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Creating and using annotation in java

I am reviewing open source spring projects. I am confused about the use of annotations around here. I want to ask to clarify this.
#Target(ElementType.METHOD)
#Retention(RUNTIME)
#Bean
public #interface Merge {
#AliasFor("targetRef")
String value() default "";
#AliasFor("value")
String targetRef() default "";
Placement placement() default Placement.APPEND;
int position() default 0;
Class<MergeBeanStatusProvider> statusProvider() default MergeBeanStatusProvider.class;
boolean early() default false;
}
An annotation has been created here named Merge. It has different parameters and default values.
#Configuration
public class LocalConfiguration {
#Merge(targetRef = "mergedList", early = true)
public List<String> blLocalMerge() {
return Arrays.asList("local-config1", "local-config2");
}
}
And this is usage of #Merge annotation in any class I choosed randomly.
When I examined the code, I could not find any class related to the implementation of Merge annotation. By the way, this problem I'm having isn't just about this annotation. Almost all the annotations I have examined are used without being implemented in any way.
I think I will understand the others if we start from this annotation.
What does this anotation do? What kind of message does it give to the place where it is used. How does the application understand what that annotation does in runtime without being implemented anywhere.
Thanks.

Annotations don't have implementations. They are processed by external classes or tools depending on the RetentionPolicy. In this case, the Merge annotation has Runtime retention so it will be available via reflection once the class is loaded. At runtime any interested party (in this case I assume the Spring Framework) can use getAnnotations on your LocalConfiguration class to detect the Merge annotation and take whatever action that needs to be taken. The possibilities are really up to the framework that defined the annotation. A lot of Spring injection works like this with annotations but they are also used by many other frameworks such as Hibernate, Jersey, etc. The main idea is that annotations act as markers on specific code points to be used by an external entity at a later point.

Create a annotation on a class to verify two dates with Kotlin/Java

I want to verity two dates parameters in a class, start day must before end day. However, I have multiple classes which have this demand, so I want to customize a generic annotation.
what I have tried:
class dataValidator<T : Any>: ConstraintValidator<DateConstraint, T>
It raised a error:
javax.validation.UnexpectedTypeException: HV000030: No validator could be found for constraint 'my.test.annotation.DateValid' validating type 'my.test.model.DateModel'. Check configuration for ''
However, if I change my validator to this, and it works
class dataValidator<T : Any>: ConstraintValidator<DateConstraint, DateModel>
So, is there any way I can write a generic annotation?

Normally this problem will happen when you are trying to use incorrect validator annotation on any bean property, you must use the right annotations for the right types.
You can find more information here

Problems searching for annotated classes using Reflections API

I'm having a very difficult time using the Reflections API to find classes that are annotated with a custom annotation at runtime. The ultimate goal is to find all classes in the project that are annotated with my custom #Job annotation, collect them, and allow each of them to be run from one location without adding each one to the page manually. However, I'm finding it extremely difficult to get the initial search to work correctly, so I cannot move on with my project.
My current approach is to use:
Reflections reflections = new Reflections(new ConfigurationBuilder()
.setUrls(ClasspathHelper.forPackage("jobs"))
.setScanners(new TypeAnnotationsScanner())
.filterInputsBy(new FilterBuilder().includePackage("jobs")));
Set<Class<?>> jobs = reflections.getTypesAnnotatedWith(Job.class);
where "jobs" is the package containing all of the job classes that I am searching for, which will be annotated with the custom #Job annotation. "jobs" is a base package in my project, but the overall url on my machine looks something like ".../(project)/app/jobs". This setup results in one url being searched, which is ".../(project)/app/" with the additional filter "+jobs.*" in the configuration object. This seems like it is working correctly, but clearly something is wrong because I do not get any classes in the set.
If it matters, the annotation is coded as:
package jobs;
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.TYPE)
public #interface Job {
String description();
}
The annotation class is located within the same "jobs" package as the job classes I am searching for. An example of a job definition with the annotation included is:
package jobs;
#Job(description = "Description of what the job will do")
public class ExampleJob extends MasterJob {...}
I cannot find what I need to change in order to get this search to function as intended. Thanks for the help, and please let me know if I can clarify anything further.
EDIT: I believe the problem is associated with how the Play Framework loads its classes. Fortunately, the framework provides its own annotation search function, which I used instead. According to a comment, the code I have listed here will work, given that you have all the dependencies to run it. Feel free to use it as a template and let me know if it works for you as well.

Bean Validation - constructor/factory parameter

I have a dto object which keeps an IP Range using first and last fields. Simple CRUD operations are made with this class using dropwizard (jersey-jackson-hibernate validator)
public class IpRangeDto {
#JsonCreator
public static IpRangeDto fromCidr(#JsonProperty("cidr") String cidr) {
//Resolve CIDR and assign first and last fields
}
#NotNull
#IpAddress // My custom validator
private String first;
#NotNull
#IpAddress
private String last;
}
For the sake of user-friendliness I had decided to add an alternative way to create this object, which is by using CIDR. So the client could send either first and last fields in JSON or only the cidr field. So the way to do it is as above, using #JsonCreator. And it works just fine.
"ipRange":{
"first": "15.0.0.1",
"last": "15.0.0.255",
}
"ipRange":{
"cidr": "15.0.0.0/24"
}
I want to validate this CIDR value that it's the right format so I can return 422 with proper error message. If I throw exception in the constructor/factory method then jersey-jackson returns 400 directly (even if I throw ConstraintViolationException, it's encapsulated by JsonProcessingException).
I could simply ignore the exceptions, and leave the fields empty which will return 422 because of #NotNull constraints but then the error message will not be as clear as it should be.
I tried adding my #Cidr validator next to the #JsonProperty parameter but that doesn't seem to be effective. My understanding is that validation occurs after Jackson is finished with creating Dtos, so with my #JsonCreator approach there might not be any solution to this problem. So I'm open to refactoring suggestions as well.

I am not an expert on the exact integration of Bean Validation into jackson, but I think it is just doing actual property validation. This means as you already pointed out, the entities are created first and then the properties are validated.
Bean Validation (as of version 1.1) also offers so called method validation, in which case you could place your Cidr constraint onto the string parameter of the method, but as said, I don't think that there is an integration in jackson for that.
And one more thing ;-) - static methods and properties are generally excluded from validation in Bean Validation (see also http://beanvalidation.org/1.1/spec/#d0e2815).
Regarding a workaround, one thing comes to mind (even though it feels a bit complicated). Write a custom class level IpRange constraint. In a class constraint you would get passed a IpRangeDto instance and it is up to you to validate the whole object and select the right error message for any violations. Provided you would add a cidr property to the dto which gets set when fromCidr is called, you would have then all information you need for the validation and selection of a proper error message.

how can i trigger my custom annotation when method called?

I want to write my own Secure module library for web but i not solution somethings. Example; I have a class and have a method. Method have my custom annotation.
class example{
#Admin
public void go(){
syso("working");
}
}
When This method called , how can i trigger my annotation.
(Example; Hibernate Validators. We write hibernate annotation on method , it working only method called)
by the way my english is bad :)

Annotations are not "triggered"... you have to write code that looks for their presence and takes action.
The "code" can either be executed at runtime, but is more commonly executed at compile time using the Annotation Processing Tool to alter the source to inject extra, typically cross-cutting, code appropriate for the annotation.