A quick Googling defines Excursion as: “a journey taken for pleasure”. Considering what I am about to blog about the title of this blog post might be a bit misleading, but you gotta give me one for the rhyme 
As you might or might not know, doing recursion in Java is simply a bad thing. This is mainly because Java can’t do tail recursion. You can use recursion in Java if you are absolutely positive that you are only going to do a very limited number of recursive calls. If you could possibly go over 100 calls you should consider making it a for or while loop instead, if the Java runtime performs somewhere around 1000 recursive calls you will get a StackOverflowError. This is really bad – you see if you read the StackOverflowError docs you will see that it is a subclass of VirtualMachineError. The docs for VirtualMachineError says:
Thrown to indicate that the Java Virtual Machine is broken or has run out of resources necessary for it to continue operating
This means that you have pretty much no choice but to log a fatal error and abort the JVM.
There are ways for making the recursion limit of the JVM bigger by setting some system properties, but that is really just a band aid and I would advise against using them.
The Real Life Case: XML Parsing
Java 6 ships with a new XML parsing library, the core class of which being XMLStreamReader (also known as the “push parser”). I must say that it is quite a nice library and a huge improvement over SAX parsing, while still keeping a blazing performance. We use it in Summa and has been very happy with it.
The problem came when we started indexing documents like this one: java-recursion-lection-1.xml. You can definitely expect to find similar structures out in the wild (as we have seen here at work). The basic document structure is as follows:
<mydocument>
<mytag>
SOME TEXT BLOCK
</mytag>
</mydocument>
If we just want to extract the text block it would be annoying with a standard SAX parser because a SAX parser splits up characters segments into arbitrary chunks and you have to collect them into one string yourself. The push parser API makes this a lot easier because it defines the property XMLInputFactory.IS_COALESCING which, when set, requires the parser to collect all the text chunks into one string. So extracting the raw text contents is easy peasy lemon squeezy:
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamReader;
import javax.xml.stream.events.XMLEvent;
import java.io.FileReader;
/**
* A small excursion in Java recursion.
*/
public class JavaRecursionLecture1 {
public static void main(String[] args) throws Exception {
XMLInputFactory inputFactory = XMLInputFactory.newInstance();
inputFactory.setProperty(XMLInputFactory.IS_COALESCING, Boolean.TRUE);
XMLStreamReader reader = inputFactory.createXMLStreamReader(
new FileReader("/home/mke/Documents/java-recursion-lection-1.xml"));
parse(reader);
}
public static void parse(XMLStreamReader reader) throws Exception {
while (reader.hasNext()) {
int event = reader.getEventType();
switch (event) {
case XMLEvent.START_DOCUMENT :
System.out.println("Document start");
break;
case XMLEvent.START_ELEMENT :
System.out.println("Element: " + reader.getLocalName() );
break;
case XMLStreamReader.CHARACTERS :
// Warning: Here be StackOverflowErrors
System.out.println("Char data:\n" + reader.getText());
break;
}
reader.next();
}
}
}
Except that this will throw a StackOverflowError if you run it on the file I linked you to. “What is up with that, there is no recursion here!” – you ask?
The problem here is that XMLStreamReader is highly recursive underneath the hood. My file contains lots of XML entities and the parser will make a recursive call each time a new entity is found in the stream. Looking at the heart of the implementation you will see that the author(s) actually where very minute about making sure that all recursive calls where tail calls. This would have been very robust had the Java runtime supported tail recursion – alas.
There are two ways to work around this misfeature. The first one is to don’t set the IS_COALESCING property, and then change the switch statement to something like this, using reader.getElementText() instead:
switch (event) {
case XMLEvent.START_DOCUMENT :
System.out.println("Document start");
break;
case XMLEvent.START_ELEMENT :
System.out.println("Element: " + reader.getLocalName() );
if ("mytag".equals(reader.getLocalName())) {
System.out.println(reader.getElementText());
}
break;
case XMLStreamReader.CHARACTERS :
// Warning: Here be StackOverflowErrors
System.out.println("Char data:\n" + reader.getText());
break;
}
This is not particularly elegant since it hard codes our <mytag> element. A more generic way is to provide your own coalescing implementation of getText():
/**
* Use this method in response to XMLEvent.CHARACTERS event instead of
* XMLStreamReader.getElementText() on a XMLEvent.START_ELEMENT. The former
* approach will
* @param reader the XMLStreamReader to pull character data out of,
* the reader is expected to be in a XMLEvent.CHARACTERS state
* @return A string containing the full character data as one string
* @throws Exception if the Jupiter aligns with Mars
*/
public static String getCoalescedText(XMLStreamReader reader)
throws XMLStreamException {
StringBuilder builder = new StringBuilder();
char[] buf = new char[1024];
while (reader.getEventType() == XMLEvent.CHARACTERS) {
int offset = 0;
int len;
while (true) {
len = reader.getTextCharacters(offset, buf, 0, buf.length);
if (len != 0) builder.append(buf, 0, len);
if (len < buf.length) break;
}
reader.next();
}
return builder.toString();
And then in the switch branch checking on character events do:
case XMLStreamReader.CHARACTERS :
// Warning: If you expect a StackOverflowError here, you are
// going to wait a long while!
System.out.println("Character data:\n"
+ getCoalescedText(reader));
break;
Anyway – this became a long an code-full post. All I really wanted to say was Avoid recursion in Java unless you know exactly what you are doing.
