在Servlet中重用Nashorn ScriptEngine

Question

我想在Servlet中执行JavaScript。是否可以在所有servlet调用中重用相同的脚本引擎？Servlet实例由多个线程共享。这是否需要为每个请求创建一个新的脚本引擎？那将是不可接受的性能损失。例如，以下代码是否保存？

public class MyServlet extends HttpServlet {

private ScriptEngineManager factory;
private ScriptEngine engine;

@Override
public void init() throws ServletException {
    factory = new ScriptEngineManager();
    engine = factory.getEngineByName("nashorn");
}

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

}

如果这样做不安全，那么避免每个请求创建引擎的最佳方法是什么？使用引擎池？

编辑：
是否可以对所有Servlet请求重用一个同一个引擎和一个同一个JavaScriptObject，作为对JS函数的求值，如果该函数不更改任何共享对象而仅使用给定的参数跟电话吗？查看上面示例的以下适应：

public class MyServlet extends HttpServlet {

private ScriptEngineManager factory;
private ScriptEngine engine;
private ScriptObjectMirror script;

@Override
public void init() throws ServletException {
    try {
        factory = new ScriptEngineManager();
        engine = factory.getEngineByName("nashorn");
        script = (ScriptObjectMirror)engine.eval("function(writer) {writer.print('Hello, World!');}");
    } catch (ScriptException ex) {
        Logger.getLogger(MyServlet.class.getName()).log(Level.SEVERE, null, ex);
    }
}

@Override
public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
    try (PrintWriter writer = res.getWriter()) {
        script.call(null, writer);
        writer.close();
    } catch (IOException ex) {
        Logger.getLogger(MyServlet.class.getName()).log(Level.SEVERE, null, ex);
    }
}

这样安全吗？

Answer 1

在javax.script.ScriptEngineFactory里面有一种方法getParameter(String key)。

使用特殊键，THREADING您可以获得该特定引擎工厂的线程信息。

这个小程序为每个注册的引擎工厂打印出此信息：

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")));
    }
  }
}

对于Java 7，它是：

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

对于Java 8：

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

null 表示引擎实现不是线程安全的。

在您的Servlet中，您可以使用ThreadLocal来为每个线程保留一个单独的引擎，从而允许将引擎重新用于同一线程所服务的后续请求。

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);
    }
  }
}