JDK5.0新特性-反射

反射:
什么是反射?
剖析Java类中的各个组成部分映射成一个个java对象
为什么用反射？
多用于框架和组件，写出复用性高的通用程序

1.Class类.
Class代表的就是我们的字节码文件.
三种方式.
1.类名.class
2.对象.getClass();
3.Class.forName();

在开发中创建使用第三种 Class.forName();为什么?
这种方式耦合度低，没有依赖性.

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2.Constructor（构造器对象）
构造器的作用就是用来创建对象的。
Constructor代表的是构造器对象，我们得到它后就可以实例化对象.

1.怎样得到Constructor对象.
getConstructor()-----得到指定的构造器
getConstructors();---得到所有的构造器


2.关于Constructor的注意事项
1.如果构造方法不是public。那么要获取其构造器对象使用 getDeclaredConstuctor()方法.

2.对于私有化权限，在使用前必须设置权限检查.
public void setAccessible(boolean flag)
值为 true 则指示反射的对象在使用时应该取消 Java 语言访问检查

3.在开发中，我们在得到Class时，一般会直接实例化类的对象.

2.Field

3.Method

1.

2.对于static方法怎样调用?
静态方法调用时不需要使用对象，直接写null
staticMethod.invoke(null,参数);
方法的参数如果是数组怎样调用?

在调用invoke时，第二个参数强制转换成Object，或在外面在包装一层数组.

1

package cn.itcast.reflect;

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

import org.junit.Test;

public class Demo1 {

    @Test
    public void demo1() throws ClassNotFoundException, InstantiationException,
            IllegalAccessException {
        Class clazz = Class.forName("cn.itcast.reflect.Teacher");
        clazz.newInstance();

        Teacher t = new Teacher();
    }

    // 获取Class对象
    @Test
    public void demo2() throws ClassNotFoundException {

        // 1.通过.class
        // Class clazz=Teacher.class;

        // 2.通过对象获取
        // Teacher t = new Teacher();
        // Class clazz = t.getClass(); //getClass方法是在Object类中定义的

        // 3.Class.forName()

        Class clazz = Class.forName("cn.itcast.reflect.Teacher");

    }

    // 获取构造器.
    @Test
    public void demo3() throws SecurityException, NoSuchMethodException,
            IllegalArgumentException, InstantiationException,
            IllegalAccessException, InvocationTargetException {

        // 1.得到Teacher类的Class对象.
        Class clazz = Teacher.class;

        // 2.得到Teacher类的构造器.
        Constructor c1 = clazz.getConstructor(); // 无参数构造
        Constructor c2 = clazz.getConstructor(String.class); // String类型参数构造
        Constructor c3 = clazz.getConstructor(String.class, int.class); // String,int类型参数构造

        // 3.通过Constructor对象创建Teacher类的实例.
        // Teacher obj1 = (Teacher) c1.newInstance(); // 相当于new Teacher();
        //
        // obj1.show();

        // Teacher obj2 = (Teacher) c2.newInstance("张三");// new Teacher("张三");
        //
        // obj2.show();

        Teacher obj3 = (Teacher) c3.newInstance("李四", 20);// new
                                                            // Teacher("李四",20);

        obj3.show();

    }

    // 关于Constructor的注意事项
    @Test
    public void demo4() throws SecurityException, NoSuchMethodException,
            IllegalArgumentException, InstantiationException,
            IllegalAccessException, InvocationTargetException {
        // 1.得到Teacher类的Class对象.
        Class clazz = Teacher.class;
        // 2.获取非public的构造器对象.
        Constructor c = clazz.getDeclaredConstructor(String.class, int.class);

        // 取消安全检查
        c.setAccessible(true);

        // 3.创建Teacher对象
        Teacher obj = (Teacher) c.newInstance("李四", 20);

        obj.show();

    }

    @Test
    public void demo5() throws InstantiationException, IllegalAccessException {
        Class clazz = Teacher.class;

        Teacher t = (Teacher) clazz.newInstance();// 要求必须存在无参构造
    }

    // 关于Field类.---它代表的是属性.
    @Test
    public void demo6() throws SecurityException, NoSuchFieldException, IllegalArgumentException, IllegalAccessException, InstantiationException {
        // Field对象的获取也是通过Class
        Class clazz = Teacher.class;
        Field fname = clazz.getDeclaredField("name"); //相当于得到了String name属性.
        
        fname.setAccessible(true);
        
        //得到属性值.
        Teacher t=(Teacher) clazz.newInstance();
        //Object msg=fname.get(t); //相当于   t.name;
        
        
        //对属性值进行赋值操作.
        
        fname.set(t, "abcdef"); //t.name="abcdef";
        
        System.out.println(t.getName());
    }
}

2

package cn.itcast.reflect;

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

import org.junit.Test;

//Method操作
public class Demo2 {

    // 得到Teacher类中的show方法的Metho对象.
    @Test
    public void fun1() throws SecurityException, NoSuchMethodException,
            IllegalArgumentException, IllegalAccessException,
            InvocationTargetException, InstantiationException {
        // 1.得到Class
        Class c = Teacher.class;

        // 2.得到Method
        // Method m1 = c.getMethod("show"); // 获取show();

        Method m2 = c.getMethod("show", String.class);

        // 3.让方法执行.
        Teacher t = (Teacher) c.newInstance(); // Teacher t=new Teacher();
        // m1.invoke(t); //相当于 t.show();

        m2.invoke(t, "good");

    }

    @Test
    public void fun2() throws SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException, InstantiationException {
        // 1.得到Class
        Class c = Teacher.class;

        // 2.得到Method
        Method m=c.getMethod("show", String.class, int.class);
        
        //3.让方法执行.
        Object returnValue=m.invoke(c.newInstance(), "tom",20); //String msg=t.show("tom",20);
        
        System.out.println(returnValue);

    }
}

3

package cn.itcast.reflect;

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

import org.junit.Test;

public class Demo3 {

    public static void main(String[] args) {

        // 要通过反射去调用main方法.并传递参数.

        System.out.println(args[0]);

        System.out.println(args[1]);

    }

    @Test
    public void fun() throws SecurityException, NoSuchMethodException,
            IllegalArgumentException, IllegalAccessException,
            InvocationTargetException {
        // 1.得到当前类的Class

        Class clazz = Demo3.class;
        // 2.得到这个类中的main方法的Method

        Method main = clazz.getMethod("main", String[].class);

        // 3.调用main方法. invoke。

        //main.invoke(null, new String[]{"hello", "world"}); // 参数个数错误
        
        main.invoke(null,new Object[]{new String[]{"hello", "world"}});
        
        main.invoke(null, (Object)(new String[]{"hello", "world"}));
    }
}

4

package cn.itcast.reflect;

public class Teacher {
    public int age = 10;
    private String name = "tom";

    public Teacher() {
        System.out.println("Teacher无参构造方法");
    }

    public Teacher(String name) {
        this.name = name;
        System.out.println("Teacher构造方法----String name");
    }

    private Teacher(String name, int age) {
        this.name = name;
        this.age = age;
        System.out.println("Teacher构造方法----String name,int age");
    }

    public String getName() {
        return name;
    }

    public void show() {
        System.out.println("name:" + name + "  age:" + age);
    }

    public void show(String msg) {
        System.out.println(msg);
    }
    
    public String show(String msg,int i){
        return msg+"   "+i;
    }

}