Java核心知识体系2：注解机制详解

class Parent {    public void rewriteMethod() {
        
    }
}class Child extends Parent {    /**
        * 重载父类的 rewriteMethod() 方法
        */
    @Override
    public void rewriteMethod() {
    }    /**
        * 被弃用的过时方法
        */
    @Deprecated
    public void oldMethod() {
    }    /**
        * 忽略告警
        * 
        * @return
        */
    @SuppressWarnings("keep run")    public List infoList() {        List list = new ArrayList();        return list;
    }
}

@Target(ElementType.METHOD)@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

@Documented@Retention(RetentionPolicy.RUNTIME)@Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
public @interface Deprecated {
}

@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})@Retention(RetentionPolicy.SOURCE)
public @interface SuppressWarnings {    String[] value();
}

public enum ElementType { 
    TYPE, // 类、接口、枚举类 
    FIELD, // 成员变量（包括：枚举常量） 
    METHOD, // 成员方法 
    PARAMETER, // 方法参数 
    CONSTRUCTOR, // 构造方法 
    LOCAL_VARIABLE, // 局部变量 
    ANNOTATION_TYPE, // 注解类 
    PACKAGE, // 可用于修饰：包 
    TYPE_PARAMETER, // 类型参数，JDK 1.8 新增 
    TYPE_USE // 使用类型的任何地方，JDK 1.8 新增 }

public enum RetentionPolicy { 
    SOURCE,    // 源文件保留
    CLASS,       // 编译期保留，默认为该值，CLASS
    RUNTIME   // 运行期保留，可通过反射去获取注解信息}

@Retention(RetentionPolicy.SOURCE)
public @interface SourcePolicy {  // 源文件保留策略}@Retention(RetentionPolicy.CLASS)
public @interface ClassPolicy {  // 编译器保留策略}@Retention(RetentionPolicy.RUNTIME)
public @interface RuntimePolicy {  // 运行期保留策略}

public class RetentionTest { 
	@SourcePolicy
	public void sourcePolicy() {
	} 
	@ClassPolicy
	public void classPolicy() {
	} 
	@RuntimePolicy
	public void runtimePolicy() {
	}
}

{  public retention.RetentionTest();
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial   #1            // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0

  public void sourcePolicy();
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 7: 0

  public void classPolicy();
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 11: 0
    RuntimeInvisibleAnnotations:
      0: #11()

  public void runtimePolicy();
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 15: 0
    RuntimeVisibleAnnotations:
      0: #14()}

import java.lang.annotation.Documented;import java.lang.annotation.ElementType;import java.lang.annotation.Target; 
@Documented@Target({ElementType.TYPE,ElementType.METHOD})public @interface DocAnnotation { 
	public String value() default "default";
}

@DocAnnotation("some method doc")public void testMethod() {  // 测试方法的文档注解}

@Inherited@Retention(RetentionPolicy.RUNTIME)@Target({ElementType.TYPE,ElementType.METHOD})
public @interface InheritedAnnotation {    String [] values();    int number();
}

@InheritedAnnotation(values = {"brand"}, number = 100)
public class UserInfo {
}class Customer extends UserInfo {	@Test
    public void testMethod(){        Class clazz = Student.class;        Annotation[] annotations = clazz.getAnnotations();        for (Annotation annotation : annotations) {            System.out.println(annotation.toString());
        }
    }
}

xxx.InheritedAnnotation(values=[brand], number=100)

public @interface Pet {     String myPet();
}public @interface Pets {    Pet[] value();
}public class RepeatAnnotationOV {    @Pets({@Pet(myPet="dog"),@Pet(myPet="cat")})
    public void workMethod(){
    }
}

@Repeatable(Pets.class)
public @interface Pet {     String myPet();
}public @interface Pets {    Pet[] value();
}public class RepeatAnnotationNV {    @Pet(role="dog")    @Pet(role="cat")
    public void workMethod(){ }
}

package com.helenlyn.common.annotation;import java.lang.annotation.Documented;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;/**
 * <p>Description: 水果供应者注解        </p>
 * <p>Copyright: Copyright (c) 2021 </p>
 * <p>Company: helenlyn Co., Ltd.             </p>
 *
 * @author brand
 * @date 2021/5/16 16:35
 * <p>Update Time:                      </p>
 * <p>Updater:                          </p>
 * <p>Update Comments:                  </p>
 */@Target(ElementType.FIELD)@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface FruitProvider {    /**
     * 供应商编号
     * @return
     */
    public int id() default -1;    /**
     * 供应商名称
     * @return
     */
    public String name() default "";    /**
     * 供应商地址
     * @return
     */
    public String address() default "";
}

package com.helenlyn.common.dto;import com.helenlyn.common.annotation.FruitColor;import com.helenlyn.common.annotation.FruitName;import com.helenlyn.common.annotation.FruitProvider;/**
 * <p>Description:               </p>
 * <p>Copyright: Copyright (c) 2021 </p>
 * <p>Company: helenlyn Co., Ltd.             </p>
 *
 * @author brand
 * @date 2021/5/16 16:28
 * <p>Update Time:                      </p>
 * <p>Updater:                          </p>
 * <p>Update Comments:                  </p>
 */public class AppleDto {   @FruitName("Apple")
    private String appleName;    @FruitColor(fruitColor=FruitColor.Color.RED)
    private String appleColor;    @FruitProvider(id=1,name="helenlyn 贸易公司",address="福州xx路xxx大楼")
    private String appleProvider;
}

