Trail: The Reflection API
Lesson: Members
Section: Methods

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~~The Java Tutorials have been written for JDK 8.~~Java教程是为JDK 8编写的。~~Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.~~本页中描述的示例和实践没有利用后续版本中引入的改进，并且可能使用不再可用的技术。
~~See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.~~有关Java SE 9及其后续版本中更新的语言特性的摘要，请参阅Java语言更改。
~~See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.~~有关所有JDK版本的新功能、增强功能以及已删除或不推荐的选项的信息，请参阅JDK发行说明。

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.

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.

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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