Java中的StringTokenizer实现字符串切割详解
作者：小徐也要努力鸭
这篇文章主要介绍了Java中的StringTokenizer实现字符串切割详解,java.util工具包提供了字符串切割的工具类StringTokenizer,Spring等常见框架的字符串工具类(如Spring的StringUtils),需要的朋友可以参考下
前言
java.util工具包提供了字符串切割的工具类StringTokenizer，Spring等常见框架的字符串工具类（如Spring的StringUtils），常见此类使用。
例如Spring的StringUtils下的方法：
public static String[] tokenizeToStringArray(
@Nullable String str, String delimiters, boolean trimTokens, boolean ignoreEmptyTokens) {
if (str == null) {
return EMPTY_STRING_ARRAY;
}
StringTokenizer st = new StringTokenizer(str, delimiters);
List<String> tokens = new ArrayList<>();
while (st.hasMoreTokens()) {
String token = st.nextToken();
if (trimTokens) {
token = token.trim();
}
if (!ignoreEmptyTokens || token.length() > 0) {
tokens.add(token);
}
}
return toStringArray(tokens);
}
又如定时任务框架Quartz中，cron表达式类CronExpression，其中的buildExpression方法是为了处理cron表达式的，cron表达式有7个子表达式，空格隔开，cron表达式字符串的切割也使用到了StringTokenizer类，方法如下：
protected void buildExpression(String expression) throws ParseException {
this.expressionParsed = true;
try {
if (this.seconds == null) {
this.seconds = new TreeSet();
}
if (this.minutes == null) {
this.minutes = new TreeSet();
}
if (this.hours == null) {
this.hours = new TreeSet();
}
if (this.daysOfMonth == null) {
this.daysOfMonth = new TreeSet();
}
if (this.months == null) {
this.months = new TreeSet();
}
if (this.daysOfWeek == null) {
this.daysOfWeek = new TreeSet();
}
if (this.years == null) {
this.years = new TreeSet();
}
int exprOn = 0;
for(StringTokenizer exprsTok = new StringTokenizer(expression, " \t", false); exprsTok.hasMoreTokens() && exprOn <= 6; ++exprOn) {
String expr = exprsTok.nextToken().trim();
if (exprOn == 3 && expr.indexOf(76) != -1 && expr.length() > 1 && expr.contains(",")) {
throw new ParseException("Support for specifying 'L' and 'LW' with other days of the month is not implemented", -1);
}
if (exprOn == 5 && expr.indexOf(76) != -1 && expr.length() > 1 && expr.contains(",")) {
throw new ParseException("Support for specifying 'L' with other days of the week is not implemented", -1);
}
if (exprOn == 5 && expr.indexOf(35) != -1 && expr.indexOf(35, expr.indexOf(35) + 1) != -1) {
throw new ParseException("Support for specifying multiple \"nth\" days is not implemented.", -1);
}
StringTokenizer vTok = new StringTokenizer(expr, ",");
while(vTok.hasMoreTokens()) {
String v = vTok.nextToken();
this.storeExpressionVals(0, v, exprOn);
}
}
if (exprOn <= 5) {
throw new ParseException("Unexpected end of expression.", expression.length());
} else {
if (exprOn <= 6) {
this.storeExpressionVals(0, "*", 6);
}
TreeSet<Integer> dow = this.getSet(5);
TreeSet<Integer> dom = this.getSet(3);
boolean dayOfMSpec = !dom.contains(NO_SPEC);
boolean dayOfWSpec = !dow.contains(NO_SPEC);
if ((!dayOfMSpec || dayOfWSpec) && (!dayOfWSpec || dayOfMSpec)) {
throw new ParseException("Support for specifying both a day-of-week AND a day-of-month parameter is not implemented.", 0);
}
}
} catch (ParseException var8) {
throw var8;
} catch (Exception var9) {
throw new ParseException("Illegal cron expression format (" + var9.toString() + ")", 0);
}
}使用方法
import com.google.common.collect.Lists;
import java.util.List;
import java.util.StringTokenizer;
/**
* @author xiaoxu
* @date 2023-10-18
* spring_boot:com.xiaoxu.boot.tokenizer.TestStringTokenizer
*/
public class TestStringTokenizer {
public static void main(String[] args) {
print("你 好 吗\t我是 \t你的\t 朋友 \t", " \t", false);
}
public static void print(String str, String delimiter, boolean isReturnDelims) {
System.out.println("切割字符串：【" + str + "】；" + "分隔符：【" + delimiter + "】。");
List<String> strs = Lists.newArrayList();
String s;
boolean x;
for (StringTokenizer strToken = new StringTokenizer(str, delimiter, false); strToken.hasMoreTokens(); x = (s != null && strs.add(s))) {
s = strToken.nextToken();
System.out.println("切割：【" + s + "】");
if(s.equals("吗"))
s = null;
}
System.out.println("字符串数组：" + strs);
