注解和反射的学习笔记（狂神说）

注解和反射的学习笔记（狂神说）
注解与反射
什么是注解
和注释⼀样，注解不是程序本⾝，⽽是对程序作出解释，⽽注解与注释不同的点在于，注解可以被其他程序⽐如编译器读取
内置注解
@Override//重写注解
@Deprecated//不推荐使⽤注解，可以使⽤但是⼜风险或者有更好的⽅式
@SuppressWarnings//“镇压”警告注解
元注解
什么是元注解？
元注解的作⽤就是负责注解其它注解，Java提供了3个meta-annotation类型，它们被⽤来提供对其它annotation类型的说明；
这些类型可以在ng.annotation包中可以找到（@Target、@Retention、@Document、@Inherited）；
元注解的作⽤：
1. @Target：⽤来描述注解的使⽤范围（即：被描述的注解可以使⽤在什么地⽅）；
2. @Retention：表⽰什么级别保存该注解信息，⽤于描述注解的⽣命周期（SOURCE < CLASS < RUNTIME)；
3. @Document：说明该注解将被包含在javadoc中；
4. @Inherited：说明⼦类可以继承⽗类中的该注解；
package iss.demo05;
import ng.annotation.ElementType;
import ng.annotation.Retention;
import ng.annotation.RetentionPolicy;
import ng.annotation.Target;
/**
* 测试元注解
*/
@MyAnnotation
public class TestAnnotation {
public static void main(String[] args) {
}
@MyAnnotation
public void test(){}
}
/**
* ⾃定义⼀个注解
* Target表⽰有效的范围，⽬前设置的是在⽅法和类上都有效
* Retention表⽰什么级别时保留该注解，⽬前设置的是运⾏级别
*/
@Target(value = {ElementType.METHOD, ElementType.TYPE})
@Retention(value = RetentionPolicy.RUNTIME)
@interface MyAnnotation{
}
⾃定义注解
package iss.demo05;
import ng.annotation.ElementType;
import ng.annotation.Retention;
import ng.annotation.RetentionPolicy;
import ng.annotation.Target;
public class Test2 {
//注解可以显⽰复制，如果没有默认值，就必须给注解复制
@MyAnnotation1(schools = {"天软","天红"})
public void test(){
}
//如果注解只有⼀个参数可以省略参数名(名字为value才可以)
@MyAnnotation2("")
public void test2(){
}
}
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation1{
//注解的参数：类型+参数名()[default 默认值];
String name() default "";
int age() default 0;
int id() default -1;
String[] schools();
}
@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation2{
//注解的参数：类型+参数名()[default 默认值];
String[] value();
}
反射
动态语⾔和静态语⾔
**动态语⾔**：在运⾏时可以改变其结构：例如新的函数、对象甚⾄代码可以被引进，已有的函数可以被删除或者是其他结构上的变化。

通俗来说就是运⾏时代码可以根据⼀些条件来改变⾃⾝的结构。

主要动态语⾔：Object-C、C#、JavaScript、PH
静态语⾔：与动态语⾔相对应的，运⾏时不能改变其结构，如Java、C、C++Java不是动态语⾔，但是java可以称为是“准动态语⾔”。

即java有⼀定的动态性，可以利⽤反射机制
获得类似动态语⾔的特性。

Java的动态性使得编程时更加灵活。

function f(){
var x="var a=3;var b=5;x=a+b;";
eval(x);
反射是Java被视为动态语⾔的关键
反射功能
优缺点
API
Class类的介绍
[
package iss.demo06;
//什么叫反射
public class Demo04_Reflection {
public static void main(String[] args) throws ClassNotFoundException {
// 通过反射获取类的class对象
Class<?> c = Class.forName("er");
System.out.println(c);
Class<?> c1 = Class.forName("er");
Class<?> c2 = Class.forName("er");
Class<?> c3 = Class.forName("er");
// 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
/**
* public native int hashCode();返回该对象的hash码值
* 注：哈希值是根据哈希算法算出来的⼀个值，这个值跟地址值有关，但不是实际地址值。

