可以弄清楚为什么ScriptEngine引擎在代码中为null

Question

这是我正在为MapLestory服务器运行的一些代码。每当应该发生像与NPC交谈之类的事件的脚本时，将运行此脚本以创建所谓的任何脚本（NPC，Portal，Event等）的脚本路径。

我还使用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 + "\nException " + e);
            FileoutputUtil.log(FileoutputUtil.ScriptEx_Log, "Error executing script. Path: " + path + 
            "\nException " + e);
            return null;
        } finally {
            try {
                if (fr != null) {
                    fr.close();
                }
            } catch (IOException ignore) {

            }
        }
    } 
}

这是我收到的蝙蝠错误：

错误执行脚本。路径：脚本/事件/someEvent.js异常 java.lang.nullpointerexception：无法调用 “ javax.script.scriptEngine.eval（java.io.reader）”，因为“引擎”为null

每当我尝试与使用此方法的某些东西进行交互时，这些错误就会抛出（即单击NPC或某些脚本时在服务器启动时运行）。

Answer 1

NullPoInterException 发生，因为您使用的Java运行时没有安装的“ JavaScript”脚本引擎，引起 sem.getEngineByName（“ JavaScript”）返回 > null 。

已经进行了验证（请参阅）通过运行以下代码：

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实现上运行此代码时，我会得到以下结果：

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JavaScript引擎，因为Mozilla Rhino图书馆不是开源的。您需要Java 7的Oracle版本才能获取JavaScript引擎。

您还可以看到JavaScript已在Java 15中删除，如 JDK 15发行说明：

已删除了Nashorn JavaScript脚本引擎，其API和 JJS 工具。发动机，API和该工具在Java 11中被删除，并在将来的版本中明确删除它们。请参阅JDK-8236933

Answer 2

我对JDK 17和 lib rhino.js.enengine-1.7.71.jar 也有同样的问题。

我

org.mozilla.javascript-1.7.5.jar

也添加了我的班级路径。解决了错误。

Answer 3

或者，只需直接使用犀牛本身，没有 scriptenginemanager ：

pom.xml：

<dependency>
  <groupId>org.mozilla</groupId>
  <artifactId>rhino</artifactId>
  <version>1.7.15</version>
</dependency>

<dependency>
  <groupId>org.mozilla</groupId>
  <artifactId>rhino-engine</artifactId>
  <version>1.7.15</version>
</dependency>

<dependency>
  <groupId>org.mozilla</groupId>
  <artifactId>rhino-runtime</artifactId>
  <version>1.7.15</version>
</dependency>

java代码：

import java.util.*;
import org.mozilla.javascript.Context;
import org.mozilla.javascript.Scriptable;


public static String executeScript(String js) {
    // Creates and enters a Context. The Context stores information
    // about the execution environment of a script.
    Context cx = Context.enter();
    try {
        // Initialize the standard objects (Object, Function, etc.)
        // This must be done before scripts can be executed. Returns
        // a scope object that we use in later calls.
        Scriptable scope = cx.initStandardObjects();


        // Now evaluate the string we've colected.
        Object result = cx.evaluateString(scope, js, "<cmd>", 1, null);

        // Convert the result to a string and print it.
        logger.debug(Context.toString(result));
        return (String) result;
    } finally {
        // Exit from the context.
        Context.exit();
    }
}