I want to load a very big XML file into a DOM tree (using JAXP), do some modifications and run XPath queries on the resulting DOM.
We use our own DOM implementation which is implemented using lazy loading (i.e. in the beginning only the first two levels of the DOM are loaded from the file, if getChildNodes()/etc. is called we go back to the file and load more levels). This is very slow, however, we are able to load much bigger files, especially if we only use parts of the file.
My question: XPath is using a different view on the XML (afaik). I'm curious if the default Oracle JDK implementation is now converting the whole DOM document into some internal XPath document (which would be really bad, as it would eagerly load the whole document) or if the XPath implementation is able to work directly on our DOM tree (i.e. no further loading if the XPath can be evaluated within the already loaded elements).
This can be tested with few lines of code, just feed your DOM to XPath evaluator and put few breakpoints/debug prints into your DOM methods. If they are get called for elements that should not been retrieved then it builds its own tree. Example: try to query only document's first child and see what it will actually try to retrieve.
And also may be you can check radically different approach if your files are so big? Called SAX.
Our DOM implementation is now finished, therefore I could test this now:
Unfortunately the official JDK implementation as well as the current Xalan-J implementation are converting the whole DOM tree to an internal data structure before evaluating the path.
This should be really bad even if you're not having a lazy DOM implementation, this is really bad in any case...
Related
Reading the docs, this is the method used in all the examples I've seen:
(Version of org.jdom.input.SAXBuilder is jdom-1.1.jar)
Document doc = new SAXBuilder().build(is);
Element root = doc.getRootElement();
Element child = root.getChild("someChildElement");
...
where is is an InputStream variable.
I'm wondering, since this is a SAX builder (as opposed to a DOM builder), does the entire inputstream get read into the document object with the build method? Or is it working off a lazy load and as long as I request elements with Element.getChildren() or similar functions (stemming from the root node) that are forward-only through the document, then the builder automatically takes care of loading chunks of the stream for me?
I need to be sure I'm not loading the whole file into memory.
Thanks,
Mike
The DOM parser similarly to the JDom parser loads the whole XML resource in memory to provide you a Document instance allowing to navigate in the elements of the XML.
Some references here :
the DOM standard is a codified standard for an in-memory document
model.
And here :
JDOM works on the logical in-memory XML tree,
Both DOM and JDom use the SAX parser internally to read the XML resource but they use it only to store the whole content in the Document instance that they return. Indeed, with Dom and JDom, the client never needs to provide a handler to intercept events triggered by the SAX parser.
Note that both DOM and JDom don't have any obligation to use SAX internally.
They use them mainly as the SAX standard is already there and so it makes sense to use it for reporting errors.
I need to be sure I'm not loading the whole file into memory.
You have two programming models to work with XML: streaming and the document object model (DOM).
You are looking for the first one.
So use the SAX parser by providing your handler to handle events generated by the SAX parser (startDocument(), startElement(), and so for) or as alternative look at a more user friendly API : STAX (Streaming API for XML) :
As an API in the JAXP family, StAX can be compared, among other APIs,
to SAX, TrAX, and JDOM. Of the latter two, StAX is not as powerful or
flexible as TrAX or JDOM, but neither does it require as much memory
or processor load to be useful, and StAX can, in many cases,
outperform the DOM-based APIs. The same arguments outlined above,
weighing the cost/benefits of the DOM model versus the streaming
model, apply here.
It eagerly parses the whole file to build the in-memory representation (i.e. Document) of the XML file.
If you want to be absolutely certain of that, you can go through the source on GitHub. More importantly the following classes: SAXBuilder, SAXHandler, and Document.
I read some articles about the XML parsers and came across SAX and DOM.
SAX is event-based and DOM is tree model -- I don't understand the differences between these concepts.
From what I have understood, event-based means some kind of event happens to the node. Like when one clicks a particular node it will give all the sub nodes rather than loading all the nodes at the same time. But in the case of DOM parsing it will load all the nodes and make the tree model.
Is my understanding correct?
Please correct me If I am wrong or explain to me event-based and tree model in a simpler manner.
Well, you are close.
In SAX, events are triggered when the XML is being parsed. When the parser is parsing the XML, and encounters a tag starting (e.g. <something>), then it triggers the tagStarted event (actual name of event might differ). Similarly when the end of the tag is met while parsing (</something>), it triggers tagEnded. Using a SAX parser implies you need to handle these events and make sense of the data returned with each event.
In DOM, there are no events triggered while parsing. The entire XML is parsed and a DOM tree (of the nodes in the XML) is generated and returned. Once parsed, the user can navigate the tree to access the various data previously embedded in the various nodes in the XML.
In general, DOM is easier to use but has an overhead of parsing the entire XML before you can start using it.
In just a few words...
SAX (Simple API for XML): Is a stream-based processor. You only have a tiny part in memory at any time and you "sniff" the XML stream by implementing callback code for events like tagStarted() etc. It uses almost no memory, but you can't do "DOM" stuff, like use xpath or traverse trees.