*/
System.out.println(c1.hashCode());
System.out.println(c2.hashCode());
System.out.println(c3.hashCode());
}
}
//实体类
class User {
private String name;
private int id;
private int age;
public User() {
}
public User(String name, int id, int age) {
= name;
this.id = id;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
= name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}
获取class的实例
package iss.demo06;
//测试class类的创建⽅式有哪些
public class Demo05_CreateClass {
public static void main(String[] args) throws ClassNotFoundException {
Person person = new Student();
System.out.println("这个⼈是:"+);
//⽅式⼀:通过对象查询
Class c1 = person.getClass();
System.out.println(c1.hashCode());
//⽅式⼆:forname获得
Class c2 = Class.forName("iss.demo06.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,基本类型的包装类都有⼀个Type
Class c4 = Integer.TYPE;
System.out.println(c4);
//获得⽗类类型
Class c5 = c1.getSuperclass();
System.out.println(c5);
}
}
class Person {
String name;
public Person() {
}
public Person(String name) {
= name;
}
@Override
public String toString() {
return "Person{" +
"name=" + name +
'}';
}
}
class Student extends Person {
public Student() {
= "学⽣";
}
}
class Teacher extends Person {
public Teacher() {
= "⽼师";
}
}
哪些类型可以有Class对象？
class:外部类、成员（成员内部类、静态内部类），局部内部类，匿名内部类。

