java反射基礎應用備註

反射機制應用很普遍。這裏簡單備註下

package com.shone.ailin;

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

public class Reflectdemo {
    
    public static void main(String[] args) throws Exception {
        
        
        printVar();
        
        createInstance();

        printAllVarName();
        
    }
    
    //打印全部成員變量名
    private static void printAllVarName() {
        // TODO Auto-generated method stub
        Point p = new Point(1,3);
        Field[] fields = p.getClass().getDeclaredFields();
        for(int i=0;i<fields.length;i++) {
            System.out.println("var["+i+"]="+fields[i].getName());
        }
        
    }

    //打印成員變量的值
    private static void printVar() throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {
        Point p = new Point(1,4);
        Field fX = p.getClass().getDeclaredField("x");
        fX.setAccessible(true); //這裏必需要加上 。否則會拋出異常。由於是私有成員變量，因此，須要暴力的反射
        int x = (int)fX.get(p);
        System.out.println("x="+x);
        
    }

    //建立實例化對象並調用方法
    private static void createInstance() throws Exception, SecurityException {
        Constructor<Point> con = Point.class.getConstructor(int.class,int.class);
        try {
            Point instance = con.newInstance(1,3);
            
            Method m = instance.getClass().getDeclaredMethod("addCount", int.class,int.class);
            m.setAccessible(true);
            try {
                int count = (int) m.invoke(instance,2,7);
                System.out.println(count);
                
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            } catch (IllegalArgumentException e) {
                e.printStackTrace();
            } catch (InvocationTargetException e) {
                e.printStackTrace();
            }
            
            
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (SecurityException e) {
            e.printStackTrace();
        }
    }
    

}

Point類

package com.shone.ailin;

public class Point {
    private int x ;
    public int y ;
    
    public Point(int x ,int y ) {
        // TODO Auto-generated constructor stub
        super();
        this.x = x;
        this.y = y;
    }
    
    private int addCount(int x,int y) {
        return x+y;
    }

}