四、反射机制

阳光穿透心脏的1/2处 2022-11-05 08:48 175阅读 0赞

# 1. 定义 #

*  反向探知，在程序运行过程中动态获取类的相关属性。
 *  对于给定的一个类，可以获取类的所有属性和方法。

# 2. 使用 #

public class User {
        public void sayHello() {
            System.out.println("hello");
        }
    }
    
    public class Test {
        public static void main(String[] args) throws Exception {
            Class<User> clazz = User.class; //获取一个类对象
            User user = clazz.newInstance(); //获取一个实例对象
            Method method = clazz.getDeclaredMethod("sayHello"); //获取类中的方法
            method.invoke(user); //通过反射执行方法
        }
    }

# 3. 反射的优缺点 #

## 优点 ##

*  增加程序的灵活性，避免固有逻辑写死到程序中
 *  代码相当简洁，可以提高程序的复用性

public class Test {
        public static void main(String[] args) throws Exception {
            Animal animal = sayHello("cat");
            animal.sayHello();
        }
        // 现在Animal下有两个实现类，两个判断，但是如果新添加一个实现类，还需要再次修改这块代码，不够灵活
        public static Animal sayHello(String name) {
            if ("dog".equals(name)) {
                return new Dog();
            }
            if ("cat".equals(name)) {
                return new Cat();
            }
            return null;
        }
    }

使用反射改进，增加灵活性

public class Test {
        public static void main(String[] args) throws Exception {
            Animal animal = sayHello(Cat.class.getName());
            animal.sayHello();
        }
        // 添加新的实现类，只需要传入新实现类的全路径名即可，不用修改这块代码
        public static Animal sayHello(String name) {
            try { // 根据类的全路径名反射获取类
                Class clazz = Class.forName(name);
                return (Animal) clazz.newInstance();
            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }
    }

上面只是举例说明反射的灵活性，实际中并不建议这样用，可以用Java多态的特性“声明接口，创建实现类”实现这一点

public class Test {
        public static void main(String[] args) throws Exception {
            Animal animal = new Cat(); //声明接口Animal，创建实现类Cat
            Animal animal2 = new Dog(); //声明接口Animal，创建实现类Dog
            animal.sayHello();
            animal2.sayHello();
        }
    }

## 缺点 ##

*  相比于直接调用，反射有比较大的性能消耗，速度比较慢
 *  内部暴露和安全隐患

性能比较

public class Test {
        public static void main(String[] args) throws Exception {
            long startTime = System.currentTimeMillis();
            int length = 10000;
            for (int i = 0; i < length; i++) {
                Animal animal = new Cat();
                animal.sayHello();
            }
            long endTime = System.currentTimeMillis();
            System.out.println("对象调用"+ length +"次耗时：" + (endTime-startTime) + " ms");
            long startTime2 = System.currentTimeMillis();
            for (int i = 0; i < length; i++) {
                Animal animal = sayHello(Cat.class.getName());
                animal.sayHello();
            }
            long endTime2 = System.currentTimeMillis();
            System.out.println("反射调用"+ length +"次耗时：" + (endTime2-startTime2) + " ms");
        }
    }

![20210305002357894.png][]

## 反射慢的原因 ##

*  调用了native方法
 *  每次newInstance都会做安全检查，比较耗时

# 4. 反射的操作 #

反射获取类对象的方式

Class clazz = Cat.class;
    Class clazz2 = Class.forName("com.lucifer.tree.Cat");
    Class clazz3 = new Cat().getClass();
    Class clazz4 = clazz3.getClassLoader().loadClass("com.lucifer.tree.Cat");

