Execute code from a String in Java

Question

I've done some searching, and it seems a few people have asked similar questions, but none of them were quite what I was looking for (some suggested some scripting stuff but i didn't have enough knowledge of what i was doing so i want to use java only if possible).

I need to be able to read lines of code from a String, and then execute them in Java (like this):

String code = "System.out.println(\"Test code\");";

A lot of people reading this post might ask why don't you just execute

System.out.println("Test code");

but I want to execute code other than the println method from Strings.

Is it possible to execute code using java alone, if so how would it compile??

@JohnD as I understand you don't need to compile anything. You only want to input a string and have it "typed" 1to1 in notepad. Is that right? — Herr Derb
– Herr Derb, Commented Aug 22, 2017 at 15:10

slim · Accepted Answer · 2017-08-22 15:14:58Z

4

You can't do this as easily as I imagine you hope to. The kind of thing you're describing is something you might see in a dynamic language. Java is very much not a dynamic language - this is part of its character.

If you have tools.jar in your classpath, you can compile Java code from within your Java program:

com.sun.tools.javac.Main javac = new com.sun.tools.javac.Main();
String[] options = new String[] {
    "-classpath", classpath, "-d", outputDir, filename
};
javac.compile(options);

From there you could wrestle with the classloader, load the class you have compiled, and run it.

However this is not an easy or mainstream way to use Java, and generally people don't do it because it's neither good practice nor necessary. It is a security feature that you can't run code supplied by the user after compilation.

If you want a more dynamic language, in the Java ecosystem, you could look at Groovy.

Alternatively, you can run user-supplied Javascript, giving it controlled access to your Java program's data, using the Java Scripting API.

answered Aug 22, 2017 at 15:14

slim

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4 Comments

Jacek Cz Over a year ago

Internal classes like com.sun ... is bad proposition, specially when official javax.tools.JavaCompiler exist

Coder Over a year ago

I don't have tools.jar in my classpath, i use IntelliJ how do i add it?

Jacek Cz Over a year ago

Full JDK (JRE has not this jar)

slim Over a year ago

@JohnD Don't. I hoped to dissuade you in the body of the answer. It's difficult enough that you need to be experienced to do it. And once you're experienced you understand why it's not wise.

Chamalo90 · Accepted Answer · 2017-08-22 15:00:37Z

1

It is possible but the java code cannot be executed as such, it's not interpreted, it needs to be compiled first. So you need to put your string into a main function of another java file, then compile it and finally run it.

You might need this : http://docs.oracle.com/javase/7/docs/api/java/lang/Runtime.html

answered Aug 22, 2017 at 15:00

Chamalo90

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Comments

Herr Derb · Accepted Answer · 2017-08-22 15:24:25Z

If I understand correct, you want to input a string into the java console. This given string then should be magically be typed-in in an external program. If I did not understand correct, I'll remove that answer again.

public class Test {

    public static void main(String[] args) throws IOException, ClassNotFoundException, Exception {
    Robot robot = new Robot();
    String text = "Hello";
    File file = File.createTempFile("tmp", ".txt");

    if (Desktop.isDesktopSupported()) {
        Desktop.getDesktop().edit(file);
        Thread.sleep(1000);
    } else {
        // dunno, up to you to handle this
    }
    for (char c : text.toCharArray()) {
        int keyEvent = KeyEvent.getExtendedKeyCodeForChar(c);
        robot.keyPress(keyEvent);
        Thread.sleep(500);
    }
    }
}

If you run this basic code, notepad should open and the text Hello is getting typed. That's what I've prepared for you. Now what you need to do is using a Scanner to get the string from the console instead from a static string.

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