java: search & replace in a Stream

How do I do on-the-fly search & replace开发者_高级运维 in a Java Stream (input or output)?

I don't want to load the stream into memory or to a file.

I just see the bytes passing by and I need to do some replacements. The sequences being replaced are short (up to 20 bytes).

You can use the class provided here if static replacement rules are enough for you.

You could implement a deterministic finite automaton which looks at each byte once only (e.g. no lookbehind is required), so that you would basically stream the input through a buffer holding max as many characters as the length of your pattern, outputting the pattern on a match or overflowing (non-matched) characters when advancing in the pattern. Runtime is linear after preparation of the pattern.

Wikipedia has some information on pattern matching and how that works in theory.

I got some good ideas from the link provided and ended up writing a small class to handle replacement of $VAR$ variables in a stream. For posterity:

public class ReplacingOutputStream extends OutputStream {
    private static final int DOLLAR_SIGN = "$".codePointAt(0);
    private static final int BACKSLASH = "\\".codePointAt(0);
    private final OutputStream delegate;
    private final Map<String, Object> replacementValues;

    private int previous = Integer.MIN_VALUE;
    private boolean replacing = false;
    private ArrayList<Integer> replacement = new ArrayList<Integer>();


    public ReplacingOutputStream(OutputStream delegate, Map<String, Object> replacementValues) {
        this.delegate = delegate;
        this.replacementValues = replacementValues;
    }

    public @Override void write(int b) throws IOException {
        if (b == DOLLAR_SIGN && previous != BACKSLASH) {
            if (replacing) {
                doReplacement();
                replacing = false;
            } else {
                replacing = true;
            }
        } else {
            if (replacing) {
                replacement.add(b);
            } else {
                delegate.write(b);
            }
        }

        previous = b;
    }

    private void doReplacement() throws IOException {
        StringBuilder sb = new StringBuilder();
        for (Integer intval : replacement) {
            sb.append(Character.toChars(intval));
        }
        replacement.clear();

        String oldValue = sb.toString();
        Object _newValue = replacementValues.get(oldValue);
        if (_newValue == null) {
            throw new RuntimeException("Could not find replacement variable for value '"+oldValue+"'.");
        }

        String newValue = _newValue.toString();
        for (int i=0; i < newValue.length(); ++i) {
            int value = newValue.codePointAt(i);
            delegate.write(value);
        }
    }
}