Create JavaType From Class with Jackson

1. Introduction

While working with Jackson, we might face situations where we’ll have to produce a JavaType from the given Class object.

In this tutorial, we’ll see how to create a JavaType from a Class with the aid of Jackson library.

2. Introduction to JavaType and Class

Before getting into details, let’s glance at JavaType and Class.

2.1. The JavaType in Java

In Jackson, the JavaType class represents Java types. It is a mechanism that enables working with generic types, such as parameterized types and arrays.

Creating the JavaType instance is important, especially when we are processing generic structures during the JSON handling.

2.2. The Class in Java

In Java, the Class class is a member of the reflection API and is used at runtime to represent a class or interface.

Moreover, it offers the class information, including its name, fields, methods, and constructors.

3. Using TypeFactory to Create JavaType

To generate a JavaType instance from a provided Class object using Jackson, we harness the TypeFactory class.

The TypeFactory provides a default instance, and as such, we can construct different types, whether generic or parameterized.

Let’s take an example using TypeFactory to generate a JavaType object from a generic class:

class MyGenericClass<T> {
    // Class Implementation
}

@Test
void givenGenericClass_whenCreatingJavaType_thenJavaTypeNotNull() {
    Class<?> myClass = MyGenericClass.class;
    JavaType javaType = TypeFactory.defaultInstance().constructType(myClass);
    assertNotNull(javaType);
}

Here, we first define a Class object named myClass, representing the generic class MyGenericClass.

Then, we use the constructType() method to create a JavaType instance based on the provided Class object (myClass).

In addition, we use the assertNotNull() assertion to ensure that the JavaType instance is successfully created, validating the correctness of the process.

4. Handling Parameterized Types

To smoothly build upon our knowledge of JavaType creation, we’ll see working with parameterized types with the TypeFactory class.

Additionally, this will be based on the section that we have just discussed, which talks about the generation of JavaType instances for generic classes. 

Let’s see an example:

class Container<T> {
    // Class Implementation
}

@Test
void givenParametricType_whenCreatingJavaType_thenJavaTypeNotNull() {
    Class<?> containerClass = Container.class;
    Class<?> elementType = String.class;
    JavaType javaType = TypeFactory.defaultInstance().constructParametricType(containerClass, elementType);
    assertNotNull(javaType);
}

In this case, we have a Container parameterized class with String elements. Moreover, an instance of JavaType is created with the constructParametricType() method representing the parameterized type.

The assertion is also used to verify that the JavaType object is successfully created, hence the routine of handling parameterized type is correct. 

5. Conclusion

In this tutorial, we learned how to build instances of JavaType from Class objects with the help of the Jackson library.

As always, the source code for the examples is available over on GitHub.