How to use DTD ENTITY external reference in log4j.xml

Question

I tried to use entities from external dtd file.

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" 
[<!ENTITY logHome SYSTEM "log4j-entity.dtd">]
>

log4j-entity.dtd

<?xml version="1.0" encoding="UTF-8"?>

<!ENTITY logHome "/root/crm_test/">

I tried to use the entity values in attribute values like this.

<param name="File" value="&logHome;log/info.log"/>

I get this errror:

The external entity reference "&logHome;" is not permitted in an attribute value.

How can I do this?

Note:

This thing works..

<!ENTITY logHome  "/root/crm_test/">

score 5 · Accepted Answer · answered Jan 17 '13 at 09:07

You need to make the entity inside the internal subset a parameter entity and then reference it.

Change:

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" 
[<!ENTITY logHome SYSTEM "log4j-entity.dtd">]
>

to:

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" [
<!ENTITY % logHome SYSTEM "log4j-entity.dtd">
%logHome;
]>

Magnus · Answer 2 · 2013-01-17T10:11:00.847

1

The XML specs specifically forbid the use of external entities in attribute values. See here: http://www.w3.org/TR/2004/REC-xml-20040204/#forbidden

The following are forbidden, and constitute fatal errors: [...] a reference to an external entity in an attribute value.

So the answer is: XML won't let you do what you're trying to do. You might, however, get a similar effect if you ran your XML through an XSLT processor and applied transformations as needed.

edited Jan 17 '13 at 10:11

answered Jan 17 '13 at 07:39

Magnus

980
4
10

I think the OP is referencing an external entity on accident. The entity he/she really wants to reference is not an external entity. This would possible in XML. – Daniel Haley Jan 17 '13 at 09:10
Ah yes, that would make sense indeed. – Magnus Jan 17 '13 at 10:10

score 1 · Answer 3 · answered Jan 19 '13 at 01:11

There are a couple things wrong here.

You are using the entity name logHome for two different things (an external entity containing declarations, which should as Daniel Haley points out be a parameter entity) and an internal entity whose replacement text names a directory.
As a consequence, your reference to &logHome; in the attribute value is understood to be a reference to the resource whose URI is "log4j-entity.dtd".

The simplest way to achieve what you want would be to declare the logHome entity in the internal subset:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" [
<!ENTITY logHome "/root/crm_test/">
]>

If you really want the declaration of logHome to be external, it might be less confusing to use a different name for the parameter entity:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" [
<!ENTITY % logHomeDeclaration SYSTEM "log4j-entity.dtd">
%logHomeDeclaration;
]>

How to use DTD ENTITY external reference in log4j.xml

3 Answers3