See bug for rationale, gist is that this is still useful.

Patch from Erik Wallin <erikw@sizeit.se>
https://bugs.gentoo.org/769794
--- a/java/CMakeLists.txt
+++ b/java/CMakeLists.txt
@@ -7,6 +7,8 @@
 
 find_package(Java)
 
+set(DATA_DIR "${CMAKE_INSTALL_PREFIX}/share")
+
 set(DEFAULT_JAVACFLAGS "-source 8 -target 8 -encoding UTF-8 -Xlint:all,-serial,-cast,-unchecked,-fallthrough,-dep-ann,-deprecation,-rawtypes")
 set(JAVACFLAGS ${DEFAULT_JAVACFLAGS} CACHE STRING
   "Java compiler flags (Default: ${DEFAULT_JAVACFLAGS})")
@@ -172,3 +174,10 @@
     -P ${SRCDIR}/cmake/SignJar.cmake)
 
 add_custom_target(java ALL DEPENDS VncViewer.jar)
+
+if(NOT WIN32)
+  install(FILES
+    ${BINDIR}/VncViewer.jar
+    ${SRCDIR}/${CLASSPATH}/README
+    DESTINATION ${DATA_DIR}/vnc/classes)
+endif()