scripting in Java via javax.script: cannot retrieve ScriptEngine for Python

Question

I have a little maven project to learn about scripting in Java using javax.script API, namely JavaScript and Python. While JavaScript works well, like e.g. in this method

@Test
public void TestPrintHelloWorldJs() throws ScriptException {
 ScriptEngineManager factory = new ScriptEngineManager();
 ScriptEngine engine = factory.getEngineByName("nashorn");
 
 engine.eval("print('Hello, World')"); // works
}

I cannot get python running at all because the returned engine is always null:

@Test
public void TestPrintHelloWorldPy() throws ScriptException {
 ScriptEngineManager factory = new ScriptEngineManager();
 ScriptEngine engine = factory.getEngineByName("python"); // returns null !!!
 engine = factory.getEngineByName("jython"); // still returns null !!!
 engine.eval("print('Hello, World')"); // NullPointerException here
}

Any ideas what I can do to make it work?

---

Here are the relevant parts of my pom:

<properties>
 <maven.compiler.release>15</maven.compiler.release> <!-- Java 15 -->
</properties>
<build>
 <plugins>
 <plugin>
 <groupId>org.apache.maven.plugins</groupId>
 <artifactId>maven-compiler-plugin</artifactId>
 <version>3.8.0</version>
 </plugin>
 </plugins>
</build>
<dependencies>
 <dependency>
 <groupId>junit</groupId>
 <artifactId>junit</artifactId> <!-- for testing -->
 <version>4.12</version>
 <scope>test</scope>
 </dependency>
 <!-- https://mvnrepository.com/artifact/org.openjdk.nashorn/nashorn-core -->
 <dependency>
 <groupId>org.openjdk.nashorn</groupId>
 <artifactId>nashorn-core</artifactId> <!-- for javascript-->
 <version>15.3</version>
 </dependency>
 <!-- https://mvnrepository.com/artifact/org.python/jython -->
 <dependency>
 <groupId>org.python</groupId>
 <artifactId>jython</artifactId> <!-- for python -->
 <version>2.7.2</version>
 </dependency>
</dependencies>

I use the method from https://stackoverflow.com/a/38039047/5333340 in order to print out available engines, and the output I get is:

ScriptEngineFactory Info
 Script Engine: OpenJDK Nashorn (15.3)
 Engine Alias: nashorn
 Engine Alias: Nashorn
 Engine Alias: js
 Engine Alias: JS
 Engine Alias: JavaScript
 Engine Alias: javascript
 Engine Alias: ECMAScript
 Engine Alias: ecmascript
 Language: ECMAScript (ECMA - 262 Edition 5.1)
ScriptEngineFactory Info
 Script Engine: jython (2.7.2)
 Engine Alias: python
 Engine Alias: jython
 Language: python (2.7)

So the engine IS found. But why can't I get it? I double-checked that the arguments in getEngineByName have no typos.

I even installed python 2.7 on my computer (although I don't think this is actually necessary; the implementation should be inside the jython package), but it did not change anything.

Answer 1

Seems I wasn't using the right dependency...

I replaced

<!-- https://mvnrepository.com/artifact/org.python/jython -->
<dependency>
 <groupId>org.python</groupId>
 <artifactId>jython</artifactId>
 <version>2.7.2</version>
</dependency>

by

<!-- https://mvnrepository.com/artifact/org.python/jython-standalone -->
<dependency>
 <groupId>org.python</groupId>
 <artifactId>jython-standalone</artifactId>
 <version>2.7.2</version>
</dependency>

and that's it.