Java reflect: getDeclaredMethod, invoke

Use the java.lang.reflect namespace to use reflection. Call getDeclaredMethod and invoke.

Reflect. A Java program can look into its own structure. It can (for example) call a method by using a string for its name. This enables many powerful abilities.

Some notes. Reflection, though powerful, is not as efficient as calling a method directly. It is harder to optimize. We should reserve it for special situations.

GetDeclaredMethod example. To begin, we have a simple Java program in a "Program" class. In main, we indicate we can throw many exceptions.

Tip: The "throws" statements are auto-generated by Eclipse. You can let Eclipse add these on its own.

GetDeclaredMethod: We use this method with 1 or more arguments. The first argument is the name of the method.

Invoke: This method calls the Method reference. So the executable code in the method is run.

Java program that uses getDeclaredMethod, invoke import java.lang.reflect.*; public class Program { static void test() { // Say hello. System.out.println("Hello world"); } static void bird(String message) { // Print the argument. System.out.print("Bird says: "); System.out.println(message); } public static void main(String[] args) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException { // Use getDeclaredMethod. // ... This gets the test method by its name. Method testMethod = Program.class.getDeclaredMethod("test"); // Invoke the test method. testMethod.invoke(null, null); // Use getDeclaredMethod. // ... Get the bird method with a first argument of String. Method birdMethod = Program.class.getDeclaredMethod("bird", String.class); // Invoke the bird method. // ... First argument is class instance, which is null for a static method. // Second argument is the actual argument String. birdMethod.invoke(null, "Seed"); } } Output Hello world Bird says: Seed

Notes, arguments. Consider the arguments to getDeclaredMethod. The first is the method name (like "test"). The second is a class argument (like String.class) if the method has an argument.

Also: If there is no argument, you can omit the class reference from the getDeclaredMethod call.

Notes, invoke. Consider now the invoke() method. It receives 2 or more arguments. For a static method, we can use null as the first argument.

And: The second argument is the actual value to be passed to the method—like a String instance.

Strings

A summary. Reflection has its place in Java programs. It can enable powerful features—like the ability for a program to analyze other programs (or itself) for flaws.

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Java reflect: getDeclaredMethod, invoke

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