TWiStErRob

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Xalan Java Extensions on Android

A saga of finding out, that I can't use them without external dependencies.

I wanted to do a simple check to see whether Xalan’s extension functions are available. This would’ve helped speeding up an XML-to-CSV transformation a bit.

<xsl:stylesheet xmlns:str="xalan://java.lang.String">
...
<xsl:variable name="quote">&quot;</xsl:variable>
<xsl:variable name="double-quote">&quot;&quot;</xsl:variable>
<xsl:choose>
    <xsl:when test="function-available('str:replaceAll')">
        <!-- Happy days, fast replacement! -->
        <xsl:value-of select="str:replaceAll(string($text), $quote, $double-quote)" />
    </xsl:when>
    <xsl:otherwise>
        <!-- Oh well, we have this nice XSLT 1.0-compatbile implementation -->
        <xsl:variable name="escaped">
            <!-- @see http://stackoverflow.com/a/10528912/253468 -->
            <xsl:call-template name="string-replace-all">
                <xsl:with-param name="text" select="$text" />
                <xsl:with-param name="replace" select="$quote" />
                <xsl:with-param name="with" select="$double-quote" />
            </xsl:call-template>
        </xsl:variable>
        <xsl:value-of select="$escaped" />
    </xsl:otherwise>
</xsl:choose>

After running it on my Android phone it just kept spewing NullPointerExceptions at me:

W/System.err: SystemId Unknown; Line #98; Column #3; Attempt to invoke virtual method ‘boolean org.apache.xalan.extensions.ExtensionsTable.functionAvailable(java.lang.String, java.lang.String)’ on a null object reference

Everything was still working, but I don’t like warnings, so I started looking for the root cause.

Xalan Version

The source code is not available in the Android SDK. This means that it’s nearly impossible to comperehend what’s going on and use the stepping facilities of the IDE. I didn’t even know where the exception is coming from, because there’s no stack trace.

So I set out to figure out which version of Xalan is running on the device, so I can get the sources. I found that calling org.apache.xml.serializer.Version.main(new String[0]) would give me the answer. It’s not possible to call this from code, not even from an Evaluate while debugging, but the problem was only that it wasn’t know to IDEA, so this worked:

Class
    .forName("org.apache.xml.serializer.Version")
    .getDeclaredMethod("main", String[].class)
    .invoke(null, new Object[] { new String[0] });

The answer was: Serializer Java 2.7.1 on both a 2.3.7 and a 5.0.0 device. So I tried to reproduce the issue on my desktop, but the function-available call worked real good when using xalan:xalan:2.7.1. The problem was only reproducible on the phone. So now, I knew the version of Xalan, and added it to my source path so the IDE can handle debugging. As usual with Android source code, there are some problems with line alignment. I still couldn’t set meaningful breakpoints, but it was possible to set method breakpoints, however slow.

Finding the Sources

So the official Xalan release is not the one I needed. A quick Google search for TransformerImpl.java revealed that it was once available as /luni/src/main/java/... in the AOSP platform/libcore repository. I tried to get the latest by simply jumping to the master branch, but it wasn’t there. Quick cloning and git log -- luni/... showed me that the whole org.apache.xalan package was moved to /apache-xml in f029395. That one still wasn’t on the master branch, so another git log revelated that it was deleted and moved to an external repository in e590b9c. At first I was puzzled by what that means, but after a quick look at the repository listing I found platform/external/apache-xml. Finally this one contained up-to-date code for Lollipop.

Finally Debugging

Adding the above sources to the IDE source path I was able to browse and debug. I found out the full stack trace:

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean org.apache.xalan.extensions.ExtensionsTable.functionAvailable(java.lang.String, java.lang.String)' on a null object reference
    at org.apache.xalan.transformer.TransformerImpl.functionAvailable(TransformerImpl.java:396)
    at org.apache.xpath.functions.FuncExtFunctionAvailable.execute(FuncExtFunctionAvailable.java:89)
    at org.apache.xpath.Expression.bool(Expression.java:186)
    at org.apache.xpath.XPath.bool(XPath.java:412)
    at org.apache.xalan.templates.ElemChoose.execute(ElemChoose.java:103)
    at org.apache.xalan.transformer.TransformerImpl.executeChildTemplates(TransformerImpl.java:2223)
    at org.apache.xalan.templates.ElemTemplate.execute(ElemTemplate.java:389)
    at org.apache.xalan.templates.ElemCallTemplate.execute(ElemCallTemplate.java:241)
    at org.apache.xalan.transformer.TransformerImpl.executeChildTemplates(TransformerImpl.java:2223)
    at org.apache.xalan.transformer.TransformerImpl.transformToRTF(TransformerImpl.java:1830)
    at org.apache.xalan.transformer.TransformerImpl.transformToRTF(TransformerImpl.java:1752)
    at org.apache.xalan.templates.ElemVariable.getValue(ElemVariable.java:302)
    at org.apache.xalan.templates.ElemVariable.execute(ElemVariable.java:245)
    at org.apache.xalan.transformer.TransformerImpl.executeChildTemplates(TransformerImpl.java:2223)
    at org.apache.xalan.templates.ElemTemplate.execute(ElemTemplate.java:389)
    at org.apache.xalan.templates.ElemCallTemplate.execute(ElemCallTemplate.java:241)
    at org.apache.xalan.templates.ElemApplyTemplates.transformSelectedNodes(ElemApplyTemplates.java:370)
    at org.apache.xalan.templates.ElemApplyTemplates.execute(ElemApplyTemplates.java:175)
    at org.apache.xalan.templates.ElemApplyTemplates.transformSelectedNodes(ElemApplyTemplates.java:370)
    at org.apache.xalan.templates.ElemApplyTemplates.execute(ElemApplyTemplates.java:175)
    at org.apache.xalan.transformer.TransformerImpl.executeChildTemplates(TransformerImpl.java:2223)
    at org.apache.xalan.transformer.TransformerImpl.applyTemplateToNode(TransformerImpl.java:2096)
    at org.apache.xalan.transformer.TransformerImpl.transformNode(TransformerImpl.java:1228)
    at org.apache.xalan.transformer.TransformerImpl.transform(TransformerImpl.java:614)
    at org.apache.xalan.transformer.TransformerImpl.transform(TransformerImpl.java:1145)
    at org.apache.xalan.transformer.TransformerImpl.transform(TransformerImpl.java:1123)

