从底层重学 Java 之 String 字符串

从底层，从原理，我们来重学一次 Java。String 是字符串类，包含比特流与字符串的互转，字符串的常用切割、定位、相等判断，他的源码及实现是怎样的呢？

本系列秉承所有结论尽量从源码中来，没有源码的尽量标明出处。相关源码会附着在文章中，读本文即可，不用再自行查找源码及资料学习，方便大家充分利用路上的碎片时间。

本篇 Chat 对 String 的属性、构造函数、常用字符串处理进行逐一源码分析，帮助大家深入理解和学习 JDK 源码的牛掰优化。

本文包含以下内容：

• 类信息
  • 类的定义
  • 类的继承结构
• 局部变量
  • value
  • hash
• 构造函数
  • String()
  • String(String original)
  • String(StringBuffer buffer)
  • String(StringBuilder builder)
  • String(char value[])
  • String(char[] value, boolean share)
  • String(char value[], int offset, int count)
  • String(int[] codePoints, int offset, int count)
    • Character.isBmpCodePoint(int codePoint)
    • Character.isValidCodePoint(int codePoint)
    • Character.toSurrogates(int codePoint, char[] dst, int index)
    • Character.highSurrogate(int codePoint)
    • Character.lowSurrogate(int codePoint)
  • String(byte bytes[], int offset, int length, String charsetName)
    • checkBounds(byte[] bytes, int offset, int length)
    • StringCoding.decode(String charsetName, byte[] ba, int off, int len)
    • StringCoding.deref(ThreadLocal<SoftReference<T>> tl)
    • StringCoding.lookupCharset(String csn)
    • StringCoding.StringDecoder
    • StringCoding.set(ThreadLocal<SoftReference<T>> tl, T ob)
• CharSequence
  • 接口定义
  • length()
  • charAt(int index)
  • subSequence(int start, int end)
  • toString()
  • chars()
  • codePoints()
• length()
• isEmpty()
• charAt(int index)
• getBytes(String charsetName)
• getBytes(Charset charset)
• getBytes()
  • StringCoding.encode(char[] ca, int off, int len)
  • System.exit(int status)
  • StringCoding.encode(String charsetName, char[] ca, int off, int len)
  • StringCoding.deref
  • StringCoding.set
  • StringCoding.lookupCharset(String csn)
  • StringCoding.set(ThreadLocal tl, T ob)
  • StringCoding.StringEncoder
  • StringCoding.safeTrim(byte[] ba, int len, Charset cs, boolean isTrusted)
  • Arrays.copyOf(byte[] original, int newLength)
  • CharsetEncoder.encode(CharBuffer in, ByteBuffer out, boolean endOfInput)
  • CharsetEncoder.flush(ByteBuffer out)
• equals(Object anObject)
  • 为什么“==”不行呢？
• contentEquals(StringBuffer sb)
• contentEquals(CharSequence cs)
• equalsIgnoreCase(String anotherString)
• regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len)
• regionMatches(int toffset, String other, int ooffset, int len)
• compareTo(String anotherString)
• CASEINSENSITIVEORDER
• compareToIgnoreCase(String str)
• startsWith(String prefix, int toffset)
• startsWith(String prefix)
• endsWith(String suffix)
• hashCode()
• indexOf(int ch)
• indexOf(int ch, int fromIndex)
• lastIndexOf(int ch)
• lastIndexOf(int ch, int fromIndex)
• indexOf(String str)
• indexOf(String str, int fromIndex)
• indexOf(char[] source, int sourceOffset, int sourceCount, String target, int fromIndex)
• indexOf(char[] source, int sourceOffset, int sourceCount, char[] target, int targetOffset, int targetCount, int fromIndex)
• lastIndexOf(String str)
• lastIndexOf(String str, int fromIndex)
• lastIndexOf(char[] source, int sourceOffset, int sourceCount, String target, int fromIndex)
• lastIndexOf(char[] source, int sourceOffset, int sourceCount, char[] target, int targetOffset, int targetCount, int fromIndex)
• substring(int beginIndex)
• substring(int beginIndex, int endIndex)
• subSequence(int beginIndex, int endIndex)
• concat(String str)
  • getChars(char dst[], int dstBegin)
• replace(char oldChar, char newChar)
• matches(String regex)
• contains(CharSequence s)
• replaceFirst(String regex, String replacement)
• replaceAll(String regex, String replacement)
• replace(CharSequence target, CharSequence replacement)
• split(String regex, int limit)
• split(String regex)
• join(CharSequence delimiter, CharSequence… elements)
• join(CharSequence delimiter, Iterable<? extends CharSequence> elements)
• StringJoiner
  • StringJoiner(CharSequence delimiter)
  • StringJoiner(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
  • setEmptyValue(CharSequence emptyValue)
  • toString()
  • add(CharSequence newElement)
  • merge(StringJoiner other)
  • prepareBuilder()
  • length()
• toLowerCase(Locale locale)
• toLowerCase()
• ConditionalSpecialCasing
  • toLowerCaseEx(String src, int index, Locale locale)
  • toUpperCaseEx(String src, int index, Locale locale)
  • toLowerCaseCharArray(String src, int index, Locale locale)
  • toUpperCaseCharArray(String src, int index, Locale locale)
  • lookUpTable(String src, int index, Locale locale, boolean bLowerCasing)
  • Entry
  • entry
  • entryTable
• toUpperCase(Locale locale)
• toUpperCase()
• trim()
• toString()
• toCharArray()
• format(String format, Object… args)
• format(Locale l, String format, Object… args)
• valueOf
• valueOf(char data[])
• valueOf(char data[], int offset, int count)
• copyValueOf(char data[], int offset, int count)
• copyValueOf(char data[])
• valueOf(boolean b)
• valueOf(char c)
• valueOf(int i)
• valueOf(long l)
• valueOf(float f)
• valueOf(double d)
• intern()

适用人群：有一些 Java 基础的人群。