Java注解實(shí)踐

標(biāo)簽 ： Java基礎(chǔ)

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注解對(duì)代碼的語意沒有直接影響, 他們只負(fù)責(zé)提供信息給相關(guān)的程序使用. 注解永遠(yuǎn)不會(huì)改變被注解代碼的含義, 但可以通過工具對(duì)被注解的代碼進(jìn)行特殊處理.

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JDK 基本Annotation

注解說明

@Override	重寫
@Deprecated	已過時(shí)
@SuppressWarnings(value = "unchecked")	壓制編輯器警告
@SafeVarargs	修飾”堆污染”警告
@FunctionalInterface	Java8特有的函數(shù)式接口

• value特權(quán)
  如果使用注解時(shí)只需要為value成員變量指定值, 則使用注解時(shí)可以直接在該注解的括號(hào)中指定value值, 而無需使用name=value的形式. 如@SuppressWarnings("unchecked")(SuppressWarnings的各種參數(shù)
  請(qǐng)參考解析 @SuppressWarnings的各種參數(shù))
• 請(qǐng)堅(jiān)持使用@Override注解: 如果在每個(gè)方法中使用Override注解來聲明要覆蓋父類聲明, 編譯器就可以替你防止大量的錯(cuò)誤.

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JDK 元Annotation

元Annotation用于修飾其他的Annotation定義.

元注解釋義

@Retention	注解保留策略
@Target	注解修飾目標(biāo)
@Documented	注解文檔提取
@Inherited	注解繼承聲明

• @Retention 注解的保留策略

@Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.ANNOTATION_TYPE) public @interface Retention {RetentionPolicy value(); }

value為SOURCE, CLASS, RUNTIME三值之一:

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 }

• @Target 指定Annotation可以放置的位置(被修飾的目標(biāo))

@Documented @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.ANNOTATION_TYPE) public @interface Target {ElementType[] value(); } public enum ElementType {/** Class, interface (including annotation type), or enum declaration */TYPE,/** Field declaration (includes enum constants) */FIELD,/** Method declaration */METHOD,/** Parameter declaration */PARAMETER,/** Constructor declaration */CONSTRUCTOR,/** Local variable declaration */LOCAL_VARIABLE,/** Annotation type declaration */ANNOTATION_TYPE,/** Package declaration */PACKAGE }

• @Documented 指定被修飾的該Annotation可以被javadoc工具提取成文檔.
• @Inherited 指定被修飾的Annotation將具有繼承性
  如果某個(gè)類使用@Xxx注解(該Annotation使用了@Inherited修飾)修飾, 則其子類自動(dòng)被@Xxx注解修飾.

Annotation

/*** Created by jifang on 15/12/22.*/ @Inherited @Target({ElementType.ANNOTATION_TYPE, ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) public @interface Testable { }

Client

public class Client {@Testpublic void client(){new SubClass();} }@Testable class SupperClass{ }class SubClass extends SupperClass{public SubClass() {for (Annotation annotation : SubClass.class.getAnnotations()){System.out.println(annotation);}} }

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自定義注解

• 根據(jù)Annotation是否包含成員變量,可以把Annotation分為兩類:
  
  • 標(biāo)記Annotation: 沒有成員變量的Annotation; 這種Annotation僅利用自身的存在與否來提供信息;
  • 元數(shù)據(jù)Annotation: 包含成員變量的Annotation; 它們可以接受(和提供)更多的元數(shù)據(jù);
• 定義新注解使用@interface關(guān)鍵字, 其定義過程與定義接口非常類似(見上面的@Testable), 需要注意的是:Annotation的成員變量在Annotation定義中是以無參的方法形式來聲明的, 其方法名和返回值類型定義了該成員變量的名字和類型, 而且我們還可以使用default關(guān)鍵字為這個(gè)成員變量設(shè)定默認(rèn)值.

