InputHandler

Overview

When a task wants to prompt a user for input, it doesn't simply read the input from the console as this would make it impossible to embed Ant in an IDE. Instead it asks an implementation of the org.apache.tools.ant.input.InputHandler interface to prompt the user and hand the user input back to the task.

To do this, the task creates an InputRequest object and passes it to the InputHandler Such an InputRequest may know whether a given user input is valid and the InputHandler is supposed to reject all invalid input.

Exactly one InputHandler instance is associated with every Ant process, users can specify the implementation using the -inputhandler command line switch.

InputHandler

The InputHandler interface contains exactly one method

    void handleInput(InputRequest request) 
        throws org.apache.tools.ant.BuildException;

with some pre- and postconditions. The main postcondition is that this method must not return unless the request considers the user input valid, it is allowed to throw an exception in this situation.

Ant comes with two built-in implementations of this interface:

DefaultInputHandler

This is the implementation you get, when you don't use the -inputhandler command line switch at all. This implementation will print the prompt encapsulated in the request object to Ant's logging system and re-prompt for input until the user enters something that is considered valid input by the request object. Input will be read from the console and the user will need to press the Return key.

PropertyFileInputHandler

This implementation is useful if you want to run unattended build processes. It reads all input from a properties file and makes the build fail if it cannot find valid input in this file. The name of the properties file must be specified in the Java system property ant.input.properties.

The prompt encapsulated in a request will be used as the key when looking up the input inside the properties file. If no input can be found, the input is considered invalid and an exception will be thrown.

Note that ant.input.properties must be a Java system property, not an Ant property. I.e. you cannot define it as a simple parameter to ant, but you can define it inside the ANT_OPTS environment variable.

InputRequest

Instances of org.apache.tools.ant.input.InputRequest encapsulate the information necessary to ask a user for input and validate this input.

The instances of InputRequest itself will accept any input, but subclasses may use stricter validations. org.apache.tools.ant.input.MultipleChoiceInputRequest should be used if the user input must be part of a predefined set of choices.