I'm trying to use Java annotations to be able to add specific fields to an object.
The need is the following : I have a class that processes configuration files where keys are associated with values with the form key=value.
The problem is that I want to be able to let the user defining himself required fields which, if not present, throws exception.
The easiest solution is to pass these fields to the constructor in a String[] but, I also want the user to be able to use these required fields as it were properties of the class, so he's able to write in the code something like :
#RequiredFields(
field1,
field2,
field3)
MyClass myObject = new MyClass(String filePath);
String value = myObject.field1;
and the field field1 is also a completion proposal ?
I'm actually developping in Groovy, so if not possible in standard Java, would it be in Groovy ?
Thanks !
What about a factory method which does the validation? Something like:
MyClass my = MyClassFactory.create("arg1", "arg2")
Or, with maps on Groovy:
def my = MyClassFactory.create arg1: "foo", arg2: "bar"
And the factory itself checks the properties file.
If you really want the annotation, maybe the Type Annotations on JDK 8 are an option.
On Groovy, you can try a local AST, which seems like an overengineered solution to me, or GContracts which is programming by contract.
You could combine a factory with GContracts to #Ensure the resulting object contains all fields according to the properties file.
Related
How can I distinguish in java graphQL if a parameter was explicitly set to null, or if it was not provided at all?
The use case that I try to achieve is the following: I have a mutation like this
updateUser(id:Int!, changes:UserChanges!)
#where UserChanges is defined as:
type UserChanges {
login:String
email:String
#etc
}
The idea here is that the user provides only the fields that he wants to change (like react setState for example).
So if email is ommited, I want to leave it unchanged.
But what if he/she wants to explicitly set email to null?
Is there any way figure this out from my resolver method and act accordingly?
(I use graphql-java-tools library)
I found the answer. In case somebody needs it:
An instance of graphql.schema.DataFetchingEnvironment is available to every resolver method.
This provides methods like getArguments(), hasArgument() etc.
Using those methods, we can find out if an argument was an explicit set to null, or if it was not provided at all.
Looks like deserialization from query/variables is handled by fasterxml Jackson, and that's proper place to deal with the issue, otherwise it becomes too complex: check every field? nested?
So: UserChanges.java should look like this:
class UserChanges {
// SHOULD NOT HAVE ALL ARGUMENT CONSTRUCTOR!
Optional<String> login;
Optional<String> email;
... getters & setters
}
in this case deserializer will use setters, ONLY FOR PROVIDED FIELDS!
And {"login":null} will become:
UserChanges.login = Optional.empty
UserChanges.email = null
I'm trying to make a custom processor in Apache NiFi that can add an attribute/string to the JSON object in the flowfile content. At the moment it works when I just use a string but it's not working when I use NiFi's expression language although I have it supported in my code.
The expression language is 100% correct as it works in another processor and I've also tried different attributes to make sure it's not the attribute.
The property:
public static final PropertyDescriptor ADD_ATTRIBUTE = new PropertyDescriptor
.Builder().name("Add Attribute")
.description("Example Property")
.required(true)
.addValidator(StandardValidators.NON_EMPTY_VALIDATOR)
.expressionLanguageSupported(true)
.build();
Later in my code when I want to get the value and put in the JSON object I use:
jsonObject.put("hostname", context.getProperty(ADD_ATTRIBUTE).evaluateAttributeExpressions().getValue());
I also made a Unit Test and it works when I assign a text value to the testrunner.setProperty. However I don't know how I can assign an attribute to the testrunner or how I can use expression language in my test.
Thanks in advance for any suggestions or a solution!
I'll put my answer from Hortonworks Community Connection here too FWIW:
If the expression refers to attributes on a flow file, you will need to pass a reference to the flow file into evaluateAttributeExpressions:
FlowFile flowFile = session.get();
jsonObject.put("hostname", context.getProperty(ADD_ATTRIBUTE).evaluateAttributeExpressions(flowFile).getValue());
If the property contains an attribute name (rather than an Expression containing an attribute name) and you want the value from the flow file:
jsonObject.put("hostname", flowFile.getAttribute(context.getProperty(ADD_ATTRIBUTE).getValue()));
If the attribute value itself contains Expression Language and you want to evaluate it, take a look at the following class:
org.apache.nifi.attribute.expression.language.Query
Regarding testing...
Assuming you are evaluating the expression language against an incoming FlowFile (evaluateAttributeExpressions(flowFile)) then you can do the following:
runner.setProperty(ADD_ATTRIBUTE, "${my.attribute}");
Then create an attribute Map that has my.attribute in it:
final Map<String,String> attributes = new HashMap<>();
attributes.put("my.attribute", myAttribute);
Then enqueue some content with the attributes:
runner.enqueue(fileIn, attributes);
runner.run();
An example from the code base:
https://github.com/apache/nifi/blob/1e56de9521e4bc0752b419ffc7d62e096db1c389/nifi-nar-bundles/nifi-solr-bundle/nifi-solr-processors/src/test/java/org/apache/nifi/processors/solr/TestPutSolrContentStream.java#L243
I have interface Resource and several classes implementing it, for example Audio, Video... Further, I have created custom annotation MyAnnotation with Class type param:
#MyAnnotation(type = Audio.class)
class Audio {
...
}
#MyAnnotation(type = Video.class)
class Video{
...
}
In some other place in code I have to use Interface Resource as a returned type:
public class Operations<T extends Resource> {
....
#OtherAnnotation(type = Audio.class (if audio), type = Video.class (if video) )
T getResource();
....
}
The question is how to appropriatelly annotate annotation #OtherAnnotation depending of what kind of Resource type will be returned ?
