Find a string inside a bytebuffer

Question

I'm switching from C to Java. I'm wondering about how to find a string inside a bytebuffer, is there something like memchr in java? The bytebuffer is only partly a string, the rest is raw bytes so any java method has to work on bytes + chars.

I am also searching for something like strsep in java to split strings.

java.lang.String - methods split() or substring can be used to split the string. You should in general read the javadocs of this class. — Scorpion, Dec 28 '11 at 07:56
Not a good question - because it has two questions in one. Please split up the two (using split? :-)) — Zordid, Nov 09 '12 at 14:25

score 5 · Answer 1 · answered Dec 28 '11 at 11:30

5

You can convert the ByteBuffer into a String and use indexOf which likely to work.

ByteBuffer bb = /* non-direct byte buffer */
String text = new String(bb.array(), 0, bb.position(), bb.remaing());
int index = text.indexOf(searchText);

This has a non-trivial overhead as it creates a String. The alternative is a brute force String search which will be faster but takes time to write.

answered Dec 28 '11 at 11:30

Peter Lawrey

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This String constructor is deprecated, because it doesn't take the character encoding into account. Suggested: `String text = new String(bb.array(), 0, bb.position(), charset);` where `charset` is the encoding to use, or the default one `Charset.defaultCharset()` – mins Aug 24 '14 at 13:11
If you are reading raw C string it is most likely ISO-8859-1 encoded in which case this method is fine. Being explicit doesn't hurt performance much so it being clear is perhaps better. – Peter Lawrey Aug 24 '14 at 16:57
2

Downside to this approach, and similar, is that you have to read the whole string - not a streaming solution. – Jmoney38 Jun 20 '18 at 23:24

score 4 · Answer 2 · answered Dec 28 '11 at 08:32

You would need to encode the character string into bytes using the correct character encoding for your application. Then use a string search algorithm like Rabin-Karp or Boyer-Moore to find the resulting byte sequence within the buffer. Or, if your buffers are small, you could just perform a brute force search.

I'm not aware of any open source implementations of these search algorithms, and they aren't part of core Java.

score 1 · Answer 3 · edited May 23 '17 at 12:33

From Fastest way to find a string in a text file with java:

The best realization I've found in MIMEParser: https://github.com/samskivert/ikvm-openjdk/blob/master/build/linux-amd64/impsrc/com/sun/xml/internal/org/jvnet/mimepull/MIMEParser.java

/**
  * Finds the boundary in the given buffer using Boyer-Moore algo.
  * Copied from java.util.regex.Pattern.java
  *
  * @param mybuf boundary to be searched in this mybuf
  * @param off start index in mybuf
  * @param len number of bytes in mybuf
  *
  * @return -1 if there is no match or index where the match starts
  */

  private int match(byte[] mybuf, int off, int len) {

Needed also:

  private void compileBoundaryPattern();

score 0 · Answer 4 · answered Dec 28 '11 at 07:56

0

The String class has a nice split method String.split

answered Dec 28 '11 at 07:56

Fortyrunner

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score 0 · Answer 5 · answered Dec 28 '11 at 08:17

One option is to use a StringTokenizer, which can split the string into an iterable collection of tokens according to given delimiter(s). The tokens collection can contain the delimiter if needed. Example:

String s = "abc:def-ghi|jkl";
StringTokenizer tokenizer = new StringTokenizer(s, ":-|");
while (tokenizer.hasMoreTokens()) {
  System.out.print(tokenizer.nextToken());
}

Expected result:

abcdefghijkl

Find a string inside a bytebuffer

5 Answers5