JAB is not enabled by default. One way to enable JAB is by modifying JRE/lib/accessibility.properties to set the assistive_technologies property to the assistive technologies [1]

  assistive_technologies=com.sun.java.accessibility.AccessBridge

or by setting the system property:
  $ java -Djavax.accessibility.assistive_technologies=com.sun.java.accessibility.AccessBridge

[1] http://docs.oracle.com/javase/7/docs/technotes/guides/access/properties.html

In JDK9, JAB is in a different module and com.sun.java.accessibility.AccessBridge is a module-private API. java.awt.ToolKit will not be able to access AccessBridge class. Looks like it also supports multiple classes to be specified.

One option is to consider using ServiceLoader such that JAB will provide the implementation to a service interface that may make sense if it's expected for 3rd party a11y provider.