Eclipse: Referencing log4j.dtd in log4j.xml

Try to add the log4j.dtd as a User Specific URI XML Catalog Entry in "Preferences -> XML -> XML Catalog". As I know this is the place where eclipse manages the references to definition/validation files (like xsd). If they can be found here eclipse needs no internet access to access them on their native (web) location.

I did it like this (for test) and eclipse does not complain:

Entry element:    URI
Location:         C:\Users\me\Desktop\log4j.dtd
URI:              file:///C:/Users/me/Desktop/log4j.dtd
Key type:         URI
Key:              http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/xml/doc-files/log4j.dtd

Maybe also ${M2_REPO} works - I did not check this.

Use the native URL in your log4j.xml afterwards

<!DOCTYPE log4j:configuration SYSTEM "http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/xml/doc-files/log4j.dtd">

EDIT

I would go with the above solution but to come back to your question, I think class path variables 'can be used in a Java Build Path'. Why should they work inside a DOCTYPE definition? "Validate" (eclipse context menu) the log4j.xml file and you will get a warning that the path can not be resolved.

I hoped classpath:org/apache/log4j/xml/log4j.dtd would do the trick but that protocol is also not support (see validation error). I am afraid it will not work out of the box.

And, as I understood, the SYSTEM "log4j.dtd" notation is no placeholder. It is a valid reference to a document that is expected to be found next to the dtd (in this case).

I added DTD folder in webcontent and then I copied the log4j dtd file in that. then i tried like bellow. Its working

<!DOCTYPE log4j:configuration SYSTEM "<Path>/DTD/log4j.dtd">

Path means here the project path like /projectname

I know this question has been answered, but I'd like to provide my slightly different alternative:

<!DOCTYPE log4j:configuration PUBLIC
  "-//APACHE//DTD LOG4J 1.2//EN" "http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/xml/doc-files/log4j.dtd">

It is similar to @FrVaBe's response, but on the plus side, does not require any further Eclipse configuration (i.e., if you're sharing your project with others, or have a large team, it's one less thing to worry about).

The down side though, is that I believe it means you'll need an Internet connection (at least at some point during development, even if it's just one time).