Invoking a static method using reflection

Question

I want to invoke the main method which is static. I got the object of type Class , but I am not able to create an instance of that class and also not able to invoke the static method main .

Answer 1

// String.class here is the parameter type, that might not be the case with you
Method method = clazz.getMethod("methodName", String.class);
Object o = method.invoke(null, "whatever");

In case the method is private use getDeclaredMethod() instead of getMethod() . And call setAccessible(true) on the method object.

Answer 2

Fromthe Javadoc of Method.invoke():

If the underlying method is static, then the specified obj argument is ignored. It may be null.

What happens when you

Class klass = ...;
Method m = klass.getDeclaredMethod(methodName, paramtypes);
m.invoke(null, args)

Answer 3

String methodName= "...";
String[] args = {};

Method[] methods = clazz.getMethods();
for (Method m : methods) {
    if (methodName.equals(m.getName())) {
        // for static methods we can use null as instance of class
        m.invoke(null, new Object[] {args});
        break;
    }
}

Answer 4

public class Add {
    static int add(int a, int b){
        return (a+b);
    }
}

In the above example, 'add' is a static method that takes two integers as arguments.

Following snippet is used to call 'add' method with input 1 and 2.

Class myClass = Class.forName("Add");
Method method = myClass.getDeclaredMethod("add", int.class, int.class);
Object result = method.invoke(null, 1, 2);

Reference link .

Answer 5

Please keep in mind, that you have to also provide argument types, when trying to get desired method.

Below an example written using Groovy.

import groovy.transform.CompileStatic
import org.springframework.util.ReflectionUtils

import java.lang.reflect.Method

@CompileStatic
class Fun {

    final static String funText() {
        return 'Have fun now!'
    }

    final static String myText(String text) {
        return text
    }

}

Method m1 = ReflectionUtils.findMethod(Fun, 'funText')
println m1.invoke(null)

Method m2 = ReflectionUtils.findMethod(Fun, 'myText', String)
println m2.invoke(null, 'Some text.')

Method m3 = ReflectionUtils.findMethod(Fun, 'myText')
println m3.invoke(null, 'Another text.')

On example below m3 will fail as there is no such method.