注解与反射

注解

可以被其他程序（比如：编译器等）读取

@SuppressWarnings("all")  //镇压警告@Override  //重写@Deprecated  //不推荐程序员使用或存在更好的方式，但可以使用

package com.company.annotation;import java.lang.annotation.*;//测试元注解@MyAnnotationpublic class Test01 {    public void test(){    }}//定义一个注解//Target 表示我们的注解可以用在哪些地方@Target(value = {ElementType.METHOD,ElementType.TYPE})//Retention  表示我们的注解在什么地方还有效@Retention(value = RetentionPolicy.RUNTIME)//Documented  表示是否将我们的注解生成在JAVAdoc中@Documented//Inherited  子类可以继承父类的注解@Inherited@interface MyAnnotation{}

自定义注解

package com.company.annotation;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;public class Test02 {    @MyAnnotation2    public void test(){}    @MyAnnotation3("YRZ")    public void test2(){}}@Target({ElementType.METHOD,ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@interface MyAnnotation2{    //注解的参数：参数类型 + 参数名();    String name() default "";    int age() default 0;    int id() default -1;  //如果默认值为-1，代表不存在    String[] school() default "清华大学";}@Target({ElementType.METHOD,ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@interface MyAnnotation3{String value(); //只有一个值的情况下}

反射

Reflection

package com.company.reflection;public class Test01 {    public static void main(String[] args) throws ClassNotFoundException { Class c1 = Class.forName("com.company.reflection.User"); System.out.println(c1); Class c2 = Class.forName("com.company.reflection.User"); Class c3 = Class.forName("com.company.reflection.User"); Class c4 = Class.forName("com.company.reflection.User"); //一个类在内存中只有一个Class对象 //一个类被加载后，类的整个结构都会被封装在Class对象中 System.out.println(c2.hashCode()); System.out.println(c3.hashCode()); System.out.println(c4.hashCode());    }}//实体类：pojo、entityclass User{    private String name;    private int age;    private int id;    public User() {    }    public User(String name, int age, int id) { this.name = name; this.age = age; this.id = id;    }    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 int getId() { return id;    }    public void setId(int id) { this.id = id;    }    @Override    public String toString() { return "User{" +  "name='" + name + '\'' +  ", age=" + age +  ", id=" + id +  '}';    }}

得到Class类的几种方式

package com.company.reflection;//测试Class类的创建方式有哪些public class Test02 {    public static void main(String[] args) throws ClassNotFoundException { Person person = new Student(); System.out.println("这个人是："+person.name); //方式一：通过对象获取 Class c1 = person.getClass(); System.out.println(c1.hashCode()); //方式二：forname获得 Class c2 = Class.forName("com.company.reflection.Student"); System.out.println(c2.hashCode()); //方式三：通过类名.class获得 Class c3 = Student.class; System.out.println(c3.hashCode()); //方式四：基本内置类型的包装类都要一个Type属性 Class c4 = Integer.TYPE; System.out.println(c4); //获得父类类型 Class c5 = c1.getSuperclass(); System.out.println(c5);    }}class Person{    public String name;    public Person() {    }    public Person(String name) { this.name = name;    }    @Override    public String toString() { return "Person{" +  "name='" + name + '\'' +  '}';    }}class Student extends Person{    public Student(){ this.name = "学生";    }}class Teacher extends Person{    public Teacher(){ this.name = "老师";    }}

分析类初始化

类加载器

package com.company.reflection;public class Test04 {    public static void main(String[] args) throws ClassNotFoundException { //获取系统类加载器 ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader(); System.out.println(systemClassLoader); //获取系统类加载器的父类加载器-->扩展类加载器 ClassLoader parent = systemClassLoader.getParent(); System.out.println(parent); //获取扩展类加载器的父类加载器-->根加载器（c/c++） ClassLoader parent1 = parent.getParent(); System.out.println(parent1); //测试当前类是被哪个加载器加载的 ClassLoader classLoader = Class.forName("com.company.reflection.Test04").getClassLoader(); System.out.println(classLoader); //测试JDK内置的类是谁加载的 ClassLoader classLoader1 = Class.forName("java.lang.Object").getClassLoader(); System.out.println(classLoader1); //如何获得系统类加载器可以加载的路径 System.out.println(System.getProperty("java.class.path")); // D:\bishe\jdk8\jre\lib\charsets.jar; // D:\bishe\jdk8\jre\lib\deploy.jar; // D:\bishe\jdk8\jre\lib\ext\access-bridge-64.jar; // D:\bishe\jdk8\jre\lib\ext\cldrdata.jar; // D:\bishe\jdk8\jre\lib\ext\dnsns.jar; // D:\bishe\jdk8\jre\lib\ext\jaccess.jar; // D:\bishe\jdk8\jre\lib\ext\jfxrt.jar; // D:\bishe\jdk8\jre\lib\ext\localedata.jar; // D:\bishe\jdk8\jre\lib\ext\nashorn.jar; // D:\bishe\jdk8\jre\lib\ext\sunec.jar; // D:\bishe\jdk8\jre\lib\ext\sunjce_provider.jar; // D:\bishe\jdk8\jre\lib\ext\sunmscapi.jar; // D:\bishe\jdk8\jre\lib\ext\sunpkcs11.jar; // D:\bishe\jdk8\jre\lib\ext\zipfs.jar; // D:\bishe\jdk8\jre\lib\javaws.jar; // D:\bishe\jdk8\jre\lib\jce.jar; // D:\bishe\jdk8\jre\lib\jfr.jar; // D:\bishe\jdk8\jre\lib\jfxswt.jar; // D:\bishe\jdk8\jre\lib\jsse.jar; // D:\bishe\jdk8\jre\lib\management-agent.jar; // D:\bishe\jdk8\jre\lib\plugin.jar; // D:\bishe\jdk8\jre\lib\resources.jar; // D:\bishe\jdk8\jre\lib\rt.jar; // D:\IDEA\net\study\out\production\study; // D:\IDEA\IntelliJ IDEA 2020.1\lib\idea_rt.jar    }}

