java反射

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Java反射

反射是java语言一个非常重要的特点，通过反射，可以在运行时获取类的信息，创建对象，操作对象的属性，执行对象的方法，获取注解等。
在日常的开发过程中，反射的使用可能并不多，所以比较的陌生，但了解熟悉后，反射还是比较简单的，只是对一些API的运用。
反射主要涉及Class类和java.lang.reflect包中类。

下面通过实际的代码来说明反射的使用：

ReflectionDemo.java

package com.example;import java.lang.reflect.Constructor;import java.lang.reflect.Field;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;/** * Created by hehelt on 16/7/30. */public class ReflectionDemo {    public static void main(String[] args) {        /*------------------获取Class,总共有三种方法--------------------------*///        Class personClass = getPersonClass(1);//        Class personClass = getPersonClass(2);//        Class personClass = getPersonClass(3);        /*----------------创建对象-----------------*///        createPerson();        /*------------------操作属性--------------------------*///        getFields();//        getField("age");//        getFieldValue();//        setFieldValue();        /*----------------操作方法----------------*///        getMethods();//        doMethod();//        getAnnotation();    }    /**     * 获取Class对象,总共三种方法     *     * @param index 方法的序号     * @return     */    private static Class getPersonClass(int index) {        Class personClass = null;        switch (index) {            case 1:                // 第一种方法,直接类名后加上.class                personClass = Person.class;                break;            case 2:                // 第二种方法,每一个对象都有getClass()方法                Person person = new Person();                personClass = person.getClass();                break;            case 3:                // 第三种方法,Class的静态方法forName(String className),className为包名+类名                try {                    personClass = Class.forName("com.example.Person");                } catch (ClassNotFoundException e) {                    e.printStackTrace();                }                break;        }        return personClass;    }    /**     * 创建对象     */    public static void createPerson() {        Class personClass = getPersonClass(1);        try {            // 无惨构造函数            Constructor constructor = personClass.getConstructor(null);            Person person = (Person) constructor.newInstance(null);            System.out.println(person.getName());            // 有参构造函数            Constructor constructor1 = personClass.getConstructor(String.class);            person = (Person) constructor1.newInstance("haha");            System.out.println(person.getName());            // 构造函数为private时            Constructor constructor2 = personClass.getDeclaredConstructor(int.class, String.class);            constructor2.setAccessible(true);            person = (Person) constructor2.newInstance(13, "hello");            System.out.print(person.getName());        } catch (NoSuchMethodException e) {            e.printStackTrace();        } catch (IllegalAccessException e) {            e.printStackTrace();        } catch (InstantiationException e) {            e.printStackTrace();        } catch (InvocationTargetException e) {            e.printStackTrace();        }    }    /**     * 获取所有的属性     */    public static void getFields() {        Class personClass = getPersonClass(3);        // 权限为public的属性        Field[] fields = personClass.getFields();        for (Field field : fields) {            System.out.println(field.getName());        }        // 所有属性        fields = personClass.getDeclaredFields();        personClass.getFields();        for (Field field : fields) {            System.out.println(field.getName());        }    }    /**     * 根据属性名称获取属性     */    public static void getField() {        Class personClass = getPersonClass(3);        try {            // 属性的权限为public            Field field = personClass.getField("name");            System.out.println(field.getName());            // 属性的权限不为public            Field ageField = personClass.getDeclaredField("age");            System.out.println(ageField.getName());        } catch (NoSuchFieldException e) {            e.printStackTrace();        }    }    /**     * 获取对象的属性值     */    private static void getFieldValue() {        Class personClass = getPersonClass(3);        Person person = new Person();        try {            // 权限不为public的属性            Field field = personClass.getDeclaredField("age");            field.setAccessible(true);// 默认为false,为true时,系统不会验证权限;为false时,系统会检验权限.当需要操作的属性不为public时,需要设置该值为true,避免权限不足导致操作失败.            