Could someone help me with the kind of configuration that i need to do for my ObjectWriter such that it produces, an enum value without quotes ?
Is this possible to do this without using a custom serializer for enum ? with simple configuration ?
Can i declare some annotations on top of my enums or some kind of configuration to my objectwriter such that, it always produces enum values without quotes ?
ObjectWriter.writeValueAsString(object) ---> This should write enum value without quotes.
For a one-off serialization you can do this in two steps.
convert the enum to a TextNode
get the text value of the text node
In the example below the CAT enum is printed with quotes, while DOG is printed without quotes.
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.TextNode;
public class WriteEnumAsString {
public enum Animal {
CAT,
DOG,
}
public static void main(String[] args) throws JsonProcessingException {
var om = new ObjectMapper();
var catString = om.writeValueAsString(Animal.CAT);
System.out.println(catString);
// "CAT"
TextNode dogNode = om.valueToTree(Animal.DOG);
String dogString = dogNode.textValue();
System.out.println(dogString);
// DOG
}
}
Related
I would like to create a calculator application that can switch between different number bases. As far as entering digits is concerned, I was thinking the following would be a flexible api:
public interface ICalculator
{
string Enter(INumberElement element);
}
public class BaseTenCalculator : ICalculator
{
public string Enter(INumberElement element)
{
...
}
}
public class BaseTwoCalculator : ICalculator
{
public string Enter(INumberElement element)
{
...
}
}
My problem is that for the BaseTenCalculator, I would like a method
Enter(BaseTenNumberElement element)
and for a BaseTwoCalculator, I would like a method
Enter(BaseTwoNumberElement element)
to make sure only valid digits for that number base get entered. However, the only way I can think of enforcing this constraint is downcasting the 'element' argument in the two different implementations, and throwing an exception if INumberElement is not of the correct type. I feel like this is 'wrong', and I'm missing something. Is there another way? Is it even possible to create a common interface for two different number base calculators?
public interface ICalculator<in T> where T : INumberElement
{
string Enter(T element);
}
public class BaseTenCalculator : ICalculator<BaseTenNumberElement>
{
public string Enter(BaseTenNumberElement element)
{
throw new NotImplementedException();
}
}
public class BaseTwoCalculator : ICalculator<BaseTwoNumberElement>
{
public string Enter(BaseTwoNumberElement element)
{
throw new NotImplementedException();
}
}
I think you're thinking of the problem incorrectly. A number is a number regardless of base. Base is only a visible representation of the number. A good example to work from might be BigInteger. It has a constructor: BigInteger(String val, int radix), and a function: toString(int radix). All the work of representing the number is done the same. The only thing that differs is parsing from a string representation into the number, and then getting back out into a number format in a particular base.
You could create a multi-base calculator by using BigInteger or BigDecimal underneath and just using a base selection to set the radix value to parse or print the number(s). You'd also want to limit the input buttons (assuming you're using buttons), but that's really just a counting problem.
I'm trying to deserialize two types of json:
{
name: "bob",
worksAt: {
name: "Bobs department store",
location: "downtown"
},
age: 46
}
and
{
name: "Tom",
worksAt: "company:Bobs department store",
age: 27
}
into these objects:
The first way creates two new objects, the second way requests the object from the database based on the contents of a string.
sort of like how jackson mapper can deserialize an arbitrary string into an object, for objects like this:
public class Company{
public String name;
public Employee[] employees
public Company(){}
public Company(String json){
//turn string into object using whatever encoding you want blah blah blah...
}
}
The trouble is I need both. I need it to handle objects and strings. Both could arrive from the same input.
The first think I tried was making a Converter
It says these create a delegate type to pass to the deserializer, but the converter is always applied even when the datatype isn't a string. So that didn't work.
I've also tried a normal deserializer, but I can't find a way to defer to the BeanDeserializer. The beanDeserializer is so complicated that I can't manually instantiate it. I also see no way to defer to a default deserializer in jackson mapper.
Do I have to re-implement jackson mappers deserialization to do this? Is there any way for a deserializer to say "I can't do this, use the default implementation."?
Edit: Some further progress. Based on the Jackson Mapper source code, it looks like you can instatiate bean deserializers like this:
DeserializationConfig config = ctxt.getConfig();
JavaType type = config.constructType(_valueClass);
BeanDescription introspect = config.introspect(type);
JsonDeserializer<Object> beanDeserializer = ctxt.getFactory().createBeanDeserializer(ctxt, type , introspect);
but for some reason all the _beanProperties have the FailingDeserializer set for their _valueDeserializer and the whole thing fails. So I have no idea why that happens...
