Locks in mirah (not tested, sorry for any compilation errors)

foo_bar.mirah

class FooBar < Lockable
  locked def foo; @foo; end
  locked def foo=(foo:String); @foo = foo; end
  def bar
    a = withLock { @foo }
    sb = StringBuilder.new
    100.times { sb.append(a) }
    sb.toString
  end
end

lockable.mirah

import java.util.concurrent.locks.ReentrantLock
class Lockable
  def initialize
    @lock = ReentrantLock.new
  end

  def lock
    @lock.lock
  end

  def unlock
    @lock.unlock
  end

  macro def withLock(&block)
    quote do
      begin
        `@call.target`.lock
        `block.body`
      ensure
        `@call.target`.unlock
      end
    end
  end

  macro def self.locked(method:MethodDefinition)
    result = MethodDefinition(method.clone)
    result.body = quote do
      begin
        `@call.target`.lock
        `method.body`
      ensure
        `@call.target`.unlock
      end
    end
    result
  end
end