Java SE（注解与反射）

注解与反射

注解

• 内置注解

  package com.java.annotation;
  
  //测试内置注解
  public class TestAnnotation extends Object{
  
      //@Override  重写的注解
      @Override
      public String toString() {
          return super.toString();
      }
  
      //@Deprecated  不推荐程序员使用，但是可以使用，或者存在更好的方式
      @Deprecated
      public void test(){
          System.out.println();
      }
  
      //@SuppressWarnings 用来抑制编译时的警告信息
      @SuppressWarnings("all")
      public void test02(){
          String str;//定义变量但不使用，系统提示警告信息
      }
  }
  
  

• 元注解

  package com.java.annotation;
  
  import java.lang.annotation.*;
  
  //测试元注解
  public class TestAnnotation02 {
  }
  
  //定义一个注解
  //@Target  表示可以在哪些地方使用注解
  @Target(value={ElementType.TYPE,ElementType.METHOD})
  
  //@Retention 表示注解在什么地方有效
  //Runtime > class > sources
  @Retention(value= RetentionPolicy.CLASS)
  
  //@Documented  表示是否将注解生成在JavaDoc文档中
  @Documented
  
  //@Inherited  表示子类可以继承父类的注解
  @Inherited
  @interface MyAnnotation{
  
  }
  

• 自定义注解

  package com.java.annotation;
  
  import java.lang.annotation.ElementType;
  import java.lang.annotation.Retention;
  import java.lang.annotation.RetentionPolicy;
  import java.lang.annotation.Target;
  
  public class TestAnnotation03 {
  
  
      @MyAnnotation02(age=1)//注解可以显示赋值，如果没有默认值，必须给注解赋值
      public void test(){
      }
  
      @MyAnnotation03("a")
      public void test02(){
      }
  }
  
  @Target({ElementType.TYPE,ElementType.METHOD})
  @Retention(RetentionPolicy.RUNTIME)
  @interface MyAnnotation02{
      //注解的参数：参数类型 + 参数名();
      String name() default "";
      int age();
      int id() default -1;//默认值为-1，代表不存在
      String[] str() default {"default"};
  
  }
  
  @Target({ElementType.TYPE,ElementType.METHOD})
  @Retention(RetentionPolicy.RUNTIME)
  @interface MyAnnotation03{
      String value();//变量名为value时，在给注解赋值时可以省略参数名，直接赋值
  }
  

反射

• 获得反射对象

  package com.java.reflection;
  
  public class Test01 {
      public static void main(String[] args) throws ClassNotFoundException {
          //通过反射获取类的class对象
          Class c1=Class.forName("com.java.reflection.User");
          System.out.println(c1);
  
          Class c2=Class.forName("com.java.reflection.User");
          Class c3=Class.forName("com.java.reflection.User");
          Class c4=Class.forName("com.java.reflection.User");
  
          //一个类在内存中只有一个class对象
          //一个类被加载后，类的整个结构会被封装在class对象中
          System.out.println(c2.hashCode());
          System.out.println(c3.hashCode());
          System.out.println(c4.hashCode());
      }
  
  }
  
  //实体类:pojo，entity
  class User{
      private String name;
      private int age;
      private int id;
  
      public User() {
      }
  
      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 int getId() {
          return id;
      }
  
      public void setId(int id) {
          this.id = id;
      }
  
      @Override
      public String toString() {
          return "User{" +
                  "name='" + name + '\'' +
                  ", age=" + age +
                  ", id=" + id +
                  '}';
      }
  }
  

• Class类的创建方式

  package com.java.reflection;
  
  //测试class类的创建方式
  public class Test02 {
      public static void main(String[] args) throws ClassNotFoundException {
          Person person=new Student();
          System.out.println("这个人是："+person.name);
  
          //方式一：通过对象获得
          Class c1=person.getClass();
          System.out.println(c1.hashCode());
  
          //方式二：forName获得
          Class c2=Class.forName("com.java.reflection.Student");
          System.out.println(c2.hashCode());
  
          //方式三：通过类名.class获得
          Class<Student> c3=Student.class;
          System.out.println(c3.hashCode());
  
          //方式四：基本内置类型的包装类都有一个Type属性
          Class c4=Integer.TYPE;
          System.out.println(c4);
  
          //获得父类类型
          Class c5=c1.getSuperclass();
          System.out.println(c5);
  
      }
  }
  class Person{
      public 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";
      }
  }
  

• 所有类型的Class类

  package com.java.reflection;
  
  import java.lang.annotation.ElementType;
  
  //所有类型的class
  public class Test03 {
      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;  //枚举
          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致，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

  package com.java.reflection;
  
  //测试类的加载
  public class Test04 {
      public static void main(String[] args) {
          A a=new A();
          System.out.println(a.m);
  