DOM (Document Object Model): You load the whole thing into memory - it's a massive memory hog. You can blow memory with even medium sized documents. But you can use xpath and traverse the tree etc.
Here in simpler words:
DOM
Tree model parser (Object based) (Tree of nodes).
DOM loads the file into the memory and then parse- the file.
Has memory constraints since it loads the whole XML file before parsing.
DOM is read and write (can insert or delete nodes).
If the XML content is small, then prefer DOM parser.
Backward and forward search is possible for searching the tags and evaluation of the
information inside the tags. So this gives the ease of navigation.
Slower at run time.
SAX
Event based parser (Sequence of events).
SAX parses the file as it reads it, i.e. parses node by node.
No memory constraints as it does not store the XML content in the memory.
SAX is read only i.e. can’t insert or delete the node.
Use SAX parser when memory content is large.
SAX reads the XML file from top to bottom and backward navigation is not possible.
Faster at run time.
You are correct in your understanding of the DOM based model. The XML file will be loaded as a whole and all its contents will be built as an in-memory representation of the tree the document represents. This can be time- and memory-consuming, depending on how large the input file is. The benefit of this approach is that you can easily query any part of the document, and freely manipulate all the nodes in the tree.
The DOM approach is typically used for small XML structures (where small depends on how much horsepower and memory your platform has) that may need to be modified and queried in different ways once they have been loaded.
SAX on the other hand is designed to handle XML input of virtually any size. Instead of the XML framework doing the hard work for you in figuring out the structure of the document and preparing potentially lots of objects for all the nodes, attributes etc., SAX completely leaves that to you.
What it basically does is read the input from the top and invoke callback methods you provide when certain "events" occur. An event might be hitting an opening tag, an attribute in the tag, finding text inside an element or coming across an end-tag.
SAX stubbornly reads the input and tells you what it sees in this fashion. It is up to you to maintain all state-information you require. Usually this means you will build up some sort of state-machine.
While this approach to XML processing is a lot more tedious, it can be very powerful, too. Imagine you want to just extract the titles of news articles from a blog feed. If you read this XML using DOM it would load all the article contents, all the images etc. that are contained in the XML into memory, even though you are not even interested in it.
With SAX you can just check if the element name is (e. g.) "title" whenever your "startTag" event method is called. If so, you know that you needs to add whatever the next "elementText" event offers you. When you receive the "endTag" event call, you check again if this is the closing element of the "title". After that, you just ignore all further elements, until either the input ends, or another "startTag" with a name of "title" comes along. And so on...
You could read through megabytes and megabytes of XML this way, just extracting the tiny amount of data you need.
The negative side of this approach is of course, that you need to do a lot more book-keeping yourself, depending on what data you need to extract and how complicated the XML structure is. Furthermore, you naturally cannot modify the structure of the XML tree, because you never have it in hand as a whole.
So in general, SAX is suitable for combing through potentially large amounts of data you receive with a specific "query" in mind, but need not modify, while DOM is more aimed at giving you full flexibility in changing structure and contents, at the expense of higher resource demand.
You're comparing apples and pears. SAX is a parser that parses serialized DOM structures. There are many different parsers, and "event-based" refers to the parsing method.
Maybe a small recap is in order:
The document object model (DOM) is an abstract data model that describes a hierarchical, tree-based document structure; a document tree consists of nodes, namely element, attribute and text nodes (and some others). Nodes have parents, siblings and children and can be traversed, etc., all the stuff you're used to from doing JavaScript (which incidentally has nothing to do with the DOM).
A DOM structure may be serialized, i.e. written to a file, using a markup language like HTML or XML. An HTML or XML file thus contains a "written out" or "flattened out" version of an abstract document tree.
For a computer to manipulate, or even display, a DOM tree from a file, it has to deserialize, or parse, the file and reconstruct the abstract tree in memory. This is where parsing comes in.
Now we come to the nature of parsers. One way to parse would be to read in the entire document and recursively build up a tree structure in memory, and finally expose the entire result to the user. (I suppose you could call these parsers "DOM parsers".) That would be very handy for the user (I think that's what PHP's XML parser does), but it suffers from scalability problems and becomes very expensive for large documents.
On the other hand, event-based parsing, as done by SAX, looks at the file linearly and simply makes call-backs to the user whenever it encounters a structural piece of data, like "this element started", "that element ended", "some text here", etc. This has the benefit that it can go on forever without concern for the input file size, but it's a lot more low-level because it requires the user to do all the actual processing work (by providing call-backs). To return to your original question, the term "event-based" refers to those parsing events that the parser raises as it traverses the XML file.
The Wikipedia article has many details on the stages of SAX parsing.
In practical: book.xml
<bookstore>
<book category="cooking">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>
</bookstore>
DOM presents the xml document as a the following tree-structure in memory.
DOM is W3C standard.
DOM parser works on Document Object Model.
