Java 反射机制详解.docx

1、Java 反射机制详解动态语言动态语言，是指程序在运行时可以改变其结构：新的函数可以被引进，已有的函数可以被删除等在结构上的变化。比如众所周知的ECMAScript(JavaScript)便是一个动态语言。除此之外如Ruby、Python等也都属于动态语言，而C、C+等语言则不属于动态语言。(引自: XX百科)var execString = alert(Math.floor(Math.random()*10);eval(execString);Class反射机制 指的是可以于运行时加载,探知和使用编译期间完全未知的类. 程序在运行状态中, 可以动态加载一个只有名称的类, 对于任意一个已经加载

2、的类,都能够知道这个类的所有属性和方法; 对于任意一个对象,都能调用他的任意一个方法和属性; 加载完类之后, 在堆内存中会产生一个Class类型的对象(一个类只有一个Class对象), 这个对象包含了完整的类的结构信息,而且这个Class对象就像一面镜子,透过这个镜子看到类的结构,所以被称之为:反射。Instances of the class Class represent classes and interfaces in a running Java application. An enum is a kind of class and an annotation is a kind o

3、f interface. Every array also belongs to a class that is reflected as a Class object that is shared by all arrays with the same element type and number of dimensions(维度). The primitive Java types (boolean, byte, char, short, int, long, float, anddouble), and the keyword void are also represented as

4、Class objects. 每个类被加载进入内存之后,系统就会为该类生成一个对应的java.lang.Class对象,通过该Class对象就可以访问到JVM中的这个类.Class对象的获取 对象的getClass()方法; 类的.class(最安全/性能最好)属性; 运用Class.forName(String className)动态加载类,className需要是类的全限定名(最常用).从Class中获取信息Class类提供了大量的实例方法来获取该Class对象所对应的详细信息,Class类大致包含如下方法,其中每个方法都包含多个重载版本,因此我们只是做简单的介绍,详细请参考JDK文档获

5、取类内信息获取内容方法签名构造器Constructor getConstructor(Class. parameterTypes)包含的方法Method getMethod(String name, Class. parameterTypes)包含的属性Field getField(String name)包含的Annotation A getAnnotation(Class annotationClass)内部类Class getDeclaredClasses()外部类Class getDeclaringClass()所实现的接口Class getInterfaces()修饰符int get

6、Modifiers()所在包Package getPackage()类名String getName()简称String getSimpleName()一些判断类本身信息的方法判断内容方法签名注解类型?boolean isAnnotation()使用了该Annotation修饰?boolean isAnnotationPresent(Class annotationClass)匿名类?boolean isAnonymousClass()数组?boolean isArray()枚举?boolean isEnum()原始类型?boolean isPrimitive()接口?boolean isIn

7、terface()obj是否是该Class的实例boolean isInstance(Object obj)使用反射生成并操作对象:Method Constructor Field这些类都实现了java.lang.reflect.Member接口,程序可以通过Method对象来执行相应的方法,通过Constructor对象来调用对应的构造器创建实例,通过Filed对象直接访问和修改对象的成员变量值.创建对象通过反射来生成对象的方式有两种: 使用Class对象的newInstance()方法来创建该Class对象对应类的实例(这种方式要求该Class对象的对应类有默认构造器). 先使用Class

8、对象获取指定的Constructor对象, 再调用Constructor对象的newInstance()方法来创建该Class对象对应类的实例(通过这种方式可以选择指定的构造器来创建实例).通过第一种方式来创建对象比较常见, 像Spring这种框架都需要根据配置文件(如applicationContext.xml)信息来创建Java对象,从配置文件中读取的只是某个类的全限定名字符串,程序需要根据该字符串来创建对应的实例,就必须使用默认的构造器来反射对象.下面我们就模拟Spring实现一个简单的对象池, 该对象池会根据文件读取key-value对, 然后创建这些对象, 并放入Map中.配置文件

9、objects: id: id1, class: com.fq.domain.User , id: id2, class: com.fq.domain.Bean ObjectPool/* * Created by jifang on 15/12/31. */public class ObjectPool private Map pool; private ObjectPool(Map pool) this.pool = pool; private static Object getInstance(String className) throws ClassNotFoundException,

10、 IllegalAccessException, InstantiationException return Class.forName(className).newInstance(); private static JSONArray getObjects(String config) throws IOException Reader reader = new InputStreamReader(ClassLoader.getSystemResourceAsStream(config); return JSONObject.parseObject(CharStreams.toString

11、(reader).getJSONArray(objects); / 根据指定的JSON配置文件来初始化对象池 public static ObjectPool init(String config) try JSONArray objects = getObjects(config); ObjectPool pool = new ObjectPool(new HashMap(); if (objects != null & objects.size() != 0) for (int i = 0; i objects.size(); +i) JSONObject object = objects

12、.getJSONObject(i); if (object = null | object.size() = 0) continue; String id = object.getString(id); String className = object.getString(class); pool.putObject(id, getInstance(className); return pool; catch (IOException | ClassNotFoundException | InstantiationException | IllegalAccessException e) t