          /*1.类的加载，生成每个类对应的class对象
          * 2.链接
          * 3.初始化
          *     (){
                      System.out.println("A类静态代码块初始化");
                       m=300;
                       m=100;
                    }
                     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");
      }
  }
  

• 类发生初始化的时间

  package com.java.reflection;
  
  //类发生初始化的时间
  public class Test05 {
      static{
          System.out.println("Main类被加载");
      }
  
      public static void main(String[] args) throws ClassNotFoundException {
          //1.主动引用
          //Son son=new Son();
  
          //反射也会产生注定引用
          //Class.forName("com.java.reflection.Son");
  
          //不会产生类的引用的方法
          //System.out.println(Son.a);
  
          //Son[] son=new Son[5];
  
          System.out.println(Son.B);
      }
  }
  class Father{
      static int a=1;
      static{
          System.out.println("父类被加载");
      }
  }
  
  class Son extends Father{
      static int b=10;
      static final int B=200;
      static{
          System.out.println("子类被加载");
          b=100;
      }
  }
  

• 类加载器

  package com.java.reflection;
  
  public class Test06 {
      public static void main(String[] args) throws ClassNotFoundException {
          //获取系统类的加载器
          ClassLoader classLoader=ClassLoader.getSystemClassLoader();
          System.out.println(classLoader);
  
          //获取系统类加载器的父类加载器-->扩展类加载器
          ClassLoader parent=classLoader.getParent();
          System.out.println(parent);
  
          //获取扩展类加载器的父类加载器
          ClassLoader parent2=parent.getParent();
          System.out.println(parent2);
  
          //测试当前类的类加载器
          ClassLoader classLoader1=Class.forName("com.java.reflection.Test06").getClassLoader();
          System.out.println(classLoader1);
  
          //测试JDK内置类的类加载器
          classLoader1=Class.forName("java.lang.Object").getClassLoader();
          System.out.println(classLoader1);
  
          //获得系统类加载器可以加载是路径
          System.out.println(System.getProperty("java.class.path"));
  
          /*E:\IdeaProjects\web-app\src\main\webapp\WEB-INF\lib;
          D:\Java\mysql-connector-java-8.0.22\mysql-connector-java-8.0.22.jar;
          E:\IdeaProjects\JavaSE\out\production\Base;
          E:\IdeaProjects\JavaSE\Bsae\src\com\lib\commons-io-2.8.0.jar
           */
      }
  }
  
  

• 获取类的运行时结构

  package com.java.reflection;
  
  import java.lang.reflect.Constructor;
  import java.lang.reflect.Field;
  import java.lang.reflect.Method;
  
  public class Test07 {
      public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
          Class c1=Class.forName("com.java.reflection.User");
  
          //获得类名
          System.out.println(c1.getName());
          System.out.println(c1.getSimpleName());
  
          //获得类的属性
          System.out.println("-----------------------");
          Field[] fields=c1.getFields();//只能获取public属性
          for(Field field:fields){
              System.out.println(field);
          }
  
          System.out.println("-----------------------");
          Field[] fields02=c1.getDeclaredFields();//获取所有属性
          for(Field field:fields02){
              System.out.println(field);
          }
  
          //获取指定属性
          System.out.println("-----------------------");
          Field field=c1.getDeclaredField("name");
          System.out.println(field);
  
          //获取类的方法
          System.out.println("-----------------------");
          Method[] methods=c1.getMethods();//获取本类及父类的全部public方法
          for (Method method:methods) {
              System.out.println("getMethods的方法："+method);
          }
          Method[] methods02=c1.getDeclaredMethods();//获取本类的全部方法
          for (Method method:methods02) {
              System.out.println("getDeclaredMethods的方法："+method);
          }
  
          //获取指定方法
          System.out.println("-----------------------");
          Method getName=c1.getDeclaredMethod("getName", null);
          Method setName=c1.getDeclaredMethod("setName", String.class);
          System.out.println(getName);
          System.out.println(setName);
  
          //获取构造器
          System.out.println("-----------------------");
          Constructor[] constructors=c1.getConstructors();
          for (Constructor constructor:constructors) {
              System.out.println(constructor);
          }
          Constructor[] constructors02=c1.getDeclaredConstructors();
          for (Constructor constructor:constructors02) {
              System.out.println("***-->"+constructor);
          }
  
          //获取指定构造器
          System.out.println("-----------------------");
          Constructor constructor03=c1.getDeclaredConstructor(String.class, int.class, i
                                                              nt.class);
          System.out.println(constructor03);
      }
  }
  
  

• 动态创建对象执行方

  package com.java.reflection;
  
  import java.lang.reflect.Constructor;
  import java.lang.reflect.Field;
  import java.lang.reflect.InvocationTargetException;
  import java.lang.reflect.Method;
  
  public class Test08 {
      public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
          //获得class对象
          Class c1=Class.forName("com.java.reflection.User");
  