反射获取类的相关结构

int clazzModifier = clazz.getModifiers(); // 获取类的修饰符 1
    Package clazzPackage = clazz.getPackage(); // package com.lucifer.tree
    String clazzName = clazz.getName(); // com.lucifer.tree.Cat
    String canonicalName = clazz.getCanonicalName(); // com.lucifer.tree.Cat
    String simpleName = clazz.getSimpleName(); // Cat
    ClassLoader classLoader = clazz.getClassLoader(); // sun.misc.Launcher$AppClassLoader@18b4aac2
    Class superClass = clazz.getSuperclass(); // class java.lang.Object
    Annotation[] annotations = clazz.getAnnotations(); // [Ljava.lang.annotation.Annotation;@677327b6
    Class<?>[] interfaces = clazz.getInterfaces(); //[Ljava.lang.Class;@14ae5a5

<table>  java.lang.reflect. 
 <a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html" rel="nofollow">Modifier</a>  
 <tbody> 
  <tr> 
   <th>Modifier and Type</th> 
   <th>Constant Field</th> 
   <th>Value</th> 
  </tr> 
 </tbody> 
 <tbody> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#ABSTRACT" rel="nofollow">ABSTRACT</a></code></td> 
   <td><code>1024</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#FINAL" rel="nofollow">FINAL</a></code></td> 
   <td><code>16</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#INTERFACE" rel="nofollow">INTERFACE</a></code></td> 
   <td><code>512</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#NATIVE" rel="nofollow">NATIVE</a></code></td> 
   <td><code>256</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#PRIVATE" rel="nofollow">PRIVATE</a></code></td> 
   <td><code>2</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#PROTECTED" rel="nofollow">PROTECTED</a></code></td> 
   <td><code>4</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#PUBLIC" rel="nofollow">PUBLIC</a></code></td> 
   <td><code>1</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#STATIC" rel="nofollow">STATIC</a></code></td> 
   <td><code>8</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#STRICT" rel="nofollow">STRICT</a></code></td> 
   <td><code>2048</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#SYNCHRONIZED" rel="nofollow">SYNCHRONIZED</a></code></td> 
   <td><code>32</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#TRANSIENT" rel="nofollow">TRANSIENT</a></code></td> 
   <td><code>128</code></td> 
  </tr> 
  <tr> 
   <td><a rel="nofollow"></a><code>public&nbsp;static&nbsp;final&nbsp;int</code></td> 
   <td><code><a href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/reflect/Modifier.html#VOLATILE" rel="nofollow">VOLATILE</a></code></td> 
   <td><code>64</code></td> 
  </tr> 
 </tbody> 
</table>

反射获取类的属性

// 获取当前类和父类的公有属性
    Field[] fields = clazz.getFields();
    // 获取当前类所有属性，包括私有属性
    Field[] fields2 = clazz.getDeclaredFields();
    // 获取当前类和父类的公有方法
    Method[] methods = clazz.getMethods();
    // 获取当前类所有方法，包括私有方法
    Method[] methods2 = clazz.getDeclaredMethods();
    
    Cat cat = new Cat();
    // 调用私有方法
    Method privateMethod = clazz.getDeclaredMethod("miaomiao");
    privateMethod.setAccessible(true); //获取私有方法需要放开访问权限，否则报错
    privateMethod.invoke(cat);
    // 调用静态方法
    Method staticMethod = clazz.getDeclaredMethod("staticMethod");
    staticMethod.invoke(null);

反射获取构造器

// 获取所有公有的构造器
    Constructor<?>[] constructors = clazz.getConstructors();
    // 获取所有构造器，包括私有的
    Constructor<?>[] declaredConstructors = clazz.getDeclaredConstructors();
    // 根据参数类型获取指定构造器
    Constructor declaredConstructor = clazz.getDeclaredConstructor(String.class);
    // 如果是私有构造器，需要放开访问权限
    declaredConstructor.setAccessible(true);
    // 可以通过构造器获取实例对象
    declaredConstructor.newInstance("小花");
    // 直接通过newInstance获取对象
    Cat cat3 = (Cat) clazz.newInstance();
    // 底层本质上还是通过构造器获取实例对象
    tmpConstructor.newInstance((Object[])null);

