Java开发之反射与动态代理

#来自ゾフィー（佐菲）

1 反射（Reflect）

运行期间，获取类的信息，进行一些操作。

• 运行时构造类的对象。
• 运行时获取类的成员变量和方法。
• 运行时调用对象的方法（属性）。

2 Class 类

Class 类封装了类的所有信息。

//1.类名.class -> Person.class
//2.对象.getClass() -> person.getClass()
//3.Class.forName(类全名) -> Class.forName("com.yoyiyi.test.Person")

3 Class 常用方法

Person.java

public class Person {String name;private int age;public Person(String name, int age) {super();this.name = name;this.age = age;System.out.println("有参数构造器");}public Person() {super();System.out.println("无参数构造器");}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}private void privateMethod() {System.out.println("私有方法");}
}
public class TestReflec {public static void main(String[] args) throws Exception {Class<Person> clazz = (Class<Person>) Class.forName("demo02.Person");/***1.构造器**///获取所有的构造器Constructor<Person>[] constructors = (Constructor<Person>[]) clazz.getConstructors();for (Constructor<Person> c : constructors) {System.out.println(c);}//获取某一个构造器Constructor<Person> constructor = clazz.getConstructor(String.class, int.class);//创建对象constructor.newInstance("Jack", 89);/**2.方法**///获取类所有方法，包括父类，私有方法不能获取到Method[] methods = clazz.getMethods();for (Method method : methods) {System.out.println(method.getName());}//获取当前类所有方法，包括私有方法Method[] methods1 = clazz.getDeclaredMethods();for (Method method : methods1) {System.out.println(method.getName());}//获取指定方法Method setName = clazz.getDeclaredMethod("setName", String.class);Method setAge = clazz.getDeclaredMethod("setAge", int.class);System.out.println(setName.getName());System.out.println(setAge.getName());//调用某一个方法Person person = clazz.newInstance();setAge.invoke(person, 89);System.out.println(person.getAge());/**3.属性**///获取当前类的属性，不包括父类Field[] fields = clazz.getDeclaredFields();for (Field field : fields) {System.out.println(field.getName());}//获取当前类的指定属性Field name = clazz.getDeclaredField("name");System.out.println(name.getName());//获取属性的值Person person1 = new Person("Maria", 7);String s = (String) name.get(person1);System.out.println(s);//设置对象的值Person person2 = new Person();Field age = clazz.getDeclaredField("age");age.setAccessible(true); //私有属性，设置 setAccessible(true)age.set(person2, 5);System.out.println(person2.getAge());//获取当前类的指定私有属性Field age2 = clazz.getDeclaredField("age");age2.setAccessible(true);System.out.println(age2.getName());}
}

4 代理模式

为其他对象提供一种代理以控制对这个对象的访问（代理类相当于一个中介）。

4.1 静态代理

//ISeller.java
public interface ISeller {void sell();
}//Factory.java
public class Factory implements ISeller {@Overridepublic void sell() {System.out.println("厂家直销");}
}//Daigou.java
public class Daigou implements ISeller {private ISeller seller;public Daigou(ISeller seller) {this.seller = seller;}@Overridepublic void buy() {doBefore();//真正调用的持有的类的方法mSeller.sell();doAfter();}private void doBefore() {System.out.println("加价899");}private void doAfter() {System.out.println("提供售后");}
}//TestProxy.java
public class TestProxy {public static void main(String[] args) {Factory factory = new Factory();Daigou daigou = new Daigou(factory);daigou.buy();}
}//假如我们买一个面膜，找到代购，代购其实也是别处买的，代购加价 899 元卖给你，我们不直接和厂家发生关系，这种就是一个代理模型

代理模式可以在不修改被代理对象的基础上，通过扩展代理类，进行一些功能的附加与增强。值得注意的是，代理类和被代理类应该共同实现一个接口，或者是共同继承某个类。

但是有弊端，假如这人不仅不仅要面膜，还要核导弹、航空母舰等等，就创建了许多代理类。

4.2 动态代理

//ISeller.java
public interface ISeller {void sell();
}//SuperDaigou.java
public class SuperDaigou implements ISeller {@Overridepublic void sell() {System.out.println("超级代购");}
}//ProxyHandler.java
public class ProxyHandler implements InvocationHandler {//声明目标对象private ISeller target;public ProxyHandler(ISeller target) {this.target = target;}@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {doBefore();Object invoke = method.invoke(target, args);doAfter();return invoke;}//得到代理对象public Object getProxyInstance() {return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this);}private void doBefore() {System.out.println("加价899");}private void doAfter() {System.out.println("提供售后");}
}public class TestProxy {public static void main(String[] args) {SuperDaigou superDaigou = new SuperDaigou();ProxyHandler handler = new ProxyHandler(superDaigou);//增强原来的方法ISeller seller = (ISeller) handler.getProxyInstance();seller.sell();}
}

