Java 反射，开发框架必备技能

反射一般开发者接触不到，反射主要用户框架的开发。例如我举一个例子你就明白了：

http://www.netkiller.cn/news/list/2.html

通过反射技术我们将上面的统一资源定位付(URL) 映射到Class 相当于

class: news

method: list

parameter: 2

差不多就是下面样子

class News{

public String list(String catagory_id){

...

}

我们只需要在框架核心中分析 url 然后调用对应的方法，于此同时将参数传递过去。

Class<?> cls = Class.forName("cn.netkiller.reflect.News");
		Object obj= cls.newInstance();
		Method method = cls.getDeclaredMethod("list", int.class);
		return method.invoke(obj, 2);

本文节选自《Netkiller Java 手札》

1.9. Reflection 反射

this.getClass().getName() //当前Class名字
Thread.currentThread().getStackTrace()[1].getMethodName()); //当前方法名

1.9.1. 获得所有变量

	Field[] fields = objClass.getFields();
	for (Field field : fields) {
		System.out.println(field.getName());
	}

注意：只能去除 public变量

1.9.2. 批量赋值

1.9.3. 方法操作

JAVA反射调用方法的步骤有三步

得到要调用类的class
得到要调用的类中的方法(Method)
方法调用(invoke)

1.9.3.1. 获得所有方法

	Class<?> objClass = a.getClass();
	Method[] methods =  objClass.getDeclaredMethods();
	for (Method method : methods) {
		System.out.println(method);
	}

1.9.3.2. set/get 方法

			package cn.netkiller.reflect;

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

public class Member {
	public String name;
	private int age;

	public String getName() {
		return name;
	}

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

	public int getAge() {
		return age;
	}

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

	@Override
	public String toString() {
		return "ClassA [name=" + name + ", age=" + age + "]";
	}

	public Member() {
		// TODO Auto-generated constructor stub
	}

	public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException {
		Class<?> cls = Class.forName("cn.netkiller.reflect.Member");
		Object member = cls.newInstance();
		Method setMethod = cls.getDeclaredMethod("setAge", int.class);
		setMethod.invoke(member, 15);

		Method getMethod = cls.getDeclaredMethod("getAge");
		System.out.println(getMethod.invoke(member));

	}

}

下面做一个稍微复杂点的例子，ClassB继承ClassA，取出ClassA的成员变量赋值到ClassA。

			package cn.netkiller.reflect;

public class ClassA {
	public String name;
	private int age;

	public String getName() {
		return name;
	}

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

	public int getAge() {
		return age;
	}

	public void setAge(int age) {
		this.age = age;
	}
	
	public ClassA() {
		// TODO Auto-generated constructor stub
	}

	@Override
	public String toString() {
		return "ClassA [name=" + name + ", age=" + age + "]";
	}
}

package cn.netkiller.reflect;

public class ClassB extends ClassA{

	public ClassB() {
		// TODO Auto-generated constructor stub
	}
	private String address;

	public String getAddress() {
		return address;
	}

	public void setAddress(String address) {
		this.address = address;
	}

	@Override
	public String toString() {
		return "ClassB [address=" + address + "]";
	}
	
}


package cn.netkiller.reflect;

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

public class ReflectionTest {

	public ReflectionTest() {
		// TODO Auto-generated constructor stub
	}

	public void testSetMethod() throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, InstantiationException {

		// ClassA a = new ClassA();

		ClassB b = new ClassB();
		b.setAddress("Shenzhen");

		Class<ClassA> classA = ClassA.class;
		ClassA a = classA.newInstance();
		a.setName("Neo");
		a.setAge(30);

		System.out.println(classA.getDeclaredMethod("getAge").invoke(a));

		Method m = classA.getDeclaredMethod("setAge", int.class);
		m.setAccessible(true); // 因为写成private 所以这里必须设置
		m.invoke(b, 26);

		System.out.println(a.toString());
		System.out.println(b.toString());

		System.out.println(b.getName());
		System.out.println(b.getAge());
	}

	public static void main(String[] args) throws InvocationTargetException {

		ReflectionTest rt = new ReflectionTest();
		try {
			rt.testSetMethod();
			
		} catch (NoSuchMethodException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (SecurityException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalArgumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InstantiationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
	}

}

set 方法

			System.out.println(classA.getDeclaredMethod("getAge").invoke(a));

get 方法

	Method m = classA.getDeclaredMethod("setAge", int.class);
	m.setAccessible(true);	//因为写成private 所以这里必须设置
	m.invoke(b, 26);

1.9.3.3. static 方法调用

			Class cls = Class.forName("cn.netkiller.reflect.Student");
Method setMethod = cls.getDeclaredMethod("setAge",int.class);
setMethod.invoke(cls.newInstance(), 15);

static 方法调用时，不必得实例化对象

			Class cls = Class.forName("cn.netkiller.reflect.Student");
Method staticMethod = cls.getDeclaredMethod("setAge",int.class);
staticMethod.invoke(cls,20); //这里不需要newInstance