从源码角度简单看StringBuilder和StringBuffer的异同(全面解析)
概述
StringBuilder和StringBuffer是两个容易混淆的概念，本文从源码入手，简单看二者的异同。
容易知道的是，这两者有一个是线程安全的，而且线程安全的那个效率低。
javadoc里面的说明
javadoc是写源码的人写的注释，先看javadoc。
StringBuilder
Amutablesequenceofcharacters.ThisclassprovidesanAPIcompatiblewithStringBuffer,butwithnoguaranteeofsynchronization.Thisclassisdesignedforuseasadrop-inreplacementforStringBufferinplaceswherethestringbufferwasbeingusedbyasinglethread(asisgenerallythecase).Wherepossible,itisrecommendedthatthisclassbeusedinpreferencetoStringBufferasitwillbefasterundermostimplementations.
TheprincipaloperationsonaStringBuilderaretheappendandinsertmethods,whichareoverloadedsoastoacceptdataofanytype.Eacheffectivelyconvertsagivendatumtoastringandthenappendsorinsertsthecharactersofthatstringtothestringbuilder.Theappendmethodalwaysaddsthesecharactersattheendofthebuilder;theinsertmethodaddsthecharactersataspecifiedpoint.
Forexample,ifzreferstoastringbuilderobjectwhosecurrentcontentsare"start",thenthemethodcallz.append("le")wouldcausethestringbuildertocontain"startle",whereasz.insert(4,"le")wouldalterthestringbuildertocontain"starlet".
Ingeneral,ifsbreferstoaninstanceofaStringBuilder,thensb.append(x)hasthesameeffectassb.insert(sb.length(),x).
Everystringbuilderhasacapacity.Aslongasthelengthofthecharactersequencecontainedinthestringbuilderdoesnotexceedthecapacity,itisnotnecessarytoallocateanewinternalbuffer.Iftheinternalbufferoverflows,itisautomaticallymadelarger.
InstancesofStringBuilderarenotsafeforusebymultiplethreads.Ifsuchsynchronizationisrequiredthenitisrecommendedthatjava.lang.StringBufferbeused.
Unlessotherwisenoted,passinganullargumenttoaconstructorormethodinthisclasswillcauseaNullPointerExceptiontobethrown.
Since:
1.5
Author:
MichaelMcCloskey
SeeAlso:
java.lang.StringBuffer
java.lang.String
StringBuffer
Athread-safe,mutablesequenceofcharacters.AstringbufferislikeaString,butcanbemodified.Atanypointintimeitcontainssomeparticularsequenceofcharacters,butthelengthandcontentofthesequencecanbechangedthroughcertainmethodcalls.
Stringbuffersaresafeforusebymultiplethreads.Themethodsaresynchronizedwherenecessarysothatalltheoperationsonanyparticularinstancebehaveasiftheyoccurinsomeserialorderthatisconsistentwiththeorderofthemethodcallsmadebyeachoftheindividualthreadsinvolved.
TheprincipaloperationsonaStringBufferaretheappendandinsertmethods,whichareoverloadedsoastoacceptdataofanytype.Eacheffectivelyconvertsagivendatumtoastringandthenappendsorinsertsthecharactersofthatstringtothestringbuffer.Theappendmethodalwaysaddsthesecharactersattheendofthebuffer;theinsertmethodaddsthecharactersataspecifiedpoint.
Forexample,ifzreferstoastringbufferobjectwhosecurrentcontentsare"start",thenthemethodcallz.append("le")wouldcausethestringbuffertocontain"startle",whereasz.insert(4,"le")wouldalterthestringbuffertocontain"starlet".
Ingeneral,ifsbreferstoaninstanceofaStringBuffer,thensb.append(x)hasthesameeffectassb.insert(sb.length(),
Wheneveranoperationoccursinvolvingasourcesequence(suchasappendingorinsertingfromasourcesequence),thisclasssynchronizesonlyonthestringbufferperformingtheoperation,notonthesource.NotethatwhileStringBufferisdesignedtobesafetouseconcurrentlyfrommultiplethreads,iftheconstructorortheappendorinsertoperationispassedasourcesequencethatissharedacrossthreads,thecallingcodemustensurethattheoperationhasaconsistentandunchangingviewofthesourcesequenceforthedurationoftheoperation.Thiscouldbesatisfiedbythecallerholdingalockduringtheoperation'scall,byusinganimmutablesourcesequence,orbynotsharingthesourcesequenceacrossthreads.
Everystringbufferhasacapacity.Aslongasthelengthofthecharactersequencecontainedinthestringbufferdoesnotexceedthecapacity,itisnotnecessarytoallocateanewinternalbufferarray.Iftheinternalbufferoverflows,itisautomaticallymadelarger.
Unlessotherwisenoted,passinganullargumenttoaconstructorormethodinthisclasswillcauseaNullPointerExceptiontobethrown.
AsofreleaseJDK5,thisclasshasbeensupplementedwithanequivalentclassdesignedforusebyasinglethread,StringBuilder.TheStringBuilderclassshouldgenerallybeusedinpreferencetothisone,asitsupportsallofthesameoperationsbutitisfaster,asitperformsnosynchronization.
