java反射详解

本篇文章采用小例子来说明，因为我始终觉的，案例驱动是最好的，要不然只看理论的话，看了也不懂，不过建议大家在看完文章之后，在回过头去看看理论，会有更好的理解。

下面是java反射的三个例子

Reflection，通过一个类名，打印出构造函数，方法和变量

 

package senior;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.Scanner;

public class ReflectionTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		String name;
		if (args.length > 0)
			name = args[0];
		else {
			System.out.println("Enter class name (e.g. java.util.Date): ");
			Scanner in = new Scanner(System.in);
			name = in.nextLine();
		}
		try {
			Class<?> cl = Class.forName(name);
			Class<?> superClass = cl.getSuperclass();
			String modifiers = Modifier.toString(cl.getModifiers());
			if (modifiers.length() > 0)
				System.out.print(modifiers + " ");
			System.out.print("class " + name);
			if (superClass != null && superClass != Object.class)
				System.out.print("extends " + superClass.getName());
			System.out.print("\n{\n");
			printConstructors(cl);// 打印构造方法
			System.out.println();
			printMethods(cl);// 打印方法
			System.out.println();
			printFields(cl);// 打印数据成员
			System.out.println("}");

		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		System.exit(0);

	}

	private static void printFields(Class<?> cl) {
		// TODO Auto-generated method stub
		Field[] fields = cl.getDeclaredFields();
		for (Field f : fields) {
			System.out.print("\t");
			Class<?> type = f.getType();
			String name = f.getName();
			String modifiers = Modifier.toString(f.getModifiers());
			if (modifiers.length() > 0)
				System.out.print(modifiers + " ");
			System.out.println(type.getName() + " " + name + ";");
		}
	}

	private static void printMethods(Class<?> cl) {
		// TODO Auto-generated method stub
		Method[] methods = cl.getDeclaredMethods();
		for (Method m : methods) {
			System.out.print("\t");
			Class<?> retType = m.getReturnType();
			String name = m.getName();
			String modifiers = Modifier.toString(m.getModifiers());
			System.out.print(modifiers + " ");
			System.out.print(retType + " " + name + "(");
			@SuppressWarnings("rawtypes")
			Class[] paramTypes = m.getParameterTypes();
			for (int j = 0; j < paramTypes.length; j++) {
				if (j > 0)
					System.out.print(", ");
				System.out.print(paramTypes[j].getName());
			}
			System.out.println(");");
		}
	}

	private static void printConstructors(Class<?> cl) {
		// TODO Auto-generated method stub
		@SuppressWarnings("rawtypes")
		Constructor[] constructors = cl.getDeclaredConstructors();
		for (Constructor<?> c : constructors) {
			System.out.print("\t");
			String name = c.getName();
			String modifiers = Modifier.toString(c.getModifiers());
			System.out.print(modifiers + " ");
			System.out.print(name + "(");
			@SuppressWarnings("rawtypes")
			Class[] paramTypes = c.getParameterTypes();
			for (int j = 0; j < paramTypes.length; j++) {
				if (j > 0)
					System.out.print(", ");
				System.out.print(paramTypes[j].getName());
			}
			System.out.println(");");
		}
	}

}

 

运行结果：

MethodPointer相当于C/C++中的回调函数

package senior;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class MethodPointerTest {

	/**
	 * @param args
	 * @throws NoSuchMethodException
	 * @throws SecurityException
	 */
	public static void main(String[] args) throws SecurityException,
			NoSuchMethodException {
		// TODO Auto-generated method stub
		Method square = MethodPointerTest.class.getMethod("square",
				double.class);
		Method sqrt = Math.class.getMethod("sqrt", double.class);
		printTable(1, 10, 10, square);
		printTable(1, 10, 10, sqrt);
	}

	public static double square(double x) {
		return x * x;
	}

	private static void printTable(int i, int j, int k, Method f) {
		// TODO Auto-generated method stub
		double x = (j - i) / (k - 1);
		for (double l = i; l <= j; l += x) {
			double y;
			try {
				y = (Double) f.invoke(null, l);
				System.out.printf("%10.4f|%10.4f%n", l, y);
			} catch (IllegalArgumentException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IllegalAccessException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (InvocationTargetException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}

		}
	}

}

运行结果：

 

泛型反射实例

 

package junior;

import java.lang.reflect.*;
import java.util.*;

public class GenericReflectionTest {
	public static void main(String[] args) {
		// read class name from command line args or user input
		String name;
		if (args.length > 0)
			name = args[0];
		else {
			Scanner in = new Scanner(System.in);
			System.out
					.println("Enter class name (e.g. java.util.Collections): ");
			name = in.next();
		}

		try {
			// print generic info for class and public methods
			Class<?> cl = Class.forName(name);
			printClass(cl);
			for (Method m : cl.getDeclaredMethods())
				printMethod(m);
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		}
	}

	public static void printClass(Class<?> cl) {
		System.out.print(cl);
		printTypes(cl.getTypeParameters(), "<", ", ", ">", true);
		Type sc = cl.getGenericSuperclass();
		if (sc != null) {
			System.out.print(" extends ");
			printType(sc, false);
		}
		printTypes(cl.getGenericInterfaces(), " implements ", ", ", "", false);
		System.out.println();
	}

	public static void printMethod(Method m) {
		String name = m.getName();
		System.out.print(Modifier.toString(m.getModifiers()));
		System.out.print(" ");
		printTypes(m.getTypeParameters(), "<", ", ", "> ", true);

		printType(m.getGenericReturnType(), false);
		System.out.print(" ");
		System.out.print(name);
		System.out.print("(");
		printTypes(m.getGenericParameterTypes(), "", ", ", "", false);
		System.out.println(")");
	}

	public static void printTypes(Type[] types, String pre, String sep,
			String suf, boolean isDefinition) {
		if (pre.equals(" extends ")
				&& Arrays.equals(types, new Type[] { Object.class }))
			return;
		if (types.length > 0)
			System.out.print(pre);
		for (int i = 0; i < types.length; i++) {
			if (i > 0)
				System.out.print(sep);
			printType(types[i], isDefinition);
		}
		if (types.length > 0)
			System.out.print(suf);
	}

	public static void printType(Type type, boolean isDefinition) {
		if (type instanceof Class) {
			Class<?> t = (Class<?>) type;
			System.out.print(t.getName());
		} else if (type instanceof TypeVariable) {
			TypeVariable<?> t = (TypeVariable<?>) type;
			System.out.print(t.getName());
			if (isDefinition)
				printTypes(t.getBounds(), " extends ", " & ", "", false);
		} else if (type instanceof WildcardType) {
			WildcardType t = (WildcardType) type;
			System.out.print("?");
			printTypes(t.getUpperBounds(), " extends ", " & ", "", false);
			printTypes(t.getLowerBounds(), " super ", " & ", "", false);
		} else if (type instanceof ParameterizedType) {
			ParameterizedType t = (ParameterizedType) type;
			Type owner = t.getOwnerType();
			if (owner != null) {
				printType(owner, false);
				System.out.print(".");
			}
			printType(t.getRawType(), false);
			printTypes(t.getActualTypeArguments(), "<", ", ", ">", false);
		} else if (type instanceof GenericArrayType) {
			GenericArrayType t = (GenericArrayType) type;
			System.out.print("");
			printType(t.getGenericComponentType(), isDefinition);
			System.out.print("[]");
		}

	}
}

运行结果如下

java核心编程中的反射笔记。欢迎评论。