无法在build.xml中添加if块

Question

我无法执行if任务。 我的代码是：

<?xml version="1.0" ?> 
<project default="main"> 

    <property name="buildsequence.property.file.fullpath" value="D:\testant\AntExample" /> 

    <target name="main" depends="compile, compress" description="Main target"> 
     <echo> 
      Building the .jar file. 
     </echo> 
    </target> 

    <target name="compile" description="Compilation target"> 
     <javac srcdir="src/org" destdir="src/org" fork="yes" executable="C:\Program Files\Java\jdk1.7.0_60\bin\javac"/> 
    </target> 

    <target name="compress" description="Compression target"> 
     <jar jarfile="Project.jar" basedir="src/org" includes="*.class" /> 

     <if> 
      <available file="${buildsequence.property.file.fullpath}" /> 
      <then>    
       <echo message="File exist"/> 
      </then> 
      <else> 
       <echo message="File do not exist" />     
      </else> 
     </if> 

    </target> 

</project> 

错误：

Buildfile: D:\projects\Self\AntExample\build.xml 
compile: 
    [javac] Compiling 1 source file to D:\projects\Self\AntExample\src\org 
compress: 

BUILD FAILED 
D:\projects\Self\AntExample\build.xml:19: Problem: failed to create task or type if 
Cause: The name is undefined. 
Action: Check the spelling. 
Action: Check that any custom tasks/types have been declared. 
Action: Check that any <presetdef>/<macrodef> declarations have taken place. 

注：文件存在于D:\testant\AntExample。

Answer 1

if是ant-contrib的一部分，它必须存在于您的类路径中。下载完成后，你可以把它放在你的Ant lib文件夹（anthome/lib目录），然后你需要通过添加下面的行构建文件的开头导入任务：

<taskdef resource="net/sf/antcontrib/antlib.xml" /> 

Answer 2

正如@解释manouti如果任务是外部扩展，而不是核心ANT的一部分。

下面的示例下载缺少的罐子，并调用所需的taskdef声明：

<project default="runif"> 

    <target name="init" description="Download dependencies and setup tasks"> 
    <mkdir dir="${user.home}/.ant/lib"/> 
    <get dest="${user.home}/.ant/lib/ant-contrib.jar" src="http://search.maven.org/remotecontent?filepath=ant-contrib/ant-contrib/1.0b3/ant-contrib-1.0b3.jar"/> 

    <taskdef resource="net/sf/antcontrib/antcontrib.properties"/> 
    </target> 

    <target name="runif" depends="init" description="Example running the ant-contrib if statement"> 
    <if> 
     <available file="file.txt" /> 
     <then>    
     <echo message="File exist"/> 
     </then> 
     <else> 
     <echo message="File do not exist" />     
     </else> 
    </if> 
    </target> 

</project> 

的ant-contrib扩展还支持更现代antlib机制。以下示例演示了它如何使用名称空间并且不需要任务定义。

<project default="runif" xmlns:contrib="antlib:net.sf.antcontrib"> 

    <available classname="net.sf.antcontrib.logic.IfTask" property="if.task.exists"/> 

    <target name="init" description="Download missing dependencies" unless="if.task.exists"> 
    <mkdir dir="${user.home}/.ant/lib"/> 
    <get dest="${user.home}/.ant/lib/ant-contrib.jar" src="http://search.maven.org/remotecontent?filepath=ant-contrib/ant-contrib/1.0b3/ant-contrib-1.0b3.jar"/> 

    <fail message="ant-contrib installed run the build again"/> 
    </target> 

    <target name="runif" depends="init" description="Example running the ant-contrib if statement"> 
    <contrib:if> 
     <available file="file.txt" /> 
     <then>    
     <echo message="File exist"/> 
     </then> 
     <else> 
     <echo message="File do not exist" />     
     </else> 
    </contrib:if> 
    </target> 

</project>