CREATE EXTENSION pljava failed on PostgreSql 9.6

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