The reason it worked even with the error is because XPath#bool catches the exception and just returns false after logging.

So the question now is: Why is the ExtensionsTable null?. It is only set in one place:

void setExtensionsTable(StylesheetRoot sroot)
		throws javax.xml.transform.TransformerException {
	try {
		if (sroot.getExtensions() != null)
			m_extensionsTable = new ExtensionsTable(sroot);
	} catch (javax.xml.transform.TransformerException te) {
		te.printStackTrace();
	}

The extensions are correctly recognized, I saw earlier during debug that sroot.getExtensions() is a non-null Vector containing one element. After a debug session, it turns out that an exception was thrown, however that printStackTrace had no effect and it wasn’t visible. Don’t fret, just use printStackTrace(PrintWriter(StringWriter)) to get the trace:

javax.xml.transform.TransformerException: java.lang.ClassNotFoundException: org.apache.xalan.extensions.ExtensionHandlerJavaClass
    at org.apache.xalan.extensions.ExtensionNamespaceSupport.launch(ExtensionNamespaceSupport.java:101)
    at org.apache.xalan.extensions.ExtensionsTable.<init>(ExtensionsTable.java:66)
    at org.apache.xalan.transformer.TransformerImpl.setExtensionsTable(TransformerImpl.java:385)
    at org.apache.xalan.transformer.TransformerImpl.transformNode(TransformerImpl.java:1184)
    at org.apache.xalan.transformer.TransformerImpl.transform(TransformerImpl.java:614)
    at org.apache.xalan.transformer.TransformerImpl.transform(TransformerImpl.java:1145)
    at org.apache.xalan.transformer.TransformerImpl.transform(TransformerImpl.java:1123)
Caused by: java.lang.ClassNotFoundException: org.apache.xalan.extensions.ExtensionHandlerJavaClass
    at java.lang.Class.classForName(Native Method)
    at java.lang.BootClassLoader.findClass(ClassLoader.java:781)
    at java.lang.BootClassLoader.loadClass(ClassLoader.java:841)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:469)
    at org.apache.xalan.extensions.ObjectFactory.findProviderClass(ObjectFactory.java:100)
    at org.apache.xalan.extensions.ExtensionHandler.getClassForName(ExtensionHandler.java:65)
    at org.apache.xalan.extensions.ExtensionNamespaceSupport.launch(ExtensionNamespaceSupport.java:76)
    ... 22 more
Caused by: java.lang.NoClassDefFoundError: Class not found using the boot class loader; no stack available

Surprise, surprise, there’s a class missing. I diffed the Android Xalan and Xalan 2.7.1 sources and indeed, the whole XSLTC and most of the extension classes are not included in Android. This exception causes m_extensionsTable to be null and that’s a problem in functionAvailable. So I went ahead tried to ensure that it’s non-null:

// transformer created by factory.newTransformer(...)
TransformerImpl xalanTransformer = (TransformerImpl)transformer;
StylesheetRoot sroot = new StylesheetRoot((ErrorListener)null);
// creates an empty Vector to be returned from getExtensions()
sroot.getExtensionNamespacesManager();
// make sure the workaround works in Marsmallow too
sroot.setSecureProcessing(false);
// sets up an empty ExtensionTable, so functionAvailable works without NPE
xalanTransformer.setExtensionsTable(sroot); // default visibility

It’s worth noting that this code cannot be executed as is, because the compile classpath doesn’t have these classes. However, it’s a trivial conversion to reflective calls.

Summary

This indeed fixed the problem, however it is not a good solution, even if you accept reflective hacks. The problem being that even though the XSLT ran, and there was no error, the test="function-available" check still evaluated to false and the <xsl:template> version was used. I now think it’s pointless to do the hack to ensure an extension table, because the only place where replaceAll could have benefit, it doesn’t work. In the end I just removed the Xalan Java Extension and swallowed the XSLT 1.0-compatible workaround.

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