Removing one of the three rules magically solves the problem.

java.lang.NullPointerException
at org.drools.base.extractors.BaseObjectClassFieldExtractor.getLongValue(BaseObjectClassFieldExtractor.java:106)
at org.drools.base.ClassFieldExtractor.getLongValue(ClassFieldExtractor.java:203)
at org.drools.reteoo.CompositeObjectSinkAdapter$HashKey.setValue(CompositeObjectSinkAdapter.java:467)
at org.drools.reteoo.CompositeObjectSinkAdapter.propagateAssertObject(CompositeObjectSinkAdapter.java:292)
at org.drools.reteoo.ObjectTypeNode.assertObject(ObjectTypeNode.java:153)
at org.drools.reteoo.Rete.assertObject(Rete.java:177)
at org.drools.reteoo.ReteooRuleBase.assertObject(ReteooRuleBase.java:201)
at org.drools.reteoo.ReteooWorkingMemory.doInsert(ReteooWorkingMemory.java:71)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:886)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:858)
at org.drools.common.AbstractWorkingMemory.insert(AbstractWorkingMemory.java:659)
at com.sample.DroolsTest.main(DroolsTest.java:21)

package com.sample;

import java.io.InputStreamReader;
import java.io.Reader;

import org.drools.RuleBase;
import org.drools.RuleBaseFactory;
import org.drools.WorkingMemory;
import org.drools.compiler.PackageBuilder;
import org.drools.rule.Package;

public class DroolsTest
{

public static final void main(String[] args)
{
try

{ RuleBase ruleBase = readRule(); WorkingMemory workingMemory = ruleBase.newStatefulSession(); workingMemory.insert(new Attribute()); workingMemory.insert(new Message()); workingMemory.fireAllRules(); }

catch (Throwable t)

{ t.printStackTrace(); }

}

private static RuleBase readRule() throws Exception

{ Reader source = new InputStreamReader(DroolsTest.class.getResourceAsStream("/Sample.drl")); PackageBuilder builder = new PackageBuilder(); builder.addPackageFromDrl(source); Package pkg = builder.getPackage(); RuleBase ruleBase = RuleBaseFactory.newRuleBase(); ruleBase.addPackage(pkg); return ruleBase; }

public static class Message

{ //empty }

public static class Attribute
{

public Integer getValue()

{ return null; }

}

}

package com.sample

import com.sample.DroolsTest.Message;
import com.sample.DroolsTest.Attribute;

rule "rule_1"

when
Message()
Attribute(value == 1)
then
System.err.println("case 1");
end

rule "rule_2"

when
Message()
Attribute(value == 2)
then
System.err.println("case 2");
end

rule "rule_3"

when
Message()
$a:Attribute(value == 3)
then
System.err.println("case 3");
end