[JEXL-241] NPE when script containing string interpolation executed in multiple threads

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 3.1
• Fix Version/s: 3.2

Environment

Centos Linux 7.2.1511

openjdk version "1.8.0_101"
OpenJDK Runtime Environment (build 1.8.0_101-b13)
OpenJDK 64-Bit Server VM (build 25.101-b13, mixed mode)

Gradle dependency for JEXL is org.apache.commons:commons-jexl3:3.1

• Assignee: Henri Biestro
• Reporter: Yang Wang
• Created: 16/Oct/17 00:28
• Updated: 07/Jun/21 13:14
• Resolved: 19/Oct/17 12:03

Description

When a script containing string interpolation is executed in multiple threads, a "NullPointerException" is sometimes thrown (other times it may hang or it may run successfully).

The following is a sample program that can produce the bug (you may need to run it for a few times to encounter the bug).

package jexldemo2;

import org.apache.commons.jexl3.JexlBuilder;
import org.apache.commons.jexl3.JexlScript;
import org.apache.commons.jexl3.MapContext;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class DemoApplication {

    private static ExecutorService pool;
    private static JexlScript script = new JexlBuilder().create().createScript("`${item}`");

    public static void main(String[] args) throws InterruptedException {
        pool = Executors.newFixedThreadPool(4);

        Map<String, Object> m1 = new HashMap<>();
        m1.put("item", "A");
        Map<String, Object> m2 = new HashMap<>();
        m2.put("item", "B");

        handle(m1);
        System.out.println(script.execute(new MapContext(m2)));
        System.out.println("Reached the end");
        pool.shutdown();
    }

    private static void handle(Map<String, Object> payload) {
        System.out.printf("START: %s\n", Thread.currentThread());
        pool.submit(() -> System.out.println(script.execute(new MapContext(payload))));
        System.out.printf("STOP: %s\n", Thread.currentThread());
    }

}

And the bug output is as follows:

START: Thread[main,5,main]
STOP: Thread[main,5,main]
Exception in thread "main" java.lang.NullPointerException
	at org.apache.commons.jexl3.internal.Interpreter.visit(Interpreter.java:1818)
	at org.apache.commons.jexl3.parser.ASTJxltLiteral.jjtAccept(ASTJxltLiteral.java:50)
	at org.apache.commons.jexl3.internal.Interpreter.visit(Interpreter.java:892)
	at org.apache.commons.jexl3.parser.ASTJexlScript.jjtAccept(ASTJexlScript.java:55)
	at org.apache.commons.jexl3.internal.Interpreter.interpret(Interpreter.java:186)
	at org.apache.commons.jexl3.internal.Script.execute(Script.java:178)
	at jexldemo2.DemoApplication.main(DemoApplication.java:26)
A