狂神-注解和反射
1、注解
是注释和程序进行解析的内容
元注解
常用的内容
public class Test1 {
@MyAnnotation1(age=18,id=520)
public void test1(String[] args) {
}
@MyAnnotation2()
public void test3(String[] args) {
}
}
//自定义一个注解
//Target:指的是作用的范围，在哪些地方生效
@Target(value ={ElementType.METHOD,ElementType.TYPE} )
//Retentions是运行是生效
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnnotation1{
//注解的参数：参数类型+参数名（）；
String name() default "大美女";
int age();
int id () default -1;//如果默认值是-1表示不存在
String[] schools() default {"清华","北大"};
}
//另外一种情况是，只有value的时候，是不用写参数名称的
@interface MyAnnotation2{
String value() default "";
}
2、反射
一个类在内存中只有一个class
一个类被加载后，类的整个结构都封装在Class 对象中。
主要的API是：Class（代表一个类）、Method（方法）、Field（成员变量）、Construction（构造函数）
2.1、Class 类创建方式
打印结果是一样，说明只能是创建一个Class 对象
public class test1 {
public static void main(String[] args) throws ClassNotFoundException {
Person person= new Teacher();
System.out.println("This is "+person.name);
//方式1 获取Class对象的方式
Class p1 = person.getClass();
System.out.println(p1.hashCode());
//方式2
Class p2 = Class.forName("com.kuang.reflection.Teacher");
System.out.println(p2.hashCode());
//方式3 推荐方式
Class<Teacher> p3 = Teacher.class;
System.out.println(p3.hashCode());
}
}
class Person{
int id;
String name;
public Person() {
}
@Override
public String toString() {
return "Person{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Person(int id, String name) {
this.id = id;
this.name = name;
}
}
class Student extends Person{
public Student() {
this.name="学生";
}
}
class Teacher extends Person{
public Teacher() {
this.name="老师";
}
}
2.1.1所有类型的Class对象
public class Test2 {
public static void main(String[] args) {
//类
Class c1 = Object.class;
//void
Class c2 = Void.class;
//一维数组
Class c3 = int[].class;
//二维数组
Class c4 = int[][].class;
//包装类型
Class c5 = Integer.class;
//注解
Class c6 = Override.class;
//枚举
Class c7 = ElementType.class;
//接口
Class c8 = Comparable.class;
//class
Class c9 = 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);
int [] a=new int[10];
int [] b=new int[100];
System.out.println(a.getClass().hashCode());
System.out.println(b.getClass().hashCode());
//只要元素类型和维度是一致的，就是同一个class
}
}
打印的结果基本都是class 和interface
2.2、类加载过程
分为3步：加载、链接、初始化
public class Test3 {
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 类对应的class对象
* 2、链接 （1、验证2、准备3、解析）结束后生成 m=0；
* 3、初始化
* <clinit> (){
* 执行静态代码块、静态方法、静态变量合并
* System.out.println("静态代码块");
* int m=30;
* static int m=100;
* }
* m=100
* */
}
class A{
static {
System.out.println("静态代码块");
int m=30;
}
static int m=100;
public A() {
System.out.println("无参构造函数");
}
}
2.2.1、分析类初始化
有一个双亲委派机制
会先找系统加载器--->扩展类加载器--->跟加载器，如果在上层找到了，就会执行上层找到的类
public class Test7 {
public static void main(String[] args) {
//系统类的加载器
ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
System.out.println(systemClassLoader);
//系统加载类的父加载器--->扩展类加载器
ClassLoader parent = systemClassLoader.getParent();
System.out.println(parent);
//扩展类加载器的父类--->根加载器（C/C++）
ClassLoader parent1 = parent.getParent();
System.out.println(parent1);
//测试当前类的加载器是哪个
ClassLoader classLoader = Test7.class.getClassLoader();
System.out.println(classLoader);
//jdk下的类
classLoader = Object.class.getClassLoader();
System.out.println(classLoader);
}
}
2.3、通过反射操作属性、方法、构造器等
public class Test8 {
public static void main(String[] args) throws IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
Class c1 = Person.class;
Object p1 = c1.newInstance();
System.out.println(p1);
//通过反射，走的是无参构造
//走有参构造
Constructor p2 = c1.getDeclaredConstructor( int.class, String.class);
Person pp = (Person)p2.newInstance( 001, "小白兔");
System.out.println(pp);
//通过反射调用普通方法
Method setName = c1.getDeclaredMethod("setName",String.class);
//invoke：激活的意思
//（对象，方法的值）
setName.invoke(pp, "小月月");
System.out.println(pp.getName());
//通过反射操作属性
Field name = c1.getDeclaredField("name");
//打破封装，提高反射效率
name.setAccessible(true);
name.set(pp,"哈哈哈~~~");
System.out.println(pp.getName());
}
}
2.3.1、验证一下access 的效率
public class Test9 {
public static void test1() {
Person person = new Person();
