Mac OS Runtime for Java (MRJ, originally Macintosh Runtime for Java) was Apple's proprietary virtual machine for Java-based applications in "Classic" Mac OS (i.e. versions prior to OS X). Both a runtime environment and a software development kit (SDK) are available. The SDK includes ports of most of the tools from Sun's Java Development Kit (in the form of MPW tools and Macintosh applications), additional tools for packaging Java applications as double-clickable Macintosh applications, libraries for Macintosh-specific functionality, and documentation for the MRJ-specific classes and tools.