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java.lang.annotation包中可以找到（@Target、@Retention、@Document、@Inherited）；
元注解的作用：
-
@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）；
-
@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)；
-
@Document：说明该注解将被包含在javadoc中；
-
@Inherited：说明子类可以继承父类中的该注解；
package iss.demo05;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.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 java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.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、PHP、Python等。
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("iss.demo06.User");
System.out.println(c);
Class<?> c1 = Class.forName("iss.demo06.User");
Class<?> c2 = Class.forName("iss.demo06.User");
Class<?> c3 = Class.forName("iss.demo06.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
/**
* 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) {
this.name = name;
this.id = id;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.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:"+person.name);
//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) {
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";
}
}
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 java.lang.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("java.lang.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
//java.lang.String -->往上推
}
}
获取类的运行时结构
概念
代码
package iss.demo06;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.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("iss.demo06.User");
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(“iss.demo06.User”); //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 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 Demo08 {
public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
//获得class对象
Class c1 = Class.forName("com.iss.demo06.User");
//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());
}
}
invoke
setAccessible(true)
性能对比分析
package com.iss.demo06;
import java.lang.reflect.InvocationTargetException;
import java.lang.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 java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.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
ORM
getAnnotation
package com.iss.demo06;
import com.sun.xml.internal.ws.api.ha.StickyFeature;
import java.lang.annotation.*;
import java.lang.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;
this.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;
}
public String getName() {
return name;
}
public void setName(String name) {
this.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();
}