java反射机制

什么是java反射

就是正在运行，动态获取这个类的所有信息。

反射机制的作用

把.class -> .java    通过反射机制访问java对象的属性，方法，构造方法等。

反射机制的应用场景

jdbc加载驱动     springioc    框架

反射创建对象的方式

public class Person {
	private String name;
	private String age;
	public Person() {}
	
	public Person(String name, String age) {
		super();
		this.name = name;
		this.age = age;
	}

	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getAge() {
		return age;
	}
	public void setAge(String age) {
		this.age = age;
	}
}

public static void main(String[] args) throws Exception {
	//反射无参创建对象
	Class<?> forName = Class.forName("com.shuwenlong.Person");
	Person person = (Person) forName.newInstance();
	//反射有参创建对象
	Class<?> forName2 = Class.forName("com.shuwenlong.Person");
	Constructor<?> constructor = forName2.getConstructor(String.class,String.class);
	Person person1 = (Person) constructor.newInstance("wenlong","21");
}

   反射创建api

方法名称	作用
getDeclaredMethods[]	获取该类的所有方法
getReturnType()	获取该类的返回值
getParameterTypes()	获取传入参数
getDeclaredFields()	获取该类的所有字段
setAccessible	允许访问私有成员

使用反射为类私有属性赋值

public static void main(String[] args) throws Exception {
		Class<?> clazz = Class.forName("com.shuwenlong.Person");
		//获取类中的所有属性
		Field[] fields = clazz.getDeclaredFields();
		for (Field f : fields) {
			System.out.println(f.getName());
		}
		//获取类中所有方法
		Method[] methods = clazz.getDeclaredMethods();
		for (Method m : methods) {
			System.out.println(m.getName());
		}
		//初始化对象
		Person person = (Person) clazz.newInstance();
		Field id = clazz.getDeclaredField("age");
		//标记为true，允许为私有变量赋值
		id.setAccessible(true);
		id.set(person, "21");
		
		System.out.println(person.getAge());
		
	}

禁止反射机制初始化

将构造函数私有化

手写SpingIOC框架

什么是springioc就是把每一个实体类与实体类之间的关系交给第三方容器管理。

//实体类
public class User {
	private String name;
	private String age;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getAge() {
		return age;
	}
	public void setAge(String age) {
		this.age = age;
	}
	public String toString() {
		return "User [name=" + name + ", age=" + age + "]";
	}	
}

//xml配置
<?xml version="1.0" encoding="UTF-8"?>
<beans>
	<bean id="user1" class="com.shuwenlong.User">
		<property name="name" value="文龙"></property>
		<property name="age" value="21"></property>
	</bean>
	<bean id="user2" class="com.shuwenlong.User">
		<property name="name" value="bug"></property>
		<property name="age" value="100"></property>
	</bean>
</beans>

//手写springioc
class MySpringIOC {
	
	private String path;
	
	public MySpringIOC(String path) {
		this.path = path;
	}
	
	public  Object getBeans(String beanName) throws Exception {
		if(beanName == null || beanName.equals("")) {
			return "传入参数不能为空";
		}
		SAXReader saxReader = new SAXReader();
		Document root = saxReader.read(this.getClass().getClassLoader().getResource(path));
		Element element = root.getRootElement();
		List<Element> elements = element.elements();
		for (Element e : elements) {
			String id = e.attributeValue("id");
			if(!id.equals(beanName)) {
				continue;
			}else {
				String c = e.attributeValue("class");
				Class<?> clazz = Class.forName(c);
				Object newInstance = clazz.newInstance();
				List<Element> el = e.elements();
				for (Element ele : el) {
					String name = ele.attributeValue("name");
					String value = ele.attributeValue("value");
					
					Field nameField = clazz.getDeclaredField(name);
					nameField.setAccessible(true);
					nameField.set(newInstance,value );
					
				}
				return newInstance;
			}
		}
		return null;
	}
	
}

//测试
public class Demo {
	public static void main(String[] args) throws Exception {
		MySpringIOC ms = new MySpringIOC("app.xml");
		User u = (User) ms.getBeans("user1");
		System.out.println(u.toString());
	}
}