I have a Mapper class which extends AppEngineMapper<Key, Entity, NullWritable, NullWritable>.
Now inside my map function (public void map(Key key, Entity value, Context context)), I need to get access to PersistenceManager. To get singleton PersistenceManager, I need ServletContext object.
Typically, in a HTTPServlet extended class, I call getServletContext() and pass it to get PersistenceManager object. But in Mapper extended class, I don't have or can call getServletContext().
Can anyone advice on how to get servletcontext in my mapper class or if there is a way to pass servlet context from my servlet to Mapper extended class? I am using auto-mapreduce (programmatically) through a cron job.
Related
I am not able to understand how/when to use sealed classes in Android using Kotlin. I have read the docs but still I am getting confused about its' structure and how to use it. It'd of great help if someone could help me understand this.
Sealed classes allow us to represent hierarchies in a more flexible way. It’s also more readable and is used for better state management.
Child-classes of Sealed class, can be of any type, like: data class, object class, any regular class, or even another sealed class.
This is how we define a sealed class in Android using Kotlin:
sealed class UiState {
object Loading : UiState()
data class Success(val successMessage: String) : UiState()
data class Error(val error: Throwable?) : UiState()
}
Let’s understand this part of the code:
object Loading : UiState() : declared this as an object, as we don’t need to know anything else at this stage.
data class Success(val successMessage: String) : UiState() : declared as data class, as we need the message when we got Success state.
data class Error(val error: Throwable?) : declared this as data class, as we need the Throwable when we got Error state.
Notice how we have used object Loading : UiState(). We are using the same sealed class name as the return type of “Loading” object.
This says, that the “Loading” object belongs to “UiState” sealed class.
This is same for Success and Error states as well.
We can then use this sealed class as the following:
fun observeUiStates(uiState: UiState) = when(uiState) {
UiState.Loading -> println("Loading...Please wait.")
is UiState.Success -> println(uiState.successMessage)
is UiState.Error -> println(uiState.errorMessage)
}
In this aforementioned block of code we are passing the UiState sealed class as a parameter to observeUiStates() method and then by using when we are checking which is the current state of the app and then we will be handling the states accordingly.
Here for the sake of this example, I am just printing the result in the console.
Sealed classes, best work with Model-View-Intent pattern in Android by using RxJava.
By using them, we will now be able to observe to the state changes.
Whenever there is a new state of the app, this observeUiStates() method will give us that specific state only.
To keep it simple for this specific example, I am calling this method from main(). See the code below:
fun main() {
observeUiStates(UiState.Loading)
observeUiStates(UiState.Success("The Ui has returned success state.”))
}
This will print the following in the console:
Loading...Please wait.
The Ui has returned success state.
I see that EF Core 2 has EF.Functions property EF Core 2.0 Announcement which can be used by EF Core or providers to define methods that map to database functions or operators so that those can be invoked in LINQ queries. It included LIKE method that gets sent to the database.
But I need a different method, SOUNDEX() that is not included. How do I write such a method that passes the function to the database the way DbFunction attribute did in EF6? Or I need to wait for MS to implement it? Essentially, I need to generate something like
SELECT * FROM Customer WHERE SOUNDEX(lastname) = SOUNDEX(#param)
Adding new scalar method to EF.Functions is easy - you simply define extension method on DbFunctions class. However providing SQL translation is hard and requires digging into EFC internals.
However EFC 2.0 also introduces a much simpler approach, explained in Database scalar function mapping section of the New features in EF Core 2.0 documentation topic.
According to that, the easiest would be to add a static method to your DbContext derived class and mark it with DbFunction attribute. E.g.
public class MyDbContext : DbContext
{
// ...
[DbFunction("SOUNDEX")]
public static string Soundex(string s) => throw new Exception();
}
and use something like this:
string param = ...;
MyDbContext db = ...;
var query = db.Customers
.Where(e => MyDbContext.Soundex(e.LastName) == MyDbContext.Soundex(param));
You can declare such static methods in a different class, but then you need to manually register them using HasDbFunction fluent API.
EFC 3.0 has changed this process a little, as per https://learn.microsoft.com/en-us/ef/core/what-is-new/ef-core-3.0/breaking-changes#udf-empty-string
Example of adding CHARINDEX in a partial context class:
public partial class MyDbContext
{
[DbFunction("CHARINDEX")]
public static int? CharIndex(string toSearch, string target) => throw new Exception();
partial void OnModelCreatingPartial(
ModelBuilder modelBuilder)
{
modelBuilder
.HasDbFunction(typeof(MyDbContext).GetMethod(nameof(CharIndex)))
.HasTranslation(
args =>
SqlFunctionExpression.Create("CHARINDEX", args, typeof(int?), null));
}
}
I found the following use of a wrapper class, and was wondering if it is a good practice or whether its just duplication of code for no reason.
