Why are most string manipulations in Java based on regexp?_问答_开发者

In Java there are a bunch of methods that all have to do with manipulating Strings. The simplest example is the String.split("something") method.

Now the actual definition of many of those methods is that they all take a regular expression as their input parameter(s). Which makes then all very powerful building blocks.

Now there are two effects you'll see in many of those methods:

They recompile the expression each time the method is invoked. As such they impose a performance impact.
I've found that in most "real-life" situations these methods are called with "fixed" texts. The most common usage of the split method is even worse: It's usually called with a single char (usually a ' ', a ';' or a '&') to split by.

So it's not only that the default methods are powerful, they also seem overpowered for what they are actually used for. Internally we've developed a "fastSplit" method that splits on fixed strings. I wrote a test at home to see how much faster I could do it if it was known to be a single char. Both are significantly faster than the "standard" split method.

So I was wondering: why was the Java API chosen the way it is now? What was the good reason to go for this instead of having a something like split(char) and split(String) and a splitRegex(String) ??

Update: I slapped together a few calls to see how much time the various ways of splitting a string would take.

Short summary: It makes a big difference!

I did 10000000 iterations for each test case, always using the input

"aap,noot,mies,wim,zus,jet,teun"

and always using ',' or "," as the split argument.

This is what I got on my Linux system (it's an Atom D510 box, so it's a bit slow):

fastSplit STRING
Test  1 : 11405 milliseconds: Split in several pieces
Test  2 :  3018 milliseconds: Split in 2 pieces
Test  3 :  4396 milliseconds: Split in 3 pieces

homegrown fast splitter based on char
Test  4 :  9076 milliseconds: Split in several pieces
Test  5 :  2024 milliseconds: Split in 2 pieces
Test  6 :  2924 milliseconds: Split in 3 pieces

homegrown splitter based on char that always splits in 2 pieces
Test  7 :  1230 milliseconds: Split in 2 pieces

String.split(regex)
Test  8 : 32913 milliseconds: Split in several pieces
Test  9 : 30072 milliseconds: Split in 2 pieces
Test 10 : 31278 milliseconds: Split in 3 pieces

String.split(regex) using precompiled Pattern
Test 11 : 26138 milliseconds: Split in several pieces 
Test 12 : 23612 milliseconds: Split in 2 pieces
Test 13 : 24654 milliseconds: Split in 3 pieces

StringTokenizer
Test 14 : 27616 milliseconds: Split in several pieces
Test 15 : 28121 milliseconds: Split in 2 pieces
Test 16 : 27739 milliseconds: Split in 3 pieces

As you can see it makes a big difference if you have a lot of "fixed char" splits to do.

To give you guys some insight; I'm currently in the Apache logfiles and Hadoop arena with the data of a big website. So to me this stuff really matters :)

Something I haven't factored in here is the garbage collector. As far as I can tell compiling a regular expression into a Pattern/Matcher/.. will allocate a lot of objects, that need to be collected some time. So perhaps in the long run the differences between these versions is even bigger .... or smaller.

My conclusions so far:

Only optimize this if you have a LOT of strings to split.
If you use the regex methods always precompile if you repeatedly use the same pattern.
Forget the (obsolete) StringTokenizer
If you want to split on a single char then use a custom method, especially if you only need to split it into a specific number of pieces (like ... 2).

P.S. I'm giving you all my homegrown split by char methods to play with (under the license that everything on this site falls under :) ). I never fully tested them .. yet. Have fun.

private static String[]
        stringSplitChar(final String input,
                        final char separator) {
    int pieces = 0;

    // First we count how many pieces we will need to store ( = separators + 1 )
    int position = 0;
    do {
        pieces++;
        position = input.indexOf(separator, position + 1);
    } while (position != -1);

    // Then we allocate memory
    final String[] result = new String[pieces];

    // And start cutting and copying the pieces.
    int previousposition = 0;
    int currentposition = input.indexOf(separator);
    int piece = 0;
    final int lastpiece = pieces - 1;
    while (piece < lastpiece) {
        result[piece++] = input.substring(previousposition, currentposition);
        previousposition = currentposition + 1;
        currentposition = input.indexOf(separator, previousposition);
    }
    result[piece] = input.substring(previousposition);

    return result;
}

private static String[]
        stringSplitChar(final String input,
                        final char separator,
                        final int maxpieces) {
    if (maxpieces <= 0) {
        return stringSplitChar(input, separator);
    }
    int pieces = maxpieces;

    // Then we allocate memory
    final String[] result = new String[pieces];

    // And start cutting and copying the pieces.
    int previousposition = 0;
    int currentposition = input.indexOf(separator);
    int piece = 0;
    final int lastpiece = pieces - 1;
    while (currentposition != -1 && piece < lastpiece) {
        result[piece++] = input.substring(previousposition, currentposition);
        previousposition = currentposition + 1;
        currentposition = input.indexOf(separator, previousposition);
    }
    result[piece] = input.substring(previousposition);

    // All remaining array elements are uninitialized and assumed to be null
    return result;
}

private static String[]
        stringChop(final String input,
                   final char separator) {
    String[] result;
    // Find the separator.
    final int separatorIndex = input.indexOf(separator);
    if (separatorIndex == -1) {
        result = new String[1];
        result[0] = input;
    }
    else {
        result = new String[2];
        result[0] = input.substring(0, separatorIndex);
        result[1] = input.substring(separatorIndex + 1);
    }
    return 开发者_开发技巧result;
}

Note that the regex need not be recompiled each time. From the Javadoc:

An invocation of this method of the form str.split(regex, n) yields the same result as the expression

Pattern.compile(regex).split(str, n)

That is, if you are worried about performance, you may precompile the pattern and then reuse it:

Pattern p = Pattern.compile(regex);
...
String[] tokens1 = p.split(str1); 
String[] tokens2 = p.split(str2); 
...

instead of

String[] tokens1 = str1.split(regex);
String[] tokens2 = str2.split(regex);
...

