invoke

Learn dynamic calls with Method.invoke.

Calling methods with Method.invoke

To call a Method object at runtime you use invoke. The first parameter is the target instance; the rest are the method's parameters.

Example: Object result = m.invoke(target, args). Here target is the instance on which the method is called and args is the array of parameters. For static methods target can be null. The return value comes back as an Object; you cast it when needed.

Calling private methods

To call a private method you first need setAccessible(true). This bypasses access checks; then invoke runs.

Example: m.setAccessible(true); m.invoke(target). This pattern is used in library and test code to reach hidden methods. The module system adds some restrictions, but in single-file applications setAccessible generally works. Use it carefully in production code; it can break encapsulation.

Exercise Invoke the String.length method

Assign "Hello" to a String variable. Find the length method with String.class.getMethod("length"). Call it with m.invoke(s) and print the result with System.out.println. Add throws Exception to main.

Show example solution

import java.lang.reflect.Method;

public class InvokeLength {
    public static void main(String[] args) throws Exception {
        String s = "Hello";
        Method m = String.class.getMethod("length");
        System.out.println(m.invoke(s));
    }
}

Test yourself

Complete the invoke call

Object result = method.____(target, args);

Show answer

Object result = method.invoke(target, args);

Match the invoke concepts

invoke

Call the method
setAccessible

Open access
getMethod

Find a method
IllegalAccessException

Access blocked
Which exception is thrown when invoking an inaccessible method?
- NullPointerException
- ClassCastException
- IOException
- IllegalAccessException
Show answer

IllegalAccessException — IllegalAccessException is thrown when access is blocked.

Next lesson: getDeclaredConstructors

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