java2_day09之注解和反射
作者：互联网
注解
注解入门
//什么是注解
public class Demo01_Annotation extends Object {
//@Override就是一个注解
@Override
public String toString() {
return super.toString();
}
}
内置注解
// 什么是注解
public class Demo01_Annotation extends Object {
// @Override就是一个注解
@Override
public String toString() {
return super.toString();
}
// @Deprecated不推荐程序员使用,但是可以使用,或者存在更好的更新方式
@Deprecated
public static void test() {
System.out.println("Deprecated");
}
// @SuppressWarnings 镇压警告
@SuppressWarnings("all")
public void test01(){
List<String> list = new ArrayList<String>();
}
public static void main(String[] args) {
test();
}
}
自定义注解,元注解
//测试元注解
@MyAnnotation
public class Demo02_MetaAnnotation {
@MyAnnotation
public void test() {
}
}
//定义一个注解
//@Target可以用在什么地方
//ElementType.METHOD方法上有效 ElementType.TYPE类上有效
@Target(value = {ElementType.METHOD, ElementType.TYPE})
//@Retention在什么地方有效
//RUNTIME>CLASS>SOURCES
@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
public class Demo03_CustomAnnotation {
//注解可以显示赋值,如果没有默认值,就必须给注解赋值
@MyAnnotation2(name = "张三")
public void test() {
}
}
@Target(value = {ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation2 {
//注解的参数:参数类型+参数名()
//String name();
String name() default "";
int age() default 0;
int id() default -1;//-1代表不存在
String[] schools() default {"西部开源","清华大学"};
反射机制
Java反射机制概念
静态 & 动态语言
反射机制概念
反射机制研究与应用
反射机制优缺点
实现
//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("cn.doris.reflection.User");
System.out.println(c);
Class<?> c1 = Class.forName("cn.doris.reflection.User");
Class<?> c2 = Class.forName("cn.doris.reflection.User");
Class<?> c3 = Class.forName("cn.doris.reflection.User");
Class<?> c4 = Class.forName("cn.doris.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
/**
* 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());
System.out.println(c4.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;
}
}
理解Class类并获取Class实例
class类介绍
一个加载的类在JVM中只会有一个Class实例，因为无论创建了多少个Class实例，他们的hascode()都是一样的。注意Class是类，class是关键字，
获取Class类的实例
//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("cn.doris.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
//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());
}
}
类的加载与ClassLoader
Java内存分析
类的加载（类的加载好好理解）
深刻理解类加载:https://blog.csdn.net/m0_38075425/article/details/81627349
//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");
}
}
分析上面代码
程序自上往下执行
什么时候会发生类初始化
//测试类什么时候会初始化
public class Demo08_ActiveReference {
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();
// 反射也会产生主动引用
//Class.forName("cn.doris.reflection.Son");
//不会产生类的引用的方法
//System.out.println(Son.b); //此时main和父类被加载，
//Son[] array = new Son[5]; // 只有main被加载
//System.out.println(Son.a); // 常量并不会引起父类和子类的初始化
}
}
class Father {
static 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;
}
static int m = 300;
static final int a = 1;
}
类加载器的作用
ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();//获取系统类的加载器
ClassLoader parent = systemClassLoader.getParent();//获取系统类加载器的父类加载器-->扩展类加载器 jre1.8.0_91\lib\ext
ClassLoader parent1 = parent.getParent();//获取扩展类加载器父类加载器-->根加载器(c/c++) jre1.8.0_91\lib\rt.jar
//类加载器
public class Demo09_ClassLoader1 {
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_91\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_91\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("cn.doris.reflection.Demo09_ClassLoader1").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"));
//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
/**
* D:\Environment\java\jdk1.8.0_91\jre\lib\charsets.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\deploy.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\access-bridge-64.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\cldrdata.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\dnsns.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\jaccess.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\jfxrt.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\localedata.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\nashorn.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\sunec.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\sunjce_provider.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\sunmscapi.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\sunpkcs11.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\ext\zipfs.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\javaws.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\jce.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\jfr.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\jfxswt.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\jsse.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\management-agent.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\plugin.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\resources.jar;
* D:\Environment\java\jdk1.8.0_91\jre\lib\rt.jar;
* E:\StudyProject\annotation_reflection\out\production\annotation_reflection;