I believe that the main reason for this API design is convenience. Since regular expressions include all "fixed" strings/chars too, it simplifies the API to have one method instead of several. And if someone is worried about performance, the regex can still be precompiled as shown above.

My feeling (which I can't back with any statistical evidence) is that most of the cases String.split() is used in a context where performance is not an issue. E.g. it is a one-off action, or the performance difference is negligible compared to other factors. IMO rare are the cases where you split strings using the same regex thousands of times in a tight loop, where performance optimization indeed makes sense.

It would be interesting to see a performance comparison of a regex matcher implementation with fixed strings/chars compared to that of a matcher specialized to these. The difference might not be big enough to justify the separate implementation.

I wouldn't say most string manipulations are regex-based in Java. Really we are only talking about split and replaceAll/replaceFirst. But I agree, it's a big mistake.

Apart from the ugliness of having a low-level language feature (strings) becoming dependent on a higher-level feature (regex), it's also a nasty trap for new users who might naturally assume that a method with the signature String.replaceAll(String, String) would be a string-replace function. Code written under that assumption will look like it's working, until a regex-special character creeps in, at which point you've got confusing, hard-to-debug (and maybe even security-significant) bugs.

It's amusing that a language that can be so pedantically strict about typing made the sloppy mistake of treating a string and a regex as the same thing. It's less amusing that there's still no builtin method to do a plain string replace or split. You have to use a regex replace with a Pattern.quoted string. And you only even get that from Java 5 onwards. Hopeless.

@Tim Pietzcker:

Are there other languages that do the same?

JavaScript's Strings are partly modelled on Java's and are also messy in the case of replace(). By passing in a string, you get a plain string replace, but it only replaces the first match, which is rarely what's wanted. To get a replace-all you have to pass in a RegExp object with the /g flag, which again has problems if you want to create it dynamically from a string (there is no built-in RegExp.quote method in JS). Luckily, split() is purely string-based, so you can use the idiom:

s.split(findstr).join(replacestr)

Plus of course Perl does absolutely everything with regexen, because it's just perverse like that.

(This is a comment more than an answer, but is too big for one. Why did Java do this? Dunno, they made a lot of mistakes in the early days. Some of them have since been fixed. I suspect if they'd thought to put regex functionality in the box marked Pattern back in 1.0, the design of String would be cleaner to match.)

I imagine a good reason is that they can simply pass the buck on to the regex method, which does all the real heavy lifting for all of the string methods. Im guessing they thought if they already had a working solution it was less efficient, from a development and maintenance standpoint, to reinvent the wheel for each string manipulation method.

Interesting discussion!

Java was not originally intended as a batch programming language. As such the API out of the box are more tuned towards doing one "replace" , one "parse" etc. except on Application initialization when the app may be expected to be parsing a bunch of configuration files.

Hence optimization of these APIs was sacrificed in the altar of simplicity IMO. But the question brings up an important point. Python's desire to keep the regex distinct from the non regex in its API, stems from the fact that Python can be used as an excellent scripting language as well. In UNIX too, the original versions of fgrep did not support regex.

I was engaged in a project where we had to do some amount of ETL work in java. At that time, I remember coming up with the kind of optimizations that you have alluded to, in your question.

I suspect that the reason why things like String#split(String) use regexp under the hood is because it involves less extraneous code in the Java Class Library. The state machine resulting from a split on something like , or space is so simple that it is unlikely to be significantly slower to execute than a statically implemented equivalent using a StringCharacterIterator.

Beyond that the statically implemented solution would complicate runtime optimization with the JIT because it would be a different block of code that also requires hot code analysis. Using the existing Pattern algorithms regularly across the library means that they are more likely candidates for JIT compilation.

Very good question..

I suppose when the designers sat down to look at this (and not for very long, it seems), they came at it from a point of view that it should be designed to suit as many different possibilities as possible. Regular Expressions offered that flexibility.

They didn't think in terms of efficiencies. There is the Java Community Process available to raise this.

Have you looked at using the java.util.regex.Pattern class, where you compile the expression once and then use on different strings.

Pattern exp = Pattern.compile(":");
String[] array = exp.split(sourceString1);
String[] array2 = exp.split(sourceString2);

In looking at the Java String class, the uses of regex seem reasonable, and there are alternatives if regex is not desired:

http://java.sun.com/javase/6/docs/api/java/lang/String.html

boolean matches(String regex) - A regex seems appropriate, otherwise you could just use equals

String replaceAll/replaceFirst(String regex, String replacement) - There are equivalents that take CharSequence instead, preventing regex.

String[] split(String regex, int limit) - A powerful but expensive split, you can use StringTokenizer to split by tokens.

These are the only functions I saw that took regex.

Edit: After seeing that StringTokenizer is legacy, I would defer to Péter Török's answer to precompile the regex for split instead of using the tokenizer.

The answer to your question is that the Java core API did it wrong. For day to day work you can consider using Guava libraries' CharMatcher which fills the gap beautifully.

...why was the Java API chosen the way it is now?

Short answer: it wasn't. Nobody ever decided to favor regex methods over non-regex methods in the String API, it just worked out that way.

I always understood that Java's designers deliberately kept the string-manipulation methods to a minimum, in order to avoid API bloat. But when regex support came along in JDK 1.4, of course they had to add some convenience methods to String's API.

So now users are faced with a choice between the immensely powerful and flexible regex methods, and the bone-basic methods that Java always offered.