@Inherited @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.METHOD, ElementType.TYPE}) public @interface Tag {String name() default "該叫啥才好呢?";String description() default "這家伙很懶, 啥也沒留下..."; }

• 自定義的Annotation繼承了Annotation這個(gè)接口, 因此自定義注解中包含了Annotation接口中所有的方法;

public interface Annotation {/*** @return true if the specified object represents an annotation* that is logically equivalent to this one, otherwise false*/boolean equals(Object obj);/*** @return the hash code of this annotation*/int hashCode();/*** @return a string representation of this annotation*/String toString();/*** Returns the annotation type of this annotation.*/Class<? extends Annotation> annotationType(); }

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提取Annotation信息

• 使用Annotation修飾了類/方法/成員變量等之后,這些Annotation不會(huì)自己生效,必須由這些注解的開發(fā)者提供相應(yīng)的工具來提取并處理Annotation信息(當(dāng)然,只有當(dāng)定義Annotation時(shí)使用了@Retention(RetentionPolicy.RUNTIME)修飾,JVM才會(huì)在裝載class文件時(shí)提取保存在class文件中的Annotation,該Annotation才會(huì)在運(yùn)行時(shí)可見,這樣我們才能夠解析).
• Java使用Annotation接口來代表程序元素前面的注解, 用AnnotatedElement接口代表程序中可以接受注解的程序元素.像Class Constructor Field Method Package這些類都實(shí)現(xiàn)了AnnotatedElement接口.

public finalclass Class<T> implements java.io.Serializable,java.lang.reflect.GenericDeclaration,java.lang.reflect.Type,java.lang.reflect.AnnotatedElement { ... } public interface AnnotatedElement {/*** Returns true if an annotation for the specified type* is present on this element, else false. This method* is designed primarily for convenient access to marker annotations.*/boolean isAnnotationPresent(Class<? extends Annotation> annotationClass);/*** Returns this element's annotation for the specified type if* such an annotation is present, else null.*/<T extends Annotation> T getAnnotation(Class<T> annotationClass);/*** Returns all annotations present on this element. */Annotation[] getAnnotations();/*** Returns all annotations that are directly present on this* element. Unlike the other methods in this interface, this method* ignores inherited annotations. (Returns an array of length zero if* no annotations are directly present on this element.) The caller of* this method is free to modify the returned array; it will have no* effect on the arrays returned to other callers.*/Annotation[] getDeclaredAnnotations(); }

這樣, 我們只需要獲取到Class Method Filed等這些實(shí)現(xiàn)了AnnotatedElement接口的類實(shí)例, 就可以獲取到我們想要的注解信息了.

/*** Created by jifang on 15/12/22.*/ public class Client {@Testpublic void client() throws NoSuchMethodException {Annotation[] annotations = this.getClass().getMethod("client").getAnnotations();for (Annotation annotation : annotations) {System.out.println(annotation.annotationType().getName());}} }

如果需要獲取某個(gè)注解中的元數(shù)據(jù),則需要強(qiáng)轉(zhuǎn)成所需的注解類型,然后通過注解對(duì)象的抽象方法來訪問這些元數(shù)據(jù):

@Tag(name = "client") public class Client {@Testpublic void client() throws NoSuchMethodException {Annotation[] annotations = this.getClass().getAnnotations();for (Annotation annotation : annotations) {if (annotation instanceof Tag) {Tag tag = (Tag) annotation;System.out.println("name: " + tag.name());System.out.println("description: " + tag.description());}}} }

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模擬Junit框架

我們用@Testable標(biāo)記哪些方法是可測(cè)試的, 只有被@Testable修飾的方法才可以被執(zhí)行.

/*** Created by jifang on 15/12/27.*/ @Inherited @Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface Testable { }

如下定義TestCase測(cè)試用例定義了6個(gè)方法, 其中有4個(gè)被@Testable修飾了:

public class TestCase {@Testablepublic void test1() {System.out.println("test1");}public void test2() throws IOException {System.out.println("test2");throw new IOException("我test2出錯(cuò)啦...");}@Testablepublic void test3() {System.out.println("test3");throw new RuntimeException("我test3出錯(cuò)啦...");}public void test4() {System.out.println("test4");}@Testablepublic void test5() {System.out.println("test5");}@Testablepublic void test6() {System.out.println("test6");} }

為了讓程序中的這些注解起作用, 必須為這些注解提供一個(gè)注解處理工具.

