Java注解实践
惠泳
Java注解实践
标签 ： Java基础
注解对代码的语意没有直接影响, 他们只负责提供信息给相关的程序使用. 注解永远不会改变被注解代码的含义, 但可以通过工具对被注解的代码进行特殊处理.
JDK 基本Annotation
| 注解 | 说明 |
|---|---|
@Override | 重写 |
@Deprecated | 已过时 |
@SuppressWarnings(value = "unchecked") | 压制编辑器警告 |
@SafeVarargs | 修饰”堆污染”警告 |
@FunctionalInterface | Java8特有的函数式接口 |
value特权
如果使用注解时只需要为value成员变量指定值, 则使用注解时可以直接在该注解的括号中指定value值, 而无需使用name=value的形式. 如@SuppressWarnings("unchecked")(SuppressWarnings的各种参数
请参考解析 @SuppressWarnings的各种参数)- 请坚持使用
@Override注解: 如果在每个方法中使用Override注解来声明要覆盖父类声明, 编译器就可以替你防止大量的错误.
JDK 元Annotation
元
Annotation用于修饰其他的Annotation定义.
| 元注解 | 释义 |
|---|---|
@Retention | 注解保留策略 |
@Target | 注解修饰目标 |
@Documented | 注解文档提取 |
@Inherited | 注解继承声明 |
@Retention注解的保留策略
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
RetentionPolicy value();
}value为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(被修饰的目标)
@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指定被修饰的该Annotation可以被javadoc工具提取成文档.@Inherited指定被修饰的Annotation将具有继承性
如果某个类使用@Xxx注解(该Annotation使用了@Inherited修饰)修饰, 则其子类自动被@Xxx注解修饰.
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 {
@Test
public void client(){
new SubClass();
}
}
@Testable
class SupperClass{
}
class SubClass extends SupperClass{
public SubClass() {
for (Annotation annotation : SubClass.class.getAnnotations()){
System.out.println(annotation);
}
}
}自定义注解
- 根据
Annotation是否包含成员变量,可以把Annotation分为两类:
- 标记
Annotation: 没有成员变量的Annotation; 这种Annotation仅利用自身的存在与否来提供信息; - 元数据
Annotation: 包含成员变量的Annotation; 它们可以接受(和提供)更多的元数据;
- 标记
- 定义新注解使用
@interface关键字, 其定义过程与定义接口非常类似(见上面的@Testable), 需要注意的是:Annotation的成员变量在Annotation定义中是以无参的方法形式来声明的, 其方法名和返回值类型定义了该成员变量的名字和类型, 而且我们还可以使用default关键字为这个成员变量设定默认值.
@Inherited
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.TYPE})
public @interface Tag {
String name() default "该叫啥才好呢?";
String description() default "这家伙很懒, 啥也没留下...";
}- 自定义的Annotation继承了
Annotation这个接口, 因此自定义注解中包含了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();
}
提取Annotation信息
- 使用
Annotation修饰了类/方法/成员变量等之后,这些Annotation不会自己生效,必须由这些注解的开发者提供相应的工具来提取并处理Annotation信息(当然,只有当定义Annotation时使用了@Retention(RetentionPolicy.RUNTIME)修饰,JVM才会在装载class文件时提取保存在class文件中的Annotation,该Annotation才会在运行时可见,这样我们才能够解析). - Java使用
Annotation接口来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ClassConstructorFieldMethodPackage这些类都实现了AnnotatedElement接口.
public final
class 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();
}这样, 我们只需要获取到Class Method Filed等这些实现了AnnotatedElement接口的类实例, 就可以获取到我们想要的注解信息了.
/**
* Created by jifang on 15/12/22.
*/
public class Client {
@Test
public void client() throws NoSuchMethodException {
Annotation[] annotations = this.getClass().getMethod("client").getAnnotations();
for (Annotation annotation : annotations) {
System.out.println(annotation.annotationType().getName());
}
}
}如果需要获取某个注解中的元数据,则需要强转成所需的注解类型,然后通过注解对象的抽象方法来访问这些元数据:
@Tag(name = "client")
public class Client {
@Test
public 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());
}
}
}
}模拟Junit框架
我们用@Testable标记哪些方法是可测试的, 只有被@Testable修饰的方法才可以被执行.
/**
* Created by jifang on 15/12/27.
*/
@Inherited
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Testable {
}如下定义TestCase测试用例定义了6个方法, 其中有4个被@Testable修饰了:
public class TestCase {
@Testable
public void test1() {
System.out.println("test1");
}
public void test2() throws IOException {
System.out.println("test2");
throw new IOException("我test2出错啦...");
}
@Testable
public void test3() {
System.out.println("test3");
throw new RuntimeException("我test3出错啦...");
}
public void test4() {
System.out.println("test4");
}
@Testable
public void test5() {
System.out.println("test5");
}
@Testable
public void test6() {
System.out.println("test6");
}
}为了让程序中的这些注解起作用, 必须为这些注解提供一个注解处理工具.
