Category: Programming

Using if:set, unless:set etc. to execute a task conditionally in Ant

I’m not sure anyone except me is still struggling on with using Ant, but just in case, here is a nice thing.

In Ant 1.9.1 they added a useful feature: instead of needing to use the <if> tag and similar from ant-contrib, you can conditionally execute “any” task.

(In fact, this appears to mean “any task except a macrodef” – see my post Using if:set, unless:set etc. with macrodefs in Ant.)

You need to add these namespaces at the top of your project:

<project
     xmlns:if="ant:if"
     xmlns:unless="ant:unless"
>

and then make any task conditional by adding one of these attributes:

if:set
if:true
if:blank
unless:set
unless:true
unless:blank

if:set and unless:set take a property name and run the task if it is set at all (or unless it is set at all).

<mytask if:set="property.name" other_attrs="" .../>

Where mytask is the name of any ordinary Ant task like echo, jar etc.

(Note there is no ${} around the property name for if:set.)

if:true and unless:true take a value and run the task if it is true (or unless it is true), so they need to be used like this:

<mytask if:true="${property}" other_attrs="" .../>

(Note the ${} here, but not above.)

Similarly, if:blank and unless:blank take a value and run the task if it is blank (or unless it is blank), e.g.:

<mytask if:blank="${property}" other_attrs="" .../>

NOTE: The example in the documentation is wrong (at this time, 2013-09-13) – it uses the property name, but this does not work – you must surround it with ${} to get its value.

The properties can be specified in your build file as normal, or supplied on the command line to ant with -Dproperty.name=value.

Here’s an example of using set:

build.xml:

<project
    xmlns:if="ant:if"
    xmlns:unless="ant:unless"
    default="build"
>

    <property name="setinxml" value="true"/>

    <target name="build">
        <echo if:set="setinxml"     message="if:set=setinxml"/>
        <echo unless:set="setinxml" message="unless:set=setinxml"/>
        <echo if:set="notset"       message="if:set=notset"/>
        <echo unless:set="notset"   message="unless:set=notset"/>
        <echo if:set="setincmd"     message="if:set=setincmd"/>
        <echo unless:set="setincmd" message="unless:set=setincmd"/>
    </target>

</project>

And here’s the output:

$ ant -version
Apache Ant(TM) version 1.9.2 compiled on July 8 2013
$ ant -Dsetincmd=true
Buildfile: build.xml

build:
     [echo] if:set=setinxml
     [echo] unless:set=notset
     [echo] if:set=setincmd

BUILD SUCCESSFUL
Total time: 0 seconds

The documentation for this, such as it is, is here: If And Unless.

Working with others in git

Series: Why git?, Basics, Branches, Merging, Remotes

Now I’ve covered almost everything you need to understand the basics of how git works. In this video I discuss how to clone, pull from and fork remote git repositories, and how to send patches and make pull requests to a project maintainer.

Slides: Working with others in git.

Merging in git

Series: Why git?, Basics, Branches, Merging, Remotes

The last video covered how to make branches and simple merges.

This time we look at what to do when you want just one change from another branch (cherry-picking) and how to merge two branches when there are conflicts between them. We also look at rebasing, which is a different way to combine the work done on two branches, which hides complexity from other people at the expense of changing history.

Slides: Merging in git.

Behaviour of Java String.split() when some answers are the empty string

Can you guess the output of this program?

class SplitTest
{
    static void split( String s )
    {
        System.out.println( s.split( ";" ).length );
    }

    public static void main( String[] args )
    {
        split("");
        split(";");
        split("x;");
        split(";y");
    }
}

Here it is:

$ javac SplitTest.java && java SplitTest
1
0
1
2

Wow. Docs: String.split.

Side note: It’s not easy to fit Java examples into tweets.