Java: Using two Readers on one InputStream

Question

My application can edit a type of text file called a TDAEntry. Each such entry is based on another type of file that describes its format, called a TDAForm. The very first line in a TDAEntry text file is the name of the form it belongs to, and then the rest of the data (in text form) follows.

For opening such a file now, here is my code, simplified:

InputStream entryInput = new FileInputStream(file);

BufferedReader entryReader = new BufferedReader(
    new InputStreamReader(entryInput)); // PROBLEMATIC #1

String formName = entryReader.readLine();

TDAForm form = new TDAForm(formName);

// create an entry with the right form and the data which follows in entryInput
TDAEntry entry = new TDAEntry(form, entryInput); // PROBLEMATIC #2

Here's the problem. When reading the source code for InputStreamReader, you'll end up reading the code of this class:

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/sun/nio/cs/StreamDecoder.java#StreamDecoder.read(char%5B%5D%2Cint%2Cint)

which is buffered, which means that it reads about 8000 bytes ahead. This causes the InputStream at line "Problematic #2" to be empty and in the wrong spot, while I want it to continue as if only the very first line of the file had been read. Inside the constructor of TDAEntry I might want to construct another kind of reader on the stream, or do something else.

One obvious solution is writing a new decoder that doesn't eat up the bytes ahead. But I'm wondering if there's a simpler way to do this. How would you tackle the problem, or do you suggest another programming structure to accomplish the same thing?

Thanks a lot!

Answer 1

Just pass in the reader. it will keep reading where it left off:

TDAEntry entry = new TDAEntry(form, entryReader); 

Answer 2

You could try wrapping your FileInputStream into a BufferedInputStream since the buffered version supports mark() and reset().

Answer 3

If you just want to limit the buffer, you don't need to write your own class. You can use the second constructor, i.e. new BufferedReader(new InputStreamReader(entryInput), 20); to limit the buffer size.

Not that this is necessarily the best way to handle this problem. Basically switching streams in mid-air is quite a bad idea. You could create your own InputStream implementation that reads both the header and the entries.