int age = field.getInt(person);            System.out.println(age);            // 权限为public的属性            Field nameField = personClass.getField("name");            String name = (String) nameField.get(person);            System.out.println(name);        } catch (NoSuchFieldException e) {            e.printStackTrace();        } catch (IllegalAccessException e) {            e.printStackTrace();        }    }    /**     * 设置属性的值     */    public static void setFieldValue() {        Class personClass = getPersonClass(3);        Person person = new Person();        try {            // 属性的权限为public            Field field = personClass.getField("name");            System.out.println(field.get(person));            field.set(person, "hehe");            System.out.println(field.get(person));            // 属性的权限不为public            Field ageField = personClass.getDeclaredField("age");            ageField.setAccessible(true);// 默认为false,为true时,系统不会验证权限;为false时,系统会检验权限.当需要操作的属性不为public时,需要设置该值为true,避免权限不足导致操作失败.            System.out.println(ageField.getInt(person) + "");            ageField.set(person, 10);            System.out.println(ageField.getInt(person) + "");        } catch (NoSuchFieldException e) {            e.printStackTrace();        } catch (IllegalAccessException e) {            e.printStackTrace();        }    }    /**     * 获取类的方法     */    public static void getMethods() {        Class personClass = getPersonClass(3);        // 权限为public的方法,包括父类中的方法        Method[] methods = personClass.getDeclaredMethods();        for (Method method : methods) {            System.out.println(method.getName());        }        System.out.println("--------------");        // 所有属性,不包括父类的方法        methods = personClass.getDeclaredMethods();        for (Method method : methods) {            System.out.println(method.getName());        }    }    /**     * 获取类的方法     */    public static void getMethod() {        Class personClass = getPersonClass(3);        try {            Method getNameMethod = personClass.getMethod("getName");            System.out.println(getNameMethod.getName());            Method setNameMethod = personClass.getMethod("setName", String.class);            System.out.println(setNameMethod.getName());            Method getAgeMethod = personClass.getDeclaredMethod("getAge");            System.out.println(getAgeMethod.getName());            Method setAgeMethod = personClass.getDeclaredMethod("setAge", int.class);            System.out.println(setAgeMethod.getName());        } catch (NoSuchMethodException e) {            e.printStackTrace();        }    }    /**     * 执行类的方法     */    public static void doMethod() {        Class personClass = getPersonClass(3);        Person person = new Person();        try {            // public 无参数方法调用            Method getNameMethod = personClass.getMethod("getName");            String name = (String) getNameMethod.invoke(person);            System.out.println(name);            // public 有参数方法调用            Method setNameMethod = personClass.getMethod("setName", String.class);            setNameMethod.invoke(person, "hehe");            System.out.println((String) getNameMethod.invoke(person));            // 非public 无参数方法调用            Method getAgeMethod = personClass.getDeclaredMethod("getAge");            getAgeMethod.setAccessible(true);            int age = (int) getAgeMethod.invoke(person);            System.out.println(age);            // 非public 有参数方法调用            Method setAgeMethod = personClass.getDeclaredMethod("setAge", int.class);            setAgeMethod.setAccessible(true);            setAgeMethod.invoke(person, 12);            System.out.println((int) getAgeMethod.invoke(person));        } catch (NoSuchMethodException e) {            e.printStackTrace();        } catch (InvocationTargetException e) {            e.printStackTrace();        } catch (IllegalAccessException e) {            e.printStackTrace();        }    }    /**     * 获取注解     */    public static void getAnnotation() {        Class personClass = getPersonClass(3);        try {            Field field = personClass.getField("name");            MyAnnotation annotation = (MyAnnotation) field.getAnnotation(MyAnnotation.class);            System.out.println(annotation.MyNameValue());            Field ageField = personClass.getDeclaredField("age");            MyAnnotation ageAnnotation = (MyAnnotation) ageField.getAnnotation(MyAnnotation.class);            System.out.println(ageAnnotation.MyNameValue());        } catch (NoSuchFieldException e) {            e.printStackTrace();        }    }}

MyAnnotation.java

package com.example;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;/** * Created by hehelt on 16/8/4. * <p> * 自定义的注解 */@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.FIELD)public @interface MyAnnotation {    String MyNameValue() default "";}

Person.java

package com.example;/** * Created by hehelt on 16/8/4. */public class Person {    @MyAnnotation(MyNameValue = "heheLT")    public String name = "LT";    @MyAnnotation(MyNameValue = "heheLT")    private int age = 0;    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    private int getAge() {        return age;    }    private void setAge(int age) {        this.age = age;    }}

代码链接

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