JSON is a lightweight language independent data-interchange format which is one of the common ways of encoding data over HTTP. This articles goes over the use of the Jackson library to serialize and deserialize Java object to JSON.
If you have not already done so download and install a latest copy of the JDK. Also download the latest Jackson Jar file. Create a folder with the jar file, and a few classes you want to serialize. I created ParentClass.java, Child1.java and Child 2.java.
The excerpt below shows the contents of Parent.java, note that we are annotating the class to help with serialization. The "JsonTypeInfo" annotation tells jackson to encode the object type in the resulting JSON output in a field called objectType. Further we are also defining the possible subtypes of Parent which any input JSON text could be serialized.
The next snippet shows the Child1 class, we have also annotated the class to encode type but we do not need to define any sub-types. The class has two fields, one of which is annotated to be a JsonProperty and the other is marked to be ignored.
Child2 is very similar except for a few differences which highlight some of the features of Jackson, first we are able serialize more complex types such as the ArrayList of type string and second, we are also able to annotate Java Bean style getters to return a JSON property even if they have no member variable to back them.
Finally lets create a driver class to test our serialization and deserialization. For this we use the ObjectMapper class which has a writeValueAsString method. We just pass an annotated class to ObjectMapper's writeValueAsString method and get an encoded JSON string. Similarly we can use the readValue method to deserialize JSON back to a java object. As the code below shows we pass the string representation of JSON as well as an Ancestor of the object we are deserializing. Notice how we pass ParentClass to the readValue method but when print the class type out you will see it outputs "Child1" and "Child2" correctly.
Compiling and running the code
Notice the fact that the class name is added as a property to the output JSON, and how the dontSerializeMe field of Child1 is ignored. Also note that the types of the objects generated from JSON are Child1 and Child2 respectively even though we told the mapper we were looking for a ParentClass type.
The source code shown here can be Downloaded Here
Note that all code and other source provided here are licensed under the BSD License.