java string源码分析_Java String源码分析

String源码分析

类结构

public final class String

implements java.io.Serializable, Comparable, CharSequence

String类实现了Serializable可以被序列化

String类实现了Comparable可以进行比较

String类实现了CharSequence可以按下标进行相关操作

并且String类使用final进行修饰，不可以被继承

属性

//用来存储字符串的每一个字符

private final char value[];

//hash值

private int hash; // Default to 0

//序列化版本号

private static final long serialVersionUID = -6849794470754667710L;

//从变量名大致可以看出和序列化有关，具体的不明白

private static final ObjectStreamField[] serialPersistentFields =

new ObjectStreamField[0];

构造方法

//无参，直接使用空字符串赋值，hash为0

public String() {

this.value = "".value;

}

//使用已有字符串初始化

public String(String original) {

this.value = original.value;

this.hash = original.hash;

}

//使用char数组初始化，hash为0

public String(char value[]) {

this.value = Arrays.copyOf(value, value.length);

}

//使用字符数组，并指定偏移、字符个数初始化

public String(char value[], int offset, int count) {

if (offset < 0) {

throw new StringIndexOutOfBoundsException(offset);

}

if (count <= 0) {

if (count < 0) {

throw new StringIndexOutOfBoundsException(count);

}

if (offset <= value.length) {

this.value = "".value;

return;

}

}

// Note: offset or count might be near -1>>>1.

if (offset > value.length - count) {

throw new StringIndexOutOfBoundsException(offset + count);

}

this.value = Arrays.copyOfRange(value, offset, offset+count);

}

//使用unicode编码数组并指定偏移和数量进行初始化

public String(int[] codePoints, int offset, int count) {

if (offset < 0) {

throw new StringIndexOutOfBoundsException(offset);

}

if (count <= 0) {

if (count < 0) {

throw new StringIndexOutOfBoundsException(count);

}

if (offset <= codePoints.length) {

this.value = "".value;

return;

}

}

// Note: offset or count might be near -1>>>1.

if (offset > codePoints.length - count) {

throw new StringIndexOutOfBoundsException(offset + count);

}

final int end = offset + count;

// Pass 1: Compute precise size of char[]计算char数组大小

int n = count;

for (int i = offset; i < end; i++) {

int c = codePoints[i];

if (Character.isBmpCodePoint(c))//判断编码是不是BMP(Basic Mutilingual Plane)

continue;

else if (Character.isValidCodePoint(c))//验证编码是否在unicode编码范围内

n++;

else throw new IllegalArgumentException(Integer.toString(c));

}

// Pass 2: Allocate and fill in char[] 申明char数组并填入编码对应char

final char[] v = new char[n];

for (int i = offset, j = 0; i < end; i++, j++) {

int c = codePoints[i];

if (Character.isBmpCodePoint(c))//如果编码是BMP直接一个字符就是接受

v[j] = (char)c;

else

Character.toSurrogates(c, v, j++);//转换成两个字符存储

}

this.value = v;

}

//使用ascii码数组进行初始化

@Deprecated

public String(byte ascii[], int hibyte, int offset, int count) {

checkBounds(ascii, offset, count);

char value[] = new char[count];

if (hibyte == 0) {

for (int i = count; i-- > 0;) {

value[i] = (char)(ascii[i + offset] & 0xff);

}

} else {

hibyte <<= 8;

for (int i = count; i-- > 0;) {

value[i] = (char)(hibyte | (ascii[i + offset] & 0xff));

}

}

this.value = value;

}

@Deprecated

public String(byte ascii[], int hibyte) {

this(ascii, hibyte, 0, ascii.length);

}

//使用字节数组+字符集名初始化

public String(byte bytes[], int offset, int length, String charsetName)

throws UnsupportedEncodingException {

if (charsetName == null)

throw new NullPointerException("charsetName");

checkBounds(bytes, offset, length);

this.value = StringCoding.decode(charsetName, bytes, offset, length);

}

//使用字节数组+字符集名初始化

public String(byte bytes[], int offset, int length, Charset charset) {

if (charset == null)

throw new NullPointerException("charset");

checkBounds(bytes, offset, length);

this.value = StringCoding.decode(charset, bytes, offset, length);

}

//使用字节数组+字符集名初始化

public String(byte bytes[], String charsetName)

throws UnsupportedEncodingException {

this(bytes, 0, bytes.length, charsetName);

}

//使用字节数组+字符集名初始化

public String(byte bytes[], Charset charset) {

this(bytes, 0, bytes.length, charset);

