评估 XPath 和 XmlResourceParser 会导致异常答案

【问题标题】：Evaluating XPath an XmlResourceParser causes Exception评估 XPath 和 XmlResourceParser 会导致异常
【发布时间】：2016-01-16 23:27:33
【问题描述】：

我的资源中有一个 XML 文件，看起来像这样（删除了其他阶段元素）：

<?xml version="1.0" encoding="utf-8"?>
<schedule name="Irgend ein besondere Zuchtname">
    <stages>
        <stage interval="0" token="3">
            <name>Name</name>
            <descrition>Some description...</descrition>
            <image>image_to_retrieve.jpg</image>
        </stage>
    </stages>
</schedule>

我想获取特定阶段的图像元素的文本（通过令牌）。我的做法是这样的：

public String GetImage(int stageToken) {

    // because this is a helper class, the context is set in the constructor
    XmlResourceParser parser = context.getResources().getXml(R.xml.schedule_default);
    XPath xPath = XPathFactory.newInstance().newXPath();

    String image;

    try {
        // error occurs here:
        image = xPath.evaluate("//stage[@token=3]/image/text()", parser);
    } catch (XPathExpressionException e) {
        Log.e(LOG_TAG, "Error evaluating xpath expression.");
        e.printStackTrace();
        return null;
    }

    parser.close();
    return image;
}

表达式应该不是问题，因为我用其他 XPath 程序对其进行了测试。 XML 的有效性也是如此。这是我得到的例外：

01-17 00:12:05.044 30417-30417/at.beekeeper W/System.err: javax.xml.xpath.XPathExpressionException: javax.xml.transform.TransformerException: Attempt to invoke interface method 'org.apache.xml.dtm.DTMAxisTraverser org.apache.xml.dtm.DTM.getAxisTraverser(int)' on a null object reference
01-17 00:12:05.045 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.jaxp.XPathImpl.evaluate(XPathImpl.java:295)
01-17 00:12:05.045 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.jaxp.XPathImpl.evaluate(XPathImpl.java:371)
01-17 00:12:05.045 30417-30417/at.beekeeper W/System.err:     at at.beekeeper.util.ScheduleParser.GetImage(ScheduleParser.java:108)
01-17 00:12:05.045 30417-30417/at.beekeeper W/System.err:     at at.beekeeper.view.StageActivity.onCreate(StageActivity.java:52)
01-17 00:12:05.045 30417-30417/at.beekeeper W/System.err:     at android.app.Activity.performCreate(Activity.java:5953)
01-17 00:12:05.045 30417-30417/at.beekeeper W/System.err:     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1128)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2267)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2388)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at android.app.ActivityThread.access$800(ActivityThread.java:148)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1292)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:102)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at android.os.Looper.loop(Looper.java:135)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:5312)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at java.lang.reflect.Method.invoke(Method.java:372)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err: Caused by: javax.xml.transform.TransformerException: Attempt to invoke interface method 'org.apache.xml.dtm.DTMAxisTraverser org.apache.xml.dtm.DTM.getAxisTraverser(int)' on a null object reference
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.XPath.execute(XPath.java:365)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.jaxp.XPathImpl.eval(XPathImpl.java:219)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.jaxp.XPathImpl.evaluate(XPathImpl.java:281)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:   ... 16 more
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err: Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'org.apache.xml.dtm.DTMAxisTraverser org.apache.xml.dtm.DTM.getAxisTraverser(int)' on a null object reference
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.axes.AxesWalker.setRoot(AxesWalker.java:221)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.axes.WalkingIterator.setRoot(WalkingIterator.java:157)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.axes.NodeSequence.setRoot(NodeSequence.java:265)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.axes.LocPathIterator.execute(LocPathIterator.java:212)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:     at org.apache.xpath.XPath.execute(XPath.java:337)
01-17 00:12:05.046 30417-30417/at.beekeeper W/System.err:   ... 18 more

【问题讨论】：

不了解 Android 编程，但尝试使用 //stage[@token='3']/image/text()。胡乱猜测，但堆栈跟踪中提到了“int”。
不，正如我的问题中提到的那样，表达不是问题，因为我也尝试过其他更简单的表达。

标签： java android xml xpath

【解决方案1】：

我找到了使用 XPath 的解决方案。您需要使用 InputSource 并将 XML 文件放入 /raw 目录。

InputSource inputSource = new InputSource(context.getResources().openRawResource(R.raw.schedule_default));
XPath xPath = XPathFactory.newInstance().newXPath();
String image;       

try {
    image = xPath.evaluate("//stage[@token=3]/image/text()", inputSource);
} catch (XPathExpressionException e) {
    Log.e(LOG_TAG, "Error evaluating xpath expression.");
    e.printStackTrace();
    return null;
}

但我仍然不确定是否可以将 XmlPullParser 与 XPath 一起使用...

【讨论】：