将toString方法暴露给nashorn中的js对象

expose toString method to js objects in nashorn

本文关键字：中的 js 对象 nashorn toString 方法暴露更新时间：2023-09-26

这个问题可能很愚蠢，但我找不到任何关于它的直接文档，所以它在这里。

我想向JS端公开一个Java"构造函数"，当实例化时，它应该创建一个带有"toString"方法的实例

// java class
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import jdk.nashorn.api.scripting.AbstractJSObject;
public class Foo extends AbstractJSObject {
    String bar;
    public Foo() {
        super();
    }
    public Foo(String b) {
        super();
        this.bar = b;
    }
    @Override
    public Object newObject(Object... args) {
         String bar = null;
        if (args[0] != null) bar = args[0].toString();
        Foo f = new Foo(bar);
        return f;
    }
    @Override
    public String toString() {
        return "Foo: " + this.bar;
    }

    public static void main(String[] args) {
        ScriptEngine engine = new  ScriptEngineManager().getEngineByName("nashorn");
        engine.put("foo", new Foo());
        // f.toString however is null
        engine.eval("var f = new foo('hi'); f.toString();")
    }
}

如果实现JSObject，nashorn将路由所有属性/索引访问，通过getMember、getSlot、setMember、setSlot等JSObject方法进行调用。我稍微修改了您的代码，将"toString"作为Foo对象的属性公开，并使其可编译&可运行代码：

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import jdk.nashorn.api.scripting.AbstractJSObject;
import java.util.function.Supplier;
public class Foo extends AbstractJSObject {
    String bar;
    public Foo() {
        super();
    }
    public Foo(String b) {
        super();
        this.bar = b;
    }
    @Override
    public Object newObject(Object... args) {
        String bar = null;
        if (args[0] != null) bar = args[0].toString();
        Foo f = new Foo(bar);
        return f;
    }
    @Override
    public Object getMember(String name) {
        if (name.equals("toString")) {
            // Returning a lambda function as value of this property.
            // nashorn code can call lambdas just like script functions!
            // You may also choose to return another JSObject that
            // returns true in isFunction() method and implements "call"
            // as well.
            return (Supplier<String>)this::toString;
        } else {
            // implement other exposed properties here.
            return null;
        }
    }
    @Override
    public String toString() {
        return "Foo: " + this.bar;
    }

    public static void main(String[] args) throws Exception {
        ScriptEngine engine = new  ScriptEngineManager().getEngineByName("nashorn");
        engine.put("foo", new Foo());
        // f.toString() would call getMember("toString") and the
        // returning lambda is called!
        engine.eval("var f = new foo('hi'); print(f.toString());");
    }
}

您可以尝试以下方式：

import javax.script.*;
    import jdk.nashorn.api.scripting.AbstractJSObject;
    public class Foo extends AbstractJSObject {
        String bar;
        public Foo() {
            super();
        }
        public Foo(String b) {
            super();
            this.bar = b;
        }
        @Override
        public Object newObject(Object... args) {
            String bar = null;
            if (args[0] != null) bar = args[0].toString();
            Foo f = new Foo(bar);
            return f;
        }
        @Override
        public String toString() {
            return "Foo: " + this.bar;
        }

        public static void main(String[] args) throws ScriptException {
            ScriptEngine engine = new ScriptEngineManager().getEngineByName("javascript");
            engine.put("foo", new Foo());
            Compilable compilable = (Compilable) engine;
            Bindings bindings = engine.createBindings();
            String script = "function add(op1,op2){return op1+op2} add(a, b)";
            CompiledScript jsFunction = compilable.compile(script);
            bindings.put("a", 1);bindings.put("b", 2); //put java object into js runtime environment
            Object result = jsFunction.eval(bindings);
            System.out.println(result);
        }
    }