<反射>
1.Class
.class文件加载到内存中就是一个Class对象
获取Class对象的方式有3种:
Class.forName();
类名.class
对象.getClass()
例如：
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public class Demo1_Reflect {
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public static void main(String[] args) throws ClassNotFoundException {
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Class<?> clazz1 = Class.forName("cn.itcast.bean.Person");
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Class<?> clazz2 = Person.class;
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Person p = new Person();
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Class<?> clazz3 = p.getClass();
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System.out.println(clazz1 == clazz2);
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System.out.println(clazz2 == clazz3);
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}
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}
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public class Demo2_Reflect {
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public static void main(String[] args) throws Exception {
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BufferedReader br = new BufferedReader(new FileReader("config.txt"));
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Class<?> clazz = Class.forName(br.readLine());
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Fruit f = (Fruit) clazz.newInstance();
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Juicer j = new Juicer();
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j.run(f);
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}
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}
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interface Fruit {
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public void squeeze();
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}
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class Apple implements Fruit {
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public void squeeze() {
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System.out.println("榨出一杯苹果汁儿");
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}
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}
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class Orange implements Fruit {
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public void squeeze() {
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System.out.println("榨出一杯桔子汁儿");
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}
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}
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class Juicer {
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public void run(Fruit f) {
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f.squeeze();
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}
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}
2.Constructor
Class类的newInstance()方法是使用该类无参的构造函数创建对象
调用Class类的getConstructor(String.class,int.class)方法获取一个指定的构造函数，然后再调用Constructor类的newInstance("张三",20)方法创建对象
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public class Demo3_Constructor {
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public static void main(String[] args) throws Exception {
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Class<?> clazz = Class.forName("cn.itcast.bean.Person");
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Person p = (Person) clazz.newInstance();
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System.out.println(p);
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Constructor con = clazz.getConstructor(String.class,int.class);
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Person p = (Person) con.newInstance("张三",23);
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System.out.println(p);
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}
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}
3.Field
Class.getField(String)方法可以获取类中的指定字段(可见的), 如果是私有的可以用getDeclaedField("name")方法获取
通过set(obj, "李四")方法可以设置指定对象上该字段的值, 如果是私有的需要先调用setAccessible(true)设置访问权限
用获取的指定的字段调用get(obj)可以获取指定对象中该字段的值
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public class Demo4_Field {
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public static void main(String[] args) throws Exception {
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Class<?> clazz = Class.forName("cn.itcast.bean.Person");
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Constructor con = clazz.getConstructor(String.class,int.class);
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Person p = (Person) con.newInstance("张三",23);
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Field f = clazz.getDeclaredField("name");
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f.setAccessible(true);
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f.set(p, "李四");
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System.out.println(p);
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}
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}
4.Method
Class.getMethod(String, Class...) 和 Class.getDeclaredMethod(String, Class...)方法可以获取类中的指定方法
调用invoke(Object, Object...)可以调用该方法
Class.getMethod("eat") invoke(obj) Class.getMethod("eat",int.class) invoke(obj,10)
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public class Demo5_Method {
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public static void main(String[] args) throws Exception {
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Class<?> clazz = Class.forName("cn.itcast.bean.Person");
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Constructor con = clazz.getConstructor(String.class,int.class);
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Person p = (Person) con.newInstance("张三",23);
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Method m = clazz.getMethod("eat");
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m.invoke(p);
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Method m2 = clazz.getMethod("eat", int.class);
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m2.invoke(p, 10);
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}
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}
5.泛型反射
创建一个集合对象,泛型为String类型,需要向这个集合中添加Integer对象
泛型在编译时存在,运行时会被擦除
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public class Test3 {
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public static void main(String[] args) throws Exception {
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ArrayList<String> list = new ArrayList<>();
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list.add("黑");
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list.add("马");
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list.add("程");
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list.add("员");
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Class<?> clazz = Class.forName("java.util.ArrayList");
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Method m = clazz.getMethod("add", Object.class);
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m.invoke(list, 521);
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m.invoke(list, true);
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System.out.println(list);
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}
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}
