jaxb解析字符串xml_一个JAXB Nuance：字符串与枚举（受限制的XSD字符串）的枚举

jaxb解析字符串xml

尽管用于XML绑定的Java体系结构（ JAXB ）在名义情况下（尤其是自Java SE 6以来）相当容易使用，但它也存在许多细微差别。一些常见的细微差别是由于无法将 XML架构定义（XSD）类型与Java 类型精确匹配（绑定）。这篇文章看一个具体的例子，它还演示了当JAXB编译器生成Java类时，实施相同XML结构的不同XSD构造如何导致不同的Java类型。

下一个代码清单（用于Food.xsd ）定义了食物类型的架构。 XSD要求有效的XML将具有一个称为“食物”的根元素，并带有三个嵌套元素“蔬菜”，“水果”和“甜点”。尽管用于指定“ Vegetable”和“ Dessert”元素的方法与用于指定“ Fruit”元素的方法不同，但是两种方法都导致相似的“有效XML”。 “ Vegetable”和“ Dessert”元素直接声明为稍后在XSD中定义的指定simpleType的元素。 “水果”元素是通过引用（ ref= ）定义到另一个包含simpleType定义元素的。

Food.xsd

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"xmlns:dustin="http://marxsoftware.blogspot.com/foodxml"targetNamespace="http://marxsoftware.blogspot.com/foodxml"elementFormDefault="qualified"attributeFormDefault="unqualified"><xs:element name="Food"><xs:complexType><xs:sequence><xs:element name="Vegetable" type="dustin:Vegetable" /><xs:element ref="dustin:Fruit" /><xs:element name="Dessert" type="dustin:Dessert" /></xs:sequence></xs:complexType></xs:element><!--Direct simple type that restricts xs:string will become enum inJAXB-generated Java class.--><xs:simpleType name="Vegetable"><xs:restriction base="xs:string"><xs:enumeration value="Carrot"/><xs:enumeration value="Squash"/><xs:enumeration value="Spinach"/><xs:enumeration value="Celery"/></xs:restriction></xs:simpleType><!--Simple type that restricts xs:string but is wrapped in xs:element(making it an Element rather than a SimpleType) will become JavaString in JAXB-generated Java class for Elements that reference it.--><xs:element name="Fruit"><xs:simpleType><xs:restriction base="xs:string"><xs:enumeration value="Watermelon"/><xs:enumeration value="Apple"/><xs:enumeration value="Orange"/><xs:enumeration value="Grape"/></xs:restriction></xs:simpleType></xs:element><!--Direct simple type that restricts xs:string will become enum inJAXB-generated Java class.        --><xs:simpleType name="Dessert"><xs:restriction base="xs:string"><xs:enumeration value="Pie"/><xs:enumeration value="Cake"/><xs:enumeration value="Ice Cream"/></xs:restriction></xs:simpleType></xs:schema>

尽管在模式中对Vegetable元素和Dessert元素的定义与对Fruit定义不同，但是生成的有效XML是相同的。接下来在food1.xml的代码清单中显示一个有效的XML文件。

food1.xml

<?xml version="1.0" encoding="utf-8"?>
<Food xmlns="http://marxsoftware.blogspot.com/foodxml"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Vegetable>Spinach</Vegetable><Fruit>Watermelon</Fruit><Dessert>Pie</Dessert>
</Food>

此时，我将使用一个简单的Groovy脚本针对上述XSD验证上述XML。接下来显示此Groovy XML验证脚本的代码（ validateXmlAgainstXsd.groovy ）。

validateXmlAgainstXsd.groovy

#!/usr/bin/env groovy// validateXmlAgainstXsd.groovy
//
// Accepts paths/names of two files. The first is the XML file to be validated
// and the second is the XSD against which to validate that XML.if (args.length < 2)
{println "USAGE: groovy validateXmlAgainstXsd.groovy <xmlFile> <xsdFile>"System.exit(-1)
}String xml = args[0]
String xsd = args[1]import javax.xml.validation.Schema
import javax.xml.validation.SchemaFactory
import javax.xml.validation.Validatortry
{SchemaFactory schemaFactory =SchemaFactory.newInstance(javax.xml.XMLConstants.W3C_XML_SCHEMA_NS_URI)Schema schema = schemaFactory.newSchema(new File(xsd))Validator validator = schema.newValidator()validator.validate(new javax.xml.transform.stream.StreamSource(xml))
}
catch (Exception exception)
{println "\nERROR: Unable to validate ${xml} against ${xsd} due to '${exception}'\n"System.exit(-1)
}
println "\nXML file ${xml} validated successfully against ${xsd}.\n"

