Vladimir wrote:

There is a sentence in specification for javax.script.ScriptEngine.getBindings():
...
Returns a scope of named values. The possible scopes are:

ScriptContext.GLOBAL_SCOPE - A set of named values shared by all ScriptEngines created by the same ScriptEngineFactory. If the ScriptEngine is created by a ScriptEngineManager, a reference to the global scope stored by the ScriptEngineManager should be returned. May return null if no global scope is associated with this ScriptEngine.
...

So, as I understand if instance of engine created by ScriptEngineManager then the engine's getBindings(ScriptContext.GLOBAL_SCOPE) should return the same Bindings object as ScriptEngineManager.getBindings(). This is not quite true for AbstractScriptEngine and embedded in Mustang JavaScript engine:

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByExtension("js");

// in this moment engine.getBindings(ScriptContext.GLOBAL_SCOPE) == manager. getBindings() as spec required

manager.setBindings(new SimpleBinding());

// and in this moment engine.getBindings(ScriptContext.GLOBAL_SCOPE) != manager. getBindings() - spec violated