Assertions type

## Assertions

The `assertions` type enables or disables the Java 1.4 assertions feature, on a whole Java program, or components of a program. It can be used in `<java>` and `<junit>` to add extra validation to code.

Assertions are covered in the J2SDK 1.4 documentation, and the Java Language Specification.

The key points to note are that a `java.lang.AssertionError` is thrown when an assertion fails, and that the facility is only available on Java 1.4 and later. To enable assertions one must set `source="1.4"` (or later) in `<javac>` when the source is being compiled, and that the code must contain `assert` statements to be tested. The result of such an action is code that neither compiles or runs on earlier versions of Java. For this reason Ant itself currently contains no assertions.

When assertions are enabled (or disabled) in a task through nested assertions elements, the class loader or command line is modified with the appropriate options. This means that the JVM executed must be a Java 1.4 or later JVM, even if there are no assertions in the code. Attempting to enable assertions on earlier VMs will result in an "Unrecognized option" error and the JVM will not start.

#### Attributes

Attribute | Description | Required
enableSystemAssertions | Flag to turn system assertions on or off. | No; default is "unspecified"

When system assertions have been neither enabled nor disabled, then the JVM is not given any assertion information - the default action of the current JVMs is to disable system assertions.

Note also that there is no apparent documentation for what parts of the JRE come with useful assertions.

### Nested elements

#### enable

Enable assertions in portions of code. If neither a package nor class is specified, assertions are turned on in all (user) code.

Attribute | Description | Required
class | The name of a class on which to enable assertions. | No
package | The name of a package in which to enable assertions on all classes. (Includes subpackages.) Use "`...`" for the anonymous package. | No

#### disable

Disable assertions in portions of code.

Attribute | Description | Required
class | The name of a class on which to disable assertions. | No
package | The name of a package in which to disable assertions on all classes. (Includes subpackages.) Use "`...`" for the anonymous package. | No

Because assertions are disabled by default, it only makes sense to disable assertions where they have been enabled in a parent package.

#### Examples

##### Example: enable assertions in all user classes

All classes not in the JRE (i.e. all non-system classes) will have assertions turned on.

    <assertions>
      <enable/>
    </assertions>

##### Example: enable a single class

Enable assertions in a class called Test

    <assertions>
      <enable class="Test"/>
    </assertions>

##### Example: enable a package

Enable assertions in the `org.apache` package and all packages starting with the `org.apache.` prefix

    <assertions>
      <enable package="org.apache"/>
    </assertions>

##### Example: System assertions

Example: enable system assertions and assertions in all `org.apache` packages except for Ant (but including `org.apache.tools.ant.Main`)

    <assertions enableSystemAssertions="true">
      <enable package="org.apache"/>
      <disable package="org.apache.tools.ant"/>
      <enable class="org.apache.tools.ant.Main"/>
    </assertions>

##### Example: disabled and anonymous package assertions

Disable system assertions; enable those in the anonymous package

    <assertions enableSystemAssertions="false">
      <enable package="..."/>
    </assertions>

##### Example: referenced assertions

This type is a datatype, so you can declare assertions and use them later

    <assertions id="project.assertions">
      <enable package="org.apache.test"/>
    </assertions>

    <assertions refid="project.assertions"/>