}

//使用字节数组初始化

public String(byte bytes[], int offset, int length) {

checkBounds(bytes, offset, length);

this.value = StringCoding.decode(bytes, offset, length);

}

public String(byte bytes[]) {

this(bytes, 0, bytes.length);

}

//使用StringBuffer初始化

public String(StringBuffer buffer) {

synchronized(buffer) {

this.value = Arrays.copyOf(buffer.getValue(), buffer.length());

}

}

//使用StringBuilder初始化

public String(StringBuilder builder) {

this.value = Arrays.copyOf(builder.getValue(), builder.length());

}

方法

静态方法

join(CharSequence，CharSequence...)使用分隔符拼接字符串

public static String join(CharSequence delimiter, CharSequence... elements) {

Objects.requireNonNull(delimiter);

Objects.requireNonNull(elements);

// Number of elements not likely worth Arrays.stream overhead.

StringJoiner joiner = new StringJoiner(delimiter);

for (CharSequence cs: elements) {

joiner.add(cs);

}

return joiner.toString();

}

join(CharSequence，Iterable extends CharSequence>)使用分隔符拼接字符串

public static String join(CharSequence delimiter,

Iterable extends CharSequence> elements) {

Objects.requireNonNull(delimiter);

Objects.requireNonNull(elements);

StringJoiner joiner = new StringJoiner(delimiter);

for (CharSequence cs: elements) {

joiner.add(cs);

}

return joiner.toString();

}

format(String，Object...)使用字符串格式指定参数进行格式化生成字符串

public static String format(String format, Object... args) {

return new Formatter().format(format, args).toString();

}

format(Local，String，Object...)根据环境使用字符串格式指定参数进行格式化生成字符串

public static String format(Locale l, String format, Object... args) {

return new Formatter(l).format(format, args).toString();

}

valueOf(Object)对象转换成字符串，如果对象为null转为为字符串“null”

public static String valueOf(Object obj) {

return (obj == null) ? "null" : obj.toString();

}

valueOf(char[])char数组转换成字符串

public static String valueOf(char data[]) {

return new String(data);

}

valueOf(xxx)xxx数据类型转换为字符串

public static String valueOf(boolean b) {

return b ? "true" : "false";

}

public static String valueOf(char c) {

char data[] = {c};

return new String(data, true);

}

public static String valueOf(int i) {

return Integer.toString(i);

}

public static String valueOf(long l) {

return Long.toString(l);

}

public static String valueOf(float f) {

return Float.toString(f);

}

public static String valueOf(double d) {

return Double.toString(d);

}

valueOf(char[],int,int)char数组按照偏移个指定字符格式转换为字符串

public static String valueOf(char data[], int offset, int count) {

return new String(data, offset, count);

}

copyValueOf(char，int，int)使用指定字符数组根据偏移和字符个数拷贝一个新字符串，同valueOf

public static String copyValueOf(char data[], int offset, int count) {

return new String(data, offset, count);

}

copyValueOf(char[])使用指定字符数组拷贝新字符串

public static String copyValueOf(char data[]) {

return new String(data);

}

成员方法

char charAt(int index)获取指定下标的字符

public char charAt(int index) {

if ((index < 0) || (index >= value.length)) {

throw new StringIndexOutOfBoundsException(index);

}

return value[index];

}

void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin)把当前字符串的char数组的指定范围拷贝到目标char数组的指定位置

public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {

if (srcBegin < 0) {

throw new StringIndexOutOfBoundsException(srcBegin);

}

if (srcEnd > value.length) {

throw new StringIndexOutOfBoundsException(srcEnd);

}

if (srcBegin > srcEnd) {

throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);

}

//System.arraycopy(Object src, int srcPos,Object dest, int destPos,int length)

//src：要拷贝的源数组

//srcPos：源数组拷贝的起始位置

//dest：目标数组

//destPost：拷贝到目标数组的起始位置

//length：要拷贝元素的个数

System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);

}

byte[] getBytes(String charsetName)根据字符集获取字符串的编码后的字节数组

public byte[] getBytes(String charsetName)

throws UnsupportedEncodingException {

if (charsetName == null) throw new NullPointerException();

return StringCoding.encode(charsetName, value, 0, value.length);

}

boolean equals(Object anObject)方法比较两个字符串，重写的Object方法

public boolean equals(Object anObject) {

if (this == anObject) {//地址相等两对象equals为true

return true;

