Call a method from a string

Question

I am making a chat. and i have everything working. But i would like to implement commands that the user can run by typing / then the command. I was thinking i would test for the '/' char at index 0. And if thats true i would run that "code". So the user would essentially be calling a method. but i cant figure out how to convert the string into java code. Help would be appreciated. And if anyone has an easier method of how to implement commands feel free to let me know.

Where

String userinput = "/test()";

Thanks

I thought about that but then i dont see a way i could pass parameters through the command — Tristan Frederick, Feb 16 '18 at 12:51
why not? what is stopping you from getting substrings from "/test(string:value1, int:2)" ? — Stultuske, Feb 16 '18 at 12:52
like i could have a command that repeates a message and it would be userinput = "/rep(5,"Hello"); and it would send Hello 5 times — Tristan Frederick, Feb 16 '18 at 12:52
Possible duplicate of [Is there an eval() function in Java?](https://stackoverflow.com/questions/2605032/is-there-an-eval-function-in-java) — Tim, Feb 16 '18 at 12:53
Are you sure you want to just blindly allow users to run any method they like insetad of only defining a specific subset that you can easily check for and implement with a switch case, a Map or something similar? Sooner or later someone will type "/System.exit()" or something worse into your chat. — OH GOD SPIDERS, Feb 16 '18 at 12:54
https://stackoverflow.com/questions/4389232/run-piece-of-code-contained-in-a-string — Tim, Feb 16 '18 at 12:54
https://stackoverflow.com/questions/17608392/run-code-from-a-string-in-java — Tim, Feb 16 '18 at 12:54
https://stackoverflow.com/questions/8661634/execute-java-code-in-java — Tim, Feb 16 '18 at 12:54
https://stackoverflow.com/questions/12719978/how-do-you-run-a-string-as-code — Tim, Feb 16 '18 at 12:55
Do you mean preset methods you define yourself or arbitrary Java methods? — Taslim Oseni, Feb 16 '18 at 12:56

Sebastian · Answer 1 · 2018-02-16T13:11:11.547

3

Letting the user actually run code via his input would require 'reflection' (you will find a lot about this by just googling e.g. 'java reflection').

BUT ATTENTION: Letting the user execute Java code this way is a big security problem!

I think the better approach for you would be something like this:

public void executeUserCommand(final String userInput) {

    String commandFromUser = // parse fromUserInput
    String param = // parse fromUserInput


    switch (commandFromUser) {
        case "command1":
            // call method with 'param'
            break;
        case "command2":
            // call method with 'param'
            break;

        default:
            // tell the user that his command was not found
            break;
    }

}

edited Feb 16 '18 at 13:11

answered Feb 16 '18 at 12:53

Sebastian

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Its just to be used by me and some friends. And thank you ill look into it – Tristan Frederick Feb 16 '18 at 12:55
and in this case i couldnt pass params could i ? – Tristan Frederick Feb 16 '18 at 13:04
yes you could, just added the answer, for the part where it says 'parse user input' you'll have to do use something like substring() and indexOf() Methods of String depending on how the user input should look like e.g. 'command:parameter' you have to use indexOf(":") to get the parameters for substring() – Sebastian Feb 16 '18 at 13:15
I will try that. What do you suggest i do for multiple params – Tristan Frederick Feb 16 '18 at 13:46

score 0 · Answer 2 · answered Feb 16 '18 at 12:55

You're going to need to look into the Java reflection API to really do anything more than very basic stuff.

At a high level, what I would do is something like this:

/test

or

/msg "John Smith" "XYZ"

You can then parse that into three tokens which gives you msg and two parameters.

What you'll want to do is use reflection to look at the object that handles the chat functions and look up any method named msg that takes two String paramters.

If you decide that allowing numbers and booleans and such is going to be part of it, you're going to get into the weeds pretty fast with the parsing side so be aware of that.

Update: saw this:

"/rep(5,"Hello");

That's going to be a lot harder to parse than /rep 5 "Hello" or /rep 5, "Hello"

i mean i could always change it to /rep("5" "Hello"). then do a integer.parseInt("5"); — Tristan Frederick, Feb 16 '18 at 13:37

Call a method from a string

2 Answers2