r'; encountering EOF without an EOL should not resolve into a valid line. So I guess my question(s) at this point are really as follows (I'll try to test these myself in greater detail when I have time, but hoping you smart folks can help me out):

• Is BufferedReader.readLine() broken/misdocumented? Or is ByteArrayInputStream returning something erroneous when its byte array is exhausted?
• Is this method of testing my Reader erroneous, and should I expect readLine() to function properly when used against System.in? I'm inclined to believe the answer to this is yes.
• Are there better ways to simulate System.in for unit testing?
• If I need to strictly discriminate against '\n' and '\r' when reading from an InputStream, am I better off writing my own readLine() method? I'd be very surprised if this is the case.

Thanks again!

Answer 1

The ByteArrayInputStream doesn't return EOL when it's exhausted. It only returns -1 which might be considered EOF.

The thing is that the BufferedReader buffers all it reads from the input stream and if the EOF(-1) is encountered before any EOL character showed up, it returns the string buffered up to that point.

So, if you want to be very strict, you can say that readLine() is either broken according to the current documentation or that it should be documented differently if this was the intended behavior.

In my opinion, considering that the last line in a stream doesn't have to end with an EOL character (EOF being enough) the current behavior of readLine is correct, ie a line was read because EOF was encountered. So, the documentation should be changed.

Answer 2

I would imagine that this would block would you be reading from a true stream (eg a network socket). But since the underlying input is an array, the reader knows that the true end of data has been reached, so blocking is unnecessary since no new data is forthcoming. So blocking would be a wrong course of action. Returning a null where actual data was read would also be a wrong thing to do.

Answer 3

I believe you want a "Robot" to emulate keystrokes for testing purposes:

• http://docs.oracle.com/javase/6/docs/api/java/awt/Robot.html

This class is used to generate native system input events for the purposes of test automation, self-running demos, and other applications where control of the mouse and keyboard is needed. The primary purpose of Robot is to facilitate automated testing of Java platform implementations.

Here's an article that discusses it further:

• http://www.developer.com/java/other/article.php/2241561/An-Automated-Test-Program-using-the-Java-Robot-Class.htm

Answer 4

What would you expect to happen with this version of your code?

String readString;
String writeString = "O\nhai\nworld.";
BufferedReader br = new BufferedReader(
    new InputStreamReader( 
        new ByteArrayInputStream(writeString.getBytes()),
        "UTF-8"),
    1024);
while (true) {
    readString = br.readLine();
    if (readString == null) break;
    System.out.println("readString: " + readString);
}

Answer 5

The only alternative to what it does now is to throw the final incomplete line away. Not desirable.