I need to parse a series of simple XML nodes (String format) as they arrive from a persistent socket connection. Is a custom Android SAX parser really the best way? It seams slightly overkill to do it in this way
I had naively hoped I could cast the strings to XML then reference the names / attributes with dot syntax or similar.
I'd use the DOM Parser. It isn't as efficient as SAX, but if it's a simple XML file that's not too large, it's the easiest way to get up and moving.
Great tutorial on how to use it here: http://tutorials.jenkov.com/java-xml/dom.html
You might want to take a look at the XPath library. This is a more simple way of parsing xml. It's similar to building SQL queries and regex's.
http://www.ibm.com/developerworks/library/x-javaxpathapi.html
I'd go for a SAX Parser:
It's much more efficient in terms of memory, especially for larger files: you don't parse an entire document into objects, instead the parser performs a single uni-directional pass over the document and triggers events as it goes through.
It's actually surprisingly easy to implement: for instance take a look at Working with XML on Android by IBM. It's only listings 5 and 6 that are the actual implementation of their SAX parser so it's not a lot of code.
You can try to use Konsume-XML: SAX/STAX/Pull APIs are too low-level and hard to use; DOM requires the XML to fit into memory and is still clunky to use. Konsume-XML is based on Pull and therefore it's extremely efficient, yet the API is higher-level and much easier to use.
Related
I've read several questions and tutorials over the internet such as
Best XML parser for Java [closed]
JAVA XML - How do I get specific elements in an XML Node?
What is the best way to create XML files in Java?
how to modify xml tag specific value in java?
Using StAX - From Oracle Tutorials
Apache Xerces Docs
Introduction to XML and XML With Java
Java DOM Parser - Modify XML Document
But since this is the very first time I have to manipulate XML documents in Java I'm still a little bit confused. The XML content is written with String concatenation and that seems to me wrong. It is the same to concatenate Strings to produce a JSON object instead of using a JSONObject class. That's the way the code is written right now:
"<msg:restenv xmlns:msg=\"http://www.b2wdigital.com/umb/NEXM_processa_nf_xml_req\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xsi:schemaLocation=\"http://www.b2wdigital.com/umb/NEXM_processa_nf_xml_req req.xsd\"><autenticacao><usuario>"
+ usuario + "</usuario><senha>" + StringUtils.defaultIfBlank(UmbrellaRestClient.PARAMETROS_INFRA_UMBRELLA.get("SENHA_UMBRELLA"), "WS.INTEGRADOR")
+ "</senha></autenticacao><parametros><parametro><p_vl_gnre>" + valorGNRE + "</p_vl_gnre><p_cnpj_destinatario>" + cnpjFilial + "</p_cnpj_destinatario><p_num_ped_compra>" + idPedido
+ "</p_num_ped_compra><p_xml_sefaz><![CDATA[" + arquivoNfe + "]]></p_xml_sefaz></parametro></parametros></msg:restenv>"
In my research I think that almost everything I've read pointed to SAX as the best solution but I never really found something really useful to read about it, almost everything states that we have to create a handler and override startElement, endElement and characters methods.
I don't have to serialize the XML file in hard disk or database or anything else, I just need to pass its content to a webservice.
So my question really is, which is the right way to do it?
Concatenate Strings the way things are done right now?
Write the XML file using a Java API like Xerces? I have no clue on how that can be done.
Read the XML file with streams and just change node texts? My XML without the files would be like that:
<msg:restenv xmlns:msg="{url}"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="{schemaLocation}">
<autenticacao>
<usuario></usuario>
<senha></senha>
</autenticacao>
<parametros>
<parametro>
<p_vl_gnre></p_vl_gnre>
<p_cnpj_destinatario></p_cnpj_destinatario>
<p_num_ped_compra></p_num_ped_compra>
<p_xml_sefaz><![CDATA[]]></p_xml_sefaz>
</parametro>
</parametros>
</msg:restenv>
I've also read something about using Apache Velocity as a template Engine since I don't actually have to serialize the XML and that's a approach that I really like because I've already worked with this framework and it's a really simple framework.
Again, I'm not looking for the best way, but the right one with tutorials and examples, if possible, on how to get things done.
It all depends on context. There is no single "right way".
The biggest issues with concatenation is the combination of escaping the XML in to a String constant (which is fiddly), but also escaping the values that you can using so that they're correct for an XML document.
For small XMLs, that's fine.
But for larger ones, it can be a pain.
If most of your XML is boilerplate with just a few values inserted, you may find that templates using something like Velocity or any of the other several libraries may be quite effective. It helps keep the template "native" (you don't have to wrap it in "'s and escape it), plus it keeps the XML out of your code, but easily lets you stamp in the parts that you need to do.
I agree that there's not just one way to do it, but I would advise you to take a look at JAXB. You can easily consume and produce XML without any of that pesky String manipulation. Look here for a simple intro: https://docs.oracle.com/javase/tutorial/jaxb/index.html
The Answer by Will Hartung is correct. There is not one right way as it depends on your situation.
