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XML merging made easy

Manipulate XML files using XPath declarations

By Laurent Bovet, JavaWorld.com, 07/10/07

Page 2 of 3

Using XPath declarations with XmlMerge

You've seen a few examples of applying XmlMerge to common Java enterprise development scenarios. l'll spend the remainder of this article explaining how the tool works. By default, you can use XPath declarations to specify how XmlMerge handles your XML sources. A sample configuration is shown in Listing 6.

Listing 6. xmlmerge.properties

action.default=COMPLETE   # By default, only add elements not
                          # already existing in first file

XPath.a=/root/a           # define a XPath named "a" and matching
                          # all <a> elements under <root>

action.a=MERGE            # configure to merge children of <a>

Listing 7 shows two more files that need to be merged, this time as specified by the above XPath declarations.

Listing 7. Two XML files waiting to be merged

<root>
<a/>
<c/>
</root>

<root>
<a>
 <b/>
</a>
<c>
 <d/>
</c>
</root>

Listing 8 shows the files merged as specified by the XPath declarations.

Listing 8. Two files merged as specified

<root>
<a>
 <b/>   # merged the content of the element <a>
</a>
<c/>    # by default, do not modify existing elements
</root>

Using XPath declarations within the XmlMerge utility lets you specify how each element in your XML files will be handled during a merge. In the next section I'll explain the actions you may have noticed in Listing 5, as well as the use of matching functions in XmlMerge.

Actions and matching functions

XmlMerge provides many built-in actions, some of them extending its functions well beyond simple merging. Consider the following actions and the various ways you could use XPath declarations to apply them to elements in your XML documents.

Table 1. Built-in actions for XmlMerge

<root>
<a>
 <b/>
 <c/>
</a>
<d/>
<e/>
</root>

<root> 
<a> 
 <c/> 
</a> 
<d/> 
<e/> 
</root>

<root> 
<a> 
 <c/> 
</a> 
<d/> 
</root>

<root> 
<a> 
 <b/> 
</a> 
<d/> 
<e/> 
</root>

<root> 
<d/> 
</root>

<root> 
<a> 
 <b/> 
</a> 
<d/> 
</root>

It is also possible to tell XmlMerge that elements from the original and the patch correspond to criteria other than the element name. For this you would use matching functions, or matchers.

Table 2. Built-in matchers for XmlMerge