DOM occupies more memory, preferred for small XML documents
DOM is Easy to navigate either forward or backward.
SAX presents the xml document as event based like start element:abc, end element:abc.
SAX is not W3C standard, it was developed by group of developers.
SAX does not use memory, preferred for large XML documents.
Backward navigation is not possible as it sequentially process the documents.
Event happens to a node/element and it gives all sub nodes(Latin nodus, ‘knot’).
This XML document, when passed through a SAX parser, will generate a sequence of events like the following:
start element: bookstore
start element: book with an attribute category equal to cooking
start element: title with an attribute lang equal to en
Text node, with data equal to Everyday Italian
....
end element: title
.....
end element: book
end element: bookstore
Both SAX and DOM are used to parse the XML document. Both has advantages and disadvantages and can be used in our programming depending on the situation
SAX:
Parses node by node
Does not store the XML in memory
We cant insert or delete a node
Top to bottom traversing
DOM
Stores the entire XML document into memory before processing
Occupies more memory
We can insert or delete nodes
Traverse in any direction.
If we need to find a node and does not need to insert or delete we can go with SAX itself otherwise DOM provided we have more memory.
If there is a very big XML and DOM parser is used to parse it.
Now there is a requirement to add/delete elements from the XML i.e edit the XML
How to edit the XML as the entire XML will not be loaded due to memory constraints ?
What could be the strategy to solve this ?
You may consider to use a SAX parser instead, which doesn't keep the whole document in memory. It will be faster and will also use much less memory.
As two other answers mentioned already, a SAX parser will do the trick. Your other alternative to DOM is a StAX parser.
Traditionally, XML APIs are either:
DOM based - the entire document is read into memory as a tree
structure for random access by the calling application
event based - the application registers to receive events as
entities are encountered within the source document.
Both have advantages; the former (for example, DOM) allows for random
access to the document, the latter (e.g. SAX) requires a small memory
footprint and is typically much faster.
These two access metaphors can be thought of as polar opposites. A
tree based API allows unlimited, random access and manipulation, while
an event based API is a 'one shot' pass through the source document.
StAX was designed as a median between these two opposites. In the StAX
metaphor, the programmatic entry point is a cursor that represents a
point within the document. The application moves the cursor forward -
'pulling' the information from the parser as it needs. This is
different from an event based API - such as SAX - which 'pushes' data
to the application - requiring the application to maintain state
between events as necessary to keep track of location within the
document.
StAX is my preferred approach for handling large documents. If DOM is a requirement, check out DOM implementations like Xerces that support lazy construction of DOM nodes:
http://xerces.apache.org/xerces-j/faq-write.html#faq-4
Your assumption of memory constraint loading the XML document may only apply to DOM. VTD-XML loads the entire XML in memory, and does it efficiently (1.3x the size of XML document)... both in memory and performance...
http://sdiwc.us/digitlib/journal_paper.php?paper=00000582.pdf
Another distinct benefit, which none other XML framework in existence has, is its incremental update capability...
http://www.devx.com/xml/Article/36379
As stivlo mentioned you can use a SAX parser for reading the XML.
But for writing the XML you can write into fileoutput stream as plain text. I am sure that you will get requirement that mentions after which tag or under which tag the new data should be inserted.
I've been looking at loading XML files with Java and I just can't seem to decipher a certain part of it.
I understand that SAX is a streaming mechanism, but when talking about DOM, various sites talk about the model "loading in the full file" or "loading in the all tags", supported by the recommendation to use SAX with large XML files.
To what degree does DOM actually load the full file? The second I access the root node, does it allocate program memory for every single byte of the file? Does it only load tags until the lowest level when it loads text contents?
I'm going to be working with large files, but random access would be useful and editing is a requirement, so I believe DOM is the best choice for me.
Thanks a lot.
It does load the entire file and constructs a tree structure in memory. Thus, every single tag, attribute and any nested tags (no matter how many levels of nesting) will be loaded. It is just that the constructed tree grows bigger the larger the XML file you have.
Yes, DOM reads the whole document, parses it, and places it in memory.
If you're parsing using DOM, you do something similar to this:
DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
Document doc = builder.parse(file);
(inside a try/catch)
The moment the parse is executed, the Document doc variable will contain the entire document represented as a DOM hierarchy.
How may I modify a XML file without any change like attributes ordering, tag expansion and encoding? (My preference is DOM API)
You could try VTD-XML.
Since this library builds an index while keeping the file content as-is, its manipulation API will allow to "patch" your file while keeping the rest intact.
Using the VTD-XML API, you will be able to navigate your XML like a DOM tree (even using XPath) and do some modifications (insert elements, insert attributes, etc.)
One nice option would be using decentxml. I've successfully used it before for programmatically changing a few attributes of a hand-writen XML config file without losing the formatting. It's not DOM, though.
This is not possible with DOM, since DOM does not know the attributes' order.
You could use a low-level API such as StAX, javax.xml.stream, but StAX is not exactly comfortable to use.