allVars field in org.mvel2.util.VariableSpaceModel class should be defined as final.

[Background]

It is reported that NPE like (*1) rarely occurs at (*2-1) in org.mvel2.util.SimpleVariableSpaceModel#createFactory(Object[]) when workload is very high.

(*1) NullPointerException

2021-09-12 03:29:44.164 ERROR [stderr] (default task-26) Caused by: java.lang.NullPointerException
2021-09-12 03:29:44.164 ERROR [stderr] (default task-26)    at org.mvel2.util.SimpleVariableSpaceModel.createFactory(SimpleVariableSpaceModel.java:18)
2021-09-12 03:29:44.164 ERROR [stderr] (default task-26)    at org.drools.core.base.mvel.MVELCompilationUnit.createFactory(MVELCompilationUnit.java:298)
2021-09-12 03:29:44.164 ERROR [stderr] (default task-26)    at org.drools.core.rule.constraint.MvelConditionEvaluator.evaluate(MvelConditionEvaluator.java:117)
2021-09-12 03:29:44.164 ERROR [stderr] (default task-26)    at org.drools.core.rule.constraint.MvelConditionEvaluator.evaluate(MvelConditionEvaluator.java:98)
2021-09-12 03:29:44.164 ERROR [stderr] (default task-26)    at org.drools.core.rule.constraint.MvelConstraint.evaluate(MvelConstraint.java:275)
2021-09-12 03:29:44.164 ERROR [stderr] (default task-26)    ... 153 more

(*2) Excerpt from org/mvel2/util/SimpleVariableSpaceModel.java

    :
    :
12  public class SimpleVariableSpaceModel extends VariableSpaceModel {
13    public SimpleVariableSpaceModel(String[] varNames) {
14      this.allVars = varNames;
15    }
16  
17    public VariableResolverFactory createFactory(Object[] vals) {
18      VariableResolver[] resolvers = new VariableResolver[allVars.length];    <---------- (*2-1) NPE occurs here.
19      for (int i = 0; i < resolvers.length; i++) {
20        if (i >= vals.length) {
21          resolvers[i] = new SimpleValueResolver(null);
22        }
23        else {
24          resolvers[i] = new IndexVariableResolver(i, vals);
25        }
26      }
27  
28      return new IndexedVariableResolverFactory(allVars, resolvers);
29    }
30  }
    :
    :

allVars field in VariableSpaceModel class is currently defined without final. According to (*3) and (*4) in [1], a non-final field is not guaranteed to be seen its expected value after the object's constructor finishes, and it may be related to the NPE issue above.

allVars is a field which does not need to be modified after initialization, so it should be defined as final to eliminate the possibility to cause the NPE issue due to being non-final field.

[1] The Java Language Specification (Java SE 11 Edition) - 17.5. final Field Semantics
https://docs.oracle.com/javase/specs/jls/se11/html/jls-17.html#jls-17.5

(Excerpt from [1])

17.5. final Field Semantics
:
:
Example 17.5-1. final Fields In The Java Memory Model
The program below illustrates how final fields compare to normal fields.

class FinalFieldExample { 
    final int x;
    int y; 
    static FinalFieldExample f;

    public FinalFieldExample() {
        x = 3; 
        y = 4; 
    } 

    static void writer() {
        f = new FinalFieldExample();
    } 

    static void reader() {
        if (f != null) {
            int i = f.x;  // guaranteed to see 3  
            int j = f.y;  // could see 0            <---------- (*3)
        } 
    } 
}

The class FinalFieldExample has a final int field x and a non-final int field y. One thread might execute the method writer and another might execute the method reader.
Because the writer method writes f after the object's constructor finishes, the reader method will be guaranteed to see the properly initialized value for f.x: it will read the value 3. However, f.y is not final; the reader method is therefore not guaranteed to see the value 4 for it. <---------- (*4)