interface：接⼝
[]：数组
enum：枚举
annotation：注解@interface
primitive type：基本数据类型
void
所有类型的class
package iss.demo06;
import ng.annotation.ElementType;
//所有类型的Class
public class Demo06_AllTypeClass {
public static void main(String[] args) {
Class c1 = Object.class; //类
Class c2 = Comparable.class; //接⼝
Class c3 = String[].class; //⼀维数组
Class c4 = int[][].class; //⼆维数组
Class c5 = Override.class; //注解
Class c6 = ElementType.class; //美剧
Class c7 = Integer.class; //基本数据类型
Class c8 = void.class; //void
Class c9 = Class.class; //class
System.out.println(c1);
System.out.println(c2);
System.out.println(c3);
System.out.println(c4);
System.out.println(c5);
System.out.println(c6);
System.out.println(c7);
System.out.println(c8);
System.out.println(c9);
//只要元素类型与维度⼀样,就是同⼀个Class
int[] a = new int[10];
int[] b = new int[100];
System.out.println(a.getClass().hashCode());
System.out.println(b.getClass().hashCode());
}
}
类加载内存分析
package iss.demo06;
//类加载
public class Demo07_ClassLoader {
public static void main(String[] args) {
A a = new A();
System.out.println(A.m);
/**
* 1. 加载到内存,会产⽣⼀个类对应Class对象
* 2. 链接,连接结束后m=0
* 3. 初始化
* <clinit>(){
* System.out.println("A类静态代码块初始化");
* m = 300;
* m = 100;
* }
*/
}
}
class A {
static {
System.out.println("A类静态代码块初始化");
m = 300;
}
static int m = 100;
public A() {
System.out.println("A类⽆参构造初始化");
}
}
分析类初始化
package iss.demo06;
//测试类什么时候会初始化
public class Demo08_Reference {
static {
System.out.println("Main类被加载");
}
public static void main(String[] args) throws ClassNotFoundException {
// 1. 主动调⽤
//Son son = new Son();
// 2.反射也会产⽣主动引⽤
Class.forName("iss.demo06.Son");
//3.不会产⽣类的引⽤的⽅法
//System.out.println(Son.b);
//4.Son[] array = new Son[5];
//5.System.out.println(Son.M);
}M
}
class Father {
static final int b = 2;
static {
System.out.println("⽗类被加载");
}
}
class Son extends Father {
static {
System.out.println("⼦类被加载");
m = 100;
}M
static int m = 300;
static final int M = 1;
}
类加载器
作⽤
ClassLoader
package iss.demo06;
//类加载器
public class Demo09_ClassLoader {
public static void main(String[] args) throws ClassNotFoundException {
//获取系统类的加载器
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 jre1.8.0_261\lib\ext
ClassLoader parent = systemClassLoader.getParent();
System.out.println(parent);
//获取扩展类加载器⽗类加载器-->根加载器(c/c++) jre1.8.0_261\lib\rt.jar
ClassLoader parent1 = parent.getParent();
System.out.println(parent1);
//测试当前类是哪个加载器加载的
ClassLoader classLoader = Class.forName("iss.demo06.Demo09_ClassLoader").getClassLoader(); System.out.println(classLoader);
//测试JDK内置的类是谁加载的
classLoader = Class.forName("ng.Object").getClassLoader();
System.out.println(classLoader);
//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:\Program Files\Java\jdk1.8.0_261\jre\lib\charsets.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\deploy.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\access-bridge-64.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\cldrdata.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\dnsns.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\jaccess.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\jfxrt.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\localedata.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\nashorn.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\sunec.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\sunjce_provider.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\sunmscapi.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\sunpkcs11.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\ext\zipfs.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\javaws.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\jce.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\jfr.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\jfxswt.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\jsse.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\management-agent.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\plugin.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\resources.jar;
D:\Program Files\Java\jdk1.8.0_261\jre\lib\rt.jar;
D:\Users\Administrator\IdeaProjects\untitled\out\production\untitled;
D:\Program Files\JetBrains\IntelliJ IDEA 2019.3.1\lib\idea_rt.jar
*/
//双亲委派机制检测安全性你写的类和跟加载器⼀样的不会⽤你写的类
//ng.String -->往上推
}
}
获取类的运⾏时结构
概念
代码
package iss.demo06;
import ng.reflect.Constructor;
import ng.reflect.Field;
import ng.reflect.Method;
//获取类的信息
public class Demo10_ClassInfo {
public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
Class c1 = Class.forName("er");
User user = new User();
c1 = user.getClass();
//获得类的名字
System.out.println(c1.getName());// 获得包名 + 类名
System.out.println(c1.getSimpleName());// 获得类名
System.out.println("=======================");
//获得类的属性
Field[] fields = c1.getFields();//只能找到public属性
for (Field field : fields) {
System.out.println("getFields:" + field);
}
fields = c1.getDeclaredFields();//找到全部的属性
for (Field field : fields) {
System.out.println("getDeclaredFields:" + field);
}
//获得指定属性的值
Field name = c1.getDeclaredField("name");
System.out.println(name);
System.out.println("=======================");
//获得类的⽅法
Method[] methods = c1.getMethods(); //获得本类及⽗类的全部public⽅法
for (Method method : methods) {
System.out.println("getMethods:" + method);
}
methods = c1.getDeclaredMethods(); //获得本类的所有⽅法(包括私有)
for (Method method : methods) {
System.out.println("getDeclaredMethods:" + method);
}
System.out.println("=======================");
//获得指定的⽅法
//重载
Method getName = c1.getMethod("getName", null);
Method setName = c1.getMethod("setName", String.class);
System.out.println(getName);
System.out.println(setName);
//获得类的构造器
System.out.println("=======================");
Constructor[] constructors = c1.getConstructors();
for (Constructor constructor : constructors) {
System.out.println("getConstructors:" + constructor);
}
constructors = c1.getDeclaredConstructors();
