java语言注解基础概念详解

/** 
 * 获取指定类型的注解 
 */
public <A extends Annotation> A getAnnotation(Class<A> annotationType);
/** 
 * 获取所有注解，如果有的话 
 */
public Annotation[] getAnnotations();
/** 
 * 获取所有注解，忽略继承的注解 
 */
public Annotation[] getDeclaredAnnotations();
/** 
 * 指定注解是否存在该元素上，如果有则返回true，否则false 
 */
public Boolean isAnnotationPresent(Class<? extends Annotation> annotationType);
/** 
 * 获取Method中参数的所有注解 
 */
public Annotation[][] getParameterAnnotations();

// 适用类、接口（包括注解类型）或枚举 
@Retention(RetentionPolicy.RUNTIME) 
@Target(ElementType.TYPE) 
public @interface ClassInfo { 
  String value(); 
} 
// 适用field属性，也包括enum常量 
@Retention(RetentionPolicy.RUNTIME) 
@Target(ElementType.FIELD) 
public @interface FieldInfo { 
  int[] value(); 
} 
// 适用方法 
@Retention(RetentionPolicy.RUNTIME) 
@Target(ElementType.METHOD) 
public @interface MethodInfo { 
  String name() default "long"; 
  String data(); 
  int age() default 27; 
} 

/** 
 * 测试运行时注解 
 */
@ClassInfo("Test Class") 
public class TestRuntimeAnnotation {
	@FieldInfo(value = {1, 2}) 
	  public String fieldInfo = "FiledInfo";
	@FieldInfo(value = {10086}) 
	  public int i = 100;
	@MethodInfo(name = "BlueBird", data = "Big") 
	  public static String getMethodInfo() {
		return TestRuntimeAnnotation.class.getSimpleName();
	}
}

/** 
 * 测试运行时注解 
 */
private void _testRuntimeAnnotation() {
	StringBuffer sb = new StringBuffer();
	Class<?> cls = TestRuntimeAnnotation.class;
	Constructor<?>[] constructors = cls.getConstructors();
	// 获取指定类型的注解 
	sb.append("Class注解：").append("\n");
	ClassInfo classInfo = cls.getAnnotation(ClassInfo.class);
	if (classInfo != null) {
		sb.append(Modifier.toString(cls.getModifiers())).append(" ") 
		        .append(cls.getSimpleName()).append("\n");
		sb.append("注解值: ").append(classInfo.value()).append("\n\n");
	}
	sb.append("Field注解：").append("\n");
	Field[] fields = cls.getDeclaredFields();
	for (Field field : fields) {
		FieldInfo fieldInfo = field.getAnnotation(FieldInfo.class);
		if (fieldInfo != null) {
			sb.append(Modifier.toString(field.getModifiers())).append(" ") 
			          .append(field.getType().getSimpleName()).append(" ") 
			          .append(field.getName()).append("\n");
			sb.append("注解值: ").append(Arrays.toString(fieldInfo.value())).append("\n\n");
		}
	}
	sb.append("Method注解：").append("\n");
	Method[] methods = cls.getDeclaredMethods();
	for (Method method : methods) {
		MethodInfo methodInfo = method.getAnnotation(MethodInfo.class);
		if (methodInfo != null) {
			sb.append(Modifier.toString(method.getModifiers())).append(" ") 
			          .append(method.getReturnType().getSimpleName()).append(" ") 
			          .append(method.getName()).append("\n");
			sb.append("注解值: ").append("\n");
			sb.append("name: ").append(methodInfo.name()).append("\n");
			sb.append("data: ").append(methodInfo.data()).append("\n");
			sb.append("age: ").append(methodInfo.age()).append("\n");
		}
	}
	System.out.print(sb.toString());
}