【java】反射基础

Class类

import java.io.*;
import java.util.Scanner;public class Main {public static void main(String[] args) throws ClassNotFoundException {Class<String> clazz = String.class;   //使用class关键字，通过类名获取Class<?> clazz2 = Class.forName("java.lang.String");//使用Class类静态方法forName()，通过包名.类名获取，注意返回值是Class<?>String c=new String("coleak");Class<?> clazz3 = c.getClass();  //通过实例对象获取System.out.println(clazz);}
}

class java.lang.String

Class类也是一个泛型类，只有第一种方法，能够直接获取到对应类型的Class对象，而以下两种方法使用了?通配符作为返回值，但是实际上都和第一个返回的是同一个对象

即：在JVM中每个类始终只存在一个Class对象，无论通过什么方法获取，都是一样的

public class Main {public static void main(String[] args) {Class<String[]> clazz = String[].class;System.out.println(clazz.getName());  //获取类名称（得到的是包名+类名的完整名称）System.out.println(clazz.getSimpleName());System.out.println(clazz.getTypeName());System.out.println(clazz.getClassLoader());   //获取它的类加载器}
}

Class对象与多态

import java.lang.reflect.Type;
public class Main {public static void main(String[] args) {String str = "";System.out.println(str.getClass());System.out.println(str instanceof String);System.out.println(str.getClass() == String.class);   //直接判断是否为这个类型Integer i = 10;System.out.println(i.getClass().asSubclass(Number.class));   //当Integer不是Number的子类时，会产生异常System.out.println(i.getClass().getSuperclass());Type type = i.getClass().getGenericSuperclass();//getGenericSuperclass()获取父类的原始类型System.out.println(type);System.out.println(type instanceof Class);}}

class java.lang.String
true
true
class java.lang.Integer
class java.lang.Number
class java.lang.Number
true

创建类对象

无参构造

public class Main {public static void main(String[] args) throws InstantiationException, IllegalAccessException{Class<Student> clazz = Student.class;Student student = clazz.newInstance();student.test();}static class Student{public void test(){System.out.println("coleak");}}}

构造器

import java.lang.reflect.InvocationTargetException;
public class Main {public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {Class<Student> clazz = Student.class;Student student = clazz.getConstructor(String.class).newInstance("coleak");student.test();}static class Student{public Student(String str){}public void test(){System.out.println("cc");}}}

非public权限

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
public class Main {public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {Class<Student> clazz = Student.class;Constructor<Student> constructor = clazz.getDeclaredConstructor(String.class);constructor.setAccessible(true);   //修改访问权限Student student = constructor.newInstance("coleak");student.test();}static class Student{private final String name;private Student(String str){this.name=str;}public void test(){System.out.println(name);}}}

调用类方法

public权限

import java.lang.reflect.Method;
public class Main {public static void main(String[] args) throws ReflectiveOperationException {Class<?> clazz = Class.forName("com.test.Student");Object instance = clazz.newInstance();   //创建出学生对象Method method = clazz.getMethod("test", String.class);   //通过方法名和形参类型获取类中的方法method.invoke(instance, "coleak");   //通过Method对象的invoke方法来调用方法}}

非public权限

package com.test;
public class Student {private void test(String str){System.out.println("coleak"+str);}
}import java.lang.reflect.Method;
public class Main {public static void main(String[] args) throws ReflectiveOperationException {Class<?> clazz = Class.forName("com.test.Student");Object instance = clazz.newInstance();   //创建出学生对象Method method = clazz.getDeclaredMethod("test", String.class);   //通过方法名和形参类型获取类中的方法method.setAccessible(true);method.invoke(instance, "coleak");   //通过Method对象的invoke方法来调用方法}}import java.lang.reflect.Method;
public class Main {public static void main(String[] args) throws ReflectiveOperationException {Class<?> clazz = Class.forName("com.test.Student");Method method = clazz.getDeclaredMethod("test", String.class);   //通过方法名和形参类型获取类中的方法System.out.println(method.getName());   //获取方法名称System.out.println(method.getReturnType());   //获取返回值类型}
}//方法的参数为可变参数时
Method method = clazz.getDeclaredMethod("test", String[].class);

可以直接通过Method对象来获取这些信息

修改类的属性

public权限

package com.test;
public class Student {public int i;public void test(){System.out.println("coleak"+i);}
}import java.lang.reflect.Field;
import java.lang.reflect.Method;
public class Main {public static void main(String[] args) throws ReflectiveOperationException {Class<?> clazz = Class.forName("com.test.Student");Object instance = clazz.newInstance();Field field = clazz.getField("i");   //获取类的成员字段ifield.set(instance, 100);   //将类实例instance的成员字段i设置为100Method method = clazz.getMethod("test");method.invoke(instance);}
}

非public权限

import java.lang.reflect.Field;
import java.lang.reflect.Method;
public class Main {public static void main(String[] args) throws ReflectiveOperationException {Class<?> clazz = Class.forName("com.test.Student");Object instance = clazz.newInstance();Field field = clazz.getDeclaredField("i");   //获取类的成员字段ifield.setAccessible(true);field.set(instance, 100);   //将类实例instance的成员字段i设置为100Method method = clazz.getMethod("test");method.invoke(instance);}
}

类加载器

public class Main {public static void main(String[] args) {System.out.println(Main.class.getClassLoader());   //查看当前类的类加载器System.out.println(Main.class.getClassLoader().getParent());  //父加载器System.out.println(Main.class.getClassLoader().getParent().getParent());  //爷爷加载器System.out.println(String.class.getClassLoader());   //String类的加载器}
}

jdk.internal.loader.ClassLoaders$AppClassLoader@2437c6dcjdk.internal.loader.ClassLoaders$PlatformClassLoader@49e4cb85
null
null
BootstarpClassLoader是C++编写的，我们在Java中是获取不到的