下一个屏幕快照展示了针对food1.xml和Food.xsd运行上述Groovy XML验证脚本。

food1ValidatedAgainstFoodXsd

到目前为止，本文的目的是展示XSD中的不同方法如何导致相同的XML有效。尽管这些不同的XSD方法规定了相同的有效XML，但是当使用JAXB生成基于XSD的类时，它们会导致不同的Java类行为。下一个屏幕快照演示了针对Food.xsd运行JDK提供的JAXB xjc编译器以生成Java类。

xjcOnFoodXsd

上面显示的JAXB生成的输出表明Java类是为“ Vegetable”和“ Dessert”元素创建的，而不是为“ Fruit”元素创建的。这是因为在XSD中，“蔬菜”和“甜点”的定义不同于“水果”。下一个代码清单是由xjc编译器生成的Food.java类的。由此可见，生成的Food.java类引用了针对Vegetable和Dessert特定生成的Java类型，但仅引用了Fruit的通用Java字符串。

Food.java（由JAXB jxc编译器生成）

//
// This file was generated by the JavaTM Architecture for XML Binding(JAXB) Reference Implementation, v2.2.8-b130911.1802 
// See <a href="http://java.sun.com/xml/jaxb">http://java.sun.com/xml/jaxb</a> 
// Any modifications to this file will be lost upon recompilation of the source schema. 
// Generated on: 2015.02.11 at 10:17:32 PM MST 
//package com.blogspot.marxsoftware.foodxml;import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlSchemaType;
import javax.xml.bind.annotation.XmlType;/*** <p>Java class for anonymous complex type.* * <p>The following schema fragment specifies the expected content contained within this class.* * <pre>* <complexType>*   <complexContent>*     <restriction base="{http://www.w3.org/2001/XMLSchema}anyType">*       <sequence>*         <element name="Vegetable" type="{http://marxsoftware.blogspot.com/foodxml}Vegetable"/>*         <element ref="{http://marxsoftware.blogspot.com/foodxml}Fruit"/>*         <element name="Dessert" type="{http://marxsoftware.blogspot.com/foodxml}Dessert"/>*       </sequence>*     </restriction>*   </complexContent>* </complexType>* </pre>* * */
@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {"vegetable","fruit","dessert"
})
@XmlRootElement(name = "Food")
public class Food {@XmlElement(name = "Vegetable", required = true)@XmlSchemaType(name = "string")protected Vegetable vegetable;@XmlElement(name = "Fruit", required = true)protected String fruit;@XmlElement(name = "Dessert", required = true)@XmlSchemaType(name = "string")protected Dessert dessert;/*** Gets the value of the vegetable property.* * @return*     possible object is*     {@link Vegetable }*     */public Vegetable getVegetable() {return vegetable;}/*** Sets the value of the vegetable property.* * @param value*     allowed object is*     {@link Vegetable }*     */public void setVegetable(Vegetable value) {this.vegetable = value;}/*** Gets the value of the fruit property.* * @return*     possible object is*     {@link String }*     */public String getFruit() {return fruit;}/*** Sets the value of the fruit property.* * @param value*     allowed object is*     {@link String }*     */public void setFruit(String value) {this.fruit = value;}/*** Gets the value of the dessert property.* * @return*     possible object is*     {@link Dessert }*     */public Dessert getDessert() {return dessert;}/*** Sets the value of the dessert property.* * @param value*     allowed object is*     {@link Dessert }*     */public void setDessert(Dessert value) {this.dessert = value;}}

