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

• 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.

• @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;
• 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Class Constructor Field Method Package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.

参考:

Effective Java

疯狂Java讲义

Java核心技术