For a beginner programmer, I suggest writing the strings manually so you get to understand XML in general and your content in particular. As for the mechanics of String concatenation, you would generally be using StringBuilder rather than String for better performance. Where thread-safety is needed, use StringBuffer.
The major issue is memory.
Abundant MemoryIf you have lots of memory and small XML documents, you can load the entire document into memory. This way you can traverse a document forwards, backwards, and jump around arbitrarily. This approach is know as Document Object Model (DOM). One better-known implementation of this approach is Apache Xerces. There are other good implementations as well.
Scarce MemoryIf you have little memory and large XML documents, then you need to plow through the document from start to finish, biting off small chunks at a time for lower memory usage. This approach is known as SAX. You can find multiple good implementations.
Another issue is validation. Do you want to validate the XML documents against a DTD or Schema? Some tools do this and some do not.
When all you need is to serialize the content of a Java object and read it back, I recommend the Simple XML Serialization library. Much simpler with a quicker learning-curve than the other tools.
We have a new requirement:
There are some BIG xml files keep coming into our system and we will need to process them immediately and quickly using Java. The file is huge but the required information for our processing is inside a element which is very small.
...
...
What is the best way to extract this small portion of the data from the huge file before we start processing. If we try to load the entire file, we will get out of memory error immediately due to size. What is the efficient way in Java that I can use to get the ..data..data..data.. data element without loading or reading the file line by line. Is there any SAX Parser that I can use to get this done?
Thank you
The SAX parsers are event based and are much faster because they do what you need: they don't read the xml document entirely. There is a SAXParser available in the Java distributions.
I had to parse huge files in a previous project (1G-2G) and didn't want to deal with using SAX. I find SAX too low-level in some instances and like keepings a traversal approach in most cases.
I have used the VTD library http://vtd-xml.sourceforge.net/. It's an EXTREMELY fast library that uses pointers to navigate through the document.
Well, if you want to read a part of a file, you will need to read each line of the file to be able to identify the part of the file of interest and then extract what you need.
If you only need a small portion of the incoming XML, you can either use SAX, or if you need to read only specific elements or attributes, you could use XPath, which would be a lot simpler to implement.
Java comes with a built-in SAXParser implementation as well as an XPath implementation. Find the javadocs for SAXParser here and for XPath here.
StAX is another option based on steaming the data, like SAX, but benefits from a more friendly approach (IMO) to processing the data by "pulling" what you want rather than having it "pushed" to you.
I'm sure this might have been discussed at length or answered before, however I need a bit more information on the best approach for my situation...
Problem:
We have some large XML data (anywhere from 100k to 5MB) which we need to inflate into Java objects. The issue is that the data doesn't really doesn't map onto an object very well at all, so we need to only pull certain parts of the data out and create the objects. Given that, solutions such as JAXB or XStream really aren't appropriate.
So, we need to pull XML data out and get it into java objects as efficiently as possible.
Possible Solutions:
The way I see it, we have 3 possible solutions:
SAX parsing
DOM parsing
XSLT
We can load the XML into any JAXP implementation and pull the data out using one of the above methods.
Question(s)
I have a few questions/concerns:
How does XSLT work under the hood? Is it just a DOM parser? I ask because XSLT seems like a good way to go, but I don't really want to consider it if it won't give us better performance than DOM.
What are some popular libraries that provide DOM, XSLT, and SAX XML parsers?
In your experience, what are the reasons for picking DOM, SAX, or XSLT? Does the ease of use of DOM or XSLT totally dominate the performance improvements SAX offers?
Any benchmarks out there? The ones I've found are old (as in, 8 years old). So some recent benchmarks would be appreciated.
Are there any other solutions besides those outlined above that I could be missing?
Edit:
A few clarifications... You can use XSLT to directly inject values into a Java object... it is normally used to transform XML into some other XML, however I'm talking from the standpoint of calling a method from XSLT into java to inject the value.
I'm still not clear on how an XSLT processor works exactly... How is it feeding the XML into the XSLT code you write?
Use XSLT to transform the large XML files into a local domain model that is mapped to java objets with JAXB.
Start with the JDK 5+ built in XML libraries (unless you absolutely need XSLT 2.0, in which case use Saxon)
Don't focus on relative performance of SAX/DOM, focus on learning how to write XPath expressions and use XSLT, and then worry about performance later if and only if you find it to be a problem.
The Eclipse XML editors are decent, but if you can afford it, spring for Oxygen XML, which will let you do XPath evaluation in realtime.
We had a similar situation and I just threw together some XPath code that parsed the stuff I needed.
It was amazingly quick even on 100k+ XML files. We went as low tech as possible. We handle around 1000 files a day of that size and parsing time is very low. We have no memory issues, leaks etc.
We wrote a quick prototype in Groovy (if my memory is accurate) - proof of concept took me about 10 minutes
JAXB, the Java API for XML Binding might be what you want. You use it to inflate an XML document into a Java object graph made up of "Java content objects". These content objects are instances of classes generated by JAXB to match the XML document's schema
But if you already have a set of Java classes, or don't yet have a schema for the document, JAXB probably isn't the best way to go. I'd suggest doing a SAX parse and then building up your Java objects during the parse. Alternatively you could try a DOM parse and then walk the resulting Document tree to pull out the parts of interest (maybe with XPath) -- but 5MB of XML might turn into 50MB of DOM tree objects in Java.
