不管是公司的项目，还是个人的日常练习，都会使用 org.apache.commons.lang3.StringUtils 这个String工具类，究其原因，是太好用了，而且太全了。比如判断是否为空，截取，拼接之类的。一直没有仔细的看过它的源码，今天我就研究一番，同时作为我“阅读源码”路上的第一步。
//todo
什么是CharSequence
//todo
‘’，null，‘ ’区别
注：本次是基于commons-lang3-3.8.1 进行阅读分析总结的，其他版本与此相差不多。
//源码 public static boolean isEmpty(final CharSequence cs) { return cs == null || cs.length() == 0; }
@Test public void isEmpty_Test() { //true StringUtils.isEmpty(null); //true StringUtils.isEmpty(""); //false StringUtils.isEmpty(" "); //false StringUtils.isEmpty("bob"); //false StringUtils.isEmpty(" bob "); }
//源码 public static boolean isNotEmpty(final CharSequence cs) { return !isEmpty(cs); }
@Test public void isNotEmpty_Test() { //false StringUtils.isNotEmpty(null); //false StringUtils.isNotEmpty(""); //true StringUtils.isNotEmpty(" "); //true StringUtils.isNotEmpty("bob"); //true StringUtils.isNotEmpty(" bob "); }
与isEmpty相比，对于“ ”，isEmpty(" ") =false，isBlank(" ") =true,
//源码 public static boolean isBlank(final CharSequence cs) { int strLen; if (cs == null || (strLen = cs.length()) == 0) { return true; } for (int i = 0; i < strLen; i++) { if (!Character.isWhitespace(cs.charAt(i))) { return false; } } return true; }
@Test public void isBlank_Test() { //true StringUtils.isBlank(null); //true StringUtils.isBlank(""); //true StringUtils.isBlank(" "); //false StringUtils.isBlank("bob"); //false StringUtils.isBlank(" bob "); }
@Test public void isNotBlank_Test() { //false StringUtils.isNotBlank(null); //false StringUtils.isNotBlank(""); //false StringUtils.isNotBlank(" "); //true StringUtils.isNotBlank("bob"); //true StringUtils.isNotBlank(" bob "); }
//源码 public static String trim(final String str) { return str == null ? null : str.trim(); }
@Test public void trim_Test() { //null StringUtils.trim(null); //"" StringUtils.trim(""); //"" StringUtils.trim(" "); //"abc" StringUtils.trim("abc"); //"abc" StringUtils.trim(" abc "); }
//源码 public static String trimToNull(final String str) { final String ts = trim(str); return isEmpty(ts) ? null : ts; }
StringUtils.trimToNull(null) = null StringUtils.trimToNull("") = null StringUtils.trimToNull(" ") = null StringUtils.trimToNull("abc") = "abc" StringUtils.trimToNull(" abc ") = "abc"
//源码 public static String trimToEmpty(final String str) { return str == null ? EMPTY : str.trim(); }
StringUtils.trimToEmpty(null) = "" StringUtils.trimToEmpty("") = "" StringUtils.trimToEmpty(" ") = "" StringUtils.trimToEmpty("abc") = "abc" StringUtils.trimToEmpty(" abc ") = "abc"
//源码 public static String truncate(final String str, final int maxWidth) { return truncate(str, 0, maxWidth); }
StringUtils.truncate(null, 0) = null StringUtils.truncate(null, 2) = null StringUtils.truncate("", 4) = "" StringUtils.truncate("abcdefg", 4) = "abcd" StringUtils.truncate("abcdefg", 6) = "abcdef" StringUtils.truncate("abcdefg", 7) = "abcdefg" StringUtils.truncate("abcdefg", 8) = "abcdefg" StringUtils.truncate("abcdefg", -1) = throws an IllegalArgumentException
可以看到，对于起始截取索引，截取长度为负数时，直接报异常；然后被截取字符串为null时，返回null；截取索引大于字符串长度时，返回“”。
//源码 public static String truncate(final String str, final int offset, final int maxWidth) { if (offset < 0) { throw new IllegalArgumentException("offset cannot be negative"); } if (maxWidth < 0) { throw new IllegalArgumentException("maxWith cannot be negative"); } if (str == null) { return null; } if (offset > str.length()) { return EMPTY; } if (str.length() > maxWidth) { final int ix = offset + maxWidth > str.length() ? str.length() : offset + maxWidth; return str.substring(offset, ix); } return str.substring(offset); }
StringUtils.truncate(null, 0, 0) = null StringUtils.truncate(null, 2, 4) = null StringUtils.truncate("", 0, 10) = "" StringUtils.truncate("", 2, 10) = "" StringUtils.truncate("abcdefghij", 0, 3) = "abc" StringUtils.truncate("abcdefghij", 5, 6) = "fghij" StringUtils.truncate("raspberry peach", 10, 15) = "peach" StringUtils.truncate("abcdefghijklmno", 0, 10) = "abcdefghij" StringUtils.truncate("abcdefghijklmno", -1, 10) = throws an IllegalArgumentException StringUtils.truncate("abcdefghijklmno", Integer.MIN_VALUE, 10) = "abcdefghij" StringUtils.truncate("abcdefghijklmno", Integer.MIN_VALUE, Integer.MAX_VALUE) = "abcdefghijklmno" StringUtils.truncate("abcdefghijklmno", 0, Integer.MAX_VALUE) = "abcdefghijklmno" StringUtils.truncate("abcdefghijklmno", 1, 10) = "bcdefghijk" StringUtils.truncate("abcdefghijklmno", 2, 10) = "cdefghijkl" StringUtils.truncate("abcdefghijklmno", 3, 10) = "defghijklm" StringUtils.truncate("abcdefghijklmno", 4, 10) = "efghijklmn" StringUtils.truncate("abcdefghijklmno", 5, 10) = "fghijklmno" StringUtils.truncate("abcdefghijklmno", 5, 5) = "fghij" StringUtils.truncate("abcdefghijklmno", 5, 3) = "fgh" StringUtils.truncate("abcdefghijklmno", 10, 3) = "klm" StringUtils.truncate("abcdefghijklmno", 10, Integer.MAX_VALUE) = "klmno" StringUtils.truncate("abcdefghijklmno", 13, 1) = "n" StringUtils.truncate("abcdefghijklmno", 13, Integer.MAX_VALUE) = "no" StringUtils.truncate("abcdefghijklmno", 14, 1) = "o" StringUtils.truncate("abcdefghijklmno", 14, Integer.MAX_VALUE) = "o" StringUtils.truncate("abcdefghijklmno", 15, 1) = "" StringUtils.truncate("abcdefghijklmno", 15, Integer.MAX_VALUE) = "" StringUtils.truncate("abcdefghijklmno", Integer.MAX_VALUE, Integer.MAX_VALUE) = "" StringUtils.truncate("abcdefghij", 3, -1) = throws an IllegalArgumentException StringUtils.truncate("abcdefghij", -2, 4) = throws an IllegalArgumentException
org.apache.commons.lang3.StringUtils 源码解读汇总
原文：https://www.cnblogs.com/DDgougou/p/13250684.html