}

if (anObject instanceof String) {

String anotherString = (String)anObject;

int n = value.length;

if (n == anotherString.value.length) {//判断两字符串的字符个数

char v1[] = value;

char v2[] = anotherString.value;

int i = 0;

while (n-- != 0) {

if (v1[i] != v2[i])//有一个字符不相等最直接为false

return false;

i++;

}

return true;

}

}

return false;

}

contentEquals(CharSequence cs)判断当前String与其他字符序列是否相等，与equals不同的是，equals只有当两个对象都是String时equals才为true，contentEquals可以用来同其他StringBuffer、StringBuilder和其他字符序列进行比较

public boolean contentEquals(CharSequence cs) {

// Argument is a StringBuffer, StringBuilder

if (cs instanceof AbstractStringBuilder) {

if (cs instanceof StringBuffer) {

synchronized(cs) {//如果是StringBuffer那么进行上锁操作

return nonSyncContentEquals((AbstractStringBuilder)cs);

}

} else {//StringBuilder不上锁

return nonSyncContentEquals((AbstractStringBuilder)cs);

}

}

// Argument is a String

if (cs instanceof String) {

return equals(cs);

}

// Argument is a generic CharSequence

char v1[] = value;

int n = v1.length;

if (n != cs.length()) {

return false;

}

for (int i = 0; i < n; i++) {//其他字符序列，一个一个字符进行比较

if (v1[i] != cs.charAt(i)) {

return false;

}

}

return true;

}

equalsIgnoreCase(String anotherString)两个字符串忽略大小写进行比较是否相等

public boolean equalsIgnoreCase(String anotherString) {

return (this == anotherString) ? true

: (anotherString != null)//不为空

&& (anotherString.value.length == value.length)//字符个数相等

&& regionMatches(true, 0, anotherString, 0, value.length);//忽略大小写比较

}

public boolean regionMatches(boolean ignoreCase, int toffset,

String other, int ooffset, int len) {

char ta[] = value;

int to = toffset;

char pa[] = other.value;

int po = ooffset;

// Note: toffset, ooffset, or len might be near -1>>>1.

if ((ooffset < 0) || (toffset < 0)

|| (toffset > (long)value.length - len)

|| (ooffset > (long)other.value.length - len)) {

return false;

}

while (len-- > 0) {

char c1 = ta[to++];

char c2 = pa[po++];

if (c1 == c2) {

continue;

}

if (ignoreCase) {

// If characters don't match but case may be ignored,

// try converting both characters to uppercase.

// If the results match, then the comparison scan should

// continue.

//把两个字符转换成大写的

char u1 = Character.toUpperCase(c1);

char u2 = Character.toUpperCase(c2);

if (u1 == u2) {

continue;

}

// Unfortunately, conversion to uppercase does not work properly

// for the Georgian alphabet, which has strange rules about case

// conversion. So we need to make one last check before

// exiting.

//转换成大写的不相等，在转换成小写的判断

if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {

continue;

}

}

return false;

}

return true;

}

int compareTo(String anotherString)进行字符串的比较

public int compareTo(String anotherString) {

int len1 = value.length;

int len2 = anotherString.value.length;

int lim = Math.min(len1, len2);

char v1[] = value;

char v2[] = anotherString.value;

int k = 0;

while (k < lim) {

char c1 = v1[k];

char c2 = v2[k];

if (c1 != c2) {//如果当前字符串的字符比参数的大返回正数，否则返回负数

return c1 - c2;

}

k++;

}

//如果两个字符串，长度小的字符串与长度大的前部分每个字符都相等，如果两字符串长度相等返回0，当前字符串长度大于参数字符串返回整数，当前字符串长度小于参数字符串返回负数

return len1 - len2;

}

compareToIgnoreCase(String str)字符串忽略大小写进行比较

public int compareToIgnoreCase(String str) {

return CASE_INSENSITIVE_ORDER.compare(this, str);

}

public int compare(String s1, String s2) {

int n1 = s1.length();

int n2 = s2.length();

int min = Math.min(n1, n2);

for (int i = 0; i < min; i++) {

char c1 = s1.charAt(i);

char c2 = s2.charAt(i);

if (c1 != c2) {

c1 = Character.toUpperCase(c1);

c2 = Character.toUpperCase(c2);

if (c1 != c2) {

c1 = Character.toLowerCase(c1);

c2 = Character.toLowerCase(c2);

if (c1 != c2) {

//如果两字符不相等，最后是转换成小写的进行比较

return c1 - c2;

