append

i = size+1
realloc(i)
set(i, val)

application.rb

config.filter_parameters += [:password]

critical.rb

data << m * n

expensive_to_create.rb

class ExpensiveToCreate
  def self.instance
    @instance ||= ExpensiveToCreate.new
  end
end

jrlint.sh

$ jrlint
…
./config/application.rb:54: [nonatomic, warning] Non-local operator assignment is not guaranteed to be atomic

jruby.sh

$ jruby threads.rb
size => 182

mri.sh

$ ruby threads.rb
size => 256

RubyArray.java

/** rb_ary_push - specialized rb_ary_store 
 *
 */
@JRubyMethod(name = "<<", required = 1)
public RubyArray append(IRubyObject item) {
    modify();
    int valuesLength = values.length - begin;
    if (realLength == valuesLength) {
        if (realLength == Integer.MAX_VALUE) throw getRuntime().newArgumentError("index too big");

        long newLength = valuesLength + (valuesLength >> 1);
        if (newLength > Integer.MAX_VALUE) {
            newLength = Integer.MAX_VALUE;
        } else if (newLength < ARRAY_DEFAULT_SIZE) {
            newLength = ARRAY_DEFAULT_SIZE;
        }

        realloc((int) newLength, valuesLength);
    }
    
    safeArraySet(values, begin + realLength++, item);
    
    return this;
}

threads.rb

data = []
threads = []
16.times do |m|
  threads << Thread.new do
    16.times do |n|
      data << m * n
    end
  end
end

threads.map(&:join)
puts "size => #{data.size}"