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  /*
   * Copyright 2010 JBoss Inc
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 
 package org.drools.runtime;
 
StatefulKnowledgeSession is the most common way to interact with the engine. A StatefulKnowledgeSession allows the application to establish an iterative conversation with the engine, where the state of the session is kept across invocations. The reasoning process may be triggered multiple times for the same set of data. After the application finishes using the session, though, it must call the dispose() method in order to free the resources and used memory.

Simple example showing a stateful session executing rules for a given collection of java objects.

 KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
 kbuilder.add( ResourceFactory.newFileSystemResource( fileName ), ResourceType.DRL );
 assertFalse( kbuilder.hasErrors() );
 if (kbuilder.hasErrors() ) {
     System.out.println( kbuilder.getErrors() );
 }
 KnowledgeBase kbase = KnowledgeBaseFactory.newKnowledgeBase();
 kbase.addKnowledgePackages( kbuilder.getKnowledgePackages() );
 
 StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
 for( Object fact : facts ) {
     ksession.insert( fact );
 }
 ksession.fireAllRules();
 ksession.dispose();
 

Simple example showing a stateful session executing processes.

 KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
 kbuilder.add( ResourceFactory.newFileSystemResource( fileName ), ResourceType.BPMN2 );
 KnowledgeBase kbase = kbuilder.newKnowledgeBase();
 StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
 ksession.startProcess("com.sample.processid");
 ksession.signalEvent("SomeEvent", null);
 ksession.startProcess("com.sample.processid");
 ksession.dispose();
 

StatefulKnowledgeSessions support globals. Globals are used to pass information into the engine (like data or service callbacks that can be used in your rules and processes), but they should not be used to reason over. If you need to reason over your data, make sure you insert it as a fact, not a global.

Globals are shared among ALL your rules and processes, so be especially careful of (and avoid as much as possible) mutable globals. Also, it is a good practice to set your globals before inserting your facts or starting your processes. Rules engines evaluate rules at fact insertion time, and so, if you are using a global to constraint a fact pattern, and the global is not set, you may receive a NullPointerException.

Globals can be resolved in two ways. The StatefulKnowledgeSession supports getGlobals() which returns the internal Globals, which itself can take a delegate. Calling of setGlobal(String, Object) will set the global on an internal Collection. Identifiers in this internal Collection will have priority over the externally supplied Globals delegate. If an identifier cannot be found in the internal Collection, it will then check the externally supplied Globals delegate, if one has been set.

Code snippet for setting a global:

 StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
 ksession.setGlobal( "hbnSession", hibernateSession ); // sets a global hibernate session, that can be used for DB interactions in the rules.
 for( Object fact : facts ) {
     ksession.insert( fact );
 }
 ksession.fireAllRules(); // this will now execute and will be able to resolve the "hbnSession" identifier.
 ksession.dispose();
 

Like StatelessKnowledgeSession this also implements CommandExecutor which can be used to script a StatefulKnowledgeSession. See CommandExecutor for more details.

See also:
org.drools.core.runtime.Globals
 
 public interface StatefulKnowledgeSession
    extends
    int getId();
    
    
Releases all the current session resources, setting up the session for garbage collection. This method must always be called after finishing using the session, or the engine will not free the memory used by the session.
    void dispose();
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