> For the complete documentation index, see [llms.txt](https://shepherd-xie.gitbook.io/be-a-javaer/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://shepherd-xie.gitbook.io/be-a-javaer/di-3-zhang-java-gao-ji-bian-cheng/section-12.md). # 第 12 节 反射与Annotation 从JDK1.5之后Java开发提供了Annotation技术支持，这种技术为项目的编写带来新的模型，而后经过十多年的发展，Annotation技术得到了非常广泛的应用，并且已经在所有的项目之中都会存在。 ## 获取Annotation信息 在进行类或方法定义的时候都可以使用一系列的Annotation进行声明，于是如果要想获取这些Annotation的信息，那么就可以直接通过反射来完成。在 `java.lang.reflect` 里面有一个 `AccessibleObject` 类，在本类中提供有获取Annotation类的方法： * 获取全部Annotation： `public Annotation[] getAnnotations()`； * 获取指定Annotation： `public T getAnnotation(Class annotationClass)`； **范例：** 定义一个接口，并且在接口上使用Annotation ```java public class Application { public static void main(String[] args) throws Exception { { // 获取接口上全部的Annotation信息 Annotation[] annotations = IMessage.class.getAnnotations(); for (Annotation annotation : annotations) { System.out.println(annotation); } } System.out.println("========================================="); { // 获取MessageImpl子类上的Annotation Annotation[] annotations = MessageImpl.class.getAnnotations(); for (Annotation annotation : annotations) { System.out.println(annotation); } } System.out.println("========================================="); { // 获取MessageImpl.send()方法上的Annotation Method method = MessageImpl.class.getDeclaredMethod("send", String.class); Annotation[] annotations = method.getAnnotations(); for (Annotation annotation : annotations) { System.out.println(annotation); } } } } @FunctionalInterface @Deprecated(since = "1.0") interface IMessage { void send(String msg); } @SuppressWarnings("serial") class MessageImpl implements IMessage, Serializable { @Override public void send(String msg) { System.out.println("[SEND] " + msg); } } ``` 不同的Annotation有它的存在范围，下面对比两个Annotation： * `FunctionalInterface` ```java @Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) public @interface FunctionalInterface {} ``` * `SuppressWarnings` ```java @Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE}) @Retention(RetentionPolicy.SOURCE) public @interface SuppressWarnings {} ``` 此时可以发现 `FunctionalInterface` 是在运行时生效的Annotation，所以在程序执行的时候可以获取此Annotation，而 `SuppressWarnings` 是在源代码编写时有效。表示生效范围的 `RetentionPolicy` 共有三种状态： ```java public enum RetentionPolicy { /** * Annotations are to be discarded by the compiler. */ SOURCE, /** * Annotations are to be recorded in the class file by the compiler * but need not be retained by the VM at run time. This is the default * behavior. */ CLASS, /** * Annotations are to be recorded in the class file by the compiler and * retained by the VM at run time, so they may be read reflectively. * * @see java.lang.reflect.AnnotatedElement */ RUNTIME } ``` ## 自定义Annotation 现在已经清楚了Annotation的获取以及Annotation的运行策略，但是最为关键的是如何自定义Annotation。为此Java里面提供了新的语法，使用 `@interface` 声明Annotation。 **范例：** 自定义Annotation ```java public class Application { public static void main(String[] args) throws Exception { Method method = Message.class.getDeclaredMethod("send", String.class); CustomizeAnnotation annotation = method.getAnnotation(CustomizeAnnotation.class); String msg = annotation.title() + " [" + annotation.url() + "]"; method.invoke(Message.class.getDeclaredConstructor().newInstance(), msg); } } @Retention(RetentionPolicy.RUNTIME) @interface CustomizeAnnotation { String title(); String url() default "www.hello.com"; } class Message { @CustomizeAnnotation(title = "Hello") public void send(String msg) { System.out.println("[SEND] " + msg); } } ``` 使用Annotation之后的最大的特点是可以结合反射机制实现程序的处理。 ## 工厂设计模式与Annotation整合 ```java public class Application { public static void main(String[] args) throws Exception { MessageService messageService = new MessageService(); messageService.send("Hello World!"); } } @Retention(RetentionPolicy.RUNTIME) @interface UseMessage { Class clazz(); } @UseMessage(clazz = NetMessageImpl.class) class MessageService { private IMessage message; public MessageService() { UseMessage annotation = MessageService.class.getAnnotation(UseMessage.class); this.message = (IMessage) Factory.getInstance(annotation.clazz()); } public void send(String msg) { this.message.send(msg); } } class Factory { private Factory() {} public static T getInstance(Class clazz) { try { return (T) new MessageProxy().bind(clazz.getDeclaredConstructor().newInstance()); } catch (Exception e) { e.printStackTrace(); } return null; } } class MessageProxy implements InvocationHandler { private Object target; public Object bind(Object target) { this.target = target; return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this); } public boolean connect() { System.out.println("[PROXY INFO] Channel connection..."); return true; } public void close() { System.out.println("[PROXY INFO] Channel closed."); } @Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { if (this.connect()) { return method.invoke(target, args); } else { throw new Exception("[ERROR] Unable to open channel."); } } finally { this.close(); } } } interface IMessage { void send(String msg); } class MessageImpl implements IMessage { @Override public void send(String msg) { System.out.println("[SEND] " + msg); } } class NetMessageImpl implements IMessage { @Override public void send(String msg) { System.out.println("[NET SEND] " + msg); } } ``` --- # Agent Instructions This documentation is published with GitBook. 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