//Class:
public class SomeClass{
public Integer someInt;
public String someString;
}
//Callout Class:
public class CalloutClass{
public SomeClass someMethod(){
//...code to do a callout to an api
SomeClass someClassObj = (SomeClass)JSON.Deserialize(APIResponse.getBody(), SomeClass.class);
return someClassObj;
}
}
//Controller:
public class SomeController {
public SomeController(){
someClassObj = calloutClassObj.someMethod();
SomeWrapper wrapperObj = new SomeWrapper();
for(SomeClass iterObj : someClassObj){
wrapperObj.someWrapperInt = iterObj.someInt;
wrapperObj.someWrapperString = iterObj.someString;
}
}
public class someWrapper{
public Integer someWrapperInt{get;set;}
public String someWrapperString{get;set;}
}
}
The wrapper class "someWrapper" could be eliminated if we just use getters and setters ({get;set;}) in "SomeClass."
Could anyone explain if there could be a reason for following this procedure?
Thanks,
James
My assumption (because, code in controller is extra pseudo) is
SomeClass is a business entity, purpose of which is to store/work with business data. By work I mean using it's values to display it (using wrapper in controller), to calculate smth in other entities or build reports... Such kind of object should be as lightweight as possible. You usually iterate through them. You don't need any methods in such kind of objects. Exception is constructor with parameter(s). You might want to have SomeObject__c as parameter or someWrapper.
someWrapper is a entity to display business entity. As for wrapper classes in controllers. Imagine, that when you display entity on edit page and enter a value for someWrapperInt property, you want to update someWrapperString property (or you can just put validation there, for example, checking if it is really Integer). Usually, as for business entity, you don't want such kind of functionality. But when user create or edit it, you may want smth like this.
I'm using Objectify and wish to have its Key<> type passed around in my API. I've created an ApiTransformer, but my questions is where to declare it, since the serialized Key<> class is not available, hence I cannot declare its transformer as a class annotation. I tried declaring it in the #Api annotation, but it doesn't work, I still get the error:
There was a problem generating the API metadata for your Cloud Endpoints classes: java.lang.IllegalArgumentException: Parameterized type com.googlecode.objectify.Key<[my package].User> not supported.
The ApiTransformer looks like:
public class KeyTransformer implements Transformer<Key<?>, String> {
public String transformTo(Key<?> in) {
return in.getString();
}
public Key<?> transformFrom(String in) {
return Key.valueOf(in);
}
}
And in my #Api I have:
#Api(name = "users", version = "v1",transformers = {KeyTransformer.class})
Unfortunately you can't. As you said you need to declare it on the Key class, your only chances to make this work are either.
1) Recompile the Key class for objectify with the #transformer annotation.
2) Extend the Key class with your own implementation and define the transformer there.
I don't really like any of those options so the way i usually resolve this is to hide the key object getter (by using #ApiResourceProperty(ignored=AnnotationBoolean.TRUE)) and only expose the id from that key.
That way you get a Endpoints frendly object, the only downside is you'll have to reconstitute the key using Key.create(YourClass.class, longId) manually whenever you need it.
You can add transforms to 3rd party classes by listing the transform in #Api annotation. I'm not dead sure it'll work parameterized class, but I don't see why not.
https://cloud.google.com/appengine/docs/java/endpoints/javadoc/com/google/api/server/spi/config/Api#transformers()
I'm using app engine datastore so I have entity like this.
#PersistenceCapable
public class Author {
#PrimaryKey
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
#JsonProperty("id")
#JsonSerialize(using = JsonKeySerializer.class)
#JsonDeserialize(using = JsonKeyDeserializer.class)
private Key key;
....
}
When the model is sent to view, it will serialize the Key object as an Id value. Then, if I send data back from view I want to deserialize the Id back to Key object by using JsonKeyDeserializer class.
public class JsonKeyDeserializer extends JsonDeserializer<Key> {
#Override
public Key deserialize(JsonParser jsonParser, DeserializationContext deserializeContext)
throws IOException, JsonProcessingException {
String id = jsonParser.getText();
if (id.isEmpty()) {
return null;
}
// Here is the problem because I have several entities and I can't fix the Author class in this deserializer like this.
// I want to know what class is being deserialized at runtime.
// return KeyFactory.createKey(Author.class.getSimpleName(), Integer.parseInt(id))
}
}
I tried to debug the value in deserialize's parameters but I can't find the way to get the target deserialized class. How can I solve this?
You may have misunderstood the role of KeySerializer/KeyDeserializer: they are used for Java Map keys, and not as generic identifiers in database sense of term "key".
So you probably would need to use regular JsonSerializer/JsonDeserializer instead.
As to type: it is assumed that handlers are constructed for specific types, and no extra type information is passed during serialization or deserialization process: expected type (if handlers are used for different types) must be passed during construction.
When registering general serializers or deserializers, you can do this when implementing Module, as one of the arguments is type for which (de)serializer is requested.
When defining handlers directly for properties (like when using annotations), this information is available on createContextual() callback of interface ContextualSerializer (and -Deserializer), if your handler implements it: BeanProperty is passed to specify property (in this case field with annotation), and you can access its type. This information needs to be stored to be used during (de)serialization.
EDIT: as author pointed out, I actually misread the question: KeySerializer is the class name, not annotation.