单例的漏洞

问题：正常调用单例类对外提供的方法，只能获取到唯一的一个实例，但是通过反射机制可以获取到不同的实例

原因：反射可以调用私有的构造器造成的

// 通过单例对外提供的方法拿到唯一的实例
    SingleInstanceClass instanceClass1 = SingleInstanceClass.getInstance();
    SingleInstanceClass instanceClass2 = SingleInstanceClass.getInstance();
    SingleInstanceClass instanceClass3 = SingleInstanceClass.getInstance();
    // 通过反射获取私有构造器，可以获取到新的实例
    Constructor declaredConstructor = instanceClass1.getClass().getDeclaredConstructor();
    declaredConstructor.setAccessible(true);
    SingleInstanceClass instanceClass4 = (SingleInstanceClass) declaredConstructor.newInstance();

解决方法：在私有构造中加入判断逻辑，结合异常处理

public class SingleInstanceClass {
        public static SingleInstanceClass instance;
        private SingleInstanceClass() {
            // 处理通过反射获取实例的漏洞
            if (instance != null) {
                throw new RuntimeException();
            }
        }
        public static SingleInstanceClass getInstance() {
            if (instance == null) {
                instance = new SingleInstanceClass();
            }
            return instance;
        }
    }

反射的应用场景

*  jdbc封装

Class.forName("com.mysql.cj.jdbc.Driver");
    String url = "jdbc:mysql://localhost:3306/mysql";
    String user = "root";
    String pwd = "root";
    Connection connection = DriverManager.getConnection(url, user, pwd);

*  SpringIoC

ApplicationContext ac = new ClassPathXmlApplicationContext("spring-ioc.xml");
    User user = (User)ac.getBean("user");
    
    // ioc的底层本质上还是反射
    public Object getBean(String name) throws BeansException {
        this.assertBeanFactoryActive();
        return this.getBeanFactory().getBean(name);
    }
    
    protected final void refreshBeanFactory() throws BeansException {
        if (this.hasBeanFactory()) {
            this.destroyBeans();
            this.closeBeanFactory();
        }
    
        try {// 创建对应IOC容器对象
            DefaultListableBeanFactory beanFactory = this.createBeanFactory();
            beanFactory.setSerializationId(this.getId());
            this.customizeBeanFactory(beanFactory);
            // 将配置文件中的<bean>解析成一个BeanDefinition对象
            this.loadBeanDefinitions(beanFactory);
            synchronized(this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        } catch (IOException var5) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + this.getDisplayName(), var5);
        }
    }
    
    // 获取文件路径
    InputStream inputStream = encodedResource.getResource().getInputStream();
    // SAX 加载配置文件
    Document doc = this.doLoadDocument(inputSource, resource);
    // 将配置文件解析成BeanDefinition对象
    this.registerBeanDefinitions(doc, resource);
    
    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        if (delegate.nodeNameEquals(ele, "import")) {
            this.importBeanDefinitionResource(ele);
        } else if (delegate.nodeNameEquals(ele, "alias")) {
            this.processAliasRegistration(ele);
        } else if (delegate.nodeNameEquals(ele, "bean")) {
            this.processBeanDefinition(ele, delegate);
        } else if (delegate.nodeNameEquals(ele, "beans")) {
            this.doRegisterBeanDefinitions(ele);
        }
    }
    
    protected abstract Object createBean(String var1, RootBeanDefinition var2, Object[] var3) throws BeanCreationException;
    
    Constructor<?>[] ctors = this.determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    
    BeanUtils.instantiateClass(constructorToUse, new Object[0]);
    public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException { 
        ReflectionUtils.makeAccessible(ctor);//放开访问权限
        return ctor.newInstance(args);//底层还是使用的反射机制，反射获取构造器获取对象实例
    }

*  jdbcTemplate
 *  Mybatis

[20210305002357894.png]: /images/20221023/9abdae39e1b443a7be5496402681e9b3.png