1.反射的原理

2.反射相关类

java.lang.Class
java.lang.reflect.Method
java.lang.reflect.Field
java.lang.reflect.Constructor

2.1 java.lang.Class

public static void main(String[] args) throws IOException, ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
  //通过Properties类 读写配置文件
  Properties properties = new Properties();
  properties.load(new FileInputStream("/Users/qixiangyang/Documents/javaCode/leecodePractice/src/hero.properties"));
  String classPath = properties.get("classpath").toString();
  String methodName = properties.get("method").toString();
  System.out.println("classPath = " + classPath);
  System.out.println("methodName = " + methodName);

  //加载类，生成Class类型的对象
  Class cls = Class.forName(classPath);
  //通过 cls 得到加载的类的对象实例
  Object o = cls.newInstance();
  System.out.println(cls);	//输出内容：class com.yang.reflection.Hero
}

2.2 java.lang.reflect.Method

public static void main(String[] args) throws IOException, ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
  Properties properties = new Properties();
  properties.load(new FileInputStream("/Users/qixiangyang/Documents/javaCode/leecodePractice/src/hero.properties"));
  String classPath = properties.get("classpath").toString();
  String methodName = properties.get("method").toString();
  System.out.println("classPath = " + classPath);
  System.out.println("methodName = " + methodName);

  Class cls = Class.forName(classPath);
  Object o = cls.newInstance();
  //通过cls 得到加载类中的方法对象
  //在反射中，将方法视为对象（万物皆对象的理解）
  Method method = cls.getMethod(methodName);
  //通过反射执行对应的方法
  method.invoke(o);
}

2.3 java.lang.reflect.Field

public static void main(String[] args) throws IOException, ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
  Properties properties = new Properties();
  properties.load(new FileInputStream("/Users/qixiangyang/Documents/javaCode/leecodePractice/src/hero.properties"));
  String classPath = properties.get("classpath").toString();
  String methodName = properties.get("method").toString();
  System.out.println("classPath = " + classPath);
  System.out.println("methodName = " + methodName);

  Class cls = Class.forName(classPath);
  Object o = cls.newInstance();
  System.out.println(cls);

  //字段的值  字段的修饰符不能是私有的，如果是私有的话，程序会报错
  Field fieldName = cls.getField("age");
  System.out.println(fieldName.get(o));
}

2.4 java.lang.reflect.Constructor

public static void main(String[] args) throws IOException, ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
  Properties properties = new Properties();
  properties.load(new FileInputStream("/Users/qixiangyang/Documents/javaCode/leecodePractice/src/hero.properties"));
  String classPath = properties.get("classpath").toString();
  String methodName = properties.get("method").toString();
  System.out.println("classPath = " + classPath);
  System.out.println("methodName = " + methodName);

  Class cls = Class.forName(classPath);
  Object o = cls.newInstance();
  System.out.println(cls);

  //获取到无参构造器
  Constructor constructor01 = cls.getConstructor();
  System.out.println(constructor01);	//	public com.yang.reflection.Hero()

  //获取到有参构造器
  Constructor constructor02 = cls.getConstructor(String.class);
  System.out.println(constructor02);	//	public com.yang.reflection.Hero(java.lang.String)
}

3. 反射调用优化

传统方式与反射的用时比较

传统方式：

public static void m1(){
  Hero hero = new Hero();
  long start = System.currentTimeMillis();
  for (int i=0;i<90000000;i++){
    hero.playHero();
  }
  long end = System.currentTimeMillis();
  System.out.println("m1()耗时：" + (end-start));	//	m1()耗时：5
}

反射：

public static void m2() throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
  Class cls = Class.forName("com.yang.reflection.Hero");
  Object o = cls.newInstance();
  long start = System.currentTimeMillis();
  for (int i=0;i<90000000;i++){
    Method playHero = cls.getMethod("playHero");
    playHero.invoke(o);
  }
  long end = System.currentTimeMillis();
  System.out.println("m2()耗时：" + (end-start));	//  m2()耗时：12364
}

根据两个方法执行的结果可以看出，通过反射的方式调用方法是非常耗时的！

反射调用的优化

关闭访问检查

public static void m3() throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
  Class cls = Class.forName("com.yang.reflection.Hero");
  Object o = cls.newInstance();
  long start = System.currentTimeMillis();
  for (int i=0;i<90000000;i++){
    Method playHero = cls.getMethod("playHero");
    //关闭访问检查 设为true
    playHero.setAccessible(true);
    playHero.invoke(o);
  }
  long end = System.currentTimeMillis();
  System.out.println("m3()耗时：" + (end-start));	//  m3()耗时：11708
}

与m2()方法相比，时间上确实缩短了一些，但是效果其实并不是很明显！

4.Class类常用方法

public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchFieldException {
  String classAllPath = "com.yang.reflection.Hero";
  //1. 获取到Hero类 对应的 Class对象
  //  <?>  表示不确定的Java类型
  Class<?> cls = Class.forName(classAllPath);

  //2. 输出cls
  System.out.println(cls);    // class com.yang.reflection.Hero
  System.out.println(cls.getClass());     // class java.lang.Class

  //3. 得到包名
  System.out.println(cls.getPackage());   // package com.yang.reflection

  //4. 得到全类名
  System.out.println(cls.getName());      // com.yang.reflection.Hero

  //5. 通过cls创建对象实例
  Hero hero = (Hero)cls.newInstance();
  System.out.println(hero);   // Hero{heroName='鲁班', orientation='射手', skillName='火力压制'}

  //6. 通过反射获取属性
  Field heroName = cls.getField("heroName");
  System.out.println(heroName.get(hero));     // 鲁班

  //7. 给属性赋值
  heroName.set(hero,"后羿");
  System.out.println(heroName.get(hero));     // 后羿

  //8. 获取所有属性
  Field[] heroFields = cls.getFields();
  System.out.println("Hero类所有的属性如下:");
  for (Field field:heroFields){
    System.out.println(field.getName());
  }


}

本篇文章参考韩顺平老师的视频所整理。