为Eclipse创建Ant的build.xml文件编辑自动提示

Eclipse中编辑XML配置文件的时候我们总是习惯于既有的自动提示和和完成功能，事实上它是因为有了DTD的句法约束，特定功能的XML文件都有特定的DTD头来验证和约束句法，而普通的XML文件则没有这种优待。

JavaEE版的Eclipse自动集成了Ant插件，但是，并没有提供Ant的DTD文件。原因在Apache官网的Ant项目下的FAQ中有解释，原文如下：

[b]Is there a DTD that I can use to validate my
build files?

[/b]


An incomplete DTD can be created by the

<antstructure>

task - but this one has a few problems:

It doesn't know about required attributes. Only manual tweaking of this file can help here.
It is not complete - if you add new tasks via

<taskdef>

it won't know about it. See this page by Michel Casabianca for a solution to this problem. Note that the DTD you can download
at this page is based on Apache Ant 0.3.1.
It may even be an invalid DTD. As Ant allows tasks writers to define arbitrary elements, name collisions will happen quite frequently - if your version of Ant contains the optional

<test>

and

<junit>

tasks, there are two XML elements
named

test

(the task and the nested child element of

<junit>

) with different attribute lists. This problem cannot be solved; DTDs don't give a syntax rich enough to support this.

一言概之就是说DTD并不完善，功能上问题多多，但是仍然给了我们自己手动生成DTD的工具<antstructure>，对我们而言，不完善的工具也好过没有。

具体的完成步骤看一下操作：

1、先在任意一个位置创建一个build.xml文件：

<?version="1.0" encoding="UTF-8"?>
<project name="antdtd">
<target name="makeantdtd" basedir=".">
<antstructure output="ant.dtd" />
</target>
</project>

2、从Apache官网的commons项目下的download找到net的jar文件下载，commons-net-3.1.jar需要jave1.5
or later的支持。将commons-net-3.1.jar拷贝到%ANT_HOIME%下的lib目录下。

3、cmd进入build.xml所在目录，执行ant命令，就会在当前目录下生成ant.dtd文件。如果没有commons-net-3.1.jar，会出现java.lang.ClassNotFoundException:
org.apache.commons.net.ftp.FTPClientConfig异常。

4、拷贝ant.dtd到任意位置保存，建议放在%ANT_HOME%下的etc目录下。在Eclipse中添加DTD：Window-Preference-XML--XML
Catalog，选择添加，location指定到保存ant.dtd的位置，Key Type：Public ID，Key：任意指定，如ant_DTD。

5、在Eclipse中某项目下新建XML文件，通过指定DTD生成build.xml，发现问题。在Problems
View中查看，有既定问题（如果用的是commons-net-1.4.jar可能问题会不同）。根据提示[Element type "target" must not be declared more than once. line2431]，找到ant.dtd的2431行，去掉<!ELEMENT
target EMPTY>即可.在build.xml中右键validate，问题消失，OK。