/**
* 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("共运行" + (failed + passed) + "个方法, 成功" + passed + "个, 失败" + failed + "个");
}
public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
TestableProcessor.process("com.feiqing.annotation.TestCase");
}
}抛出特定异常
前面介绍的只是一个标记Annotation,程序通过判断Annotation是否存在来决定是否运行指定方法,现在我们要针对只在抛出特殊异常时才成功添加支持,这样就用到了具有成员变量的注解了:
/**
* 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出错啦...");
}
public void test4() {
System.out.println("test4");
}
@TestableException({ArithmeticException.class, IOException.class})
public void test5() throws FileNotFoundException {
FileInputStream stream = new FileInputStream("xxxx");
}
@Testable
public void test6() {
System.out.println("test6");
}
}- 注解处理器
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);
// 没有抛出异常(失败)
++failed;
} catch (InvocationTargetException e) {
// 获取异常的引发原因
Throwable cause = e.getCause();
int oldPassed = passed;
for (Class excType : method.getAnnotation(TestableException.class).value()) {
// 是我们期望的异常类型之一(成功)
if (excType.isInstance(cause)) {
++passed;
break;
}
}
// 并不是我们期望的异常类型(失败)
if (oldPassed == passed) {
++failed;
System.out.printf("Test <%s> failed <%s> %n", method, e);
}
}
}
}
System.out.println("共运行" + (failed + passed) + "个方法, 成功" + passed + "个, 失败" + failed + "个");
}
public static void main(String[] args) throws IllegalAccessException, InstantiationException, ClassNotFoundException {
process("com.feiqing.annotation.TestCase");
}
}
注解添加监听器
下面通过使用Annotation简化事件编程, 在传统的代码中总是需要通过addActionListener方法来为事件源绑定事件监听器:
/**
* Created by jifang on 15/12/27.
*/
public class SwingPro {
private JFrame mainWin = new JFrame("使用注解绑定事件监听器");
private JButton ok = new JButton("确定");
private JButton cancel = new JButton("取消");
public void init() {
JPanel jp = new JPanel();
// 为两个按钮设置监听事件
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 {
@Override
public void actionPerformed(ActionEvent e) {
JOptionPane.showMessageDialog(null, "你点击了确认按钮!");
}
}
class CancelListener implements ActionListener {
@Override
public void actionPerformed(ActionEvent e) {
JOptionPane.showMessageDialog(null, "你点击了取消按钮!");
}
}下面我们该用注解绑定监听器:
- 首先, 我们需要自定义一个注解
/**
* Created by jifang on 15/12/27.
*/
@Inherited
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface ActionListenerFor {
Class<? extends ActionListener> listener();
}- 然后还要一个注解处理器
/**
* 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()) {
// 如果该成员变量被ActionListenerFor标记了
if (field.isAnnotationPresent(ActionListenerFor.class)) {
// 设置访问权限
field.setAccessible(true);
// 获取到成员变量的值
AbstractButton targetButton = (AbstractButton) field.get(targetObject);
// 获取到注解中的Listener
Class<? extends ActionListener> listener = field.getAnnotation(ActionListenerFor.class).listener();
// 添加到成员变量中
targetButton.addActionListener(listener.newInstance());
}
}
}
}- 主程序(注意注释处)
public class SwingPro {
private JFrame mainWin = new JFrame("使用注解绑定事件监听器");
/**
* 使用注解设置Listener
*/
@ActionListenerFor(listener = OkListener.class)
private JButton ok = new JButton("确定");
@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
}重复注解
在Java5到Java7这段时间里, 同一个程序元素前只能使用一个相同类型的Annotation; 如果需要在同一个元素前使用多个相同的Annotation, 则必须使用Annotation容器(在Java8中, 对这种情况做了改善, 但其实也只是一种写法上的简化, 其本质还是一样的).由于在实际开发中,Java8还未大面积的使用, 因此在此只介绍Java7中重复注解定义与使用.
- Table Annotation定义(代表数据库表)
/**
* Created by jifang on 15/12/27.
*/
@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
String name() default "表名是啥?";
String description() default "这家伙很懒, 啥也没留下...";
}- Table 容器
@Inherited
@Retention(RetentionPolicy.RUNTIME)
public @interface Tables {
Table[] value();
}注意: 容器注解的保留期必须比它所包含的注解的保留期更长, 否则JVM会丢弃容器, 相应的注解也就丢失了.
- Client
使用时需要用Table容器来盛装Table注解
@Tables({
@Table(name = "t_user", description = "用户表"),
@Table(name = "t_feed", description = "动态表")
})
public class Client {
@Test
public 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 = "用户表")
@Table(name = "t_feed", description = "动态表")的形式来注解Client, 但@Tables还是需要开发者来写的, 由此可以看出, 重复注解只是一种简化写法, 这种写法只是一种假象: 多个重复注解其实会被作为容器注解的value成员.
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