If I create the sample project from Eclipse, and modify DroolsTest to contain:

KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
kbuilder.add(ResourceFactory.newClassPathResource("Sample.drl"), ResourceType.DRL);
KnowledgeBuilderErrors errors = kbuilder.getErrors();
KnowledgeBaseConfiguration conf = KnowledgeBaseFactory.newKnowledgeBaseConfiguration();
conf.setOption( EventProcessingOption.STREAM );

KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase( conf );
kbase.addKnowledgePackages(kbuilder.getKnowledgePackages());
StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
KnowledgeRuntimeLogger logger = KnowledgeRuntimeLoggerFactory.newFileLogger(ksession, "test");

ArrayList<Message> a=new ArrayList<Message>();
Message a1=new Message(); a1.setId(1); a1.setMessage("mesC:7"); a1.setStatus(Message.HELLO); a.add(a1);
Message a2=new Message(); a2.setId(2);a2.setMessage("mesC:8"); a2.setStatus(Message.HELLO);a.add(a2);
ksession.setGlobal("data", a);

Message message = new Message();
message.setId(3);
message.setMessage("Hello World");
message.setStatus(Message.HELLO);
ksession.insert(message);

Message message2 = new Message();
message2.setId(4);
message2.setMessage("mesC:7");
message2.setStatus(Message.HELLO);
ksession.insert(message2);

ksession.fireAllRules();
logger.close();

and then have a rule:

package com.sample

import com.sample.DroolsTest.Message;
global java.util.ArrayList data;

declare Message
@role( event )
end
rule "Hello World"
when
a : Message(id==4)
m : Message(id!=a.id, message==a.message) from data
then
System.out.println( "a is:"a" m is:"+m );
end

The result is:
a is:Message:[4] msg:mesC:7 m is:null

Notice, m is NULL

if you change this to fact based, m will contain the correct value.