是否可以在运行时向对象（特别是在我的情况下，方法）添加注释？
关于更多解释：我有两个模块，模块A和模块B。模块B依赖于模块A，模块A不依赖于任何东西。（modA是我的核心数据类型和接口等，modB是db/data层）modB也依赖于外部库。在我的例子中，modB正在将一个类从modA移交给外部库，这需要某些方法来进行注释。特定的注释都是ex externalLib的一部分，正如我所说，modA不依赖于exdocialLib，我想保持这种方式。
那么，这是否可能，或者您对解决此问题的其他方法有什么建议？
这可以通过Javassist等字节码检测库来实现。
特别是，请查看 AnnotationsAttribute 类，了解如何创建/设置注释的示例，以及有关如何操作类文件的一般准则的字节码 API 教程部分。
不过，这绝不是简单明了的 - 我不建议使用这种方法，并建议您考虑Tom的答案，除非您需要为大量类执行此操作（或者所述类在运行时之前不可用，因此编写适配器是不可能的）。
还可以在运行时使用 Java 反射 API 向 Java 类添加注释。从本质上讲，必须重新创建在类中定义的内部注释映射（或者对于在内部类中定义的Java 8）。当然，这种方法非常棘手，对于较新的Java版本，可能随时会中断。但是对于快速和肮脏的测试/原型设计，这种方法有时很有用。java.lang.Classjava.lang.Class.AnnotationData
Java 8 的概念示例：
public final class RuntimeAnnotations {
private static final Constructor<?> AnnotationInvocationHandler_constructor;
private static final Constructor<?> AnnotationData_constructor;
private static final Method Class_annotationData;
private static final Field Class_classRedefinedCount;
private static final Field AnnotationData_annotations;
private static final Field AnnotationData_declaredAnotations;
private static final Method Atomic_casAnnotationData;
private static final Class<?> Atomic_class;
static{
// static initialization of necessary reflection Objects
try {
Class<?> AnnotationInvocationHandler_class = Class.forName("sun.reflect.annotation.AnnotationInvocationHandler");
AnnotationInvocationHandler_constructor = AnnotationInvocationHandler_class.getDeclaredConstructor(new Class[]{Class.class, Map.class});
AnnotationInvocationHandler_constructor.setAccessible(true);
Atomic_class = Class.forName("java.lang.Class$Atomic");
Class<?> AnnotationData_class = Class.forName("java.lang.Class$AnnotationData");
AnnotationData_constructor = AnnotationData_class.getDeclaredConstructor(new Class[]{Map.class, Map.class, int.class});
AnnotationData_constructor.setAccessible(true);
Class_annotationData = Class.class.getDeclaredMethod("annotationData");
Class_annotationData.setAccessible(true);
Class_classRedefinedCount= Class.class.getDeclaredField("classRedefinedCount");
Class_classRedefinedCount.setAccessible(true);
AnnotationData_annotations = AnnotationData_class.getDeclaredField("annotations");
AnnotationData_annotations.setAccessible(true);
AnnotationData_declaredAnotations = AnnotationData_class.getDeclaredField("declaredAnnotations");
AnnotationData_declaredAnotations.setAccessible(true);
Atomic_casAnnotationData = Atomic_class.getDeclaredMethod("casAnnotationData", Class.class, AnnotationData_class, AnnotationData_class);
Atomic_casAnnotationData.setAccessible(true);
} catch (ClassNotFoundException | NoSuchMethodException | SecurityException | NoSuchFieldException e) {
throw new IllegalStateException(e);
}
}
public static <T extends Annotation> void putAnnotation(Class<?> c, Class<T> annotationClass, Map<String, Object> valuesMap){
putAnnotation(c, annotationClass, annotationForMap(annotationClass, valuesMap));
}
public static <T extends Annotation> void putAnnotation(Class<?> c, Class<T> annotationClass, T annotation){
try {
while (true) { // retry loop
int classRedefinedCount = Class_classRedefinedCount.getInt(c);
Object /*AnnotationData*/ annotationData = Class_annotationData.invoke(c);
// null or stale annotationData -> optimistically create new instance
Object newAnnotationData = createAnnotationData(c, annotationData, annotationClass, annotation, classRedefinedCount);
// try to install it
if ((boolean) Atomic_casAnnotationData.invoke(Atomic_class, c, annotationData, newAnnotationData)) {
// successfully installed new AnnotationData
break;
}
}
} catch(IllegalArgumentException | IllegalAccessException | InvocationTargetException | InstantiationException e){
throw new IllegalStateException(e);
}
}
@SuppressWarnings("unchecked")
private static <T extends Annotation> Object /*AnnotationData*/ createAnnotationData(Class<?> c, Object /*AnnotationData*/ annotationData, Class<T> annotationClass, T annotation, int classRedefinedCount) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
Map<Class<? extends Annotation>, Annotation> annotations = (Map<Class<? extends Annotation>, Annotation>) AnnotationData_annotations.get(annotationData);
Map<Class<? extends Annotation>, Annotation> declaredAnnotations= (Map<Class<? extends Annotation>, Annotation>) AnnotationData_declaredAnotations.get(annotationData);
Map<Class<? extends Annotation>, Annotation> newDeclaredAnnotations = new LinkedHashMap<>(annotations);
newDeclaredAnnotations.put(annotationClass, annotation);
Map<Class<? extends Annotation>, Annotation> newAnnotations ;
if (declaredAnnotations == annotations) {
newAnnotations = newDeclaredAnnotations;
} else{
newAnnotations = new LinkedHashMap<>(annotations);
newAnnotations.put(annotationClass, annotation);
}
return AnnotationData_constructor.newInstance(newAnnotations, newDeclaredAnnotations, classRedefinedCount);
}
@SuppressWarnings("unchecked")
public static <T extends Annotation> T annotationForMap(final Class<T> annotationClass, final Map<String, Object> valuesMap){
return (T)AccessController.doPrivileged(new PrivilegedAction<Annotation>(){
public Annotation run(){
InvocationHandler handler;
try {
handler = (InvocationHandler) AnnotationInvocationHandler_constructor.newInstance(annotationClass,new HashMap<>(valuesMap));
} catch (InstantiationException | IllegalAccessException
| IllegalArgumentException | InvocationTargetException e) {
throw new IllegalStateException(e);
}
return (Annotation)Proxy.newProxyInstance(annotationClass.getClassLoader(), new Class[] { annotationClass }, handler);
}
});
}
}
用法示例：
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface TestAnnotation {
String value();
}
public static class TestClass{}
public static void main(String[] args) {
TestAnnotation annotation = TestClass.class.getAnnotation(TestAnnotation.class);
System.out.println("TestClass annotation before:" + annotation);
Map<String, Object> valuesMap = new HashMap<>();
valuesMap.put("value", "some String");
RuntimeAnnotations.putAnnotation(TestClass.class, TestAnnotation.class, valuesMap);
annotation = TestClass.class.getAnnotation(TestAnnotation.class);
System.out.println("TestClass annotation after:" + annotation);
}
输出：
TestClass annotation before:null
TestClass annotation after:@RuntimeAnnotations$TestAnnotation(value=some String)
此方法的局限性：
