寒假学习笔记之Java注解与反射
狂神说注解和反射笔记整理
笔记总结来自狂神说 Java 注解和反射详解
文章目录
- 狂神说注解和反射笔记整理
- 一、注解
- 二、反射机制
- 注解和反射笔记. md, 练习项目链接
一、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"};
二、反射机制
1.Java 反射机制概念
1.1 静态 & 动态语言
1.2 反射机制概念
1.3 反射机制研究与应用
1.4 反射机制优缺点
1.5 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
2.1class 类介绍
2.2 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";
}
}
2.3 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
3.1Java 内存分析
3.2 类的加载
//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
3.3 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); //Son[] array = new Son[5]; //System.out.println(Son.a); }}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; } static int m = 300; static final int a = 1;}
3.4 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
6.1 有 Class 对象, 能做什么
6.2 方法及使用
// 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()); }}
6.3 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(); 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("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(); test02(); test03(); }}
- 反射操作泛型
- 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 + '\'' + '}'; } 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();}
注解和反射笔记. md, 练习项目链接
链接：https://pan.baidu.com/s/16kvMC5jeMWQvMP1fripWxQ
提取码：tyui
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