通过将类名和方法名作为参数传递来运行java方法

Question

I am trying to make a program that executes a particular method when the class name and method name are passed as String parameter to the caller. Consider the code below. I have a class CarBean :

public class CarBean {

    private String brand;
    private String color;

    /**
     * @return the brand
     */
    public String getBrand() {
        return brand;
    }

    /**
     * @param the brand to set
     */
    public void setBrand(String brand) {
        this.brand= brand;
    }

    /**
     * @return the color
     */
    public String getColor() {
        return color;
    }

    /**
     * @param the color to set
     */
    public void setColor(String color) {
        this.color= color;
    }

}

Now I want to run this via a method as below :

runTheMehod("CarBean","getColor");

The implementation of runTheMethod would be like this :

public runTheMethod(String className, String methodName){
try {
            Object carObj = Class.forName(className).newInstance();
            //What to do now???
        } catch (InstantiationException | IllegalAccessException
                | ClassNotFoundException e) {
            e.printStackTrace();
        }
}

I can get an object using the class name. Now I need to cast it to a CarBean object and then I can run its method. So wondering how to cast it at runtime as the classname would be different for each call. Also, can I check whether the class has specific method before I try call it?

Any suggestion on the problem would be appreciated. Also, I'm all ears to know if there is a better approach to do this.

Answer 1

What to do now???

You can now call the method on your carObj like this:

Method method = carObj.getClass().getMethod(methodName);
method.invoke(carObj);

See also:

Method.invoke

Answer 2

If all you are trying to do is call a method by name, you don't need to cast it to anything. It is sufficient to use the Object with further reflection.

See Class.getDeclaredMethod() , as in:

Object carObj = ...;
Method method = carObj.getClass().getDeclaredMethod(methodName, ...);
Object retObj = method.invoke(carObj, ...);

Note that we don't care what type carObj actually is, although if you wanted to check you could always use instanceof or Class.isAssignableFrom or Class.isInstance .

It is a bit weird, though, that you are instantiating a new object then calling one of its methods all in one go. That object goes away once your runTheMethod returns.

---

By the way, it looks like you're just trying to get and set bean properties. You might want to have a look at Apache Commons BeanUtils instead, then your example becomes simply:

CarBean bean = ...;
String color = (String)PropertyUtils.getSimpleProperty(bean, "color"); // calls getter.

Answer 3

I doubt you really need to do this. I suspect you have an XY Problem . Your question is somewhat similar to asking whether Java has an eval function and the correct answer is similar :

First ask yourself, where did these String come from? Did another part of your program generate them, or was it input provided by the user?

• Another part of my program generated it : so, you want one part of your program to decide the kind of operation to perform, but not perform the operation, and a second part that performs the chosen operation. Instead of generating and then evaluating String s, use the Strategy , Command or Builder design pattern, as appropriate for your particular case.

• It is user input : the user could input anything , including class and method names that, when executed, could cause your program to misbehave, crash, expose information that should be secret, damage persistent information (such as the content of a database), and other such nastiness. The only way to prevent that would be to parse the String s and then either immediately execute the method on a successful parse, or have the parser create a Command object for later execution.