为什么需要特别告知 JAXBContext 一个已经在包中的类? [英] Why does JAXBContext need to be told specifically about a class that is already in the package?
问题描述
这个程序:
import foo.bar.baz.ClassSpecificallyIncluded;
import javax.xml.bind.JAXBContext;
public class A {
public static void main(String[] args) throws Exception {
System.out.println(JAXBContext.newInstance(ClassSpecificallyIncluded.class).toString());
System.out.println(JAXBContext.newInstance("foo.bar.baz").toString());
}
}
为第一个和第二个 JAXBContext 产生不同的输出:
produces different output for the first and second JAXBContext:
jar:file:/C:/dev/trunk/rt/tomcat/shared/lib/webservices-rt-2.0.1.jar!/com/sun/xml/bind/v2/runtime/JAXBContextImpl.class Build-ID:1.0
此上下文中已知的类:
[乙
布尔值
字节
字符
foo.bar.baz.Class1
foo.bar.baz.Class1$NestedClass
foo.bar.baz.Class2
foo.bar.baz.ClassSpecificallyIncluded
foo.bar.baz.AnotherClass
com.sun.xml.bind.api.CompositeStructure
双
浮动
整数
java.awt.Image
java.io.文件
java.lang.Boolean
java.lang.Byte
java.lang.Character
java.lang.Class
java.lang.Double
java.lang.Float
java.lang.Integer
java.lang.Long
java.lang.Object
java.lang.Short
java.lang.String
java.lang.Void
java.math.BigDecimal
java.math.BigInteger
java.net.URI
java.net.URL
java.util.Calendar
java.util.Date
java.util.GregorianCalendar
java.util.UUID
javax.activation.DataHandler
javax.xml.bind.JAXBElement
javax.xml.datatype.Duration
javax.xml.datatype.XMLGregorianCalendar
javax.xml.namespace.QName
javax.xml.transform.Source
长
短
无效
jar:file:/C:/dev/trunk/rt/tomcat/shared/lib/webservices-rt-2.0.1.jar!/com/sun/xml/bind/v2/runtime/JAXBContextImpl.class Build-Id: 1.0
Classes known to this context:
[B
boolean
byte
char
foo.bar.baz.Class1
foo.bar.baz.Class1$NestedClass
foo.bar.baz.Class2
foo.bar.baz.ClassSpecificallyIncluded
foo.bar.baz.AnotherClass
com.sun.xml.bind.api.CompositeStructure
double
float
int
java.awt.Image
java.io.File
java.lang.Boolean
java.lang.Byte
java.lang.Character
java.lang.Class
java.lang.Double
java.lang.Float
java.lang.Integer
java.lang.Long
java.lang.Object
java.lang.Short
java.lang.String
java.lang.Void
java.math.BigDecimal
java.math.BigInteger
java.net.URI
java.net.URL
java.util.Calendar
java.util.Date
java.util.GregorianCalendar
java.util.UUID
javax.activation.DataHandler
javax.xml.bind.JAXBElement
javax.xml.datatype.Duration
javax.xml.datatype.XMLGregorianCalendar
javax.xml.namespace.QName
javax.xml.transform.Source
long
short
void
jar:file:/C:/dev/trunk/rt/tomcat/shared/lib/webservices-rt-2.0.1.jar!/com/sun/xml/bind/v2/runtime/JAXBContextImpl.class Build-ID:1.0
此上下文中已知的类:
[乙
布尔值
字节
字符
foo.bar.baz.Class1
foo.bar.baz.Class1$NestedClass
foo.bar.baz.Class2
<强><<<此处缺少类>>>
foo.bar.baz.AnotherClass
com.sun.xml.bind.api.CompositeStructure
双
浮动
整数
java.awt.Image
java.io.文件
java.lang.Boolean
java.lang.Byte
java.lang.Character
java.lang.Class
java.lang.Double
java.lang.Float
java.lang.Integer
java.lang.Long
java.lang.Object
java.lang.Short
java.lang.String
java.lang.Void
java.math.BigDecimal
java.math.BigInteger
java.net.URI
java.net.URL
java.util.Calendar
java.util.Date
java.util.GregorianCalendar
java.util.UUID
javax.activation.DataHandler
javax.xml.bind.JAXBElement
