Prior to Java 9, the layout engine was used to handle font complexities, specifically fonts that have rendering behaviors beyond what the common Latin fonts have. Java used the uniform client interface, also referred to as ICU, as the de facto text rendering tool. The ICU layout engine has been depreciated and, in Java 9, has been replaced with the HarfBuzz font layout engine.
HarfBuzz is an OpenType text rendering engine. This type of layout engine has the characteristic of providing script-aware code to help ensure text is laid out as desired.
The impetus for the change from the ICU layout engine to the HarfBuzz font layout engine was IBM's decision to cease supporting the ICU layout engine. Therefore, the JDK was updated to contain the HarfBuzz font layout engine.
