Invoking Methods (The Java™ Tutorials > The Reflection API

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Invoking Methods

Reflection provides a means for invoking methods on a class. Typically, this would only be necessary if it is not possible to cast an instance of the class to the desired type in non-reflective code. Methods are invoked with java.lang.reflect.Method.invoke(). The first argument is the object instance on which this particular method is to be invoked. (If the method is static, the first argument should be null.) Subsequent arguments are the method's parameters. If the underlying method throws an exception, it will be wrapped by an java.lang.reflect.InvocationTargetException. The method's original exception may be retrieved using the exception chaining mechanism's InvocationTargetException.getCause() method.

Finding and Invoking a Method with a Specific Declaration

Consider a test suite which uses reflection to invoke private test methods in a given class. The Deet example searches for public methods in a class which begin with the string "test", have a boolean return type, and a single Locale parameter. It then invokes each matching method.

/*
 * Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Sun Microsystems nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */ 

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Type;
import java.util.Locale;
import static java.lang.System.out;
import static java.lang.System.err;

public class Deet<T> {
    private boolean testDeet(Locale l) {
	// getISO3Language() may throw a MissingResourceException
	out.format("Locale = %s, ISO Language Code = %s%n", l.getDisplayName(), l.getISO3Language());
	return true;
    }

    private int testFoo(Locale l) { return 0; }
    private boolean testBar() { return true; }

    public static void main(String... args) {
	if (args.length != 4) {
	    err.format("Usage: java Deet <classname> <langauge> <country> <variant>%n");
	    return;
	}

	try {
	    Class<?> c = Class.forName(args[0]);
	    Object t = c.newInstance();

	    Method[] allMethods = c.getDeclaredMethods();
	    for (Method m : allMethods) {
		String mname = m.getName();
		if (!mname.startsWith("test")
		    || (m.getGenericReturnType() != boolean.class)) {
		    continue;
		}
 		Type[] pType = m.getGenericParameterTypes();
 		if ((pType.length != 1)
		    || Locale.class.isAssignableFrom(pType[0].getClass())) {
 		    continue;
 		}

		out.format("invoking %s()%n", mname);
		try {
		    m.setAccessible(true);
		    Object o = m.invoke(t, new Locale(args[1], args[2], args[3]));
		    out.format("%s() returned %b%n", mname, (Boolean) o);

		// Handle any exceptions thrown by method to be invoked.
		} catch (InvocationTargetException x) {
		    Throwable cause = x.getCause();
		    err.format("invocation of %s failed: %s%n",
			       mname, cause.getMessage());
		}
	    }

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

Deet invokes getDeclaredMethods() which will return all methods explicitly declared in the class. Also, Class.isAssignableFrom() is used to determine whether the parameters of the located method are compatible with the desired invocation. Technically the code could have tested whether the following statement is true since Locale is final:

Locale.class == pType[0].getClass()

However, Class.isAssignableFrom() is more general.

$ java Deet Deet ja JP JP
invoking testDeet()
Locale = Japanese (Japan,JP), ISO Language Code = jpn
testDeet() returned true
$ java Deet Deet xx XX XX
invoking testDeet()
invocation of testDeet failed: Couldn't find 3-letter language code for xx

First, note that only testDeet() meets the declaration restrictions enforced by the code. Next, when testDeet() is passed an invalid argument it throws an unchecked java.util.MissingResourceException. In reflection, there is no distinction in the handling of checked versus unchecked exceptions. They are all wrapped in an InvocationTargetException

Invoking Methods with a Variable Number of Arguments

Method.invoke() may be used to pass a variable number of arguments to a method. The key concept to understand is that methods of variable arity are implemented as if the variable arguments are packed in an array.

The InvokeMain example illustrates how to invoke the main() entry point in any class and pass a set of arguments determined at runtime.

/*
 * Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Sun Microsystems nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */ 

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();
	}
    }
}

First, to find the main() method the code searches for a class with the name "main" with a single parameter that is an array of String Since main() is static, null is the first argument to Method.invoke(). The second argument is the array of arguments to be passed.

$ java InvokeMain Deet Deet ja JP JP
invoking Deet.main()
invoking testDeet()
Locale = Japanese (Japan,JP), ISO Language Code = jpn
testDeet() returned true

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