在运行时给注解传值
需求：假设DeviceData类有
@Entity这个注解，需要在运行时修改value的值。
@Entity(value="data")
public class DeviceData {
private String deviceId;
private Long timestamp;
private String data;
//省略setter、getter...
}
以下代码经测试可行。该代码只是测试，并非完全针对上述需求。
/**
* Changes the annotation value for the given key of the given annotation to newValue and returns
* the previous value.
*/
@SuppressWarnings("unchecked")
public static Object changeAnnotationValue(Annotation annotation, String key, Object newValue){
Object handler = Proxy.getInvocationHandler(annotation);
Field f;
try {
f = handler.getClass().getDeclaredField("memberValues");
} catch (NoSuchFieldException | SecurityException e) {
throw new IllegalStateException(e);
}
f.setAccessible(true);
Map<String, Object> memberValues;
try {
memberValues = (Map<String, Object>) f.get(handler);
} catch (IllegalArgumentException | IllegalAccessException e) {
throw new IllegalStateException(e);
}
Object oldValue = memberValues.get(key);
if (oldValue == null || oldValue.getClass() != newValue.getClass()) {
throw new IllegalArgumentException();
}
memberValues.put(key,newValue); return oldValue;}
使用例子
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface ClassAnnotation {
String value() default "";
}
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface FieldAnnotation {
String value() default "";
}
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface MethodAnnotation {
String value() default "";
}
@ClassAnnotation("class test")
public static class TestClass{
@FieldAnnotation("field test")
public Object field;
@MethodAnnotation("method test")
public void method(){ }
}
public static void main(String[] args) throws Exception {
final ClassAnnotation classAnnotation = TestClass.class.getAnnotation(ClassAnnotation.class);
System.out.println("old ClassAnnotation = " + classAnnotation.value());
changeAnnotationValue(classAnnotation, "value", "another class annotation value");
System.out.println("modified ClassAnnotation = " + classAnnotation.value());
Field field = TestClass.class.getField("field");
final FieldAnnotation fieldAnnotation =
field.getAnnotation(FieldAnnotation.class);
System.out.println("old FieldAnnotation = " +
fieldAnnotation.value());
changeAnnotationValue(fieldAnnotation, "value", "another field annotation value");
System.out.println("modified FieldAnnotation = " + fieldAnnotation.value());
Method method = TestClass.class.getMethod("method");
final MethodAnnotation methodAnnotation = method.getAnnotation(MethodAnnotation.class);
System.out.println("old MethodAnnotation = " + methodAnnotation.value());
changeAnnotationValue(methodAnnotation, "value", "another method annotation value");
System.out.println("modified MethodAnnotation = " + methodAnnotation.value());}
