java 中的反射

简介

java 中的反射机制 -- 检查类的结构 (猜测主要作用于未运行的类)
可以作为调试手段

code

package com;

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

public class ReflectionTest {
	public static void main(String[] args) {
		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.Date): ");
			name = in.next();
		}
		
		try{
			Class cl = Class.forName(name);
			Class supercl = cl.getSuperclass();
			String modifiers = Modifier.toString(cl.getModifiers());
			if (modifiers.length() > 0) System.out.print(modifiers + " ");
			System.out.print("class " + name);
			if (supercl != null && supercl != Object.class) System.out.print(" extends " + supercl.getName());
			
			System.out.print("
{
");
			printConstructors(cl);
			System.out.println();
			printMethods(cl);
			System.out.println();
			printFields(cl);
			System.out.println("}");
		}
		catch(ClassNotFoundException e){
			e.printStackTrace();
		}
	}
	/**
	 * Prints all constructors of a class
	 * @param cl a class
	 * */
	public static void printConstructors(Class cl) {
		Constructor[] constructors = cl.getDeclaredConstructors();
		
		for(Constructor c: constructors){
			String name = c.getName();
			System.out.print(" ");
			String modifiers = Modifier.toString(c.getModifiers());
			if(modifiers.length() > 0) System.out.print(modifiers + " ");
			System.out.print(name + "(");
			
			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(");");
		}
	}
	
	/**
	 * Prints all methods of a class
	 * @param cl a class
	 * */
	public static void printMethods(Class cl){
		Method[] methods = cl.getDeclaredMethods();
		for(Method m : methods) {
			Class retType = m.getReturnType();
			String name = m.getName();
			
			System.out.print("   ");
			String modifiers = Modifier.toString(m.getModifiers());
			if (modifiers.length() > 0) System.out.print(modifiers + " ");
			System.out.print(retType.getName() + " " + name + "(");
			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(");");
		}
	}
	
	/**
	 * Prints all fields of a class
	 * @param cl a class
	 * */
	public static void printFields(Class cl) {
		Field[] fields = cl.getDeclaredFields();
		
		for(Field f : fields){
			Class type = f.getType();
			String name = f.getName();
			System.out.print("  ");
			String modifiers = Modifier.toString(f.getModifiers()); // 获取修饰符
			if (modifiers.length() > 0) System.out.print(" [modifiers] " + modifiers + " [modifiers] ");
			System.out.println(type.getName() + " " + name + ";");
		}
	}
}

Answer

Enter class name (e.g. java.util.Date): 
java.lang.Double
public final class java.lang.Double extends java.lang.Number
{
 public java.lang.Double(double);
 public java.lang.Double(java.lang.String);

   public boolean equals(java.lang.Object);
   public static java.lang.String toString(double);
   public java.lang.String toString();
   public int hashCode();
   public static int hashCode(double);
   public static double min(double, double);
   public static double max(double, double);
   public static native long doubleToRawLongBits(double);
   public static long doubleToLongBits(double);
   public static native double longBitsToDouble(long);
   public volatile int compareTo(java.lang.Object);
   public int compareTo(java.lang.Double);
   public byte byteValue();
   public short shortValue();
   public int intValue();
   public long longValue();
   public float floatValue();
   public double doubleValue();
   public static java.lang.Double valueOf(java.lang.String);
   public static java.lang.Double valueOf(double);
   public static java.lang.String toHexString(double);
   public static int compare(double, double);
   public static boolean isNaN(double);
   public boolean isNaN();
   public static boolean isFinite(double);
   public static boolean isInfinite(double);
   public boolean isInfinite();
   public static double sum(double, double);
   public static double parseDouble(java.lang.String);

   [modifiers] public static final [modifiers] double POSITIVE_INFINITY;
   [modifiers] public static final [modifiers] double NEGATIVE_INFINITY;
   [modifiers] public static final [modifiers] double NaN;
   [modifiers] public static final [modifiers] double MAX_VALUE;
   [modifiers] public static final [modifiers] double MIN_NORMAL;
   [modifiers] public static final [modifiers] double MIN_VALUE;
   [modifiers] public static final [modifiers] int MAX_EXPONENT;
   [modifiers] public static final [modifiers] int MIN_EXPONENT;
   [modifiers] public static final [modifiers] int SIZE;
   [modifiers] public static final [modifiers] int BYTES;
   [modifiers] public static final [modifiers] java.lang.Class TYPE;
   [modifiers] private final [modifiers] double value;
   [modifiers] private static final [modifiers] long serialVersionUID;
}