Reuse Nashorn ScriptEngine in Servlet [duplicate]

Answer 1

In javax.script.ScriptEngineFactory there is a method getParameter(String key).

With the special key THREADING you get threading information for this specific engine factory.

This small program prints out this information for every registered engine factory:

import javax.script.ScriptEngineFactory;
import javax.script.ScriptEngineManager;

public class ScriptEngineTest {
  public static void main(String[] args) {
    final ScriptEngineManager mgr = new ScriptEngineManager();
    for(ScriptEngineFactory fac: mgr.getEngineFactories()) {
      System.out.println(String.format("%s (%s), %s (%s), %s", fac.getEngineName(),
          fac.getEngineVersion(), fac.getLanguageName(),
          fac.getLanguageVersion(), fac.getParameter("THREADING")));
    }
  }
}

For Java 7 it is:

Mozilla Rhino (1.7 release 3 PRERELEASE), ECMAScript (1.8), MULTITHREADED

For Java 8:

Oracle Nashorn (1.8.0_25), ECMAScript (ECMA - 262 Edition 5.1), null

null means the engine implementation is not thread safe.

In your servlet you can use a ThreadLocal to hold a seperate engine for each thread allowing to reuse the engine for subsequent requests served by the same thread.

public class MyServlet extends HttpServlet {

  private ThreadLocal<ScriptEngine> engineHolder;

  @Override
  public void init() throws ServletException {
    engineHolder = new ThreadLocal<ScriptEngine>() {
      @Override
      protected ScriptEngine initialValue() {
        return new ScriptEngineManager().getEngineByName("nashorn");
      }
    };
  }

  @Override
  public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
    try (PrintWriter writer = res.getWriter()) {
      ScriptContext newContext = new SimpleScriptContext();
      newContext.setBindings(engineHolder.get().createBindings(), ScriptContext.ENGINE_SCOPE);
      Bindings engineScope = newContext.getBindings(ScriptContext.ENGINE_SCOPE);
      engineScope.put("writer", writer);
      Object value = engineHolder.get().eval("writer.print('Hello, World!');", engineScope);
      writer.close();
    } catch (IOException | ScriptException ex) {
      Logger.getLogger(MyServlet.class.getName()).log(Level.SEVERE, null, ex);
    }
  }
}