java反射机制学习

java反射介绍

JAVA反射机制是在运行状态中，对于任意一个类，都能够知道这个类的所有属性和方法；对于任意一个对象，都能够调用它的任意一个方法和属性；这种动态获取的信息以及动态调用对象的方法的功能称为java语言的反射机制。主要类包括以下

java.lang.Class;                

java.lang.reflect.Constructor; java.lang.reflect.Field;        

java.lang.reflect.Method;

java.lang.reflect.Modifier;

Class作用是通过类加载机制把相关类加载到虚拟机并返回Object对象句柄
Field对Object的所有属性进行修改和访问
Method对Object的所有方法进行访问
Modifier返回类或者其成员的访问修饰符
直接建立例子学习，说明在代码注释上，哈哈

package reflect;

public class Student {

	private int id = 1;

	private int age = 23;

	private String name = "Lin";

	protected int gade = 80;

	private Funning mFun = new Funning("pingpong");

	String dec = "good student";

	public static Student getInstance() {
		return new Student();
	}

	public Student(int age, String name, Funning mFun) {
		super();
		this.age = age;
		this.name = name;
		this.mFun = mFun;
	}

	public void show() {
		System.out.print(" id=" + id + " name = " + name + " age = " + age + " funning = " + mFun.getName() + " gede = "
				+ gade + " " + " dec=" + dec);
	}

	public Student() {
		super();

	}

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Funning getmFun() {
		return mFun;
	}

	public void setmFun(Funning mFun) {
		this.mFun = mFun;
	}

	private class Funning {
		private String name;

		public Funning(String fun) {
			name = fun;
		}

		public String getName() {
			return name;
		}

		public void setName(String name) {
			this.name = name;
		}

	}

}

基本的测试代码为：

public static void main(String[] args) {
		// TODO Auto-generated method stub
		try {
		     //通过绝对的类名路劲加载类
			Class student_class = Class.forName("reflect.Student");  
			//获取类的指定构造函数，其中int类型的为int.class而不Integer.class，而对于构造函数里面含有内部类，Funning为Student的内部类，所以完整的类名路劲为Student的绝对路径“reflect.Student”+“$”+内部类名
			Constructor student_constructor = student_class.getConstructor(int.class, String.class,Class.forName("reflect.Student$Funning"));  
			//内部类的加载
			Class funning_class = Class.forName("reflect.Student$Funning");
			//内部类的构造函数默认都有对其外部类的指针，所以构造函数的第一个参数默认值为其寄存的类
			Constructor funning_constructor = funning_class.getConstructor(student_class,String.class);
			//先实例化内部类还是先实例外部类需要根据实际情况分析，若外部类需要根据内部类的数据提供支撑先就先实例内部类，否则实例外部类
			Object funning = funning_constructor.newInstance(null,"basketball");
			Object student = student_constructor.newInstance(23,"LZH",funning);
			if (student != null) {
			    //反射Sudent的方法show,getMethod返回的是修饰符为public的方法，getDeclaredMethod不区分修饰符
				Method show = Student.class.getDeclaredMethod("show");
				//运行Sudent的公有方法show
				show.invoke(student);
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

默认输出结果为

id=1 name = Lin age = 23 funning = pingpong gede = 80 dec=good student

1 访问私有类并改变其属性，把funning的输出由pingpong改成football

public static void main(String[] args) {
		// TODO Auto-generated method stub
		try {
			// Student.getInstance().show()
			Class student_class = Class.forName("reflect.Student"); 
			Method method = student_class.getDeclaredMethod("getInstance");
			Object student = method.invoke(null);
			if(student != null){
			 Method show = Student.class.getDeclaredMethod("show");
			 //Begin
			    Field mFun_field = student_class.getDeclaredField("mFun");
				mFun_field.setAccessible(true);   //设置私有变量为可读
				Object mFun = mFun_field.get(student);
				Field name_field = funning_class.getDeclaredField("name");
				name_field.setAccessible(true);
				name_field.set(mFun,new String("football")); //由默认的pingpong改为football
			 //End
			 show.invoke(student);
			}
		} catch (Exception e) {
            e.printStackTrace();
		}
	}

输出结果为：

id=1 name = LZH age = 23 funning = football gede = 80 dec=good student

注意：
1 静态变量或方法的反射，直接在set(Object,Object)或者get(Object,Object)或者invoke(Object,…parameter)等第一个表示实例对象的参数设置为null就行
2 final变量的修饰由于在代码编译过程会直接被替换，比如

final int i = 1;
int j = i + i;

编译会回直接变成
final int i = 1;
int j = 1 + 1;

android 通过反射修改Toast的显示层级，由WindowManager.LayoutParams.TYPE_TOAST 提升到WindowManager.LayoutParams.TYPE_SYSTEM_ERROR，即使锁屏也可以看到Toast提示

try{
           Class toast_class = Class.forName("android.widget.Toast");
           Class TN_class = Class.forName("android.widget.Toast$TN");
           Method maketext = toast_class.getDeclaredMethod("makeText",Context.class,CharSequence.class,int.class);
           Object mToast = maketext.invoke(null,getActivity(),tips,Toast.LENGTH_LONG);
           Method show = toast_class.getDeclaredMethod("show");
           Field mTN_field = toast_class.getDeclaredField("mTN");
           mTN_field.setAccessible(true);
           Object mTN = mTN_field.get(mToast);

           Field lp_field = TN_class.getDeclaredField("mParams");
           lp_field.setAccessible(true);
           WindowManager.LayoutParams lp = (WindowManager.LayoutParams )lp_field.get(mTN);
           lp.type = WindowManager.LayoutParams.TYPE_SYSTEM_ERROR;
           show.invoke(mToast);


       }catch (Exception e){
          e.printStackTrace();
       }

所以final的属性的反射暂时发现是不能修改的，若有更好的方法欢迎大家提出，谢谢