如何使用 JAXBContext 将单例对象保存到 XML 文件？

Question

我希望使用 JAXBContext 将单例对象保存到 XML 文件。有问题的对象称为 Player，它正在扩展抽象 class 实体。实体也扩展线程。由于 Player 是一个单例对象，它的构造函数是私有的，并且无法从 class 外部访问。我怎样才能正确地做到这一点？

这是我用来保存播放器的代码片段：

public static void savePlayer() throws JAXBException {
        JAXBContext context = JAXBContext.newInstance(Player.class);
        Marshaller m = context.createMarshaller();
        m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        OutputStream os = null;
        try {
            os = new FileOutputStream(StringConstants.PLAYER_SAVE_FILE);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
        m.marshal(Player.getInstance(), os);
    }

再读一遍这段代码：

public static Player readPlayer() throws JAXBException {
        JAXBContext context = JAXBContext.newInstance(Player.class);
        Unmarshaller u = context.createUnmarshaller();
        File f = new File(StringConstants.PLAYER_SAVE_FILE);
        return (Player) u.unmarshal(f);
    }

在 classes 实体和播放器上，我使用了以下注释：

@XmlRootElement

Richt 在 classes 的开头。

这是我得到的错误输出：

com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 1 counts of IllegalAnnotationExceptions
java.lang.Thread$UncaughtExceptionHandler is an interface, and JAXB can't handle interfaces.
    this problem is related to the following location:
        at java.lang.Thread$UncaughtExceptionHandler
        at public java.lang.Thread$UncaughtExceptionHandler java.lang.Thread.getUncaughtExceptionHandler()
        at java.lang.Thread
        at model.entities.Entity
        at model.entities.Player

    at com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException$Builder.check(IllegalAnnotationsException.java:91)
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.getTypeInfoSet(JAXBContextImpl.java:445)
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.<init>(JAXBContextImpl.java:277)
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.<init>(JAXBContextImpl.java:124)
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl$JAXBContextBuilder.build(JAXBContextImpl.java:1123)
    at com.sun.xml.internal.bind.v2.ContextFactory.createContext(ContextFactory.java:147)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at javax.xml.bind.ContextFinder.newInstance(ContextFinder.java:247)
    at javax.xml.bind.ContextFinder.newInstance(ContextFinder.java:234)
    at javax.xml.bind.ContextFinder.find(ContextFinder.java:462)
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:641)
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:584)
    at model.io.SaveWriter.savePlayer(SaveWriter.java:20)
    at tests.SaveTest.main(SaveTest.java:17)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 1 counts of IllegalAnnotationExceptions
java.lang.Thread$UncaughtExceptionHandler is an interface, and JAXB can't handle interfaces.
    this problem is related to the following location:
        at java.lang.Thread$UncaughtExceptionHandler
        at public java.lang.Thread$UncaughtExceptionHandler java.lang.Thread.getUncaughtExceptionHandler()
        at java.lang.Thread
        at model.entities.Entity
        at model.entities.Player

    at com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException$Builder.check(IllegalAnnotationsException.java:91)
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.getTypeInfoSet(JAXBContextImpl.java:445)
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.<init>(JAXBContextImpl.java:277)
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.<init>(JAXBContextImpl.java:124)
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl$JAXBContextBuilder.build(JAXBContextImpl.java:1123)
    at com.sun.xml.internal.bind.v2.ContextFactory.createContext(ContextFactory.java:147)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at javax.xml.bind.ContextFinder.newInstance(ContextFinder.java:247)
    at javax.xml.bind.ContextFinder.newInstance(ContextFinder.java:234)
    at javax.xml.bind.ContextFinder.find(ContextFinder.java:462)
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:641)
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:584)
    at model.io.SaveReader.readPlayer(SaveReader.java:18)
    at tests.SaveTest.main(SaveTest.java:23)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)

Answer 1

终于找到问题了。如问题中所述，class 播放器扩展了 class 实体。 class 实体扩展了公共线程 class。显然，无法为线程对象正确实例化 JAXBContext。我通过让 class 实体实现 接口 Runnable 解决了这个问题，这让我没有再出现任何错误。

如何使用 JAXBContext 将单例对象保存到 XML 文件？

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