String.replace实现真的很有效吗？

Question

我以前认为String.replace比String.replaceAll更快,因为后者使用Pattern正则表达式而前者不使用.但事实上,在性能或实施方面没有显着差异.就是这个:

public String replace(CharSequence target, CharSequence replacement) {
    return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
        this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
}

这里有什么需要使用Pattern？我写了一个非正则表达式替换版本

static String replace(String s, String target, String replacement) {
    StringBuilder sb = new StringBuilder(s);
    for (int i = 0; (i = sb.indexOf(target, i)) != -1; i += replacement.length()) {
        sb.replace(i, i + target.length(), replacement);
    }
    return sb.toString();
}

并比较了表现

    public static void main(String args[]) throws Exception {
        String s1 = "11112233211";
        for (;;) {
            long t0 = System.currentTimeMillis();
            for (int i = 0; i < 1000000; i++) {
//              String s2 = s1.replace("11", "xxx");
                 String s2 = replace(s1, "11", "22");
            }
            System.out.println(System.currentTimeMillis() - t0);
        }
    }

基准:我的版本 - 400ms; JDK版本 - 1700ms.

我的测试是错误的还是String.replace真的效率低下？

Answer 1

让你知道String.replace是多么低效

来自Java 7更新11的源代码.

public String replace(CharSequence target, CharSequence replacement) {
    return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
        this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
}

AFAIK,使用Pattern和Matcher.quiteReplacement等是尝试清晰而不是有效.我怀疑它可以追溯到许多内部库是在没有性能考虑的情况下编写的.

恕我直言Java 7已经看到许多内部库提高了性能,特别是减少了不必要的对象创建.这种方法显然是改进的候选者.

您可以通过执行一次复制来提高性能,而不是尝试插入现有的StringBuilder.

static String replace2(String s, String target, String replacement) {
    StringBuilder sb = null;
    int start = 0;
    for (int i; (i = s.indexOf(target, start)) != -1; ) {
        if (sb == null) sb = new StringBuilder();
        sb.append(s, start, i);
        sb.append(replacement);
        start = i + target.length();
    }
    if (sb == null) return s;
    sb.append(s, start, s.length());
    return sb.toString();
}

public static void main(String... ignored) {
    String s1 = "11112233211";
    for (; ; ) {
        timeReplace(s1);
        timeReplace2(s1);
        timeStringReplaceRefactored(s1);
        timeStringReplace(s1);
    }
}

private static void timeStringReplace(String s1) {
    long start0 = System.currentTimeMillis();
    for (int i = 0; i < 1000000; i++) {
        String s2 = s1.replace("11", "xxx");
        if (s2.length() <= s1.length()) throw new AssertionError();
    }
    System.out.printf("String.replace %,d ns avg%n", System.currentTimeMillis() - start0);
}

private static void timeStringReplaceRefactored(String s1) {
    long start0 = System.currentTimeMillis();
    Pattern compile = Pattern.compile("11", Pattern.LITERAL);
    String xxx = Matcher.quoteReplacement("xxx");
    for (int i = 0; i < 1000000; i++) {
        String s2 = compile.matcher(s1).replaceAll(xxx);
        if (s2.length() <= s1.length()) throw new AssertionError();
    }
    System.out.printf("String.replace %,d ns avg (Refactored)%n", System.currentTimeMillis() - start0);
}
private static void timeReplace(String s1) {
    long start0 = System.currentTimeMillis();
    for (int i = 0; i < 1000000; i++) {
        String s2 = replace(s1, "11", "xxx");
        if (s2.length() <= s1.length()) throw new AssertionError();
    }
    System.out.printf("Replace %,d ns avg%n", System.currentTimeMillis() - start0);
}

private static void timeReplace2(String s1) {
    long start0 = System.currentTimeMillis();
    for (int i = 0; i < 1000000; i++) {
        String s2 = replace2(s1, "11", "xxx");
        if (s2.length() <= s1.length()) throw new AssertionError();
    }
    System.out.printf("My replace %,d ns avg%n", System.currentTimeMillis() - start0);
}

static String replace(String s, String target, String replacement) {
    StringBuilder sb = new StringBuilder(s);
    for (int i = 0; (i = sb.indexOf(target, i)) != -1; i += replacement.length()) {
        sb.replace(i, i + target.length(), replacement);
    }
    return sb.toString();
}

版画

Replace 177 ns avg
My replace 108 ns avg
String.replace 436 ns avg (Refactored)
String.replace 598 ns avg

捕获模式并替换文本有一点帮助,但不如使用自定义例程进行替换.