An XML schema uses XML syntax to describe the relationships among elements, attributes and entities in an XML document. The purpose of an XML schema is to define a class of XML documents that must adhere to a particular set of structural rules and data constraints. For example, you may want to define separate schemas for chapter-oriented books, for an online purchase order system, or for a personnel database. In the context of JAXB, an XML document containing data that is constrained by an XML schema is referred to as a document instance, and the structure and data within a document instance is referred to as a content tree.

By default, the JAXB binding compiler binds Java classes and packages to a source XML schema based on rules defined in Section 5, "Binding XML Schema to Java Representations," in the JAXB Specification. In most cases, the default binding rules are sufficient to generate a robust set of schema-derived classes from a wide range of schemas. There may be times, however, when the default binding rules are not sufficient for your needs. JAXB supports customizations and overrides to the default binding rules by means of binding customizations made either inline as annotations in a source schema, or as statements in an external binding customization file that is passed to the JAXB binding compiler. Note that custom JAXB binding customizations also allow you to customize your generated JAXB classes beyond the XML-specific constraints in an XML schema to include Java-specific refinements such as class and package name mappings.

The JAXB binding compiler is the core of the JAXB processing model. Its function is to transform, or bind, a source XML schema to a set of JAXB content classes in the Java programming language. Basically, you run the JAXB binding compiler using an XML schema (optionally with custom binding declarations) as input, and the binding compiler generates Java classes that map to constraints in the source XML schema.

The JAXB binding framework implementation is a runtime API that provides interfaces for unmarshalling, marshalling, and validating XML content in a Java application. The binding framework comprises interfaces in the javax.xml.bind package.

These are the schema-derived classes generated by the binding JAXB compiler. The specific classes will vary depending on the input schema.

In the context of JAXB, a Java application is a client application that uses the JAXB binding framework to unmarshal XML data, validate and modify Java content objects, and marshal Java content back to XML data. Typically, the JAXB binding framework is wrapped in a larger Java application that may provide UI features, XML transformation functions, data processing, or whatever else is desired.

XML content that is unmarshalled as input to the JAXB binding framework -- that is, an XML instance document, from which a Java representation in the form of a content tree is generated. In practice, the term "document" may not have the conventional meaning, as an XML instance document does not have to be a completely formed, selfstanding document file; it can instead take the form of streams of data passed between applications, or of sets of database fields, or of XML infosets, in which blocks of information contain just enough information to describe where they fit in the schema structure.

In JAXB, the unmarshalling process supports validation of the XML input document against the constraints defined in the source schema. This validation process is optional, however, and there may be cases in which you know by other means that an input document is valid and so you may choose for performance reasons to skip validation during unmarshalling. In any case, validation before (by means of a third-party application) or during unmarshalling is important, because it assures that an XML document generated during marshalling will also be valid with respect to the source schema. Validation is discussed more later in this chapter.

XML content that is marshalled out to an XML document. In JAXB, marshalling involves parsing an XML content object tree and writing out an XML document that is an accurate representation of the original XML document, and is valid with respect the source schema. JAXB can marshal XML data to XML documents, SAX content handlers, and DOM nodes.