CREATE EXTENSION pljava failed on PostgreSql 9.6 - postgresql

Question

I try to install pljava for postgesql 9.6 in Ubuntu 16.04, but it fails with error.
A gradlew script makes an installation, but it fails with the following error"
Failed to execute: CREATE EXTENSION pljava; because: ERROR: could not access file "pljava-so-1.5.1-BETA1": No such file or directory
A gradlew source:
Sql sql = establishConnection()
try {
sql.execute("SET pljava.libjvm_location TO '${javaNative}';")
sql.execute("SET pljava.vmoptions TO '-Xshare:on -XX:+DisableAttachMechanism';")
sql.execute("ALTER DATABASE postgres SET pljava.libjvm_location FROM CURRENT;")
sql.execute("ALTER DATABASE postgres SET pljava.vmoptions FROM CURRENT;")
sql.execute("CREATE EXTENSION pljava;")
} finally {
sql.close()
}
As i understand, it fails during a creation of extension pljava, commands above was executed without any errors.
According to #ChapmanFlack advice:
I have no any mentions about pljava-so-1.5.1-BETA1 file at pg_config --libdir directory. Also i didn't find that file at pg_config --sharedir
grep LOAD pljava--1.5.1-BETA1.sql output following:
touched off by the LOAD command, making possible a decent installation
LOAD command, but finds the CREATE EXTENSION command instead). So,
temporarily LOAD 'pljava-so-1.5.1-BETA1'; Ok, the LOAD succeeded, so
everything happened ... unless ... the same PostgreSQL turns LOAD into
a (successful) no-op in that case, meaning To fail fast in that case,
expect that the LOAD actions should have
I think PL/Java builds from a prebuilt, because of that line at gradlew:
commandLine 'java', '-jar', '../installer/build/dependencies/pljava-pg9.6.jar'
I would be appreciate for any advice