for (Constructor constructor : constructors) {
System.out.println("getDeclaredConstructors:" + constructor);
}
//获得指定的构造器
Constructor declaredConstructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
System.out.println("指定构造器" + declaredConstructor);
}
}
//********以下笔记**************
Class c1 = Class.forName(“er”); //获取当前对象的Class
//获得类的名字
c1.getName();// 获得包名 + 类名
c1.getSimpleName();// 获得类名
//获得类的⽅法
c1.getMethods(); //获得本类及⽗类的全部public⽅法
c1.getDeclaredMethods(); //获得本类的所有⽅法
c1.getMethod(“getName”, null);//获得指定的⽅法
//获得类的构造器
c1.getConstructors();
c1.getDeclaredConstructors();
c1.getDeclaredConstructor(String.class, int.class, int.class);//获得指定的构造器
动态创建对象执⾏⽅法
newInstance()
getDeclaredConstructor
package com.iss.demo06;
import ng.reflect.Constructor;
import ng.reflect.Field;
import ng.reflect.InvocationTargetException;
import ng.reflect.Method;
//动态创建对象，通过反射
public class Demo08 {
public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
//获得class对象
Class c1 = Class.forName("er");
//创建⼀个对象
System.out.println("============================");
User user = (User)c1.newInstance();//本质是调⽤了类的⽆参构造器
System.out.println(user);
//通过构造器创建对象
System.out.println("============================");
Constructor constructor = c1.getDeclaredConstructor(String.class,int.class,int.class);
User user2 = (User)constructor.newInstance("天天",22,55);
System.out.println(user2);
//通过反射调⽤普通⽅法
//通过反射获取⼀个⽅法
System.out.println("============================");
Method setName = c1.getDeclaredMethod("setName",String.class);
//invoke:激活的意思
//参数：对象，⽅法的值
setName.invoke(user,"⽼婆");
System.out.println(user.getName());
System.out.println("============================");
//通过反射操作属性
User user3 = (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监测，属性或⽅法的setAccessible(true)
name.setAccessible(true);
name.set(user3,"苏苏");
System.out.println(user3.getName());
}
}
性能对⽐分析
package com.iss.demo06;
import ng.reflect.InvocationTargetException;
import ng.reflect.Method;
public class Demo09 {
public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
test1();
test2();
test3();
}
public static void test1() {
User user = new User();
long start = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
user.getName();
}
long end = System.currentTimeMillis();
System.out.println(end - start + "ms");
}
public static void test2() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
User user = new User();
Class c1 = user.getClass();
Method getName = c1.getDeclaredMethod("getName", null);
long start = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
getName.invoke(user, null);
}
long end = System.currentTimeMillis();
System.out.println(end - start + "ms");
}
public static void test3() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
User user = new User();
Class c1 = user.getClass();
Method getName = c1.getDeclaredMethod("getName", null);
getName.setAccessible(true);
long start = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
getName.invoke(user, null);
}
long end = System.currentTimeMillis();
System.out.println(end - start + "ms");
}
}
反射操作泛型
package com.iss.demo06;
import ng.reflect.Method;
import ng.reflect.ParameterizedType;
import ng.reflect.Type;
import java.util.List;
import java.util.Map;
public class Demo10 {
public void test01(Map<String, User> map, List<User> list) {
System.out.println("test01");
}
public Map<String, User> test02() {
System.out.println("test02");
return null;
}
public static void main(String[] args) throws NoSuchMethodException {
Method method = Demo10.class.getMethod("test01", Map.class, List.class);
Type[] genericParameterTypes = method.getGenericParameterTypes();
for (Type genericParameterType : genericParameterTypes) {
System.out.println("参数范型" + genericParameterType);
if (genericParameterType instanceof ParameterizedType) {
Type[] actualTypeAnguments = ((ParameterizedType) (ParameterizedType) genericParameterType).getActualTypeArguments(); for (Type actualTypeAngument : actualTypeAnguments) {
System.out.println("实际参数范型" + actualTypeAngument);
}
}
}
Method method1 = Demo10.class.getMethod("test02", null);
Type getGenericReturnType = method1.getGenericReturnType();
if (getGenericReturnType instanceof ParameterizedType) {
Type[] actualTypeArguments = ((ParameterizedType) getGenericReturnType).getActualTypeArguments(); for (Type actualTypeArgument : actualTypeArguments) {
System.out.println("返回值范型" + actualTypeArgument);
}
}
}
}
获取注解信息
getAnnotation
package com.iss.demo06;
import com.sun.xml.internal.ws.api.ha.StickyFeature;
import ng.annotation.*;
import ng.reflect.Field;
import java.security.cert.CRLReason;
public class Demo11 {
public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
Class c1 = Class.forName("com.iss.demo06.Student");
//通过反射获取注解
Annotation[] annotations = c1.getAnnotations();
for (Annotation annotation : annotations) {
System.out.println(annotation);//@com.reflection.Table(value=db_student)
}
//获得注解中的value值
Table table = (Table) c1.getAnnotation(Table.class);
String value = table.value();
System.out.println(value);//db_student
//获得类指定的注解
Field f = c1.getDeclaredField("name");
Filed annotation = f.getAnnotation(Filed.class);
System.out.println(annotation.columnName());//db_name
System.out.println(annotation.length());//3
System.out.println(annotation.type());//varchar
}
}
@Table("db_student")
class Student {
@Filed(columnName = "db_id", type = "int", length = 10)
private int id;
@Filed(columnName = "db_age", type = "int", length = 10)
private int age;
@Filed(columnName = "db_name", type = "varchar", length = 3)
private String name;
public Student() {
}
public Student(int id, int age, String name) {
this.id = id;
this.age = age;
= name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
= name;
}
}
//类名的注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Table {
String value();
}
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Filed {
String columnName();
String type();
int length();
}。