/**
 * <p>Description: FruitInfoUtil注解实现 </p>
 * <p>Copyright: Copyright (c) 2021 </p>
 * <p>Company: helenlyn Co., Ltd.             </p>
 *
 * @author brand
 * @date 2021/5/16 16:37
 * <p>Update Time:                      </p>
 * <p>Updater:                          </p>
 * <p>Update Comments:                  </p>
 */
public class FruitInfoUtil {
    public static String getFruitInfo(Class<?> clazz) {
        String strFruitName = " 水果名称：";
        String strFruitColor = " 水果颜色：";
        String strFruitProvicer = "供应商信息：";

        Field[] fields = clazz.getDeclaredFields();        for (Field field : fields) {            if (field.isAnnotationPresent(FruitName.class)) {
                FruitName fruitName = (FruitName) field.getAnnotation(FruitName.class);
                strFruitName += fruitName.value();
                System.out.println(strFruitName);
            } else if (field.isAnnotationPresent(FruitColor.class)) {
                FruitColor fruitColor = (FruitColor) field.getAnnotation(FruitColor.class);
                strFruitColor += fruitColor.fruitColor().toString();
                System.out.println(strFruitColor);
            } else if (field.isAnnotationPresent(FruitProvider.class)) {
                FruitProvider fruitProvider = (FruitProvider) field.getAnnotation(FruitProvider.class);
                strFruitProvicer = " 供应商编号：" + fruitProvider.id() + " 供应商名称：" + fruitProvider.name() + " 供应商地址：" + fruitProvider.address();
                System.out.println(strFruitProvicer);
            }
        }        return String.format("%s;%s;%s;", strFruitName, strFruitColor, strFruitProvicer);
    }
}

2022-07-09 11:33:41.688  INFO 5895 --- [TaskExecutor-35] o.s.a.r.c.CachingConnectionFactory       : Attempting to connect to: cl-debug-rabbitmq-erp-service-7w0cpa.docker.sdp:9146Hibernate: update UserBasicInfo set personName=? where personCode=?
 水果名称：Apple
 水果颜色：RED
 供应商编号：1 供应商名称：helenlyn 贸易公司 供应商地址：福州xx路xxx大楼

// 自定义ElementType.TYPE_PARAMETER注解@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.TYPE_PARAMETER)public @interface MyNotEmpty {
}// 自定义ElementType.TYPE_USE注解@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.TYPE_USE)public @interface MyNotNull {
}// 测试类public class TypeParameterAndTypeUseAnnotation<@MyNotEmpty T>{  //使用TYPE_PARAMETER类型，会编译不通过//		public @MyNotEmpty T test(@MyNotEmpty T a){//			new ArrayList<@MyNotEmpty String>();//				return a;//		}

  //使用TYPE_USE类型，编译通过
  public @MyNotNull T test2(@MyNotNull T a){    new ArrayList<@MyNotNull String>();    return a;
  }
}

/**
 * @author brand
 * @Description: 数据源切换注解
 * @Copyright: Copyright (c) 2021
 * @Company: Helenlyn, Inc. All Rights Reserved.
 * @date 2021/12/15 7:36 下午
 */@Target({ ElementType.TYPE, ElementType.METHOD})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface DataSource {
    String name() default "";
}

/**
 * @author brand
 * @Description:
 * @Copyright: Copyright (c) 2021
 * @Company: Helenlyn, Inc. All Rights Reserved.
 * @date 2021/12/15 7:49 下午
 */@Aspect@Componentpublic class DataSourceAspect implements Ordered  {    /**
     * 定义一个切入点，匹配到上面的注解DataSource
     */
    @Pointcut("@annotation(com.helenlyn.dataassist.annotation.DataSource)")
    public void dataSourcePointCut() {
    }    /**
     * Around 环绕方式做切面注入
     * @param point
     * @return
     * @throws Throwable
     */
    @Around("dataSourcePointCut()")
    public Object around(ProceedingJoinPoint point) throws Throwable {        MethodSignature signature = (MethodSignature) point.getSignature();        Method method = signature.getMethod();        DataSource ds = method.getAnnotation(DataSource.class);        String routeKey = ds.name();  // 从头部中取出注解的name（basic 或 cloudoffice 或 attend），用这个name进行数据源查找。
        String dataSourceRouteKey = DynamicDataSourceRouteHolder.getDataSourceRouteKey();        if (StringUtils.isNotEmpty(dataSourceRouteKey)) {            // StringBuilder currentRouteKey = new StringBuilder(dataSourceRouteKey);
            routeKey = ds.name();
        }
        DynamicDataSourceRouteHolder.setDataSourceRouteKey(routeKey);        try {            return point.proceed();
        } finally { // 最后做清理，这个步骤很重要，因为我们的配置中有一个默认的数据源，执行完要回到默认的数据源。
            DynamicDataSource.clearDataSource();
            DynamicDataSourceRouteHolder.clearDataSourceRouteKey();
        }
    }    @Override
    public int getOrder() {        return 1;
    }
}

/**
     * 无注解默认情况：数据源指向basic
     * @return
     */
    @RequestMapping(value = "/default/{user_code}", method = RequestMethod.GET)
    public UserInfoDto getUserInfo(@PathVariable("user_code") String userCode) {        return userInfoService.getUserInfo(userCode);
    }    /**
     * 数据源指向attend
     * @return
     */
    @DataSource(name= Constant.DATA_SOURCE_ATTEND_NAME)
    @RequestMapping(value = "/attend/{user_code}", method = RequestMethod.GET)    public UserInfoDto getUserInfoAttend(@PathVariable("user_code") String userCode) {        return userInfoService.getUserInfo(userCode);
    }    /**
     * 数据源指向cloud
     * @return
     */
    @DataSource(name= Constant.DATA_SOURCE_CLOUD_NAME)
    @RequestMapping(value = "/cloud/{user_code}", method = RequestMethod.GET)    public UserInfoDto getUserInfoCloud(@PathVariable("user_code") String userCode) {        return userInfoService.getUserInfo(userCode);
    }