StringUtils # split 的坑

背景

public static void main(String[] args) {
    String netmask = "103.140.147.0/24";
    System.out.println(org.springframework.util.StringUtils.split(netmask, ","));
    System.out.println(org.apache.commons.lang3.StringUtils.split(netmask, ","));
}

有這么一段常見的代碼，可以想象結果嗎？ 今天在項目測試中就出現了 誤用StringUtils.split. 方法導致的 NullPointException。

根據 異常 NullPointException 提示，我定位了到 StringUtils.split 方法返回null，但不符合 邏輯啊， 不滿足 分片，應該直接返回 本身 才對。況且以前也在用的，點進去查看了 jar，並看了方法。

發現引用了 org.springframework.util.StringUtils

/**
	 * Split a {@code String} at the first occurrence of the delimiter.
	 * Does not include the delimiter in the result.
	 * @param toSplit the string to split
	 * @param delimiter to split the string up with
	 * @return a two element array with index 0 being before the delimiter, and
	 * index 1 being after the delimiter (neither element includes the delimiter);
	 * or {@code null} if the delimiter wasn't found in the given input {@code String}
	 */
	@Nullable
	public static String[] split(@Nullable String toSplit, @Nullable String delimiter) {
		if (!hasLength(toSplit) || !hasLength(delimiter)) {
			return null;
		}
		int offset = toSplit.indexOf(delimiter);
		if (offset < 0) {
			return null;
		}

		String beforeDelimiter = toSplit.substring(0, offset);
		String afterDelimiter = toSplit.substring(offset + delimiter.length());
		return new String[] {beforeDelimiter, afterDelimiter};
	}

其實也解釋的很清楚了，分隔符 不存在，就直接返回了 null

分析

在改成 org.apache.commons.lang3.StringUtils， 才是符合我們要求的

public static String[] split(final String str, final String separatorChars) {
        return splitWorker(str, separatorChars, -1, false);
    }
    

/**
     * Performs the logic for the {@code split} and
     * {@code splitPreserveAllTokens} methods that return a maximum array
     * length.
     *
     * @param str  the String to parse, may be {@code null}
     * @param separatorChars the separate character
     * @param max  the maximum number of elements to include in the
     *  array. A zero or negative value implies no limit.
     * @param preserveAllTokens if {@code true}, adjacent separators are
     * treated as empty token separators; if {@code false}, adjacent
     * separators are treated as one separator.
     * @return an array of parsed Strings, {@code null} if null String input
     */
    private static String[] splitWorker(final String str, final String separatorChars, final int max, final boolean preserveAllTokens) {
        // Performance tuned for 2.0 (JDK1.4)
        // Direct code is quicker than StringTokenizer.
        // Also, StringTokenizer uses isSpace() not isWhitespace()

        if (str == null) {
            return null;
        }
        final int len = str.length();
        if (len == 0) {
            return ArrayUtils.EMPTY_STRING_ARRAY;
        }
        final List<String> list = new ArrayList<>();
        int sizePlus1 = 1;
        int i = 0, start = 0;
        boolean match = false;
        boolean lastMatch = false;
        if (separatorChars == null) {
            // Null separator means use whitespace
            while (i < len) {
                if (Character.isWhitespace(str.charAt(i))) {
                    if (match || preserveAllTokens) {
                        lastMatch = true;
                        if (sizePlus1++ == max) {
                            i = len;
                            lastMatch = false;
                        }
                        list.add(str.substring(start, i));
                        match = false;
                    }
                    start = ++i;
                    continue;
                }
                lastMatch = false;
                match = true;
                i++;
            }
        } else if (separatorChars.length() == 1) {//我的場景
            // Optimise 1 character case
            final char sep = separatorChars.charAt(0);
            while (i < len) {
                if (str.charAt(i) == sep) {
                    if (match || preserveAllTokens) {
                        lastMatch = true;
                        if (sizePlus1++ == max) {
                            i = len;
                            lastMatch = false;
                        }
                        list.add(str.substring(start, i)); //這里 分割
                        match = false;
                    }
                    start = ++i;
                    continue;
                }
                lastMatch = false;
                match = true;
                i++;
            }
        } else {
            // standard case    這里像是有滿足 分隔符中的字符 就進行分割
            while (i < len) {
                if (separatorChars.indexOf(str.charAt(i)) >= 0) {
                    if (match || preserveAllTokens) {
                        lastMatch = true;
                        if (sizePlus1++ == max) {
                            i = len;
                            lastMatch = false;
                        }
                        list.add(str.substring(start, i));
                        match = false;
                    }
                    start = ++i;
                    continue;
                }
                lastMatch = false;
                match = true;
                i++;
            }
        }
        if (match || preserveAllTokens && lastMatch) {
            list.add(str.substring(start, i));  //遍歷完自符，如果分隔符不存在，則返回 str 本身
        }
        return list.toArray(new String[list.size()]);
    }

擴展

org.apache.commons.lang3.StringUtils@split 方法能滿足一般的情況，但是 他不會 對 空格做處理，如果有特殊要求，可以用 guava的 工具類

List<String> hostNameList = Splitter.on(",")
                    .trimResults()
                    .omitEmptyStrings()//可以 選擇是否對 空字符串 做處理
                    .splitToList(hostName);

guava 是一種很強大的工具，能想到的功能基本都有，大家有空可以多看看它里面的實現和 設計思路。