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, *includinganefficientreadLine
methodforreadingdataoneline *atatime.Withsomeprotocols,suchasHTTPPOSTandPUT,a *ServletInputStream
objectcanbeusedtoreaddatasentfromthe *client. **A
ServletInputStream
objectisnormallyretrievedviathe *{@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
true
ifallthedatahasbeenreadfromthestream, *elsefalse
. * *@sinceServlet3.1 */ publicabstractbooleanisFinished(); /** *Returnstrue
ifdatacanbereadwithoutblocking,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); }
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