Have you tried writing a custom deserializer? This gives you the most control on how Jackson deserializes the object. You may be able to try to deserialize one way, and if there's an error, try another way.
Jackson can also handle polymorphic deserialization, though this would require a small change to the json to include type information, and it sounds like your problem constraints might not allow that.
If I understand the problem correctly, I would recommend using JsonNode. You can define a setter in your top-level type like this:
setWorksAt(JsonNode node) {
if (node.getNodeType == JsonNodeType.STRING) {
String name = node.getText();
name = name.substring(name.lastIndexOf(':'));
this.company = new Company(name);
} else if (node.getNodeType == JsonNodeType.OBJECT) {
this.company = mapper.treeToValue(node, Company.class);
}
}
That allows you to handle the two separate worksFor inputs, while still allowing the standard mapper to handle any substructures for the OBJECT case.
With recent versions of Jackson (2.8+ I think, definitely works with 2.9) you can use multiple #JsonCreator and do something like this:
public class Company {
private String name;
private String location;
private Company(String name, String location) {
this.name = name;
this.location = location;
}
private Company(String stringRepresentation) {
// add code here to parse string and extract name and location
}
#JsonCreator
private static Company fromJson(
#JsonProperty("name") String name,
#JsonProperty("location") String location)
{
return new Company(name, location);
}
#JsonCreator
private static Company fromJson(String str) {
return Company(str);
}
}
Im trying to write an aplication with uses hibernate to write to database, however in some actions i have to use JDBC on data in tables made by HB.
JDBS is requred to give administrator ability to create SQL queries with will return statistic info about data in database like number of processed document of specified type, numbers of success/failed log in attempts or total value of products in orders.
To do that i've done an from that allows to create class that has override toString() with return nice sql query string.
All works but now im trying to make administrator live easier by hiving him an ability to choose of table/column names. And here is an problem, because they are created by hibernate. some by #column annotation other by field name.
How can i check how field mapping?
I know its all about reflections but didnt do much of that in java yet.
example
#Entity
#Table(name= "my_table_name" )
public class TableOFSomething implements Serializable{
//This field isn't mapped into database and info about it is not requred.
//In fact, info about it may cause an error.
private static final long serialVersionUID = 7L;
#Id
#Column(name="id")
private String id;
private String fieldOne;
#Column(name="field_two")
private String fieldTwo;
#Column(name="renamed_just_for_fun")
private int Number;
//code with getters & setters
}
How to write methods that will have definition like
public <T> String tableName(Class<T> Target); //returns name of table in database
public <T> ArrayList<String> tabelFields(Class<T> Target); //returns name of fields in database
Hibernate has API - getClassMetadata that can explore the mapping. The API might change and is now located in another place , but i will use it and not in reflection for this.
look on this post for more details:
Get the table name from the model in Hibernate
if you want reflection , so use this link
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import javax.persistence.Column;
import javax.persistence.Table;
import odi.beans.statistic.QueryBean;
public class ReflectionTest {
public static void main(String[] args) {
ReflectionTest test=new ReflectionTest();
System.out.println("Table name of "+QueryBean.class.getName()+" is "+test.getTableName(QueryBean.class));
System.out.println("Column names in this table are:");
for(String n: test.getColumnNames(QueryBean.class)){
System.out.println("\t"+n);
}
System.out.println("Good bye ;)");
}
public <T> ArrayList<String> getColumnNames(Class<T> target) {
ArrayList<String> ret=new ArrayList<>();
Field[] fields = target.getDeclaredFields();
String fieldName =null;
for (Field f : fields) {
//jump to next if if field is static
if (Modifier.isStatic(f.getModifiers()))
continue;
if (f.isAnnotationPresent(Column.class)) {
Column a = f.getAnnotation(Column.class);
fieldName = a.name();
} else {
fieldName = f.getName();
}
ret.add(fieldName);
}
return ret;
}
public <T> String getTableName(Class<T> target){
String ret=target.getSimpleName();
if (target.isAnnotationPresent(Table.class))
{
Table t=target.getAnnotation(Table.class);
ret=t.name();
}
return ret;
}
}
Is it cover all possibilities?
I know now that Hibernate way would be easier, but this is also about learning of very useful reflection mechanism :)
EDIT:
Important question:
Will this work only on annotations or also on xml mapping?
I have the problem described here:
http://social.msdn.microsoft.com/Forums/en-AU/csharplanguage/thread/b310c71a-2479-4a93-888a-29294cecbe09
They give a solution using a SerializationBinder. Is there another alternative?? Like decorating my classes with a different namespace and assembly?? The reason is that I have some classes with this problem used many times, and I have to add the line "formatter.Binder = ..." in each part of the code. It would be easier to apply my hipothetic second solution.