          //构造一个对象
          System.out.print("构造一个对象:");
          User user=(User) c1.newInstance();
          System.out.println(user);
  
          //通过构造器创建对象
          System.out.print("通过构造器创建对象:");
          Constructor constructor=c1.getDeclaredConstructor(String.class,int.class,int.class);
          User user1=(User) constructor.newInstance("xuexi",002,23);
          System.out.println(user1);
  
          //通过反射调用普通方法
          System.out.print("反射调用普通方法：");
          User user2=(User) c1.newInstance();
          //通过反射获取方法
          Method setName=c1.getDeclaredMethod("setName", String.class);
  
          //invoke(对象,"方法值"): 激活
          setName.invoke(user2,"芝士");
          System.out.println(user2.getName());
  
          //通过反射获取属性
          System.out.print("反射获取属性：");
          User user3=(User) c1.newInstance();
          Field name = c1.getDeclaredField("name");
  
          //不能直接操作私有属性，需要关闭程序的安全检测，属性或方法的setAccessible(true)
          name.setAccessible(true);
          name.set(user3,"圣灵");
          System.out.println(user3.getName());
      }
  }
  
  

• 性能对比分析

  package com.java.reflection;
  
  import java.lang.reflect.InvocationTargetException;
  import java.lang.reflect.Method;
  
  public class Test09 {
      //普通方法调用
      public static void test01(){
          User user=new User();
  
          long startTime=System.currentTimeMillis();
  
          for (int i = 0; i < 10_0000_0000; i++) {
              user.getName();
          }
  
          long endTime=System.currentTimeMillis();
  
          System.out.println("普通方法执行10亿次："+(endTime-startTime)+"ms");
      }
  
      //反射方法调用
      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 < 10_0000_0000; i++) {
              getName.invoke(user,null);
          }
  
          long endTime=System.currentTimeMillis();
  
          System.out.println("反射方法执行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 < 10_0000_0000; i++) {
              getName.invoke(user,null);
          }
  
          long endTime=System.currentTimeMillis();
  
          System.out.println("反射方法(关闭检测)执行10亿次："+(endTime-startTime)+"ms");
      }
  
      public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
          test01();
          test02();
          test03();
      }
  }
  
  

• 获取泛型信息

  package com.java.reflection;
  
  import java.lang.reflect.MalformedParametersException;
  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 Test10 {
      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=Test10.class.getMethod("test01", Map.class, List.class);
  
          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("------------------");
          method=Test10.class.getMethod("test02",null);
  
          Type genericReturnType = method.getGenericReturnType();
          if(genericReturnType instanceof ParameterizedType){
              Type[] actualTypeArguments=((ParameterizedType) genericReturnType).getActualTypeArguments();
              for (Type actualTypeArgument : actualTypeArguments) {
                  System.out.println(actualTypeArgument);
              }
          }
      }
  }
  
  

• 获取注解信息

  package com.java.reflection;
  
  import java.lang.annotation.*;
  import java.lang.reflect.Field;
  
  //练习：ORM-->对象关系映射
  //利用注解和反射完成类和表结构的映射关系
  public class Test11 {
      public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
          Class c1=Class.forName("com.java.reflection.Student01");
          
          //通过反射获取注解
          Annotation[] annotations = c1.getAnnotations();
          for (Annotation annotation:annotations) {
              System.out.println(annotation);
          }
  
          //获得注解的value值
          TableName tableName=(TableName) c1.getAnnotation(TableName.class);
          String value = tableName.value();
          System.out.println(value);
  
          //获得类指定的注解
          Field f=c1.getDeclaredField("name");
          FieldInterface annotation=f.getAnnotation(FieldInterface.class);
          System.out.println(annotation.colName());
          System.out.println(annotation.type());
          System.out.println(annotation.length());
      }
  }
  
  @TableName("db-student01")
  class Student01{
      @FieldInterface(colName = "db-name",type = "varchar",length = 20)
      private String name;
      @FieldInterface(colName = "db-id",type = "int",length = 10)
      private int id;
      @FieldInterface(colName = "db-age",type = "int",length = 10)
      private int age;
  
      public Student01() {
      }
  
      public Student01(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;
      }
  
      @Override
      public String toString() {
          return "student{" +
                  "name='" + name + '\'' +
                  ", id=" + id +
                  ", age=" + age +
                  '}';
      }
  }
  
  //类名的注解
  @Target(ElementType.TYPE)
  @Retention(RetentionPolicy.RUNTIME)
  @interface TableName{
      String value();
  }
  
  //属性的注解
  @Target(ElementType.FIELD)
  @Retention(RetentionPolicy.RUNTIME)
  @interface FieldInterface{
      String colName();
      String type();
      int length();
  }