}

}

}

}

return n1 - n2;

}

startsWith(String prefix)判断字符串是否以指定字符串开头

public boolean startsWith(String prefix) {

return startsWith(prefix, 0);

}

public boolean startsWith(String prefix, int toffset) {

char ta[] = value;

int to = toffset;

char pa[] = prefix.value;

int po = 0;

int pc = prefix.value.length;

// Note: toffset might be near -1>>>1.

if ((toffset < 0) || (toffset > value.length - pc)) {

return false;

}

while (--pc >= 0) {//循环给定前缀字符串长度

if (ta[to++] != pa[po++]) {//前缀字符串字符和当前字符串字符比较

return false;

}

}

return true;

}

boolean endsWith(String suffix)判断字符串是否以指定字符串结尾

public boolean endsWith(String suffix) {

return startsWith(suffix, value.length - suffix.value.length);

}

int hashCode()获取字符串的hashCode

public int hashCode() {

//默认字符串hash为0，如果是用另一个字符串就等于另一个字符串的hash

int h = hash;

if (h == 0 && value.length > 0) {

char val[] = value;

for (int i = 0; i < value.length; i++) {//一个一个字符的变量

//前面字符的hash*31+当前字符的ascii码

h = 31 * h + val[i];

}

hash = h;

}

return h;

}

int indexOf(int ch)根据unicode编码获取下标

public int indexOf(int ch) {

return indexOf(ch, 0);

}

public int indexOf(int ch, int fromIndex) {

final int max = value.length;

if (fromIndex < 0) {

fromIndex = 0;

} else if (fromIndex >= max) {//如果查找的起始位置超过了数组下标

// Note: fromIndex might be near -1>>>1.

return -1;

}

if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {

//编码是一个基本多语言编码

// handle most cases here (ch is a BMP code point or a

// negative value (invalid code point))

final char[] value = this.value;

for (int i = fromIndex; i < max; i++) {

if (value[i] == ch) {

return i;

}

}

return -1;

} else {

//获取需要使用两个char存储的编码的下标

return indexOfSupplementary(ch, fromIndex);

}

}

private int indexOfSupplementary(int ch, int fromIndex) {

if (Character.isValidCodePoint(ch)) {//是一个合法的unicode编码

final char[] value = this.value;

final char hi = Character.highSurrogate(ch);

final char lo = Character.lowSurrogate(ch);

final int max = value.length - 1;

for (int i = fromIndex; i < max; i++) {

if (value[i] == hi && value[i + 1] == lo) {

return i;

}

}

}

return -1;

}

int lastIndexOf(int ch)获取指定编码从后往前搜索的第一个下标

public int lastIndexOf(int ch) {

return lastIndexOf(ch, value.length - 1);

}

public int lastIndexOf(int ch, int fromIndex) {

if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) {//编码是一个基本多语言unicode编码，使用一个char存储

// handle most cases here (ch is a BMP code point or a

// negative value (invalid code point))

final char[] value = this.value;

int i = Math.min(fromIndex, value.length - 1);

for (; i >= 0; i--) {

if (value[i] == ch) {

return i;

}

}

return -1;

} else {

//编码使用两个char存储

return lastIndexOfSupplementary(ch, fromIndex);

}

}

private int lastIndexOfSupplementary(int ch, int fromIndex) {

if (Character.isValidCodePoint(ch)) {

final char[] value = this.value;

char hi = Character.highSurrogate(ch);

char lo = Character.lowSurrogate(ch);

int i = Math.min(fromIndex, value.length - 2);

for (; i >= 0; i--) {

if (value[i] == hi && value[i + 1] == lo) {

return i;

}

}

}

return -1;

}

int indexOf(String str)获取指定字符串的的第一个字符在当前字符串的下标

public int indexOf(String str) {

return indexOf(str, 0);

}

public int indexOf(String str, int fromIndex) {

return indexOf(value, 0, value.length,

str.value, 0, str.value.length, fromIndex);

}

static int indexOf(char[] source, int sourceOffset, int sourceCount,

char[] target, int targetOffset, int targetCount,

int fromIndex) {

if (fromIndex >= sourceCount) {

return (targetCount == 0 ? sourceCount : -1);

}

if (fromIndex < 0) {

fromIndex = 0;

}

if (targetCount == 0) {

return fromIndex;

