yes, jruby-1.7.x defaults to 1.9

via commandline you can use the switch '--1.8'

or use a .jrubyrc file: see jruby -jar jruby-complete-1.7.20.jar --properties which dumps a sample .jrubyrc file - look for compat.version

or use -Djruby.compat.version=1.8 or set this system properties on the java
side before you "execute" jruby

I didn't get that properly.
Suppose say we have an already built jruby-complete.17.20.jar file in .m2 repository. And we build & package Apache Hbase which takes this jruby-complete jar and adds it to its classpath. How can we make the entire jar to work in ruby 1.8 mode?

For Hbase shell. it has the below script

if [ "$COMMAND" = "shell" ] ; then
  # eg export JRUBY_HOME=/usr/local/share/jruby
  if [ "$JRUBY_HOME" != "" ] ; then
    CLASSPATH="$JRUBY_HOME/lib/jruby.jar:$CLASSPATH"
    HBASE_OPTS="$HBASE_OPTS -Djruby.home=$JRUBY_HOME -Djruby.lib=$JRUBY_HOME/lib"
  fi
        #find the hbase ruby sources
  if [ -d "$HBASE_HOME/lib/ruby" ]; then
    HBASE_OPTS="$HBASE_OPTS -Dhbase.ruby.sources=$HBASE_HOME/lib/ruby"
  else
    HBASE_OPTS="$HBASE_OPTS -Dhbase.ruby.sources=$HBASE_HOME/hbase-shell/src/main/ruby"
  fi
  HBASE_OPTS="$HBASE_OPTS $HBASE_SHELL_OPTS"
  CLASS="org.jruby.Main -X+O ${JRUBY_OPTS} ${HBASE_HOME}/bin/hirb.rb"

For the hbase (maven) testcase "TestShell"
we have this error,
Running org.apache.hadoop.hbase.client.TestShell
Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 16.006 sec <<< FAILURE! - in org.apache.hadoop.hbase.client.TestShell
testRunShellTests(org.apache.hadoop.hbase.client.TestShell) Time elapsed: 1.359 sec <<< ERROR!
org.jruby.embed.EvalFailedException: (LoadError) no such file to load -- src/test/ruby/hbase/hbase_test.rb
at org.jruby.embed.internal.EmbedEvalUnitImpl.run(EmbedEvalUnitImpl.java:132)
at org.jruby.embed.ScriptingContainer.runUnit(ScriptingContainer.java:1341)
at org.jruby.embed.ScriptingContainer.runScriptlet(ScriptingContainer.java:1386)
at org.apache.hadoop.hbase.client.TestShell.testRunShellTests(TestShell.java:81)
Caused by: org.jruby.exceptions.RaiseException: (LoadError) no such file to load -- src/test/ruby/hbase/hbase_test.rb
at org.jruby.RubyKernel.load(org/jruby/RubyKernel.java:1091)
at RUBY.(root)(src/test/ruby/tests_runner.rb:50)
at org.jruby.RubyArray.each(org/jruby/RubyArray.java:1613)
at RUBY.(root)(src/test/ruby/tests_runner.rb:48)

replace this line from your script
CLASS="org.jruby.Main -X+O ${JRUBY_OPTS} ${HBASE_HOME}/bin/hirb.rb"
with (to force 1.8 mode)
CLASS="org.jruby.Main --1.8 -X+O ${JRUBY_OPTS} ${HBASE_HOME}/bin/hirb.rb"
or (adding "." to LOAD_PATH)
CLASS="org.jruby.Main -I. -X+O ${JRUBY_OPTS} ${HBASE_HOME}/bin/hirb.rb"

since from the error it looks like the 1.9 feature not to find files on current working directory unless this directory is on the LOAD_PATH

I think i didn't convey clearly about the hbase shell problem and hbase testcase issue. Both got mixed up here. But the root cause is same (Jruby is used in 1.9 mode)

1) Shell problem

The script loads the hbase shell when invoked.
I modified the script to
CLASS="org.jruby.Main --1.8 -X+O ${JRUBY_OPTS} ${HBASE_HOME}/bin/hirb.rb"
but still when invoked it throws out

[ayappan@soe07-vm3 bin]$ ./hbase shell
include_class is deprecated. Use java_import.
include_class is deprecated. Use java_import.
include_class is deprecated. Use java_import.
NoMethodError: undefined method `getTerminal' for Java::Jline::Terminal:Module
refresh_width at /home/ayappan/hbase/bin/../hbase-shell/src/main/ruby/shell/formatter.rb:33
initialize at /home/ayappan/hbase/bin/../hbase-shell/src/main/ruby/shell/formatter.rb:46
(root) at /home/ayappan/hbase/bin/../bin/hirb.rb:128

contents of /home/ayappan/hbase/bin/../hbase-shell/src/main/ruby/shell/formatter.rb
def refresh_width()
if $stdout.tty?
@max_width = Java::jline.Terminal.getTerminal().getTerminalWidth() ----->33
else
@max_width = 0
end
end

2)Testcase issue

Hbase has this testcase code.

package org.apache.hadoop.hbase.client;

import java.io.IOException;

import org.apache.hadoop.hbase.testclassification.ClientTests;
import org.apache.hadoop.hbase.testclassification.LargeTests;
import org.jruby.embed.PathType;
import org.junit.Test;
import org.junit.experimental.categories.Category;

@category({ ClientTests.class, LargeTests.class })
public class TestShell extends AbstractTestShell {

@test
public void testRunShellTests() throws IOException {
System.setProperty("shell.test.exclude", "replication_admin_test.rb");
// Start all ruby tests
jruby.runScriptlet(PathType.ABSOLUTE, "src/test/ruby/tests_runner.rb");
}

}

Your 1) issue above doesn't look like a 1.8/1.9 problem. It's failing to find a method in jline's Terminal class. This method does not exist on the Terminal in the jline that JRuby 1.7.20 ships, at least. I guess you are attempting to update hbase? You will need to figure out an alternative to this call.

The 2) issue could be a 1.8/1.9 issue if the file in question is in CWD. Normally 1.9 mode will not look in CWD unless it is explicitly added to the load path, using -I. as @mkristian suggested above. You can also force JRuby to use 1.8 mode throughout a given JVM by setting property jruby.compat.version to 1.8.

I'm not seeing a JRuby issue yet :-)

Oh, I would also suggest that the hbase update just use 1.9 mode and deal with the (mostly minor) issues that will result. 1.8 mode is completely gone in JRuby 9000, and 1.8.7 has been EOL in the Ruby world for years.

closing since its a JRuby 1.7.x (EOL) report against its (non preferred) --1.8 mode