浅谈request.getinputstream只能读取一次的问题
首先，我们复习一下InputStreamread方法的基础知识，
javaInputStreamread方法内部有一个，postion，标志当前流读取到的位置，每读取一次，位置就会移动一次，如果读到最后，InputStream.read方法会返回-1，标志已经读取完了，如果想再次读取，可以调用inputstream.reset方法，position就会移动到上次调用mark的位置，mark默认是0，所以就能从头再读了。
当然，能否reset是有条件的，它取决于markSupported,markSupported()方法返回是否可以mark/reset
我们再回头看request.getInputStream
request.getInputStream返回的值是ServletInputStream,查看ServletInputStream源码发现，没有重写reset方法，所以查看InputStream源码发现marksupported返回false，并且reset方法，直接抛出异常。
InputStream.java
publicbooleanmarkSupported(){
returnfalse;
}
publicsynchronizedvoidreset()throwsIOException{
thrownewIOException("mark/resetnotsupported");
}
综上所述，在request.getinputstream读取一次后position到了文件末尾，第二次就读取不到数据，由于无法reset()，所以，request.getinputstream只能读取一次。
总结：
这个问题最根本还是对javaIO的read、reset方法的深入理解，尤其是read方法的内部实现原理。
附ServletInputStream.java源码
/* *LicensedtotheApacheSoftwareFoundation(ASF)underoneormore *contributorlicenseagreements.SeetheNOTICEfiledistributedwith *thisworkforadditionalinformationregardingcopyrightownership. *TheASFlicensesthisfiletoYouundertheApacheLicense,Version2.0 *(the"License");youmaynotusethisfileexceptincompliancewith *theLicense.YoumayobtainacopyoftheLicenseat * *http://www.apache.org/licenses/LICENSE-2.0 * *Unlessrequiredbyapplicablelaworagreedtoinwriting,software *distributedundertheLicenseisdistributedonan"ASIS"BASIS, *WITHOUTWARRANTIESORCONDITIONSOFANYKIND,eitherexpressorimplied. *SeetheLicenseforthespecificlanguagegoverningpermissionsand *limitationsundertheLicense. */ packagejavax.servlet; importjava.io.IOException; importjava.io.InputStream; /** *Providesaninputstreamforreadingbinarydatafromaclientrequest, *includinganefficientreadLinemethodforreadingdataoneline *atatime.Withsomeprotocols,suchasHTTPPOSTandPUT,a *ServletInputStreamobjectcanbeusedtoreaddatasentfromthe *client. **A
ServletInputStreamobjectisnormallyretrievedviathe *{@linkServletRequest#getInputStream}method. **Thisisanabstractclassthataservletcontainerimplements.Subclassesof *thisclassmustimplementthe
java.io.InputStream.read()method. * *@seeServletRequest */ publicabstractclassServletInputStreamextendsInputStream{ /** *Doesnothing,becausethisisanabstractclass. */ protectedServletInputStream(){ //NOOP } /** *Readstheinputstream,onelineatatime.Startingatanoffset,reads *bytesintoanarray,untilitreadsacertainnumberofbytesorreaches *anewlinecharacter,whichitreadsintothearrayaswell. **Thismethodreturns-1ifitreachestheendoftheinputstreambefore *readingthemaximumnumberofbytes. * *@paramb *anarrayofbytesintowhichdataisread *@paramoff *anintegerspecifyingthecharacteratwhichthismethod *beginsreading *@paramlen *anintegerspecifyingthemaximumnumberofbytestoread *@returnanintegerspecifyingtheactualnumberofbytesread,or-1if *theendofthestreamisreached *@exceptionIOException *ifaninputoroutputexceptionhasoccurred */ publicintreadLine(byte[]b,intoff,intlen)throwsIOException{ if(len<=0){ return0; } intcount=0,c; while((c=read())!=-1){ b[off++]=(byte)c; count++; if(c=='\n'||count==len){ break; } } returncount>0?count:-1; } /** *Returns
trueifallthedatahasbeenreadfromthestream, *elsefalse. * *@sinceServlet3.1 */ publicabstractbooleanisFinished(); /** *Returnstrueifdatacanbereadwithoutblocking,else *false.Ifthismethodiscalledandreturnsfalse,the *containerwillinvoke{@linkReadListener#onDataAvailable()}whendatais *available. * *@sinceServlet3.1 */ publicabstractbooleanisReady(); /** *Setsthe{@linkReadListener}forthis{@linkServletInputStream}and *therebyswitchestonon-blockingIO.Itisonlyvalidtoswitchto *non-blockingIOwithinasyncprocessingorHTTPupgradeprocessing. * *@paramlistenerThenon-blockingIOreadlistener * *@throwsIllegalStateExceptionIfthismethodiscalledifneither *asyncnorHTTPupgradeisinprogressor *ifthe{@linkReadListener}hasalready *beenset *@throwsNullPointerExceptionIflistenerisnull * *@sinceServlet3.1 */ publicabstractvoidsetReadListener(ReadListenerlistener); }
以上这篇浅谈request.getinputstream只能读取一次的问题就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持毛票票。