}

char first = target[targetOffset];

int max = sourceOffset + (sourceCount - targetCount);

for (int i = sourceOffset + fromIndex; i <= max; i++) {

/* Look for first character. */

if (source[i] != first) {

while (++i <= max && source[i] != first);

}

/* Found first character, now look at the rest of v2 */

if (i <= max) {

int j = i + 1;

//计算终止下标

int end = j + targetCount - 1;

for (int k = targetOffset + 1; j < end && source[j]

== target[k]; j++, k++);

if (j == end) {

/* Found whole string. */

return i - sourceOffset;

}

}

}

return -1;

}

String substring(int beginIndex)获取指定下标到最末下标的字符串

public String substring(int beginIndex) {

if (beginIndex < 0) {

throw new StringIndexOutOfBoundsException(beginIndex);

}

//计算长度

int subLen = value.length - beginIndex;

if (subLen < 0) {

throw new StringIndexOutOfBoundsException(subLen);

}

return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);

}

String substring(int beginIndex, int endIndex)根据起始下标和结束下标获取字符串，包含起始下标字符，不包含结束下标字符

public String substring(int beginIndex, int endIndex) {

if (beginIndex < 0) {

throw new StringIndexOutOfBoundsException(beginIndex);

}

if (endIndex > value.length) {

throw new StringIndexOutOfBoundsException(endIndex);

}

//计算字符个数

int subLen = endIndex - beginIndex;

if (subLen < 0) {

throw new StringIndexOutOfBoundsException(subLen);

}

return ((beginIndex == 0) && (endIndex == value.length)) ? this

: new String(value, beginIndex, subLen);

}

String concat(String str)把参数字符串拼接到当前字符串

public String concat(String str) {

int otherLen = str.length();

if (otherLen == 0) {

return this;

}

//获取拼接字符串的长度

int len = value.length;

//把原字符串的字符拷贝到一个大小为原字符串大小+参数字符串大小的新数组中

char buf[] = Arrays.copyOf(value, len + otherLen);

//把拼接字符串的字符拷贝到数组中

str.getChars(buf, len);

return new String(buf, true);

}

String replace(char oldChar, char newChar)把指定字符替换为新字符

public String replace(char oldChar, char newChar) {

if (oldChar != newChar) {

int len = value.length;

int i = -1;

char[] val = value; /* avoid getfield opcode */

while (++i < len) {

if (val[i] == oldChar) {//找到需要替换字符的位置

break;

}

}

if (i < len) {

char buf[] = new char[len];

for (int j = 0; j < i; j++) {

buf[j] = val[j];

}

while (i < len) {

char c = val[i];

buf[i] = (c == oldChar) ? newChar : c;//把原字符替换为新字符

i++;

}

return new String(buf, true);

}

}

return this;

}

boolean matches(String regex)判断正则表达式是否比配当前字符串

public boolean matches(String regex) {

return Pattern.matches(regex, this);

}

boolean contains(CharSequence s)判断当前字符串是否包含另一个字符序列

public boolean contains(CharSequence s) {

return indexOf(s.toString()) > -1;

}

String trim()去掉字符串前后的空格

public String trim() {

int len = value.length;

int st = 0;

char[] val = value; /* avoid getfield opcode */

//找到字符串由前往后第一个不是空格的位置

while ((st < len) && (val[st] <= ' ')) {

st++;

}

//找到字符串由后往前第一个不是空格的位置

while ((st < len) && (val[len - 1] <= ' ')) {

len--;

}

return ((st > 0) || (len < value.length)) ? substring(st, len) : this;

}

char[] toCharArray()把字符串转换成字符数组

public char[] toCharArray() {

// Cannot use Arrays.copyOf because of class initialization order issues

char result[] = new char[value.length];

//使用System.arraycopy方法拷贝

System.arraycopy(value, 0, result, 0, value.length);

return result;

}

本地方法

native String intern();获取字符串所指向的字符串常量池中对象的地址

@Test

public void test8() {

String s1="abc";

String s2=new String("abc");

System.out.println(s1==s2);//false

System.out.println(s1==s2.intern());//true

}

使用s1="abc"这种方式栈中变量s1直接指向字符串常量池中的常量“abc”，而s2=new String("abc")这种方式，栈中变量s2指向的是对中一个变量t，t指向字符串常量池中的“abc”，所以s1和s2指向的地址不相同

s2.intern()获取的是字符串的常量池中的地址，也就是如果变量直接指向常量池，那么就是变量的地址，如果变量指向堆，那么会获取堆所指向字符串常量池中的地址