Obtaining Method Type Information

A method declaration includes the name, modifiers, parameters, return type, and list of throwable exceptions. The java.lang.reflect.Method class provides a way to obtain this information.

The MethodSpy example illustrates how to enumerate all of the declared methods in a given class and retrieve the return, parameter, and exception types for all the methods of the given name.


import java.lang.reflect.Method;
import java.lang.reflect.Type;
import static java.lang.System.out;

public class MethodSpy {
    private static final String  fmt = "%24s: %s%n";

    // for the morbidly curious
    <E extends RuntimeException> void genericThrow() throws E {}

    public static void main(String... args) {
	try {
	    Class<?> c = Class.forName(args[0]);
	    Method[] allMethods = c.getDeclaredMethods();
	    for (Method m : allMethods) {
		if (!m.getName().equals(args[1])) {
		    continue;
		}
		out.format("%s%n", m.toGenericString());

		out.format(fmt, "ReturnType", m.getReturnType());
		out.format(fmt, "GenericReturnType", m.getGenericReturnType());

		Class<?>[] pType  = m.getParameterTypes();
		Type[] gpType = m.getGenericParameterTypes();
		for (int i = 0; i < pType.length; i++) {
		    out.format(fmt,"ParameterType", pType[i]);
		    out.format(fmt,"GenericParameterType", gpType[i]);
		}

		Class<?>[] xType  = m.getExceptionTypes();
		Type[] gxType = m.getGenericExceptionTypes();
		for (int i = 0; i < xType.length; i++) {
		    out.format(fmt,"ExceptionType", xType[i]);
		    out.format(fmt,"GenericExceptionType", gxType[i]);
		}
	    }

        // production code should handle these exceptions more gracefully
	} catch (ClassNotFoundException x) {
	    x.printStackTrace();
	}
    }
}

Here is the output for Class.getConstructor() which is an example of a method with parameterized types and a variable number of parameters.

$ java MethodSpy java.lang.Class getConstructor
public java.lang.reflect.Constructor<T> java.lang.Class.getConstructor
  (java.lang.Class<?>[]) throws java.lang.NoSuchMethodException,
  java.lang.SecurityException
              ReturnType: class java.lang.reflect.Constructor
       GenericReturnType: java.lang.reflect.Constructor<T>
           ParameterType: class [Ljava.lang.Class;
    GenericParameterType: java.lang.Class<?>[]
           ExceptionType: class java.lang.NoSuchMethodException
    GenericExceptionType: class java.lang.NoSuchMethodException
           ExceptionType: class java.lang.SecurityException
    GenericExceptionType: class java.lang.SecurityException

This is the actual declaration of the method in source code:

public Constructor<T> getConstructor(Class<?>... parameterTypes)

First note that the return and parameter types are generic. Method.getGenericReturnType() will consult the Signature Attribute in the class file if it's present. If the attribute isn't available, it falls back on Method.getReturnType() which was not changed by the introduction of generics. The other methods with name getGenericFoo() for some value of Foo in reflection are implemented similarly.

Next, notice that the last (and only) parameter, parameterType, is of variable arity (has a variable number of parameters) of type java.lang.Class. It is represented as a single-dimension array of type java.lang.Class. This can be distinguished from a parameter that is explicitly an array of java.lang.Class by invoking Method.isVarArgs(). The syntax for the returned values of Method.get*Types() is described in Class.getName().

The following example illustrates a method with a generic return type.

$ java MethodSpy java.lang.Class cast
public T java.lang.Class.cast(java.lang.Object)
              ReturnType: class java.lang.Object
       GenericReturnType: T
           ParameterType: class java.lang.Object
    GenericParameterType: class java.lang.Object

The generic return type for the method Class.cast() is reported as java.lang.Object because generics are implemented via type erasure which removes all information regarding generic types during compilation. The erasure of T is defined by the declaration of Class:

public final class Class<T> implements ...

Thus T is replaced by the upper bound of the type variable, in this case, java.lang.Object.

The last example illustrates the output for a method with multiple overloads.

$ java MethodSpy java.io.PrintStream format
public java.io.PrintStream java.io.PrintStream.format
  (java.util.Locale,java.lang.String,java.lang.Object[])
              ReturnType: class java.io.PrintStream
       GenericReturnType: class java.io.PrintStream
           ParameterType: class java.util.Locale
    GenericParameterType: class java.util.Locale
           ParameterType: class java.lang.String
    GenericParameterType: class java.lang.String
           ParameterType: class [Ljava.lang.Object;
    GenericParameterType: class [Ljava.lang.Object;
public java.io.PrintStream java.io.PrintStream.format
  (java.lang.String,java.lang.Object[])
              ReturnType: class java.io.PrintStream
       GenericReturnType: class java.io.PrintStream
           ParameterType: class java.lang.String
    GenericParameterType: class java.lang.String
           ParameterType: class [Ljava.lang.Object;
    GenericParameterType: class [Ljava.lang.Object;

If multiple overloads of the same method name are discovered, they are all returned by Class.getDeclaredMethods(). Since format() has two overloads (with with a Locale and one without), both are shown by MethodSpy.

Note: Method.getGenericExceptionTypes() exists because it is actually possible to declare a method with a generic exception type. However this is rarely used since it is not possible to catch a generic exception type.

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