iText中PdfStream对象map为空原因分析

一段代码，是为了遍历每一页中的图片

            PdfDictionary resources = pageDict.GetAsDictionary(PdfName.Resources);
            PdfDictionary xObjects = resources.GetAsDictionary(PdfName.XObject);

            if (xObjects == null)
                return;

            var keySet = xObjects.KeySet();

            foreach (var pdfName in keySet.ToList())
            {
                PdfStream stream = xObjects.GetAsStream(pdfName);
                PdfObject subtype = stream.Get(PdfName.Subtype);

结果对某个pdf进行处理的时候，在第三页出现了运行时的报错。原因是stream.Get的时候，stream对象的内部成员map是null，因此 map.Get(key) 必然不可运行！但是为什么会出现这种情况呢？先跟踪一下 xObjects.GetAsStream(pdfName) 是怎么处理的

        public virtual PdfObject Get(PdfName key, bool asDirect) {
            if (!asDirect) {
                return map.Get(key);
            }
            else {
                PdfObject obj = map.Get(key);
                if (obj != null && obj.GetObjectType() == INDIRECT_REFERENCE) {
                    return ((PdfIndirectReference)obj).GetRefersTo(true);
                }
                else {
                    return obj;
                }
            }
        }

每一页的xobject都是Indirect的object，因此还需要从整个pdf文档中找到这个object的真正定义，通过 PdfIndirectReference::GetRefersTo

        public virtual PdfObject GetRefersTo(bool recursively) {
            if (!recursively) {
                if (refersTo == null && !CheckState(FLUSHED) && !CheckState(MODIFIED) && !CheckState(FREE) && GetReader() 
                    != null) {
                    refersTo = GetReader().ReadObject(this);
                }
                return refersTo;
            }
            else {
                PdfObject currentRefersTo = GetRefersTo(false);
                for (int i = 0; i < LENGTH_OF_INDIRECTS_CHAIN; i++) {
                    if (currentRefersTo is iText.Kernel.Pdf.PdfIndirectReference) {
                        currentRefersTo = ((iText.Kernel.Pdf.PdfIndirectReference)currentRefersTo).GetRefersTo(false);
                    }
                    else {
                        break;
                    }
                }
                return currentRefersTo;
            }
        }

一般每一页的新对象，其 refersTo == null，需要通过 GetReader().ReadObject(this) 从整个文档中找到真正定义。但是在出错的一页，refersTo 已经不为空，指向一个 {10 0 R Flushed;}。我们再跟踪一下，对第一页的对象，GetReader().ReadObject(this) 是怎么处理的
PdfReader::ReadObject

        private PdfObject ReadObject(PdfIndirectReference reference, bool fixXref) {
            if (reference == null) {
                return null;
            }
            if (reference.refersTo != null) {
                return reference.refersTo;
            }
            try {
                currentIndirectReference = reference;
                if (reference.GetObjStreamNumber() > 0) {
                    PdfStream objectStream = (PdfStream)pdfDocument.GetXref().Get(reference.GetObjStreamNumber()).GetRefersTo(
                        false);
                    ReadObjectStream(objectStream);
                    return reference.refersTo;
                }
                else {
                    if (reference.GetOffset() > 0) {
                        PdfObject @object;
                        try {
                            tokens.Seek(reference.GetOffset());
                            tokens.NextValidToken();
                            if (tokens.GetTokenType() != PdfTokenizer.TokenType.Obj || tokens.GetObjNr() != reference.GetObjNumber() ||
                                 tokens.GetGenNr() != reference.GetGenNumber()) {
                                tokens.ThrowError(KernelExceptionMessageConstant.INVALID_OFFSET_FOR_THIS_OBJECT, reference.ToString());
                            }
                            @object = ReadObject(false);
                        }

会根据对象的offset，从原始文档中读出对象。正好第一页中要读的一个对象就是10，它读出来的状态是{10 0 R}，是没有Flushed在后面的！那什么时候变成flushed的呢？继续跟踪程序，发现是在每一页处理结束后，程序都会调用一次 page.Flush(true);，对象10就变成了 {10 0 R Flushed;}。而 Flush 函数的说明是
Flushes page dictionary, its content streams, annotations and thumb image. If <c>flushResourcesContentStreams</c> is true, all content streams that are rendered on this page (like FormXObjects, annotation appearance streams, patterns) and also all images associated with this page will also be flushed.

这说明了，由于这个图片同时被第1页和第3页使用了，而因为调用Flush的时候，后面的参数加了true，因此处理完第一页的时候被写出去了。继续跟踪一下，为什么Flush之后，stream对象的map变空了

PdfPage类
        private void FlushResourcesContentStreams(PdfDictionary resources) {
            if (resources != null && !resources.IsFlushed()) {
                FlushWithResources(resources.GetAsDictionary(PdfName.XObject));
                FlushWithResources(resources.GetAsDictionary(PdfName.Pattern));
                FlushWithResources(resources.GetAsDictionary(PdfName.Shading));
            }
        }

        private void FlushWithResources(PdfDictionary objsCollection) {
            if (objsCollection == null || objsCollection.IsFlushed()) {
                return;
            }
            foreach (PdfObject obj in objsCollection.Values()) {
                if (obj.IsFlushed()) {
                    continue;
                }
                FlushResourcesContentStreams(((PdfDictionary)obj).GetAsDictionary(PdfName.Resources));
                FlushMustBeIndirectObject(obj);
            }
        }

        private void FlushMustBeIndirectObject(PdfObject obj) {
            // TODO DEVSIX-744
            obj.MakeIndirect(GetDocument()).Flush();
        }

PdfObject类
       public void Flush(bool canBeInObjStm) {
            try {
                PdfDocument document = GetIndirectReference().GetDocument();
                if (document != null) {
                    if (document.IsAppendMode() && !IsModified()) {
                        ILogger logger = ITextLogManager.GetLogger(typeof(PdfObject));
                        logger.LogInformation(iText.IO.Logs.IoLogMessageConstant.PDF_OBJECT_FLUSHING_NOT_PERFORMED);
                        return;
                    }
                    document.CheckIsoConformance(this, IsoKey.PDF_OBJECT);
                    document.FlushObject(this, canBeInObjStm && GetObjectType() != STREAM && GetObjectType() != INDIRECT_REFERENCE
                         && GetIndirectReference().GetGenNumber() == 0);
                }
            }


PdfWriter类
        protected internal virtual void FlushObject(PdfObject pdfObject, bool canBeInObjStm) {
            switch (pdfObject.GetObjectType()) {
                case PdfObject.STREAM:
                case PdfObject.DICTIONARY: {
                    PdfDictionary dictionary = ((PdfDictionary)pdfObject);
                    MarkDictionaryContentToFlush(dictionary);
                    dictionary.ReleaseContent();
                    break;
                }

PdfDictionary类
        protected internal virtual void ReleaseContent() {
            map = null;
        }