具有特定的Vegetable和Dessert类的优点是，与一般的Java String相比，它们具有附加的类型安全性。 Vegetable.java和Dessert.java实际上都是枚举，因为它们来自XSD中的枚举值。接下来的两个代码清单中显示了这两个生成的枚举。

Vegetable.java（使用JAXB xjc编译器生成）

//
// This file was generated by the JavaTM Architecture for XML Binding(JAXB) Reference Implementation, v2.2.8-b130911.1802 
// See <a href="http://java.sun.com/xml/jaxb">http://java.sun.com/xml/jaxb</a> 
// Any modifications to this file will be lost upon recompilation of the source schema. 
// Generated on: 2015.02.11 at 10:17:32 PM MST 
//package com.blogspot.marxsoftware.foodxml;import javax.xml.bind.annotation.XmlEnum;
import javax.xml.bind.annotation.XmlEnumValue;
import javax.xml.bind.annotation.XmlType;/*** <p>Java class for Vegetable.* * <p>The following schema fragment specifies the expected content contained within this class.* <p>* <pre>* <simpleType name="Vegetable">*   <restriction base="{http://www.w3.org/2001/XMLSchema}string">*     <enumeration value="Carrot"/>*     <enumeration value="Squash"/>*     <enumeration value="Spinach"/>*     <enumeration value="Celery"/>*   </restriction>* </simpleType>* </pre>* */
@XmlType(name = "Vegetable")
@XmlEnum
public enum Vegetable {@XmlEnumValue("Carrot")CARROT("Carrot"),@XmlEnumValue("Squash")SQUASH("Squash"),@XmlEnumValue("Spinach")SPINACH("Spinach"),@XmlEnumValue("Celery")CELERY("Celery");private final String value;Vegetable(String v) {value = v;}public String value() {return value;}public static Vegetable fromValue(String v) {for (Vegetable c: Vegetable.values()) {if (c.value.equals(v)) {return c;}}throw new IllegalArgumentException(v);}}

Dessert.java（使用JAXB xjc编译器生成）

//
// This file was generated by the JavaTM Architecture for XML Binding(JAXB) Reference Implementation, v2.2.8-b130911.1802 
// See <a href="http://java.sun.com/xml/jaxb">http://java.sun.com/xml/jaxb</a> 
// Any modifications to this file will be lost upon recompilation of the source schema. 
// Generated on: 2015.02.11 at 10:17:32 PM MST 
//package com.blogspot.marxsoftware.foodxml;import javax.xml.bind.annotation.XmlEnum;
import javax.xml.bind.annotation.XmlEnumValue;
import javax.xml.bind.annotation.XmlType;/*** <p>Java class for Dessert.* * <p>The following schema fragment specifies the expected content contained within this class.* <p>* <pre>* <simpleType name="Dessert">*   <restriction base="{http://www.w3.org/2001/XMLSchema}string">*     <enumeration value="Pie"/>*     <enumeration value="Cake"/>*     <enumeration value="Ice Cream"/>*   </restriction>* </simpleType>* </pre>* */
@XmlType(name = "Dessert")
@XmlEnum
public enum Dessert {@XmlEnumValue("Pie")PIE("Pie"),@XmlEnumValue("Cake")CAKE("Cake"),@XmlEnumValue("Ice Cream")ICE_CREAM("Ice Cream");private final String value;Dessert(String v) {value = v;}public String value() {return value;}public static Dessert fromValue(String v) {for (Dessert c: Dessert.values()) {if (c.value.equals(v)) {return c;}}throw new IllegalArgumentException(v);}}

为XML元素生成枚举可确保只能用Java表示这些元素的有效值。

结论

JAXB使将Java映射到XML相对容易，但是由于Java和XML类型之间没有一对一的映射，因此在某些情况下，为特定XSD规定元素生成的Java类型并不明显。这篇文章显示了两种不同的构建XSD来强制使用相同的基本XML结构的方法如何导致用JAXB xjc编译器生成的Java类产生截然不同的结果。在本文中显示的示例中，直接声明XSD上的元素在simpleType将XSD的string限制为一组特定的枚举值比将元素声明为引用其他封装了simpleType的限制字符串枚举值的元素更可取，因为类型安全这是在生成枚举而不是使用常规Java String的。