Since:
JDK1.0
Author:
ArthurvanHoff
SeeAlso:
java.lang.StringBuilder
java.lang.String
javadoc小结
从上面可以看出：
StringBuffer和StringBuilder都可以认为是可变的String。
StringBuffer是线程安全的，先出现，在JDK1.0的时候就有了。
StringBuilder是非线程安全的，后出现，在JDK1.5才有。
二者的接口完全一样，StringBuilder更快。
其实，正常的用，知道这几点就好了，不过还是想看看源码里面怎么实现的。
源码
如何实现线程安全
看源码可以知道，这二者都继承了一个抽象类AbstractStringBuilder
publicfinalclassStringBuffer extendsAbstractStringBuilder implementsjava.io.Serializable,CharSequence publicfinalclassStringBuilder extendsAbstractStringBuilder implementsjava.io.Serializable,CharSequence
代码量也不是很大，StringBuilder440行代码，StringBuffer718行代码，最多的是AbstractStringBuilder，一共1440行代码。
看代码从几个角度看，一是一些关键的内部结构是怎么样的，二是我们常用的函数是怎么实现的。因为String是不可变的，是放在常量池里面的。猜测StringBuilder和StringBuffer应该是用char数组实现的。
/** *Thevalueisusedforcharacterstorage. */ char[]value; /** *Thecountisthenumberofcharactersused. */ intcount;
可以看出，用value存数据，用一个count表示长度。
看几个常见方法的不同实现。
StringBuffer
@Override
publicsynchronizedStringBufferappend(Stringstr){
toStringCache=null;
super.append(str);
returnthis;
}
/**
*@throwsStringIndexOutOfBoundsException{@inheritDoc}
*/
@Override
publicsynchronizedStringBufferinsert(intoffset,Stringstr){
toStringCache=null;
super.insert(offset,str);
returnthis;
}
@Override
publicsynchronizedStringtoString(){
if(toStringCache==null){
toStringCache=Arrays.copyOfRange(value,0,count);
}
returnnewString(toStringCache,true);
}
StringBuilder
@Override
publicStringBuilderappend(Stringstr){
super.append(str);
returnthis;
}
/**
*@throwsStringIndexOutOfBoundsException{@inheritDoc}
*/
@Override
publicStringBuilderinsert(intoffset,Stringstr){
super.insert(offset,str);
returnthis;
}
@Override
publicStringtoString(){
//Createacopy,don'tsharethearray
returnnewString(value,0,count);
}
由代码可知，大部分情况想，StrinbBuffer只是增加了一个synchronized关键字来保证线程安全。但toString方法不同，这个后面再说。
初始化大小和如何增长
既然实际上是一个数组，那么数组开始的大小和增长的方法就很重要，我们通过代码看一下。
/**
*Constructsastringbufferwithnocharactersinitandan
*initialcapacityof16characters.
*/
publicStringBuffer(){
super(16);
}
/**
*Constructsastringbuilderwithnocharactersinitandan
*initialcapacityof16characters.
*/
publicStringBuilder(){
super(16);
}
可以看到，这两个默认构造函数都说明默认的数组大小是16。
为什么是16呢？我没想明白。
接下来关心如何增长的？我们看看append的实现
publicAbstractStringBuilderappend(Stringstr){
if(str==null)
returnappendNull();
intlen=str.length();
ensureCapacityInternal(count+len);
str.getChars(0,len,value,count);
count+=len;
returnthis;
}
/**
*ThismethodhasthesamecontractasensureCapacity,butis
*neversynchronized.
*/
privatevoidensureCapacityInternal(intminimumCapacity){
//overflow-consciouscode
if(minimumCapacity-value.length>0)
expandCapacity(minimumCapacity);
}
/**
*ThisimplementstheexpansionsemanticsofensureCapacitywithno
*sizecheckorsynchronization.
*/
voidexpandCapacity(intminimumCapacity){
intnewCapacity=value.length*2+2;
if(newCapacity-minimumCapacity<0)
newCapacity=minimumCapacity;
if(newCapacity<0){
if(minimumCapacity<0)//overflow
thrownewOutOfMemoryError();
newCapacity=Integer.MAX_VALUE;
}
value=Arrays.copyOf(value,newCapacity);
}
上面三个方法说明了如何扩容。
把当前的容量*2+2
如果新增加的长度大于这个值，则设为新增加的值
如果溢出，则抛出OutOfMemoryError
StringBuffer中toString的实现
/**
*AcacheofthelastvaluereturnedbytoString.Cleared
*whenevertheStringBufferismodified.
*/
privatetransientchar[]toStringCache;
@Override
publicsynchronizedStringBufferappend(Stringstr){
toStringCache=null;
super.append(str);
returnthis;
}
@Override
publicsynchronizedStringtoString(){
if(toStringCache==null){
toStringCache=Arrays.copyOfRange(value,0,count);
}
returnnewString(toStringCache,true);
}
可以看到，定义了一个数组toStringCache，每次数据有变化，都把这个设置为null。在toString的时候，再重新从当前数据里面拿。
transient关键字是为了避免这个数组被序列化。
小结
其实java本身的源码写的还是比较简单的，学习知识如果能从源码入手，能够更深入的理解很多原理。本文只是简单的列举了一部分源码，简单说明StringBuffer和StringBuilder的异同。有兴趣的朋友可以自己看一下。
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