Java Annotation例子

https://www.baeldung.com/java-custom-annotation

阅读了baeldung里的例子，把code抽取出来，作为参考

package anno; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface Init { }

package anno; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD) public @interface JsonElement { public String key() default ""; }

package anno; import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) public @interface JsonSerializable { }

package anno; @JsonSerializable public class Person { @JsonElement private String firstName; @JsonElement private String lastName; @JsonElement(key = "personAge") private String age; private String address; public Person(String firstName, String lastName, String age) { this.firstName = firstName; this.lastName=lastName; this.age=age; } @Init private void initNames() { this.firstName = this.firstName.substring(0, 1).toUpperCase() + this.firstName.substring(1); this.lastName = this.lastName.substring(0, 1).toUpperCase() + this.lastName.substring(1); } // Standard getters and setters }

package anno; public class JsonSerializationException extends Exception { public JsonSerializationException(String message) { super(message); } }

package anno; import java.lang.reflect.Field; import java.lang.reflect.Method; import java.util.HashMap; import java.util.Map; import java.util.Objects; import java.util.stream.Collectors; public class ObjectToJsonConverter { public String convertToJson(Object object) throws JsonSerializationException { try { checkIfSerializable(object); initializeObject(object); return getJsonString(object); } catch (Exception e) { throw new JsonSerializationException(e.getMessage()); } } private void checkIfSerializable(Object object) throws JsonSerializationException { if (Objects.isNull(object)) { throw new JsonSerializationException("The object to serialize is null"); } Class<?> clazz = object.getClass(); if (!clazz.isAnnotationPresent(JsonSerializable.class)) { throw new JsonSerializationException("The class " + clazz.getSimpleName() + " is not annotated with JsonSerializable"); } } private void initializeObject(Object object) throws Exception { Class<?> clazz = object.getClass(); for (Method method : clazz.getDeclaredMethods()) { if (method.isAnnotationPresent(Init.class)) { method.setAccessible(true); method.invoke(object); } } } private String getJsonString(Object object) throws Exception { Class<?> clazz = object.getClass(); Map<String, String> jsonElementsMap = new HashMap<>(); for (Field field : clazz.getDeclaredFields()) { field.setAccessible(true); if (field.isAnnotationPresent(JsonElement.class)) { jsonElementsMap.put(getKey(field), (String) field.get(object)); } } String jsonString = jsonElementsMap.entrySet() .stream() .map(entry -> "\"" + entry.getKey() + "\":\"" + entry.getValue() + "\"") .collect(Collectors.joining(",")); return "{" + jsonString + "}"; } private String getKey(Field field) { JsonElement je= field.getAnnotation(JsonElement.class); if(je.key()!=null && je.key().length()>0) { return je.key(); }else { return field.getName(); } } }