由于使用了反射，效率比较低。

5 动态代理原理解析

//Proxy.java
public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)throws IllegalArgumentException{Objects.requireNonNull(h);final Class<?>[] intfs = interfaces.clone();final SecurityManager sm = System.getSecurityManager();if (sm != null) {checkProxyAccess(Reflection.getCallerClass(), loader, intfs);}//1.动态生成 class 文件字节流，然后通过 loader 加载此字节流创建代理类 classClass<?> cl = getProxyClass0(loader, intfs);/** Invoke its constructor with the designated invocation handler.*/try {if (sm != null) {checkNewProxyPermission(Reflection.getCallerClass(), cl);}final Constructor<?> cons = cl.getConstructor(constructorParams);final InvocationHandler ih = h;if (!Modifier.isPublic(cl.getModifiers())) {AccessController.doPrivileged(new PrivilegedAction<Void>() {public Void run() {cons.setAccessible(true);return null;}});}//2.获取代理类的类构造对象return cons.newInstance(new Object[]{h});} catch (IllegalAccessException|InstantiationException e) {throw new InternalError(e.toString(), e);} catch (InvocationTargetException e) {Throwable t = e.getCause();if (t instanceof RuntimeException) {throw (RuntimeException) t;} else {throw new InternalError(t.toString(), t);}} catch (NoSuchMethodException e) {throw new InternalError(e.toString(), e);}}//Proxy.java
private static Class<?> getProxyClass0(ClassLoader loader,Class<?>... interfaces) {if (interfaces.length > 65535) {throw new IllegalArgumentException("interface limit exceeded");}// If the proxy class defined by the given loader implementing// the given interfaces exists, this will simply return the cached copy;// otherwise, it will create the proxy class via the ProxyClassFactoryreturn proxyClassCache.get(loader, interfaces);}// WeakCache.javapublic V get(K key, P parameter) {Objects.requireNonNull(parameter);expungeStaleEntries();Object cacheKey = CacheKey.valueOf(key, refQueue);// 根据cachekey获取键值对valuesMap, valuesMap的key是接口列表的包装类，value是动态生成代理类的包装类ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);if (valuesMap == null) {ConcurrentMap<Object, Supplier<V>> oldValuesMap= map.putIfAbsent(cacheKey,valuesMap = new ConcurrentHashMap<>());if (oldValuesMap != null) {valuesMap = oldValuesMap;}}// create subKey and retrieve the possible Supplier<V> stored by that//核心：获取代理类  ProxyClassFactory#applyObject subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));Supplier<V> supplier = valuesMap.get(subKey);Factory factory = null;while (true) {if (supplier != null) {// supplier might be a Factory or a CacheValue<V> instanceV value = supplier.get();if (value != null) {return value;}}// else no supplier in cache// or a supplier that returned null (could be a cleared CacheValue// or a Factory that wasn't successful in installing the CacheValue)// 如果动态生成代理类的工厂类为空，则创建新的工厂类if (factory == null) {factory = new Factory(key, parameter, subKey, valuesMap);}if (supplier == null) {//工厂类的包装类为空，则创建新的包装类supplier supplier = valuesMap.putIfAbsent(subKey, factory);if (supplier == null) {// successfully installed Factorysupplier = factory;}// else retry with winning supplier} else {if (valuesMap.replace(subKey, supplier, factory)) {// successfully replaced// cleared CacheEntry / unsuccessful Factory// with our Factorysupplier = factory;} else {// retry with current suppliersupplier = valuesMap.get(subKey);}}}}
//以上代码：通过 map 来存储动态生成的代理类，其中 key 是接口的包装类，value 是动态代理类的包装类//Proxy.java#ProxyClassFactoryprivate static final class ProxyClassFactoryimplements BiFunction<ClassLoader, Class<?>[], Class<?>>{// prefix for all proxy class namesprivate static final String proxyClassNamePrefix = "$Proxy";// next number to use for generation of unique proxy class namesprivate static final AtomicLong nextUniqueNumber = new AtomicLong();@Overridepublic Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);for (Class<?> intf : interfaces) {/** Verify that the class loader resolves the name of this* interface to the same Class object.*/Class<?> interfaceClass = null;try {interfaceClass = Class.forName(intf.getName(), false, loader);} catch (ClassNotFoundException e) {}if (interfaceClass != intf) {throw new IllegalArgumentException(intf + " is not visible from class loader");}/** Verify that the Class object actually represents an* interface.*/if (!interfaceClass.isInterface()) {throw new IllegalArgumentException(interfaceClass.getName() + " is not an interface");}/** Verify that this interface is not a duplicate.*/if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {throw new IllegalArgumentException("repeated interface: " + interfaceClass.getName());}}String proxyPkg = null;     // package to define proxy class inint accessFlags = Modifier.PUBLIC | Modifier.FINAL;/** Record the package of a non-public proxy interface so that the* proxy class will be defined in the same package.  Verify that* all non-public proxy interfaces are in the same package.*/for (Class<?> intf : interfaces) {int flags = intf.getModifiers();if (!Modifier.isPublic(flags)) {accessFlags = Modifier.FINAL;String name = intf.getName();int n = name.lastIndexOf('.');String pkg = ((n == -1) ? "" : name.substring(0, n + 1));if (proxyPkg == null) {proxyPkg = pkg;} else if (!pkg.equals(proxyPkg)) {throw new IllegalArgumentException("non-public interfaces from different packages");}}}if (proxyPkg == null) {// if no non-public proxy interfaces, use com.sun.proxy packageproxyPkg = ReflectUtil.PROXY_PACKAGE + ".";}/** Choose a name for the proxy class to generate.*/long num = nextUniqueNumber.getAndIncrement();//代理类名称String proxyName = proxyPkg + proxyClassNamePrefix + num;//代理类字节码byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);try {//最终生成代理类的 class 对象是本地方法 defineClass0 方法//原理是根据类名、接口、类加载器、方法列表、异常列表，按照 class 文件格式先生成字节流，再生成动态代理类return defineClass0(loader, proxyName,proxyClassFile, 0, proxyClassFile.length);} catch (ClassFormatError e) {/** A ClassFormatError here means that (barring bugs in the* proxy class generation code) there was some other* invalid aspect of the arguments supplied to the proxy* class creation (such as virtual machine limitations* exceeded).*/throw new IllegalArgumentException(e.toString());}}}