/*** Created by jifang on 15/12/27.*/ public class TestableProcessor {public static void process(String clazz) throws ClassNotFoundException, IllegalAccessException, InstantiationException {int passed = 0;int failed = 0;Object obj = Class.forName(clazz).newInstance();for (Method method : Class.forName(clazz).getMethods()) {if (method.isAnnotationPresent(Testable.class)) {try {method.invoke(obj);++passed;} catch (IllegalAccessException | InvocationTargetException e) {System.out.println("method " + method.getName() + " execute error: < " + e.getCause() + " >");e.printStackTrace(System.out);++failed;}}}System.out.println("共運(yùn)行" + (failed + passed) + "個(gè)方法, 成功" + passed + "個(gè), 失敗" + failed + "個(gè)");}public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {TestableProcessor.process("com.feiqing.annotation.TestCase");} }

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拋出特定異常

前面介紹的只是一個(gè)標(biāo)記Annotation,程序通過判斷Annotation是否存在來決定是否運(yùn)行指定方法,現(xiàn)在我們要針對(duì)只在拋出特殊異常時(shí)才成功添加支持,這樣就用到了具有成員變量的注解了:

/*** Created by jifang on 15/12/28.*/ @Inherited @Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface TestableException {Class<? extends Throwable>[] value(); }

• TestCase

/*** Created by jifang on 15/12/27.*/ public class TestCase {public void test1() {System.out.println("test1");}@TestableException(ArithmeticException.class)public void test2() throws IOException {int i = 1 / 0;System.out.println(i);}@TestableException(ArithmeticException.class)public void test3() {System.out.println("test3");throw new RuntimeException("我test3出錯(cuò)啦...");}public void test4() {System.out.println("test4");}@TestableException({ArithmeticException.class, IOException.class})public void test5() throws FileNotFoundException {FileInputStream stream = new FileInputStream("xxxx");}@Testablepublic void test6() {System.out.println("test6");} }

• 注解處理器

public class TestableExceptionProcessor {public static void process(String clazz) throws ClassNotFoundException, IllegalAccessException, InstantiationException {int passed = 0;int failed = 0;Object obj = Class.forName(clazz).newInstance();for (Method method : Class.forName(clazz).getMethods()) {if (method.isAnnotationPresent(TestableException.class)) {try {method.invoke(obj, null);// 沒有拋出異常(失敗)++failed;} catch (InvocationTargetException e) {// 獲取異常的引發(fā)原因Throwable cause = e.getCause();int oldPassed = passed;for (Class excType : method.getAnnotation(TestableException.class).value()) {// 是我們期望的異常類型之一(成功)if (excType.isInstance(cause)) {++passed;break;}}// 并不是我們期望的異常類型(失敗)if (oldPassed == passed) {++failed;System.out.printf("Test <%s> failed <%s> %n", method, e);}}}}System.out.println("共運(yùn)行" + (failed + passed) + "個(gè)方法, 成功" + passed + "個(gè), 失敗" + failed + "個(gè)");}public static void main(String[] args) throws IllegalAccessException, InstantiationException, ClassNotFoundException {process("com.feiqing.annotation.TestCase");} }

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注解添加監(jiān)聽器

下面通過使用Annotation簡(jiǎn)化事件編程, 在傳統(tǒng)的代碼中總是需要通過addActionListener方法來為事件源綁定事件監(jiān)聽器:

/*** Created by jifang on 15/12/27.*/ public class SwingPro {private JFrame mainWin = new JFrame("使用注解綁定事件監(jiān)聽器");private JButton ok = new JButton("確定");private JButton cancel = new JButton("取消");public void init() {JPanel jp = new JPanel();// 為兩個(gè)按鈕設(shè)置監(jiān)聽事件ok.addActionListener(new OkListener());cancel.addActionListener(new CancelListener());jp.add(ok);jp.add(cancel);mainWin.add(jp);mainWin.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);mainWin.pack();mainWin.setVisible(true);}public static void main(String[] args) {new SwingPro().init();} }class OkListener implements ActionListener {@Overridepublic void actionPerformed(ActionEvent e) {JOptionPane.showMessageDialog(null, "你點(diǎn)擊了確認(rèn)按鈕!");} }class CancelListener implements ActionListener {@Overridepublic void actionPerformed(ActionEvent e) {JOptionPane.showMessageDialog(null, "你點(diǎn)擊了取消按鈕!");} }

