Drools compiler does not recognize Java 9

The Drools compiler class org.drools.compiler.rule.builder.dialect.java.JavaDialectConfiguration
does not include Java 9 as a valid language level. Instead, when run using Java 9, the assumed java version is set to the default value of "1.7".

As a consequence, when compiling a rules file requiring Java 8 specific language features, the compilation will fail. For example, if a rule RHS contains the statement:

Arrays.asList("a","b","c").forEach(System.out::println);

The rule compilation fails with the error:

[ERROR] Unable to build KieBaseModel:default
Rule Compilation error : [Rule name='test']
	defaultpkg/Rule_test959974052.java (7:387) : Method references are allowed only at source level 1.8 or above

The problem can be worked around by adding a configuration file META-INF/kie.properties.conf containing the line:

drools.dialect.java.compiler.lnglevel = 1.8

The Drools compiler code will use this value instead of the System "java.version" value, avoiding the problem.