Thanks.
If the assembly version changes, serialized objects become invalid. I once made changes to the source code of Protobuf-Net to avoid the version check, and it was fairly easy to do so. However, it can lead to unexpected results (data ending up in the wrong fields), unless you avoid the implicit fields, and set an index to each field manually using annotations. That's the advantage or Protobuf-Net, that you have control over the order of the fields in the serialized stream.
Another solution is to use custom serialization? Something like:
[Serializable]
public class MyObject : ISerializable
{
public int n1;
public int n2;
public String str;
public MyObject()
{
}
protected MyObject(SerializationInfo info, StreamingContext context)
{
n1 = info.GetInt32("i");
n2 = info.GetInt32("j");
str = info.GetString("k");
}
[SecurityPermissionAttribute(SecurityAction.Demand,
SerializationFormatter =true)]
public virtual void GetObjectData(SerializationInfo info, StreamingContext context)
{
info.AddValue("i", n1);
info.AddValue("j", n2);
info.AddValue("k", str);
}
}
I have the following:
An interface I1 extends Ia, Ib, Ic
An interface I2.
A class C implements I1, I2. And this class has its own setters and getters as well.
C cInstance = new C():
//Jackson
ObjectMapper mapper = new ObjectMapper();
mapper.writeValue(new File("somefile.json"), cInstance);
//Gson
Gson gson = new Gson();
String json = gson.toJson(cInstance);
The output will be cInstance serialized according to the properties of C and what it inherited.
However, I like the properties are being serialized to be according to the setters/getters in I1 (only the cInstance properties represented in the I1 interface).
How can I do this with Jackson knowing that I have too many classes with the same problem and I can't modify the class definition or add annotations.
And the same issue applies to Deserialization (Deserializing according to an interface)
Thanks
First of all, you can always attach "mix-in annotations" even without adding annotations directly (see wiki page). With this, annotation to use would be:
#JsonSerialize(as=MyInterface.class)
but if you do not want to use mix-ins, you can force specific type to use with
objectMapper.typedWriter(MyInterface.class).writeValue(....)
Jackson's VisibilityChecker provides an easy way for filtering certain properties, especially because it allows you to test for visibility (equals "will be serialized or not") for each method/field individually.
At least this helps for the serialization phase.
Here is what I did (using Jackson version 1.9.11):
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.map.introspect.AnnotatedMethod;
import org.codehaus.jackson.map.introspect.VisibilityChecker;
public static class InterfaceVisibilityChecker extends VisibilityChecker.Std {
private final Set<Method> visibleMethods;
public InterfaceVisibilityChecker(Class<?>... clazzes) {
super(JsonAutoDetect.Visibility.PUBLIC_ONLY);
this.visibleMethods = new HashSet<>();
for (Class<?> clz : clazzes) {
this.visibleMethods.addAll(Arrays.asList(clz.getMethods()));
}
}
#Override
public boolean isGetterVisible(Method m) {
return super.isGetterVisible(m) && isVisible(m);
}
#Override
public boolean isGetterVisible(AnnotatedMethod m) {
return isGetterVisible(m.getAnnotated());
}
private boolean isVisible(Method m) {
for (Method visiMthd : visibleMethods) {
if (isOverwriteMethod(m, visiMthd)) return true;
}
return false;
}
private boolean isOverwriteMethod(Method subMethod, Method superMethod) {
// names must be equal
if (! subMethod.getName().equals(superMethod.getName())) return false;
// return types must be assignable
if (! superMethod.getReturnType().isAssignableFrom(subMethod.getReturnType())) return false;
// parameters must be equal
if (! Arrays.equals(subMethod.getParameterTypes(), superMethod.getGenericParameterTypes())) return false;
// classes must be assignable
return superMethod.getDeclaringClass().isAssignableFrom(subMethod.getDeclaringClass());
}
}
The main idea is to use the standard VisibilityChecker and extend it by a check whether the method is declared in one of the given interfaces.
The checker is applied to an ObjectMapper instance using the following snippet:
ObjectMapper om = new ObjectMapper();
om.setVisibilityChecker(new InterfaceVisibilityChecker(
I1.class,
I2.class,
Ia.class,
Ib.class,
Ic.class
));
Some comments on the solution above:
The checker is not complete, methods like isIsGetterVisible or isFieldVisible can be handled in a similar manner if needed.
isOverwriteMethod is not optimized at all, it's checks could be cached.