How do I invoke a private static method using reflection (Java)?
I would like to invoke a private static method. I have its name. I've heard it can be done using Java reflection mechanism. How can I do it?
EDIT: One problem I encountered when trying to invoke the method is how to specify the type of its argument. My method receives one argument and its type is Map. Therefore I cannot do Map<User, String>.TYPE
(In run time there's no such a thing as Map because of Java Type erasure). Is there another way to get the method?
Let's say you want to call MyClass.myMethod(int x);
Method m = MyClass.class.getDeclaredMethod("myMethod", Integer.TYPE);
m.setAccessible(true); //if security settings allow this
Object o = m.invoke(null, 23); //use null if the method is static
Invoke main from reflection tutorial
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Arrays;
public class InvokeMain {
public static void main(String... args) {
try {
Class<?> c = Class.forName(args[0]);
Class[] argTypes = new Class[] { String[].class };
Method main = c.getDeclaredMethod("main", argTypes);
String[] mainArgs = Arrays.copyOfRange(args, 1, args.length);
System.out.format("invoking %s.main()%n", c.getName());
main.invoke(null, (Object)mainArgs);
// production code should handle these exceptions more gracefully
} catch (ClassNotFoundException x) {
x.printStackTrace();
} catch (NoSuchMethodException x) {
x.printStackTrace();
} catch (IllegalAccessException x) {
x.printStackTrace();
} catch (InvocationTargetException x) {
x.printStackTrace();
}
}
}
No, you can't say Map<K,V>.class
. This is because of type erasure. At runtime, there's no such thing.
Luckily, you can say just plain old Map.class
. It's all the same at runtime.
If the warnings bother you, search for other questions related to generics and type erasure, there's a wealth of information on the subject here.
I use a single method that encapsulates getting the target method and then invoking it. Probably has some limitations, of course. Here is the method put into a class and its JUnit test:
public class Invoker {
/**
* Get method and invoke it.
*
* @author jbetancourt
*
* @param name of method
* @param obj Object to invoke the method on
* @param types parameter types of method
* @param args to method invocation
* @return return value
* @throws Exception for unforseen stuff
*/
public static final <T> Object invokeMethod(final String name, final T obj,
final Class<?>[] types, final Object... args) throws Exception {
Method method = obj.getClass().getDeclaredMethod(name, types);
method.setAccessible(true);
return method.invoke(obj, args);
}
/**
* Embedded JUnit tests.
*/
@RunWith(JUnit4.class)
public static class InvokerTest {
/** */
@Test
public void testInvoke() throws Exception {
class TestTarget {
private String hello() {
return "Hello world!";
}
}
String actual = (String) Invoker.invokeMethod("hello",
new TestTarget(), new Class<?>[] {});
String expected = "Hello world!";
assertThat(actual, is(expected));
}
}
}