}
}执行结果：
切割字符串：【你 好 吗 我是 你的 朋友 】；分隔符：【 】。
切割：【你】
切割：【好】
切割：【吗】
切割：【我是】
切割：【你的】
切割：【朋友】
字符串数组：[你, 好, 我是, 你的, 朋友]
源码片段分析
public StringTokenizer(String str, String delim, boolean returnDelims) {
currentPosition = 0;
newPosition = -1;
delimsChanged = false;
this.str = str;
maxPosition = str.length();
delimiters = delim;
retDelims = returnDelims;
setMaxDelimCodePoint();
}private void setMaxDelimCodePoint() {
if (delimiters == null) {
maxDelimCodePoint = 0;
return;
}
int m = 0;
int c;
int count = 0;
for (int i = 0; i < delimiters.length(); i += Character.charCount(c)) {
c = delimiters.charAt(i);
if (c >= Character.MIN_HIGH_SURROGATE && c <= Character.MAX_LOW_SURROGATE) {
c = delimiters.codePointAt(i);
hasSurrogates = true;
}
if (m < c)
m = c;
count++;
}
maxDelimCodePoint = m;
if (hasSurrogates) {
delimiterCodePoints = new int[count];
for (int i = 0, j = 0; i < count; i++, j += Character.charCount(c)) {
c = delimiters.codePointAt(j);
delimiterCodePoints[i] = c;
}
}
}调用setMaxDelimCodePoint()方法，源码可知，切割时设置int maxDelimCodePoint，是为了优化分隔符的检测（取的是分隔字符串中char的ASCII码值最大的字符的ASCII值，存入maxDelimCodePoint中。在方法int scanToken(int startPos)中，若满足条件(c <= maxDelimCodePoint) && (delimiters.indexOf© >= 0)，意即该字符的ASCII码值小于等于最大的maxDelimCodePoint，那么这个字符可能存在于分隔字符串中，再检测delimiters分隔字符串中是否包含该字符，反之，若ASCII码值大于分隔字符串中最大的maxDelimCodePoint，也就是说该字符一定不存在于分隔字符串里，&&直接跳过delimiters.indexOf的检测，也就达到了优化分隔符检测的效果了）。
private int scanToken(int startPos) {
int position = startPos;
while (position < maxPosition) {
if (!hasSurrogates) {
char c = str.charAt(position);
if ((c <= maxDelimCodePoint) && (delimiters.indexOf(c) >= 0))
break;
position++;
} else {
int c = str.codePointAt(position);
if ((c <= maxDelimCodePoint) && isDelimiter(c))
break;
position += Character.charCount(c);
}
}
if (retDelims && (startPos == position)) {
if (!hasSurrogates) {
char c = str.charAt(position);
if ((c <= maxDelimCodePoint) && (delimiters.indexOf(c) >= 0))
position++;
} else {
int c = str.codePointAt(position);
if ((c <= maxDelimCodePoint) && isDelimiter(c))
position += Character.charCount(c);
}
}
return position;
}scanToken方法即跳过分隔字符串，只要某此循环时，该字符包含在分隔字符串里，那么position不再自增，以此时的position值作为实际切割获取字符串的末索引， 因为subString方法是左闭右开的，该值是实际获取字符串的末索引值+1，所以可以截取到完整的不包含分隔符的字符串片段。
skipDelimiters方法类似，即过滤连续包含于分隔字符串中的字符，获取实际需要切割获取的字符串的开始索引值。
private int skipDelimiters(int startPos) {
if (delimiters == null)
throw new NullPointerException();
int position = startPos;
while (!retDelims && position < maxPosition) {
if (!hasSurrogates) {
char c = str.charAt(position);
if ((c > maxDelimCodePoint) || (delimiters.indexOf(c) < 0))
break;
position++;
} else {
int c = str.codePointAt(position);
if ((c > maxDelimCodePoint) || !isDelimiter(c)) {
break;
}
position += Character.charCount(c);
}
}
return position;
}上述分析可知，只要待切割字符串中的字符，在分隔字符串中出现，那么就会做一次切割（也就是不论分隔字符串中的每个char或字符串片段的顺序，只要连续包含在分隔字符串里，就切割）。
演示如下（注意countTokens()方法不要在循环中和nextToken()一同使用）：
public static void print2(String str, String delimiter, boolean isReturnDelims) {
StringTokenizer strTokenizer = new StringTokenizer(str, delimiter);
System.out.println("总数目:" + strTokenizer.countTokens());
int count;
String[] strs = new String[count = strTokenizer.countTokens()];
// 注意：不要在循环里写 int i = 0; i < strTokenizer.countTokens();
// 因为 countTokens方法需要使用currentPosition，而每次执行nextToken方法时，currentPosition会一直往下偏移计算，
// 会导致循环中， i < strTokenizer.countTokens();发生改变，这里应该是常量总数目
for (int i = 0; i < count; i++) {
String s = strTokenizer.nextToken();
strs[i] = s;
}
System.out.println(Arrays.toString(strs));
}countTokens源码如下：
public int countTokens() {
int count = 0;
int currpos = currentPosition;
while (currpos < maxPosition) {
currpos = skipDelimiters(currpos);
if (currpos >= maxPosition)
break;
currpos = scanToken(currpos);
count++;
}
return count;
}执行：
print2("1a2b3c4ca5bc6ba7abc8acbbaba9", "abc", false);
结果如下所示：
总数目:9
[1, 2, 3, 4, 5, 6, 7, 8, 9]
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