JavetProxyConverter and instanceof not working

[tristanlins]

When using the JavetProxyConverter, any instanceof call of proxied Java classes fails.
Simple example that throws an error:

final var javetProxyConverter = new JavetProxyConverter();
v8Runtime.setConverter(javetProxyConverter);

final var globalObject = v8Runtime.getGlobalObject();
globalObject.set("URL", URI.class);

v8Runtime.getExecutor(
  // language=javascript
  "let url = new URL('https://example.com');\n"
    + "if (!(url instanceof URL)) {\n"
    + "    throw new Error('url is not instance of URL');\n"
    + "}"
).executeVoid();

[caoccao]

Thank you for raising this issue. It is not supported, so far. You may use Class.isAssignableFrom() instead.

But, that is technically doable and could be a feature request.

[caoccao]

Update

I just reviewed the source code. It's technically impossible to make instanceof work because the right hand is a Proxy and the left hand cannot be a Proxy. So, the result is always false.

Why cannot the left hand be a Proxy? V8 is designed to recursively traverse the proxied object for its original prototype. If the underlying API always returns a Proxy, V8 will hang because of OOM (not stack overflow).

So, please try the workaround in my previous comment. Please let me know if you have any questions.

[tristanlins]

The instanceof is part of a third-party library. I cannot modify the source.

I'm a bit confused about the proxy thing.
The script runs in V8 Mode (I have missed this info in my initial post), so URL does not exist.
I tried to "polyfill" the URL with globalObject.set("URL", URI.class);.
Now I can do something like new URL(..) in JS. and this creates a java URI object?!
In my expectation, the left side is a proxy to Javas URI class?

Currently, I have worked around by implementing a JS based polyfill URL class.

[caoccao]

I suggest you review this slide for how things work behind the scene in Javet. Actually, the one you create in V8 is a Proxy. The pseudo logic in V8 is as follows:

function getPrototypeOf(obj) {
  while (obj is proxy) {
    obj = obj.target;
  } 
  return obj.prototype;
}

As the object created by Javet is a proxy, the prototype is also a proxy. In your case, that is Proxy(URL.class). That while loop will never end. That causes OOM in V8.

[aiselp]

I suggest you review this slide for how things work behind the scene in Javet. Actually, the one you create in V8 is a Proxy. The pseudo logic in V8 is as follows:

function getPrototypeOf(obj) {
  while (obj is proxy) {
    obj = obj.target;
  } 
  return obj.prototype;
}

As the object created by Javet is a proxy, the prototype is also a proxy. In your case, that is Proxy(URL.class). That while loop will never end. That causes OOM in V8.

instanceof运算符右侧是proxy似乎也能正常工作，只需将右侧的proxy对象设置一个prototype属性，然后将此属性加入到实例对象的原型链中

let U = function(){}
let p = {
  constructor: U
}
U.prototype = p
let c = new Proxy(U, {});

let r = Object.create(p);

console.log(r instanceof c);//true

[caoccao]

Hi @tristanlins, good news! I was inspired by @aiselp 's comment and spent a week investigating this issue. It's now supported at the dev branch. This new feature passes the following assertions.

v8Runtime.getGlobalObject().set("StringBuilder", StringBuilder.class);
assertTrue(v8Runtime.getExecutor("StringBuilder instanceof StringBuilder;").executeBoolean());
assertTrue(v8Runtime.getExecutor("new StringBuilder() instanceof StringBuilder;").executeBoolean());
assertTrue(v8Runtime.getExecutor("Object.getPrototypeOf(new StringBuilder()) === Object.getPrototypeOf(StringBuilder);").executeBoolean());
v8Runtime.getGlobalObject().delete("StringBuilder");

That's a really tough issue. You may try the latest snapshot build and let me know if you have any questions. If my works pleases you, please consider Buy Me a Cup of Coffee at the donation link. Thank you.

[aiselp]

@caoccao 感谢作者辛苦研究，上面的test包含以下内容或许更符合js标准

v8Runtime.getGlobalObject().set("StringBuilder", StringBuilder.class);
assertTrue(v8Runtime.getExecutor("new StringBuilder() instanceof StringBuilder;").executeBoolean());
assertTrue(v8Runtime.getExecutor("Object.getPrototypeOf(new StringBuilder()) === StringBuilder.prototype;").executeBoolean());
assertTrue(v8Runtime.getExecutor(
"let a = new StringBuilder() \n"+
"StringBuilder.prototype.abc= function(){
retun this === a
};\n"+
"a.abc()"
).executeBoolean());
v8Runtime.getGlobalObject().delete("StringBuilder");

Object.getPrototypeOf(StringBuilder)
should retun null or Function.prototype

[caoccao]

Nice catch. There's a flaw in the implementation. It's now corrected as follows.

v8Runtime.getGlobalObject().set("StringBuilder", StringBuilder.class);
assertTrue(v8Runtime.getExecutor("new StringBuilder() instanceof StringBuilder;").executeBoolean());
assertTrue(v8Runtime.getExecutor("Object.getPrototypeOf(new StringBuilder()) === StringBuilder.prototype;").executeBoolean());
assertFalse(v8Runtime.getExecutor("Object.getPrototypeOf(new StringBuilder()) === Object.getPrototypeOf(StringBuilder);").executeBoolean());
assertFalse(v8Runtime.getExecutor("StringBuilder instanceof StringBuilder;").executeBoolean());
v8Runtime.getGlobalObject().delete("StringBuilder");