13、hrow new RuntimeException(e); public Object getObject(String id) return pool.get(id); public void putObject(String id, Object object) pool.put(id, object); public void clear() pool.clear(); Clientpublic class Client Test public void client() ObjectPool pool = ObjectPool.init(config.json); User user

14、= (User) pool.getObject(id1); System.out.println(user); Bean bean = (Bean) pool.getObject(id2); System.out.println(bean); Userpublic class User private int id; private String name; private String password; public int getId() return id; public void setId(Integer id) this.id = id; public String getNam

15、e() return name; public void setName(String name) this.name = name; public String getPassword() return password; public void setPassword(String password) this.password = password; Override public String toString() return User + id= + id + , name= + name + / + , password= + password + / + ; Beanpubli

16、c class Bean private Boolean usefull; private BigDecimal rate; private String name; public Boolean getUsefull() return usefull; public void setUsefull(Boolean usefull) this.usefull = usefull; public BigDecimal getRate() return rate; public void setRate(BigDecimal rate) this.rate = rate; public Strin

17、g getName() return name; public void setName(String name) this.name = name; Override public String toString() return Bean + usefull= + usefull + , rate= + rate + , name= + name + / + ; 注意: 需要在pom.xml中添加如下依赖: com.alibaba fastjson 1.2.7 com.google.guava guava 18.0调用方法当获取到某个类对应的Class对象之后, 就可以通过该Class对象

18、的getMethod来获取一个Method数组或Method对象.每个Method对象对应一个方法,在获得Method对象之后,就可以通过调用invoke方法来调用该Method对象对应的方法.CallerSensitivepublic Object invoke(Object obj, Object. args) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException .下面我们对上面的对象池加强:可以看到Client获取到的对象的成员变量全都是默认值,既然我们已经使用了JSON这么

19、优秀的工具,我们又学习了动态调用对象的方法,那么我们就通过配置文件来给对象设置值(在对象创建时), 新的配置文件形式如下: objects: id: id1, class: com.fq.domain.User, fields: name: id, value: 101 , name: name, value: feiqing , name: password, value: ICy5YqxZB1uWSwcVLSNLcA= , id: id2, class: com.fq.domain.Bean, fields: name: usefull, value: true , name: rate,

20、 value: 3.14 , name: name, value: bean-name , id: id3, class: com.fq.domain.ComplexBean, fields: name: name, value: complex-bean-name , name: refBean, ref: id2 其中fields代表该Bean所包含的属性,name为属性名称,value为属性值(属性类型为JSON支持的类型),ref代表引用一个对象(也就是属性类型为Object,但是一定要引用一个已经存在了的对象)/* * author jifang * since 15/12/31下午

21、4:00 */public class ObjectPool private Map pool; private ObjectPool(Map pool) this.pool = pool; private static JSONArray getObjects(String config) throws IOException Reader reader = new InputStreamReader(ClassLoader.getSystemResourceAsStream(config); return JSONObject.parseObject(CharStreams.toStrin

22、g(reader).getJSONArray(objects); private static Object getInstance(String className, JSONArray fields) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InstantiationException, InvocationTargetException / 配置的Class Class clazz = Class.forName(className); / 目标Class的实例对象 Obj

23、ect targetObject = clazz.newInstance(); if (fields != null & fields.size() != 0) for (int i = 0; i fields.size(); +i) JSONObject field = fields.getJSONObject(i); / 需要设置的成员变量名 String fieldName = field.getString(name); / 需要设置的成员变量的值 Object fieldValue; if (field.containsKey(value) fieldValue = field.ge

24、t(value); else if (field.containsKey(ref) String refBeanId = field.getString(ref); fieldValue = OBJECTPOOL.getObject(refBeanId); else throw new RuntimeException(neither value nor ref); String setterName = set + fieldName.substring(0, 1).toUpperCase() + fieldName.substring(1); / 需要设置的成员变量的setter方法 Me

25、thod setterMethod = clazz.getMethod(setterName, fieldValue.getClass(); / 调用setter方法将值设置进去 setterMethod.invoke(targetObject, fieldValue); return targetObject; private static ObjectPool OBJECTPOOL; / 创建一个对象池的实例(保证是多线程安全的) private static void initSingletonPool() if (OBJECTPOOL = null) synchronized (Obj

26、ectPool.class) if (OBJECTPOOL = null) OBJECTPOOL = new ObjectPool(new ConcurrentHashMap(); / 根据指定的JSON配置文件来初始化对象池 public static ObjectPool init(String config) / 初始化pool initSingletonPool(); try JSONArray objects = getObjects(config); for (int i = 0; objects != null & i objects.size(); +i) JSONObject

27、 object = objects.getJSONObject(i); if (object = null | object.size() = 0) continue; String id = object.getString(id); String className = object.getString(class); / 初始化bean并放入池中 OBJECTPOOL.putObject(id, getInstance(className, object.getJSONArray(fields); return OBJECTPOOL; catch (IOException | ClassNotFoundException | InstantiationException | IllegalAccessException | NoSuchMethodException | InvocationTargetException e) throw new RuntimeException(e); public Object getObject(String id) return pool.get(id); public void putObject(String id, Object object) pool.put(id, object); p