Java：从ScriptEngine javascript返回一个对象 [英] Java: Returning an object from ScriptEngine javascript

问题描述

我正在尝试使用 ScriptEngine 课程。以下是我想要做的一个简短示例：

I'm trying to evaluate javascript in Java by using the ScriptEngine class. Here is a short example of what I am trying to do:

import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;

public class Test {
    public static void main(String[] args) {
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("js"); //Creates a ScriptEngine
        Object obj = engine.eval("var obj = { value: 1 }; return obj; "); // Evals the creation of a simple object
        System.out.println(obj.value); // I get an invalid token error when trying to print a property of the object
    }
}

我很确定这个应该工作......但是我很难过，我会得到任何帮助。

I'm pretty sure that this should work... but I'm stumped, and I'll take any help I can get.

推荐答案

注意：以下内容适用于Java 8，使用 Nashorn 引擎。

首先，让代码编译，从 println（）语句中删除 .value 。 obj 声明为对象，对象不对没有值字段。

First, to make the code compile, remove the .value from the println() statement. obj is declared to be type Object, and Object doesn't have a value field.

执行此操作后，运行代码时会出现以下异常：

Once you do that, you get the following exception when running the code:

Exception in thread "main" javax.script.ScriptException: <eval>:1:25 Invalid return statement
var obj = { value: 1 };  return obj; 
                         ^ in <eval> at line number 1 at column number 25

这是因为你没有功能，所以你无法调用返回。脚本的返回值是最后一个表达式的值，所以只需说 obj 。

That is because you don't have a function, so you cannot call return. The return value of the script is the value of the last expression, so just say obj.

现在它将会运行并打印 [object Object] 。要查看您返回的对象类型，请更改为 println（obj.getClass（）。getName（））。这将打印 jdk.nashorn.api.scripting.ScriptObjectMirror 。为方便起见，我已链接到javadoc。

Now it will run and print [object Object]. To see what type of object you got back, change to println(obj.getClass().getName()). That will print jdk.nashorn.api.scripting.ScriptObjectMirror. I've linked to the javadoc for your convenience.

ScriptObjectMirror 实现 Bindings 反过来实现 Map< String，Object> ，因此你可以调用 get（value）。

ScriptObjectMirror implements Bindings which in turn implements Map<String, Object>, so you can call get("value").

工作代码是：

import javax.script.*;

public class Test {
    public static void main(String[] args) throws ScriptException {
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");
        Bindings obj = (Bindings)engine.eval("var obj = { value: 1 };  obj; ");
        Integer value = (Integer)obj.get("value");
        System.out.println(value); // prints: 1
    }
}

UPDATE

关键在于创建一个带有函数的对象，这个引擎可能吗？没有Function对象。

The whole point was to create an object with functions, is that possible with this engine? There isn't a Function object.

如何执行此操作的示例：

Example for how to do that:

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

import jdk.nashorn.api.scripting.ScriptObjectMirror;

public class Test {
    public static void main(String[] args) throws Exception {
        String script = "var f = {\n" +
                        "  value: 0,\n" +
                        "  add: function(n) {\n" +
                        "    this.value += n;\n" +
                        "    return this.value;\n" +
                        "  }\n" +
                        "};\n" +
                        "f; // return object to Java\n";
        ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");
        ScriptObjectMirror obj = (ScriptObjectMirror)engine.eval(script);
        System.out.println("obj.value = " + obj.getMember("value"));
        System.out.println("obj.add(5): " + obj.callMember("add", 5));
        System.out.println("obj.add(-3): " + obj.callMember("add", -3));
        System.out.println("obj.value = " + obj.getMember("value"));
    }
}

OUTPUT

obj.value = 0
obj.add(5): 5.0
obj.add(-3): 2.0
obj.value = 2.0

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