These instructions are a way to do conditional compilation with Java like the C/C++ #ifdef. In Java there is no preprocessor and so we need to work around this missing feature. This work around is using Ant's copy and filter feature to create the source with the different code in it. In this example I wanted to accomplish the following:

• The Java classes must be valid Java code so that any editor/IDE can read it as regular Java code
• The conditional compilation must also work outside Java methods so a constant boolean if statement does not work
• The selection of the version to compile to must be settable when the build of the project is started
• The compilation must work properly without cleaning the project first

1. Add into your Java classes a start tag (like //[ifdef]) and an end tag (like //[enddef]) around the code in question. To include code for version 1:

//[ifdef]
import java.sql.ParameterMetaData;
//[enddef]

//[ifdef]
    public byte[] getBytes(String parameterName)
            throws SQLException {
        ...
    }
//[enddef]

/* //[endef]
    public byte[] getBytes(String parameterName) {
        ...
    }
//[ifdef] */

2. To swap the used code for version 2 the start and end tag just have to be set to /* respectively */. In this case the included code is excluded and the excluded code in included.

3. Add a copy-n-filter task in your Ant build script. There the source coded will be copied away and the filter tags are replace with the actual value. For version 1 the start / end tag will vanish and for version 2 they will be substituted with /* and */. Attention: the source code is copied into two different root directory one for version 1 and the other for version 2. Because Ant tries to avoid copies if the file does not have changed this can lead to having the wrong version of code in your target directory. This is an Ant script snippet:

        <copy todir="${src.dir}/${version}">
            <fileset dir="${src.dir}/java">
                <include name="**/*.java"/>
            </fileset>
            <filterset begintoken="//[" endtoken="]">
                <filter token="ifdef" value="${ifdef.token}"/>
                <filter token="enddef" value="${enddef.token}"/>
            </filterset>
        </copy>

4. Set the start / end tag token. Note that the start / end tag token are set at the end to their default value only when not already set.

        <condition property="ifdef.token" value="/*">
            <equals arg1="${compile.to.version.1}" arg2="false"/>
        </condition>
        <condition property="enddef.token" value="*/">
            <equals arg1="${compile.to.version.1}" arg2="false"/>
        </condition>
        <!-- If not set already make sure that they are defined but empty -->
        <property name="ifdef.token" value=""/>
        <property name="enddef.token" value=""/>

ant -Dcompile.to.version.1=false build

5. Make sure that you compile from the right directory:

        <javac ...
        >
            <src path="${src.dir}/${version}"/>
        </javac>

I hope this will help you in the future – Andy