What you are asking is for dynamic values for annotation attributes.
However annotations can only be set at compile time which is the reason why their values can only be compile time constants. You may only read them at runtime.
There was a similar question in which someone tried to generate the annotation value , it's answer explains why there is no way to dynamically generate a value used in annotation in a bit more detail. In that question there was an attempt to use a final class variable generated with a static method.
There are annotation processors which offer a bit more flexibility by handling placeholders. However i don't think this fits your case, as you want the dynamic values at runtime.
This answer refers to spring's use of the expression language for the Value annotation in which the placeholder (#Value("#{systemProperties.dbName})") gets overrided with the data from one of the property sources defined ( example in spring boot )
In any case, you will have to rethink your architecture a bit.
I need to create a method that takes in argument any attribute of any class. But i dont want it to be of type String, to avoid refactoring problems while renaming an attribute and to get the errors in Markers Tab of eclipse, and not while running my application.
Having a class Person :
public class Person {
private String name;
// other attributes...
// getters and setters...
}
Now the needed method :
void getAnAttributeOfAClass( <which_type_or_class_here?> attr_as_arg){
// Now I need to get the name of attribute that would be of class Strin...
}
Is there a function or a method, by which we can specify an attribute?
For example :
Person.class.name
Would it be of class Property ?
EDIT
More exactly (#Smallhacker answer helped me), I need to verify at compile time if the argument is really an attribute of the specified class.
Person.class.name // no compile time error
Person.class.nameXXX // compile time error
The closest to what you want is Reflection API's Field or JavaBeans Introspector API's PropertyDescriptor.
But usually things like that are not needed in Java projects because there are libraries which handle these concerns.
You could pass a Class object along with a String name, then let your method use Introspector internally to read that property.
Not sure I understand you well, but there is a class java.lang.reflect.Field, that has a method getName() that would give your the name of the field.
In your example, to get field name, you would do: Person.class.getDeclaredField("name").
EDIT: to get the value of a field in an object, you would do: field.get(obj);
OK, let's say You have the following variables:
Person person = ...; // initialized with some Person
Field nameField = Person.class.getDeclaredField("name");
Now to get the name of person, you would do:
String personName = (String)nameField.get(person);
Actually, this would throw an exception because name is a private field. You can however bypass the protection by doing:
nameField.setAccessible(true);
Unfortunately, Java lacks an ability to reference member variables in a way that can be analyzed at compile time.
There may be some kind of library to simplify this somewhat, but it wouldn't provide a full solution due to limitations in the language itself.
Maybe java generics can help you with this.
You can do something like:
class YourClass<E> {
void getAnAttributeOfAClass(E attr_as_arg){
// some code
}
}
someVariable = new YourClass<Person>();
someVariable.getAnAtributeOfAClass(someObject); //this will not compile if someObject is not an instance of Person
But I still don't know what you want to do exactly inside the method.
Typically I could copy values between two java beans, which have identical property names, using BeanUtils with java reflection e.g. PropertyUtils.setProperty(....)
In protobuf Message, we use the message builder class to set the value. This works but I would rather use reflection to automatically copy properties from the bean to the message as both have identical property names and type.
When I invoke the PropertyUtils.setProperty on the builder object ( got from message.newBuilder()), I get this message.
java.lang.NoSuchMethodException: Property 'testProp' has no setter method in class 'class teststuff.TestBeanProtos$TestBeanMessage$Builder'
How do I automatically copy values from java bean to protobuf message object (and vice-versa) using java reflection?
I hate to answer my question but I cant believe that I am the only one who ran into this problem. Documenting solution here in case other people are also getting started with protobuf and java. Using reflection saves wrting dozens of getter and setters.
Ok , I managed to get it to work using some of example test code shipping with protobuf. This is a very simple use case; typically a message would be a lot more complex. This code does not handle nested messages or repeated messages.
public static void setMessageBuilder(com.google.protobuf.GeneratedMessage.Builder message,Descriptors.Descriptor descriptor,Object srcObject) throws Exception {
String cname = srcObject.getClass().getName();
/*BeanMapper.getSimpleProperties -- this is a warpper method that gets the list of property names*/
List<String> simpleProps = BeanMapper.getSimpleProperties(srcObject.getClass());
Map map = new HashMap();
for (String pName : simpleProps) {
System.out.println(" processing property "+ pName);
Object value= PropertyUtils.getProperty(srcObject, pName);
if(value==null) continue;
Descriptors.FieldDescriptor fd=descriptor.findFieldByName(pName) ;
System.out.println(" property "+ pName+" , found fd :"+ (fd==null ? "nul":"ok"));
message.setField(fd, value);
System.out.println(" property "+ pName+" set ok,");
}
return ;
}
I may be off, but would protostuff help? It has nice extended support for working with other data formats, types. And even if it didn't have direct conversion support, if you go to/from JSON there are many choices for good data binding.
You can go throw all properties getClass().getFields() and make copy using reflection. It will be smt like:
for(Field f : to.getClass().getFields()){
f.set(to, from.getClass().getField(f.getName()).get(from));
}
+ probably you might be use field.setAccessible(true) invocation.
I don't know the size of your project but you may want to try Dozer, a mapper that recursively copies data from one object to another of the same type or between different complex types. Supports implicit and explicit mapping as well. I used it in a big project and worked very well. It could be as simple as
Mapper mapper = new DozerBeanMapper();
DestinationObject destObject = mapper.map(sourceObject, DestinationObject.class);
I've got the same issue, the solution is a little bit tricky.
Please use
MethodUtils.invokeMethod
instead.
where the method name is "setXXX".