How to write java.util.Properties to XML with sorted keys?

Here's a way to produce sorted output for both store Properties.store(OutputStream out, String comments) and Properties.storeToXML(OutputStream os, String comment):

Properties props = new Properties() {
    @Override
    public Set<Object> keySet(){
        return Collections.unmodifiableSet(new TreeSet<Object>(super.keySet()));
    }

    @Override
    public synchronized Enumeration<Object> keys() {
        return Collections.enumeration(new TreeSet<Object>(super.keySet()));
    }
};
props.put("B", "Should come second");
props.put("A", "Should come first");
props.storeToXML(new FileOutputStream(new File("sortedProps.xml")), null);
props.store(new FileOutputStream(new File("sortedProps.properties")), null);

You could sort the keys first, then loop through the items in the properties file and write them to the xml file.

public static void main(String[] args){
        String propFile = "/tmp/test2.xml";
        Properties props = new Properties();
        props.setProperty("key", "value");
        props.setProperty("key1", "value1");
        props.setProperty("key2", "value2");
        props.setProperty("key3", "value3");
        props.setProperty("key4", "value4");

        try {
            BufferedWriter out = new BufferedWriter(new FileWriter(propFile));
            List<String> list = new ArrayList<String>();
            for(Object o : props.keySet()){
                list.add((String)o);
            }
            Collections.sort(list);
            out.write("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n");
            out.write("<!DOCTYPE properties SYSTEM \"http://java.sun.com/dtd/properties.dtd\">\n");
            out.write("<properties>\n");
            out.write("<comment/>\n");
            for(String s : list){
                out.write("<entry key=\"" + s + "\">" + props.getProperty(s) + "</entry>\n");
            }
            out.write("</properties>\n");
            out.flush();
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

Here's a quick and dirty way to do it:

String propFile = "/path/to/file";
Properties props = new Properties();

/* Set some properties here */

Properties tmp = new Properties() {
  @Override
  public Set<Object> keySet() {
    return Collections.unmodifiableSet(new TreeSet<Object>(super.keySet()));
  }
};

tmp.putAll(props);

try {
    FileOutputStream xmlStream = new FileOutputStream(propFile);
    /* This comes out SORTED! */
    tmp.storeToXML(xmlStream,"");
} catch (IOException e) {
    e.printStackTrace();
}

Here are the caveats:

• The tmp Properties (an anonymous subclass) doesn't fulfill the contract of Properties.

For example, if you got its keySet and tried to remove an element from it, an exception would be raised. So, don't allow instances of this subclass to escape! In the snippet above, you are never passing it to another object or returning it to a caller who has a legitimate expectation that it fulfills the contract of Properties, so it is safe.

• The implementation of Properties.storeToXML could change, causing it to ignore the keySet method.

For example, a future release, or OpenJDK, could use the keys() method of Hashtable instead of keySet. This is one of the reasons why classes should always document their "self-use" (Effective Java Item 15). However, in this case, the worst that would happen is that your output would revert to unsorted.

• Remember that the Properties storage methods ignore any "default" entries.

The simplest hack would be to override keySet. A bit of a hack, and not guaranteed to work in future implementations:

new Properties() {
    @Override Set<Object> keySet() {
        return new TreeSet<Object>(super.keySet());
    }
}

(Disclaimer: I have not even tested that it compiles.)

Alternatively, you could use something like XSLT to reformat the produced XML.