Java基础：反射篇2

1.类的加载器

package com.cl.reflection;


import sun.reflect.generics.scope.ClassScope;

//类的加载器
public class Test07 {
    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
        ClassLoader parent = systemClassLoader.getParent();
        System.out.println(parent);

        //获取扩展类加载器的父类加载器-->根加载器（c/++）
        ClassLoader parent1 = parent.getParent();
        System.out.println(parent1);//null，因为跟加载器是Java类库的核心类库的，不可以直接获取

        //测试当前类是那个加载器加载的
        ClassLoader classLoader = Class.forName("com.cl.reflection.Test07").getClassLoader();
        System.out.println(classLoader);

        //测试JDK内置的类是谁加载
        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:\java\jdk8\jre\lib\charsets.jar;
        D:\java\jdk8\jre\lib\deploy.jar;
        D:\java\jdk8\jre\lib\ext\access-bridge-64.jar;
        D:\java\jdk8\jre\lib\ext\cldrdata.jar;
        D:\java\jdk8\jre\lib\ext\dnsns.jar;
        D:\java\jdk8\jre\lib\ext\jaccess.jar;
        D:\java\jdk8\jre\lib\ext\jfxrt.jar;
        D:\java\jdk8\jre\lib\ext\localedata.jar;
        D:\java\jdk8\jre\lib\ext\nashorn.jar;
        D:\java\jdk8\jre\lib\ext\sunec.jar;
        D:\java\jdk8\jre\lib\ext\sunjce_provider.jar;
        D:\java\jdk8\jre\lib\ext\sunmscapi.jar;
        D:\java\jdk8\jre\lib\ext\sunpkcs11.jar;
        D:\java\jdk8\jre\lib\ext\zipfs.jar;
        D:\java\jdk8\jre\lib\javaws.jar;
        D:\java\jdk8\jre\lib\jce.jar;
        D:\java\jdk8\jre\lib\jfr.jar;
        D:\java\jdk8\jre\lib\jfxswt.jar;
        D:\java\jdk8\jre\lib\jsse.jar;
        D:\java\jdk8\jre\lib\management-agent.jar;
        D:\java\jdk8\jre\lib\plugin.jar;
        D:\java\jdk8\jre\lib\resources.jar;
        D:\java\jdk8\jre\lib\rt.jar;
        D:\Java笔记\注解和 反射\out\production\注解和 反射;
        C:\Program Files\JetBrains\IntelliJ IDEA 2019.2.4\lib\idea_rt.jar



        * */



    }
}

2.获取类的信息

package com.cl.reflection;


import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

//获取类的信息
public class Test08 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
        Class c1= Class.forName("com.cl.reflection.User");

        User user=new User();
        c1=user.getClass();

        //获得类的名字
        System.out.println(c1.getName());//获得类名加包名
        System.out.println(c1.getSimpleName());//获得类名

        //获得类的属性
        System.out.println("==========================");
        Field[] fields=c1.getFields();//只能找到public属性


        fields = c1.getDeclaredFields();//找到全部属性
        for (Field field : fields) {
            System.out.println(field);
        }

        //获得指定属性的值
        Field name = c1.getDeclaredField("name");
        System.out.println(name);

        //获得类的方法
        System.out.println("============================");
        Method[] methods = c1.getMethods();
        for (Method method : methods) {
            System.out.println("正常的"+method);
        }
        methods=c1.getDeclaredMethods();
        for (Method method : methods) {
            System.out.println("getDeclareMethods:"+method);
        }

        //获得指定方法
        //重载
        Method getName=c1.getMethod("getName",null);
        Method setName=c1.getMethod("setName",String.class);
        System.out.println(getName);
        System.out.println(setName);

        //获得所有构造器
        System.out.println("=================================");
        Constructor[] constructors=c1.getConstructors();
        for (Constructor constructor : constructors) {
            System.out.println(constructor);
        }
        constructors=c1.getDeclaredConstructors();
        for (Constructor constructor : constructors) {
            System.out.println("s"+constructor);
        }

        //获得指定构造器
        Constructor declaredConstructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        System.out.println("指定"+declaredConstructor);
    }

}
/*
小结：1.在实际操作中，取得类的信息的操作代码，并不会经常开发
     2.但一定要熟悉java.lang.reflect包的作用，反射机制
     3.如何取得属性，方法，构造器的名称，修饰符等

* */

动态创建对象，通过反射

package com.cl.reflection;


import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

//动态创建对象，通过反射
public class Test09 {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
        //获得class对象
        Class c1=Class.forName("com.cl.reflection.User");

        //构造一个对象
        //User user=(User)c1.newInstance();//本质调用类的无参构造器
        //System.out.println(user);

        //通过构造器创建对象
        Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        User user2=(User)constructor.newInstance("king",001,18);
        System.out.println(user2);

        //通过反射调用普通方法
        User user3=(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，"方法的值"）
        setName.invoke(user3,"king");
        System.out.println(user3.getName());


    }
}

/*
//实体类:pojo,entity
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;
    }
* */

**分析性能问题，通过普通方式调用，反射方式调用，关闭检测的反射调用

package com.cl.reflection;


import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

//分析性能问题
public class Test10 {

    //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.getMethod("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.getMethod("getName", null);
        getName.setAccessible(true);//设置权限检查为true，就是关闭

        long startTime=System.currentTimeMillis();

        for (int i = 0; i < 1000000000; i++) {
            getName.invoke(user,null);
        }


        long endTime=System.currentTimeMillis();

        System.out.println("关闭检查执行10亿次："+(endTime-startTime)+"ms");
    }

    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        test01();//5ms
        test02();//2256ms
        test03();//1048ms

        /*
        总结；1.普通方式调用类中方法所需时间要远远小于利用反射调用
              2.到项目中的反射调用很多时，可以尝试关闭检测，用来提高效率
        * */
    }
}

***练习反射操作注解

package com.cl.reflection;


import java.lang.annotation.*;
import java.lang.reflect.Field;

//练习反射操作注解
public class Text12 {

    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("com.cl.reflection.Student2");

        //通过反射获得注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation);
        }

        //获得注解的value的值
        TableKing tableKing = (TableKing)c1.getAnnotation(TableKing.class);
        String value = tableKing.value();
        System.out.println(value);

        //获得类指定的注解
        Field f = c1.getDeclaredField("name");
        FieldKing annotation = f.getAnnotation(FieldKing.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());
    }
}

@TableKing("db_student")
class Student2{

    @FieldKing(columnName = "db_id",type="int",length = 10)
    private int id;
    @FieldKing(columnName = "db_age",type="int",length = 10)
    private int age;
    @FieldKing(columnName = "db_name",type="int",length = 3)
    private String name;

    public Student2() {
    }

    public Student2(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;
    }

    @Override
    public String toString() {
        return "Student2{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}

//类名的注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableKing{
    String value();
}

//属性的注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldKing{
    String columnName();
    String type();
    int length();

}