long starTime = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
person.getName();
}
long endTime = System.currentTimeMillis();
System.out.println("new 方式创建对象耗时" + (endTime - starTime) + "ms");
}
public static void test2() throws NoSuchMethodException, IllegalAccessException, InstantiationException, InvocationTargetException {
Class<Person> personClass = Person.class;
Person person = personClass.newInstance();
Method getName = personClass.getDeclaredMethod("getName", null);
long starTime = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
getName.invoke(person, null);
}
long endTime = System.currentTimeMillis();
System.out.println("反射方式创建对象耗时" + (endTime - starTime) + "ms");
}
public static void test3() throws NoSuchMethodException, IllegalAccessException, InstantiationException, InvocationTargetException {
Class<Person> personClass = Person.class;
Person person = personClass.newInstance();
Method getName = personClass.getDeclaredMethod("getName", null);
getName.setAccessible(true);
long starTime = System.currentTimeMillis();
for (int i = 0; i < 1000000000; i++) {
getName.invoke(person, null);
}
long endTime = System.currentTimeMillis();
System.out.println("Accessible方式创建对象耗时" + (endTime - starTime) + "ms");
}
public static void main(String[] args) throws InvocationTargetException, NoSuchMethodException, InstantiationException, IllegalAccessException {
test1();
test2();
test3();
}
}
2.4、通过反射获取泛型
public class Test10 {
public static void test01(Map<String,Student> map, List<Person> list){
System.out.println("test01");
}
public static Map<String,Student> test02(){
System.out.println("test01");
return null;
}
public static void main(String[] args) throws NoSuchMethodException {
Method method = Test10.class.getDeclaredMethod("test01", Map.class, List.class);
//泛型参数类型
Type[] genericParameterTypes = method.getGenericParameterTypes();
for (Type genericParameterType : genericParameterTypes) {
System.out.println(genericParameterType);
if (genericParameterType instanceof ParameterizedType){
Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments();
for (Type actualTypeArgument : actualTypeArguments) {
System.out.println(actualTypeArgument);
}
}
}
System.out.println("8888888888");
method = Test10.class.getDeclaredMethod("test02");
//返回值类型
Type genericReturnType = method.getGenericReturnType();
if (genericReturnType instanceof ParameterizedType){
Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();
for (Type actualTypeArgument : actualTypeArguments) {
System.out.println(actualTypeArgument);
}
}
}
2.5、通过反射获取注
public class Test11 {
public static void main(String[] args) throws NoSuchFieldException {
Class<Dog> c1 = Dog.class;
//通过反射获取注解
Annotation[] annotations = c1.getAnnotations();
for (Annotation annotation : annotations) {
System.out.println(annotation);
}
//获取注解的value值
DogTable dogTable = c1.getDeclaredAnnotation(DogTable.class);
System.out.println(dogTable.value());
//获得类指定的注解
Field name = c1.getDeclaredField("name");
DogFile annotation = name.getAnnotation(DogFile.class);
System.out.println(annotation.length());
System.out.println(annotation.name());
System.out.println(annotation.type());
}
}
//ORM:对象关系映射，在数据库有一个表是叫Dog
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface DogTable{
//表名
String value();
}
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface DogFile{
//字段名
String name();
int length();
String type();
}
@DogTable("db_dog")
class Dog {
@DogFile(name="db_name",length=10,type="varchar")
private String name;
@DogFile(name="db_age",length=10,type="int")
private int age;
@DogFile(name="db_color",length=10,type="varchar")
private String color;
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 String getColor() {
return color;
}
public void setColor(String color) {
this.color = color;
}
public Dog() {
}
public Dog(String name, int age, String color) {
this.name = name;
this.age = age;
this.color = color;
}
}
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