下面我們?cè)撚米⒔饨壎ūO(jiān)聽器:

• 首先, 我們需要自定義一個(gè)注解

/*** Created by jifang on 15/12/27.*/ @Inherited @Target(ElementType.FIELD) @Retention(RetentionPolicy.RUNTIME) public @interface ActionListenerFor {Class<? extends ActionListener> listener(); }

• 然后還要一個(gè)注解處理器

/*** Created by jifang on 15/12/27.*/ public class ActionListenerInstaller {public static void install(Object targetObject) throws IllegalAccessException, InstantiationException {for (Field field : targetObject.getClass().getDeclaredFields()) {// 如果該成員變量被ActionListenerFor標(biāo)記了if (field.isAnnotationPresent(ActionListenerFor.class)) {// 設(shè)置訪問權(quán)限field.setAccessible(true);// 獲取到成員變量的值AbstractButton targetButton = (AbstractButton) field.get(targetObject);// 獲取到注解中的ListenerClass<? extends ActionListener> listener = field.getAnnotation(ActionListenerFor.class).listener();// 添加到成員變量中targetButton.addActionListener(listener.newInstance());}}} }

• 主程序(注意注釋處)

public class SwingPro {private JFrame mainWin = new JFrame("使用注解綁定事件監(jiān)聽器");/*** 使用注解設(shè)置Listener*/@ActionListenerFor(listener = OkListener.class)private JButton ok = new JButton("確定");@ActionListenerFor(listener = CancelListener.class)private JButton cancel = new JButton("取消");public SwingPro init() {JPanel jp = new JPanel();// 使得注解生效try {ActionListenerInstaller.install(this);} catch (IllegalAccessException | InstantiationException e) {e.printStackTrace(System.out);}jp.add(ok);jp.add(cancel);mainWin.add(jp);mainWin.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);mainWin.pack();mainWin.setVisible(true);return this;}//下同 }

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重復(fù)注解

在Java5到Java7這段時(shí)間里, 同一個(gè)程序元素前只能使用一個(gè)相同類型的Annotation; 如果需要在同一個(gè)元素前使用多個(gè)相同的Annotation, 則必須使用Annotation容器(在Java8中, 對(duì)這種情況做了改善, 但其實(shí)也只是一種寫法上的簡(jiǎn)化, 其本質(zhì)還是一樣的).由于在實(shí)際開發(fā)中,Java8還未大面積的使用, 因此在此只介紹Java7中重復(fù)注解定義與使用.

• Table Annotation定義(代表數(shù)據(jù)庫表)

/*** Created by jifang on 15/12/27.*/ @Inherited @Retention(RetentionPolicy.RUNTIME) public @interface Table {String name() default "表名是啥?";String description() default "這家伙很懶, 啥也沒留下..."; }

• Table 容器

@Inherited @Retention(RetentionPolicy.RUNTIME) public @interface Tables {Table[] value(); }

注意: 容器注解的保留期必須比它所包含的注解的保留期更長, 否則JVM會(huì)丟棄容器, 相應(yīng)的注解也就丟失了.

• Client
  使用時(shí)需要用Table容器來盛裝Table注解

@Tables({@Table(name = "t_user", description = "用戶表"),@Table(name = "t_feed", description = "動(dòng)態(tài)表") }) public class Client {@Testpublic void client() {Tables tableArray = this.getClass().getAnnotation(Tables.class);Table[] tables = tableArray.value();for (Table table : tables) {System.out.println(table.name() + " : " + table.description());}} }

在Java8中, 可以直接使用

@Table(name = "t_user", description = "用戶表") @Table(name = "t_feed", description = "動(dòng)態(tài)表")

的形式來注解Client, 但@Tables還是需要開發(fā)者來寫的, 由此可以看出, 重復(fù)注解只是一種簡(jiǎn)化寫法, 這種寫法只是一種假象: 多個(gè)重復(fù)注解其實(shí)會(huì)被作為容器注解的value成員.

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參考:

Effective Java

瘋狂Java講義

Java核心技術(shù)

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