获取运行时类的结构

package com.company.reflection;import java.lang.reflect.Constructor;import java.lang.reflect.Field;import java.lang.reflect.Method;//获得类的信息public class Test05 {    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException { Class c1 = Class.forName("com.company.reflection.User"); //获得类的名字 System.out.println(c1.getName()); System.out.println(c1.getSimpleName()); //获得类的属性 System.out.println("======================================="); Field[] fields1 = c1.getFields(); //只能找到public属性 Field[] fields = c1.getDeclaredFields(); //找到全部的属性 for (Field field : fields) {     System.out.println(field); } Field name = c1.getDeclaredField("name"); //获得指定属性的值 System.out.println(name); //获得类的方法 System.out.println("======================================="); Method[] methods = c1.getMethods(); //获得本类及其父类的全部public方法 for (Method method : methods) {     System.out.println("正常的："+method); } Method[] declaredMethods = c1.getDeclaredMethods(); //获得本类的全部方法 for (Method declaredMethod : declaredMethods) {     System.out.println("declaredMethod: "+declaredMethod); } //获得指定方法 Method method = c1.getMethod("getName",null); Method method1 = c1.getMethod("setName",String.class); System.out.println(method); System.out.println(method1); //获得指定的构造器 System.out.println("======================================="); Constructor[] constructors = c1.getConstructors(); for (Constructor constructor : constructors) {     System.out.println(constructor); } Constructor[] declaredConstructors = c1.getDeclaredConstructors(); for (Constructor declaredConstructor : declaredConstructors) {     System.out.println(declaredConstructor); } //获得指定构造器 Constructor declaredConstructor = c1.getDeclaredConstructor(String.class,int.class,int.class); System.out.println(declaredConstructor);     }}

有了Class对象，能做什么？

package com.company.reflection;import java.lang.reflect.Constructor;import java.lang.reflect.Field;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;//动态创建对象，通过反射public class Test06 {    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException { //获得class对象 Class c1 = Class.forName("com.company.reflection.User"); //构造一个对象 User user = (User) c1.newInstance(); //本质是调用了类的无参构造 System.out.println(user); //通过构造器创建对象 Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class); User user2 = (User)constructor.newInstance("未激活", 01, 18); System.out.println(user2); //通过反射调用普通方法 User user3 = (User) c1.newInstance(); //通过反射获取一个方法 Method setname = c1.getDeclaredMethod("setName", String.class); //invoke:激活的意思 Object invoke = setname.invoke(user3, "激活成功"); //(对象,"方法的值") System.out.println(user3.getName()); //通过反射操作属性 System.out.println("==========================="); User user4 = (User) c1.newInstance(); Field name = c1.getDeclaredField("name"); //不能直接操作私有属性，我们需要关闭程序的安全检测，属性或者方法的setAccessible(true) name.setAccessible(true); name.set(user2,"激活2"); System.out.println(user2.getName());    }}

性能对比分析

package com.company.reflection;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;public class Test07 {    //普通方式调用    public static void test01(){ User user = new User(); long startTime = System.currentTimeMillis(); for (int i = 0; i < 1000000000; i++) {     user.getName(); } long endTime = System.currentTimeMillis(); System.out.println("普通方式执行10亿次："+(endTime-startTime)+"ms");    }    //反射方式调用    public static void test02() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException { User user = new User(); Class c1 = user.getClass(); Method getName = c1.getDeclaredMethod("getName", null); long startTime = System.currentTimeMillis(); for (int i = 0; i < 1000000000; i++) {     getName.invoke(user,null); } long endTime = System.currentTimeMillis(); System.out.println("反射方式执行10亿次："+(endTime-startTime)+"ms");    }    //反射方式调用  关闭检测    public static void test03() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException { User user = new User(); Class c1 = user.getClass(); Method getName = c1.getDeclaredMethod("getName", null); getName.setAccessible(true); long startTime = System.currentTimeMillis(); for (int i = 0; i < 1000000000; i++) {     getName.invoke(user,null); } long endTime = System.currentTimeMillis(); System.out.println("关闭检测执行10亿次："+(endTime-startTime)+"ms");    }    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException { test01(); test02(); test03();    }}