Faq/General/015/en

What is Java used for in LibreOffice?
LibreOffice is written primarily in C / C++, a language that generates programs called "native" designed for specific platforms. There are versions for Windows, Linux or macOS, but not for all three at the same time. However, some modules can be written in other languages, including Java.

Specifically, currently (as of version 6.3) at least these components/functionality require Java:
 * HSQLDB (optionally used for embedded database in Base; default is Firebird that doesn't depend on Java)
 * JDBC
 * Some wizards (particularly, Table/Query/Form/Report Wizards in Base)
 * Report Builder (used to generate actual reports from report templates in Base)
 * Non-Linear solvers built-in extension (DEPS and SCO) in Calc (there is an experimental Swarm solver that doesn't depend on Java)
 * MediaWiki extension (Wiki Publisher)
 * Support for scripts and extensions written in Java/Beans
 * Support for scripts and extensions written in JavaScript (since it is implemented using an ancient version (1.5R5 from 2004) of Mozilla Rhino)

See also: Development/Java