jPod是否通过数据流合并PDF？

Question

I am using jPod to Merge my PDF Documents. I merged 400 PDFs of each 20 Pages resulting in file of 190 MB, whereas the size of a single pdf is 38 KB. I checked for heap status in my IDE. I didn't get any Out of Memory Error. I ran the same in Apache Tomcat with almost 30 Clients. My Tomcat stopped serving the requests. Is it because, jPod doesn't use Streaming Or due to some other reasons??

private void run() throws Throwable {
String sOutFileFullPathAndName = "/Users/test/Downloads/" + UUID.randomUUID().toString().replace("-", "");
PDDocument dstDocument = PDDocument.createNew();

for (int i = 0;i < 400; i++) {
    //System.out.println(Runtime.getRuntime().freeMemory());
    PDDocument srcDocument = PDDocument.createFromLocator(new FileLocator("/Users/test/Downloads/2.pdf") );   
    mergeDocuments(dstDocument, srcDocument);
}
FileLocator destinationLocator = new FileLocator(sOutFileFullPathAndName);
dstDocument.save(destinationLocator, null);
dstDocument.close();
}

private void mergeDocuments(PDDocument dstDocument, PDDocument srcDocument) {
PDPageTree pageTree = srcDocument.getPageTree();
int pageCount = pageTree.getCount();
for (int index = 0; index < pageCount; index++) {
    PDPage srcPage = pageTree.getPageAt( index );
    appendPage(dstDocument, srcPage);

    srcPage = null;
}
}

private void appendPage(PDDocument document, PDPage page) {
PDResources srcResources = page.getResources();
CSContent cSContent = page.getContentStream();
PDPage newPage = (PDPage) PDPage.META.createNew();

// copy resources from source page to the newly created page

PDResources newResources = (PDResources) PDResources.META
    .createFromCos(srcResources.cosGetObject().copyDeep());
newPage.setResources(newResources);
newPage.setContentStream(cSContent);

// add that new page to the destination document

document.addPageNode(newPage);
}

Answer 1

PDF is not simply a "stream" of page data. It is a complex data structure containing objects referencing each other. In this concrete case page trees/nodes, content streams, resources,...

jPod keeps persistent object in memory using weak references only - they can always be refreshed from the random access data. If you start updating the object structure, objects get "locked" in memory, simply because the change is not persistent and cannot longer be refreshed.

Making lots of changes without peridodically saving the result will keep the complete structure in memory - i assume that's your problem here. Saving every now and then should reduce memory footprint.

In addition, this algorithm will create a poor page tree, containing in a linear array with thousands of pages. You should try to create a balanced tree structure. Another point for optimization is resource handling. Merging resources like fonts or images may dramatically reduce target size.