在 GraalVM Polyglot 上下文中从 JavaScript 访问 Java 对象

Question

运行在 GraalVM CE 上。

openjdk version "11.0.5" 2019-10-15
OpenJDK Runtime Environment (build 11.0.5+10-jvmci-19.3-b05-LTS)
OpenJDK 64-Bit GraalVM CE 19.3.0 (build 11.0.5+10-jvmci-19.3-b05-LTS, mixed mode, sharing)

案例一：

import org.graalvm.polyglot.Context;

public class Test {

    static class Data {
        public String name = "HelloWorld";
        public String getName() {
            return this.name;
        }
    }

    public static void main(String[] args) {
        Context context = Context.newBuilder("js").allowHostAccess(true).build();
        context.getBindings("js").putMember("d", new Data());

        context.eval("js", "var x = d.name");

        System.out.println(
                context.getBindings("js").getMember("x").asString()
        );
    }
}

结果：

null

为什么？

据我了解，d 正确通过：

((Data) context.getBindings("js").getMember("d").as(Data.class)).name

returns "HelloWorld".

案例二：

context.eval("js", "d.getName()");

异常

Exception in thread "main" TypeError: invokeMember (getName) 
on JavaObject[task.Test$Data@35a3d49f (task.Test$Data)] failed due to: 
Unknown identifier: getName

但是 getName 是 public... 怎么了？

Answer 1

GraalVM JavaScript 默认执行严格的沙盒规则，其中之一是 JavaScript 代码无法访问主机 Java 对象，除非用户明确允许。允许您的代码访问 context.eval("js", "d.getName()") 的最简单方法是传递选项 polyglot.js.allowAllAccess=true 如下所述 link:

GraalVM JavaScript ScriptEngine implementation

查看示例：

import org.graalvm.polyglot.Context;

public class Test {

    static class Data {
        public String name = "HelloWorld";
        public String getName() {
            return this.name;
        }
    }

    public static void main(String[] args) {
        Context context = Context.newBuilder("js").allowHostAccess(true).build();
        context.getBindings("js").putMember("d", new Data());

        context.eval("js", "var x = d.getName()");

        System.out.println(
                context.getBindings("js").getMember("d").as(Data.class)).name
        );
    }
}

Answer 2

您需要用 @HostAccess.Export

注释 class 字段和方法

By default only public classes, methods, and fields that are annotated with @HostAccess.Export are accessible to the guest language. This policy can be customized using Context.Builder.allowHostAccess(HostAccess) when constructing the context.

Example using a Java object from JavaScript:
 public class JavaRecord {
     @HostAccess.Export public int x;    
     @HostAccess.Export
     public String name() {
         return "foo";
     }
 }

或者，您可以使用 GraalVM JSR-223 ScriptEngine

GraalVM JavaScript provides a JSR-223 compliant javax.script.ScriptEngine implementation. Note that this is feature is provided for legacy reasons in order to allow easier migration for implementations currently based on a ScriptEngine. We strongly encourage users to use the org.graalvm.polyglot.Context interface

To set an option via Bindings, use Bindings.put(, true) before the engine's script context is initialized. Note that even a call to Bindings#get(String) may lead to context initialization. The following code shows how to enable polyglot.js.allowHostAccess via Bindings:
ScriptEngine engine = new ScriptEngineManager().getEngineByName("JavaScript");
Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
bindings.put("polyglot.js.allowHostAccess", true);
bindings.put("polyglot.js.allowHostClassLookup", (Predicate<String>) s -> true);
bindings.put("javaObj", new Object());
engine.eval("(javaObj instanceof Java.type('java.lang.Object'));"); // would not work 
without allowHostAccess and allowHostClassLookup This example would not work if the user would call e.g. engine.eval("var x = 1;") before calling bindings.put("polyglot.js.allowHostAccess", true);, since any call to eval forces context initialization.

Answer 3

当您使用上下文并向其添加 Java 对象时，在幕后，TruffleApi 中的 IntropLibrary 会创建一个 HostObject 并将其与该对象相关联。这意味着您不使用对象本身，而是使用包装器对象。

当您调用 getMember() 方法时，IntropLibrary 只能访问 public 仅可用的托管对象的字段和方法。由于您的内部 class 具有默认访问权限（无访问修饰符），因此 API 无法找到其成员，即使它们是 public。（class 的成员不能拥有比其 class 本身更广泛的访问权限。

要解决这个问题，你要做的就是让你内心classpublic

import org.graalvm.polyglot.Context;

public class Test {

  public static class Data {
    public String name = "HelloWorld";
    public String getName() {
        return this.name;
    }
  }

  public static void main(String[] args) {
    Context context = Context.newBuilder("js").allowHostAccess(true).build();
    context.getBindings("js").putMember("d", new Data());

    context.eval("js", "var x = d.name;");

    System.out.println(
        context.getBindings("js").getMember("x").asString()
    );
  }
}

Answer 4

要以通常的 js 方式完全访问 java 对象，您可以使用 sj4js 库。

这个例子取自文档...

public class TestObject extends JsProxyObject {
    
    // the property of the object
    private String name = "";
    
    // the constructor with the property 
    public TestObject (String name) {
        super ();
        
        this.name = name;
        
        // we hvae to initialize the proxy object
        // with all properties of this object
        init(this);
    }

    // this is a mandatory override, 
    // the proxy object needs this method 
    // to generate new objects if necessary
    @Override
    protected Object newInstance() {
        return new TestClass(this.name);
    }
    
    // the setter for the property name
    public void setName (String s) {
        this.name = s;
    }
    
    // the getter for the property name
    public String getName () {
        return this.name;
    }
}

您可以像访问 java 对象一样访问此对象。

try (JScriptEngine engine = new JScriptEngine()) {
    engine.addObject("test", new TestClass("123"));
            
    engine.exec("test.name");
    // returns "123"

    engine.exec("test['name']")
    // returns "123"

    engine.exec("test.name = '456'")   
    engine.exec("test.name");
    // returns "456"
}

在 GraalVM Polyglot 上下文中从 JavaScript 访问 Java 对象

Access Java object from JavaScript in GraalVM Polyglot context

java

graalvm

graaljs