Ant and the available task - what if something is not available?

Question

When I use the task, the property is only set to TRUE if the resource (say file) is available. If not, the property is undefined.

When I print the value of the property, it gives true if the resource was available, but otherwise just prints the property name.

Is there a way to set the property to some value if the resource is not available? I have tried setting the property explicitly before the available check, but then ant complains:

[available] DEPRECATED -  used to override an existing property.
[available]   Build file should not reuse the same property name for different values.

Answer 1

You can use a condition in combination with not:

http://ant.apache.org/manual/Tasks/condition.html

  <condition property="fooDoesNotExist">
    <not>
      <available filepath="path/to/foo"/>
    </not>
  </condition>

Answer 2

<available filepath="/path/to/foo" property="foosThere" value="true"/>
<property name="foosThere" value="false"/>

The assignment of foosThere will only be successful if it has not already been set to true by your availability check.

Answer 3

The reason for this behaviour are the if/unless-attributes in targets. The target with such an attribute will be executed if/unless a property with the name is set. If it is set to false or set to true makes no difference. So you can use the available-task to set (or not) a property and based on this execute (or not) a task. Setting the property before the available-task is no solution, as properties in ant are immutable, they cannot be changed once set.

There are three possible solutions, to set a property to a value if unset before:

1. You use the available-task in combination with not.
2. You create a task setting the property, that will be executed only if the property is unset (unless-attribute of task).
3. You simply set the property after the call to available. As the property will only be changed if unset, this will do what you want.