谈谈Java反射：从入门到实践，再到原理

Reflection is commonly used by programs which require the ability to examine or
modify the runtime behavior of applications running in the Java virtual machine.
This is a relatively advanced feature and should be used only by developers who
have a strong grasp of the fundamentals of the language. With that caveat in
mind, reflection is a powerful technique and can enable applications to perform
operations which would otherwise be impossible.
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Student student = new Student();
student.doHomework("数学");
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Class clazz = Class.forName("reflection.Student");
 Method method = clazz.getMethod("doHomework", String.class);
 Constructor constructor = clazz.getConstructor();
 Object object = constructor.newInstance();
 method.invoke(object, "语文");
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Class class1 = Class.forName("reflection.TestReflection");
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Class class2 = TestReflection.class;
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TestReflection testReflection = new TestReflection();
Class class3 = testReflection.getClass();
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//方式一
Class class1 = Class.forName("reflection.Student");
Student student = (Student) class1.newInstance();
System.out.println(student);
//方式二
Constructor constructor = class1.getConstructor();
Student student1 = (Student) constructor.newInstance();
System.out.println(student1);
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Class class1 = Class.forName("reflection.Student");
Constructor[] constructors = class1.getDeclaredConstructors();
for (int i = 0; i < constructors.length; i++) {
    System.out.println(constructors[i]);
 }
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// student 一个私有属性age，一个公有属性email
public class Student {
    private Integer age;
    public String email;
}
public class TestReflection {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class class1 = Class.forName("reflection.Student");
        Field email = class1.getField("email");
        System.out.println(email);
        Field age = class1.getField("age");
        System.out.println(age);
    }
}
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public class Student {
    private void testPrivateMethod() {
        
    }
    public void testPublicMethod() {
        
    }
}
public class TestReflection {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class class1 = Class.forName("reflection.Student");
        Method[] methods = class1.getMethods();
        for (int i = 0; i < methods.length; i++) {
            System.out.println(methods[i]);
        }
    }
}
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public class TestReflection {
    public static void main(String[] args) throws Exception {
        Class clazz = Class.forName("reflection.TestReflection");
        Method method = clazz.getMethod("target", String.class);
        method.invoke(null, "666");
    }
    public static void target(String str) {
        //打印堆栈信息
        new Exception("#" +str).printStackTrace();
        System.out.println("invoke target method");
    }
}
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java.lang.Exception: #666
invoke target method
  at reflection.TestReflection.target(TestReflection.java:17)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at reflection.TestReflection.main(TestReflection.java:11)
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public Object invoke(Object obj, Object... args)
    throws IllegalAccessException, IllegalArgumentException,
       InvocationTargetException
{
    //校验权限
    if (!override) {
        if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
            Class<?> caller = Reflection.getCallerClass();
            checkAccess(caller, clazz, obj, modifiers);
        }
    }
    MethodAccessor ma = methodAccessor;             // read volatile
    if (ma == null) {
        ma = acquireMethodAccessor(); //获取MethodAccessor
    }
    //返回MethodAccessor.invoke
    return ma.invoke(obj, args);
}
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public class Student {
    private String name;
    private Student(String name) {
        System.out.println("我是私有构造器，我被实例化了");
        this.name = name;
    }
    public void doHomework(String subject) {
        System.out.println("我的名字是" + name);
        System.out.println("我在做"+subject+"作业");
    }
}
public class TestReflection {
    public static void main(String[] args) throws Exception {
        Class clazz = Class.forName("reflection.Student");
        // 获取私有构造方法对象
        Constructor constructor = clazz.getDeclaredConstructor(String.class);
        // true指示反射的对象在使用时应该取消Java语言访问检查。
        constructor.setAccessible(true);
        Student student = (Student) constructor.newInstance("jay@huaxiao");
        student.doHomework("数学");
    }
}
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