DOM, SAX and XSLT are different animals.
DOM parsing loads the entire document into memory, which for 100K to 5MB (very small by today's standards) would work.
SAX is a stream parser which reads the XML and delivers events to your code for each tag.
XSLT is a system for transforming one XML tree into another. Even if you wrote a transform that converts the input to a more suitable format, you'd still have to write something using DOM or SAX to convert it into Java objects.
You can use the #XmlPath extension in EclipseLink JAXB (MOXy) to easily handle this use case. For a detailed example see:
http://bdoughan.blogspot.com/2010/09/xpath-based-mapping-geocode-example.html
Sample Code:
package blog.geocode;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;
import org.eclipse.persistence.oxm.annotations.XmlPath;
#XmlRootElement(name="kml")
#XmlType(propOrder={"country", "state", "city", "street", "postalCode"})
public class Address {
#XmlPath("Response/Placemark/ns:AddressDetails/ns:Country/ns:AdministrativeArea/ns:SubAdministrativeArea/ns:Locality/ns:Thoroughfare/ns:ThoroughfareName/text()")
private String street;
#XmlPath("Response/Placemark/ns:AddressDetails/ns:Country/ns:AdministrativeArea/ns:SubAdministrativeArea/ns:Locality/ns:LocalityName/text()")
private String city;
#XmlPath("Response/Placemark/ns:AddressDetails/ns:Country/ns:AdministrativeArea/ns:AdministrativeAreaName/text()")
private String state;
#XmlPath("Response/Placemark/ns:AddressDetails/ns:Country/ns:CountryNameCode/text()")
private String country;
#XmlPath("Response/Placemark/ns:AddressDetails/ns:Country/ns:AdministrativeArea/ns:SubAdministrativeArea/ns:Locality/ns:PostalCode/ns:PostalCodeNumber/text()")
private String postalCode;
}
I'm trying to write a basic RSS reader for a class project. Our book shows an example by walking the DOM tree. Is that a decent approach for an RSS reader? Would I just ignore certain tags that are of uninterest to me and not to be used by the RSS Reader? Thanks.
For inspiration you can look at ROME, an open source tool for handling RSS & Atom feeds.
It's one of two common approaches, so yes. And yes, ignoring tags that are not of interest is a good way to handle it. If you don't need it, no need to take note of it. If you know in advance exactly what tags you need, you probably don't need to walk the entire DOM tree.
You could also use a SAX parser which would probably be faster and less memory intensive, though probably not necessary in this case, depending on how many results you wish to have in the feed.
Well, the beauty of RSS feed is they always come in some standard structure, even though some feeds contain non-standard fields, like Google Picasa's RSS feed. The most straightforward approach, in my opinion, is to use a tool that allows you to unmarshall the RSS XML feed into your RSS bean. This way, you don't need to write too much code, and you can pick what fields you want and ignore fields you don't want.
In my case, I use Castor to perform the unmarshalling process where I read the Google Picasa RSS feed and gather only the fields I want. Hope this helps.
Processing Atom Feeds with JAXB
You could also map your XML to objects using JAXB. You could then use these objects in your RSS reader.
http://bdoughan.blogspot.com/2010/09/processing-atom-feeds-with-jaxb.html
The JAXB reference implementation is included in Java SE 6, there are also other implementations such as MOXy (I'm the tech lead):
http://wiki.eclipse.org/EclipseLink/Examples/MOXy/GettingStarted
You only need to map the portions you are interested in.
Processing Atom Feeds with SDO
You could also use Service Data Objects (SDO) to do this:
http://bdoughan.blogspot.com/2010/09/processing-atom-feeds-with-sdo.html
I have both parsed and produced RSS with the JDOM library. Its been around a long time and is updated in-frequently, but my experience is that it hasn't needed updating. You may want to look into it but since its quite powerful, you may find that it offers too much functionality.
http://jdom.org/
I've a big XML data returned by SAP. Of this, I need only few nodes, may be 30% of the returned data.
After googling, I got to know that I can filter the nodes in either of the ways:
Apply XSLT templates - Have seen some nice solutions, which i want, in this site only.
Using a parser - use JDOM or SAX parser.
which is the efficient way to "filter XML nodes"?
thanks
SAX parser will be the fastest and most efficient (in that you don't need to read the entire document into memory and process it).
XSLT will be probably a terser solution since all you need is an identity transform (to copy the input document) with a few templates to copy out the bits you want.
Personally I'd go with the SAX parser.
The StAX API may suit your needs - have a look at StreamFilter or EventFilter. It has an advantage over SAX in that its pull model makes it is easier to quit processing when you've parsed all the data you want without resorting to artificial mechanisms like throwing an exception.
If your employer can afford SAP, then they certainly can afford Saxon, which is an XSLT processor that can process streams of arbitrary length.