Spring uses oxm for object XML Mapping parsing

1. Understanding XML parsing technology

1.1 XML related concepts

(1) DTD: XML syntax rule, which is the verification mechanism of XML files. You can compare XML documents and DTD files to see whether the documents comply with the specifications and whether the elements and labels are used correctly.

(2) XML is the foundation of SOA.

1.2 XML processing technology

(1) In order to use XML, we need to access data through XML processor or XMLAPI. At present, jaxp provides two methods to process XML: Dom and Sax.

① DOM: DOM accesses the data and structure in XML documents programmatically, based on the tree structure of XML documents in memory. The disadvantage is that loading the entire XML document into memory requires a lot of overhead.

② Sax: Based on event driven, it uses one segment to parse one segment, which solves the problem of large memory occupied by Dom, but its disadvantage is that it is unable to access documents randomly.

(2) In order to solve the problems of DOM and sax, a stream API for XML (Stax) based on stream has appeared. At present, it has been added to jaxp1.4 of JDK6. Stax is also based on event driven.

(3) Dom, sax and Stax all process XML from the document structure, but many applications only focus on the document data itself, so XML data binding technology came into being.

Data binding: refers to the process of extracting data from storage media (XML documents and databases) and expressing these data through programs, that is, binding the data to a memory structure that can be understood and operated by the virtual machine.

XML binding framework: castor, JAXB, jibx, quick, Zeus, etc.

2. XML processor: xStream

2.1 overview of xStream

(1) XStream is a concise and easy-to-use open source framework for serializing Java objects into XML or deserializing XML into Java objects. The main features of xStream are as follows:

(2) XStream Architecture Composition:

Converters: when xStream encounters an object that needs to be converted, it will be delegated to the appropriate converter implementation.

IO (input / output): xStream is abstracted from underlying XML data through hierarchalstreamwriter and hierarchalstreamreader, and is used for serialization and deserialization operations respectively.

Context: when xStream serializes and deserializes objects, it will create two classes, marshallingcontext and unmarshallingcontext. The tower gate will process the data and delegate it to the appropriate converter.

Facade (unified entrance): integrate the above three points and open them to users with a unified interface.

2.2. Quick start

(1) Create an xStream and specify the XML parser

XStreamxstream=newXStream(newDomDriver());

If no parser is specified, xStream will use XPP (xmlpullparser) parser by default. XPP is a high-speed parser.

(2) Examples are as follows:

2.3. Use xStream alias

(1) In the above example, the full class name of the Java object corresponds to the root element of the XML file, and the attribute name corresponds to the node element of the XML file. However, in practice, the names of the Java object and the XML object may have been defined, so alias mapping is required.

XStream has three alias configurations:

Class alias: alias (stringname, classtype) is used.

Class member alias: use aliasfield (stringalias, classdefined in, stringfieldname).

Class members are used as attribute aliases: aliasattribute (classdefined in, stringattributename, stringalias) is used. Naming alone is meaningless. It should also be applied to a class through useattributefor (classdefined in, stringfieldname).

(2) Modify the example in 2.2 by alias:

2.4. XStream converter

In the development process, sometimes some custom types need to be converted. Just implement the converter interface and call the registerconverter () method of xStream to register the converter.

2.5. XStream annotation

(1) Common notes of xStream are as follows:

(2) Use

XStreamxstream=newXStream(newDomDriver());

There are two ways to load objects:

① Mode 1:

xstream. processAnnotations(AAA.class);

xstream. processAnnotations(BBB.class);

② Mode 2:

xstream. autodetectAnnotations(true);// Annotated beans are loaded automatically, and annotated objects are cached

2.6 fluidization object

(1) XStream provides an alternative implementation for objectinputstream and objectoutputstream, allowing XML serialization or deserialization in the form of object stream. The previous is the XML read by Dom Based XML parser. Here, it is obvious that we should parse in the form of stream.

Examples are as follows:

(2) The difference between using prettywriter and compactwriter is that prettywriter formats the generated XML, while compactwriter compresses the generated XML.

2.7 persistence API

(1) XStream provides a set of simple ways to persist the objects in the collection to files, such as xmlarraylist, xmlset, xmlmap, etc.

(2) Before creating a collection, you also need to specify a persistence strategy.

2.8. Handle JSON

(1) XML has an unshakable position in Web services, but in most web applications, lightweight JSON is often used as the data exchange format.

(2) XStream provides jettisonmapped xmldriver and jsonhierarchical streamdriver to complete the conversion of Java objects and JSON.

Examples are as follows:

(3) Differences between jettisonmapped xmldriver and jsonhierarchalstreamdriver:

① The jettisonmapped xmldriver generates compressed JSON, while the jsonhierarchical streamdriver generates formatted JSON.

② To convert JSON to an object, you can only use the jettisonmapped xmldriver.

3. Other common open source o / xmapping projects

Comparison of JAXB, xmlbeans, cstor and jibx:

4. Integration with spring oxm

4.1 overview of springoxm

(1) Spring oxm makes a unified abstraction and encapsulation of the mainstream O / xmapping framework. Marshall and unmarshaller are the two core interfaces of spring oxm. Marshall is used to convert objects into XML and unmarshaller is used to convert XML into objects.

(2) The O / xmapping component wrappers are as follows:

4.2. Configure in spring

(1) Xstreammarshaller configuration instance:

5. Summary

(1) XML data binding of Java applications can be summarized into two ways:

Generate Java language code from XML documents (such as JAXB, xmlbeans, castor).

Use some form of mapping binding method, that is, set how Java classes are associated with XML (such as xStream, castor, jibx).

(2) Comparison of two methods:

If you use a stable document structure defined by a schema or DTD, and the structure is suitable for the needs of your application, the code generation method may be the best choice.

If you use an existing Java class, or if you want to use the structure of the class that reflects the application's usage of the data, rather than the XML structure, the mapping method is the best choice.

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