* D:\WorkingSoftware\IntelliJ IDEA 2020.3.1\lib\idea_rt.jar
*/
}
}
创建运行时类的对象
获取运行类的完整结构
方法:
Class c1 = Class.forName("cn.doris.reflection.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
//获得类的属性
c1.getFields();//只能找到public属性
c1.getDeclaredFields();//找到全部的属性
c1.getDeclaredField("name"); //获得指定属性的值
//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
//获取类的信息
public class Demo10_ClassInfo {
public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
Class c1 = Class.forName("cn.doris.reflection.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
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);
}
}
调用运行时类的指定结构
有Class对象,能做什么
方法
//获得Class对象
Class c1 = Class.forName("cn.doris.reflection.User");
//本质上调用了类的无参构造器
User user = (User) c1.newInstance();
//构造器创建对象
Constructor constructor=c1.getDeclaredConstructor(String.class, int.class, int.class); User user1 = (User) constructor.newInstance("长歌",001,17);
//invoke:激活 // (对象,"方法值") setName.invoke(user2, "doris");
//设置安全检测 name.setAccessible(true);
//动态的创建对象,通过反射
public class Demo11_DynamicCreateObject {
public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
//获得Class对象
Class c1 = Class.forName("cn.doris.reflection.User");
//构造一个对象
/*User user = (User) c1.newInstance();//本质上调用了类的无参构造器
System.out.println(user);*/
//通过构造器创建对象
/*Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
User user1 = (User) constructor.newInstance("长歌",001,17);
System.out.println(user1);*/
//通过反射调用普通方法
User user2 = (User) c1.newInstance();
//通过反射获取一个方法
Method setName = c1.getDeclaredMethod("setName", String.class);
//invoke:激活
// (对象,"方法值")
setName.invoke(user2, "doris");
System.out.println(user2.getName());
//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检测,属性或方法的setAccessible(true)
//设置安全检测
name.setAccessible(true);
name.set(user3, "doris2");
System.out.println(user3.getName());
}
}
性能检测:
//分析性能问题
public class Demo12_Performance {
//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(); // 新建对象
Class c1 = user.getClass(); // 利用反射，找到其对象的类
Method getName = c1.getDeclaredMethod("getName", null); // 通过反射找到这个类的相应方法
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("反射方式执行10亿次,关闭检测:" + (endTime - startTime) + "ms");
}
public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
test01(); // 9ms
test02(); // 5699ms
test03(); // 1599ms
}
}
##反射操作泛型
//通过反射获取泛型
public class Demo13_Generic {
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 {
//根据反射获取方法
Method method = Demo13_Generic.class.getMethod("test01", Map.class, List.class);
//获取方法的参数
Type[] genericExceptionTypes = method.getGenericParameterTypes();
for (Type genericExceptionType : genericExceptionTypes) {
//输出
System.out.println("#" + genericExceptionType);
//判断参数是否是参数化类型
if (genericExceptionType instanceof ParameterizedType){
//强转后获取参数化类型
Type[] actualTypeArguments = ((ParameterizedType) genericExceptionType).getActualTypeArguments();
for (Type actualTypeArgument : actualTypeArguments) {
System.out.println(actualTypeArgument);
}
}
}
System.out.println("====================================");
Method method2 = Demo13_Generic.class.getMethod("test02",null);
Type genericReturnType = method2.getGenericReturnType();
if (genericReturnType instanceof ParameterizedType){
Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();
for (Type actualTypeArgument : actualTypeArguments) {
System.out.println(actualTypeArgument);
}
}
}
}
反射操作注解
//练习反射操作注解
public class Demo14_ORM {
public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
Class c1 = Class.forName("cn.doris.reflection.Student2");
//通过反射获取注解
Annotation[] annotations = c1.getAnnotations();
for (Annotation annotation : annotations) {
System.out.println(annotation);
}
//获得注解value
TableDoris tableDoris = (TableDoris) c1.getAnnotation(TableDoris.class);
String value = tableDoris.value();
System.out.println(value);
//获得类指定的注解
Field name = c1.getDeclaredField("name");
FiledDoris annotation = name.getAnnotation(FiledDoris.class);
System.out.println(annotation.columnName());
System.out.println(annotation.type());
System.out.println(annotation.length());
}
}
@TableDoris("db_student")
class Student2 {
@FiledDoris(columnName = "db_id", type = "int", length = 10)
private int id;
@FiledDoris(columnName = "db_age", type = "int", length = 3)
private int age;
@FiledDoris(columnName = "db_name", type = "varchar", length = 200)
private String name;
public Student2() {
}
public Student2(int id, int age, String name) {
this.id = id;
this.age = age;
this.name = name;
}
@Override
public String toString() {
return "Student2{" +
"id=" + id +
", age=" + age +
", 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;
}
}
//类名注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableDoris {
String value();
}
//属性注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FiledDoris {
String columnName();
String type();
int length();
}
标签：Class,day09,System,class,java2,println,注解,public,out 来源： https://www.cnblogs.com/xinxueqi/p/16335369.html