javax.xml.datatype.Duration
javax.xml.datatype.XMLGregorianCalendar
javax.xml.namespace.QName
javax.xml.transform.Source
长
短
无效
jar:file:/C:/dev/trunk/rt/tomcat/shared/lib/webservices-rt-2.0.1.jar!/com/sun/xml/bind/v2/runtime/JAXBContextImpl.class Build-Id: 1.0
Classes known to this context:
[B
boolean
byte
char
foo.bar.baz.Class1
foo.bar.baz.Class1$NestedClass
foo.bar.baz.Class2
<<< CLASS IS MISSING HERE>>>
foo.bar.baz.AnotherClass
com.sun.xml.bind.api.CompositeStructure
double
float
int
java.awt.Image
java.io.File
java.lang.Boolean
java.lang.Byte
java.lang.Character
java.lang.Class
java.lang.Double
java.lang.Float
java.lang.Integer
java.lang.Long
java.lang.Object
java.lang.Short
java.lang.String
java.lang.Void
java.math.BigDecimal
java.math.BigInteger
java.net.URI
java.net.URL
java.util.Calendar
java.util.Date
java.util.GregorianCalendar
java.util.UUID
javax.activation.DataHandler
javax.xml.bind.JAXBElement
javax.xml.datatype.Duration
javax.xml.datatype.XMLGregorianCalendar
javax.xml.namespace.QName
javax.xml.transform.Source
long
short
void
然而，ClassSpecificallyIncluded 在 foo.bar.baz 包中:
And yet, ClassSpecificallyIncluded is in the foo.bar.baz package:
//
// This file was generated by the JavaTM Architecture for XML Binding(JAXB) Reference Implementation, vJAXB 2.1.10 in JDK 6
// 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: 2012.05.14 at 10:47:17 PM IST
//
package foo.bar.baz;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "", propOrder = {
"applicationArea",
"dataArea"
})
@XmlRootElement(name = "ClassSpecificallyIncluded")
public class ClassSpecificallyIncluded {...
两个 JAXBContext 的类路径和类加载器是相同的.
The classpath and classloader are the same for both JAXBContexts.
那么为什么第二个 JAXBContext 不知道 ClassSpecificallyIncluded 呢?
So why doesn’t the second JAXBContext know about ClassSpecificallyIncluded?
推荐答案
这里有效地回答了您的问题:
The answer to your question is effectively answered here:
答案:否 按照设计，java 不知道也无法找到包中的每个类.Java 建立在及时"的概念之上.出于这个目的，这意味着 Java 不知道它得到了什么，直到它被特别要求.
Answer: No By design, java doesn't know and can't find every class in a package. Java is built on the concept of "Just in time". Which for this purpose means that java doesn't know what it's got until it's asked for something specifically.
因此，从这个角度来看，JAXB 无法找到包中的类.
So from that stand point, JAXB has no way to find what classes are in a package.
如果每次需要上下文时都必须准确地告诉 JAXB，那会很不方便.因此，作为一种方便的方法，JAXB 为您提供了提供包名称的选项.
It would be inconvenient if JAXB had to be told precisely every class every time you needed context. So as a convenience method JAXB offers you the option of providing a package name.
为了绕过 java 的限制，它尝试从该包加载一个 ObjectFactory 和一个 jaxb.index 文件.它可以这样做，因为它们是特定的名称.如果找到，则将它们用作包的清单.如果两者都没有找到，它就别无选择，只能中止，因为它不可能知道包中的内容.
In order to get round the limitations of java it attempts to load from that package an ObjectFactory and a jaxb.index file. It can do this because they are specific names. If one is found, then it uses them as a manifest for the package. If neither is found it has no option but to abort because it can't possibly know what's in the package.
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