我找不出为什么ScriptEngine引擎在代码中为空

这是我为MapleStory服务器运行的一些代码。每当与NPC对话等事件的脚本应该发生时，都会运行此脚本来为所调用的任何脚本(NPC、门户、事件等)创建脚本路径。

我也在使用jdk1.7.0_80

lang-java
package scripting;

import java.io.File; 
import java.io.FileReader; 
import java.io.IOException;
import javax.script.Invocable; 
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import client.MapleClient; 
import tools.FileoutputUtil;

public abstract class AbstractScriptManager {

    private static final ScriptEngineManager sem = new ScriptEngineManager(null);

    protected Invocable getInvocable(String path, MapleClient c) {
        return getInvocable(path, c, false);
    }

    protected Invocable getInvocable(String path, MapleClient c, boolean npc) {
        FileReader fr = null;
        try {
            path = "scripts/" + path;
            ScriptEngine engine = null;

            if (c != null) {
                engine = c.getScriptEngine(path);
            }
            if (engine == null) {
                File scriptFile = new File(path);
                if (!scriptFile.exists()) {
                    return null;
                }
                engine = sem.getEngineByName("JavaScript");
                if (c != null) {
                    c.setScriptEngine(path, engine);
                }
                fr = new FileReader(scriptFile);
                engine.eval(fr);
            } else if (c != null && npc) {
                c.getPlayer().dropMessage(-1, "You already are talking to this NPC. Use @ea if this is 
                not intended.");
            }
            return (Invocable) engine;
        } catch (Exception e) {
            System.err.println("Error executing script. Path: " + path + "
Exception " + e);
            FileoutputUtil.log(FileoutputUtil.ScriptEx_Log, "Error executing script. Path: " + path + 
            "
Exception " + e);
            return null;
        } finally {
            try {
                if (fr != null) {
                    fr.close();
                }
            } catch (IOException ignore) {

            }
        }
    } 
}

这是我收到的BAT错误：

执行脚本时出错。路径：脚本/事件/某些Event.js异常 Java.lang.NullPointerException：无法调用 "；javax.script.ScriptEngine.eval(java.io.Reader)"；，因为引擎为空

每当我尝试与使用此方法的对象交互时(例如，在运行某些脚本时单击NPC或启动服务器)，都会引发这些错误。

JAVA

出现NullPointerException是因为您使用的推荐答案运行时没有安装脚本引擎，导致sem.getEngineByName("JavaScript")返回null。

通过运行以下代码进行了验证(请参阅comment)：

ScriptEngineManager sem = new ScriptEngineManager();
List<ScriptEngineFactory> factories = sem.getEngineFactories();
for (ScriptEngineFactory factory : factories)
    System.out.println(factory.getEngineName() + " " + factory.getEngineVersion() + " " + factory.getNames());
if (factories.isEmpty())
    System.out.println("No Script Engines found");

当我在Windows 10上的各种Java实现上运行它时，我得到以下结果：

OpenJDK jdk1.7.0_75:
  No Script Engines found

Oracle jdk1.7.0_80:
  Mozilla Rhino 1.7 release 3 PRERELEASE [js, rhino, JavaScript, javascript, ECMAScript, ecmascript]

Oracle jdk1.8.0_181:
  Oracle Nashorn 1.8.0_181 [nashorn, Nashorn, js, JS, JavaScript, javascript, ECMAScript, ecmascript]

Oracle jdk-9.0.4:
  Oracle Nashorn 9.0.4 [nashorn, Nashorn, js, JS, JavaScript, javascript, ECMAScript, ecmascript]

OpenJDK jdk-11.0.2:
  Oracle Nashorn 11.0.2 [nashorn, Nashorn, js, JS, JavaScript, javascript, ECMAScript, ecmascript]

AdoptOpenJDK jdk-14.0.2+12:
  Oracle Nashorn 14.0.2 [nashorn, Nashorn, js, JS, JavaScript, javascript, ECMAScript, ecmascript]

AdoptOpenJDK jdk-15.0.1+9:
  No Script Engines found

如您所见，Java 7的OpenJDK版本没有Java引擎，因为Mozilla Rhino库不是开源的。您需要安装Oracle版本的Java 7才能获得Java引擎。

您还可以看到Java 15中删除了JavaScript，如JDK 15 Release Notes：

中所述

已删除Nashorn脚本引擎、其API和jjs工具。不建议在Java 11中删除引擎、API和工具，并明确希望在将来的版本中删除它们。请参阅jdk-8236933