Sunday, June 5, 2016

JSON? What is this, Friday the 13th?

JSON stands for "JavaScript Object Notation", and it is nothing more than a standardized format for pushing object properties around in text format.  This is handy for many purposes, especially for transmitting objects over networks.  I can create objects in Java, convert them to JSON, and then read them in Javascript on a browser.  That's all kinds of handy.

There are plenty of tools for working with JSON,  I personally like Jackson from fasterxml.com.  This package provides a great deal of functionality in an easy to use form.  It's not the only JSON library out there, not by a long shot:  But it's simple, quick, and widely used.  It's also one of the core technologies selected for the 'DropWizard' framework, which I'll be talking about in another post.

Have an object and want to make a JSON representation?  Then ObjectMapper is the tool for you:

First of all, let's create a simple object called 'Person'.  For the sake of simplicity, we'll assume that all we care about is the name.

public class Person {
    private String givenName;
    private String surName;

    // This object has to be a Bean, which means it needs a no-argument constructor    

    public Person() {}

    public Person(String surName, String givenName) {
        this.surName = surName;
        this.givenName = givenName;
    }

    public String getGivenName() {
        return givenName;
    }

    public void setGivenName(String givenName) {
        this.givenName = givenName;
    }

    public String getSurName() {
        return surName;
    }

    public void setSurName(String surName) {
        this.surName = surName;
    }

    @Override    public String toString() {
        return "Person{" +
                "givenName='" + givenName + '\'' +
                ", surName='" + surName + '\'' +
                '}';
    }
}

This import statement is important, and of course you'll have to get the .jar file to support it.  I would suggest just using maven to deal with that.

import com.fasterxml.jackson.databind.ObjectMapper;

...
    ObjectMapper objectMapper = new ObjectMapper();
    Person p = new Person("Doe", "John");
    String jsonRepresentation = objectMapper.writeValueAsString(p);
    System.out.println(jsonRepresentation);

Going back the other way is even easier:

Person p2 = objectMapper.readValue(jsonRepresentation, Person.class);
This works very well within a single project of course.  Let's say you  need to persist your objects to disk for later use.  Well, the JSON representation gives you an excellent way to do this without running into the various issues that straight Java object serialization can raise.  Your files will be human-readable, human-editable, and they're much less likely to become unusable because you changed an object definition.  What's not to love about that?
In fact, reading and writing files is an extremely simple operation with Jackson.  The ObjectMapper does most of the hard lifting for you and turns basic (or not so basic) persistence into one line operations.  If you wanted to take that Person object up above and stash it in a file on your hard drive for later use all you'd need to do is this:
objectMapper.writeValue(new File("JohnDoe.txt"), p);
Reversing the operation is almost as easy:  You just need to use one special little piece of syntax to tell ObjectMapper what kind of object you are creating:

Person p2 = objectMapper.readValue(new File("JohnDoe.txt"), Person.class);
We've basically now recreated what we did in the first couple of examples, but we're able to run one example today and the next one tomorrow after restarting the computer if we want, since instead of just sitting in a String in our program's memory the important information is now sitting on your hard drive.  The file looks like this, by the way:
{"givenName":"John","surName":"Doe"}
JSON is also great for transmitting data over networks.  In fact, if you take a look at RESTful services (which I will visit in a future post) JSON is a standard way of communicating with them.  You don't even have to care what language the remote system uses.  Just so long as you both agree on the set of properties to be sent, the other end could be written in C, .NET, JavaScript (hey, look at what the 'J' stand for in the first place) or any other language.  Maybe the other end is also written in Java, but it was developed by someone else who doesn't have access to your library with your definition of Person.  That's perfectly OK, they can roll their own very easily and so long as the properties in your JSON are all supported everything will just work:
Let's pretend we are on the other side now, and the file created above was sent to us.  We want to bring the data into our system using our home grown "AnotherPerson" class.  For demonstration purposes, the class is exactly the same as the "Person" class above, except for the property names.  So it contains code like this:
public class AnotherPersonNoAnnotations {
    private String firstName;
    private String lastName;

    // This object has to be a Bean, which means it needs a no-argument constructor    

public AnotherPersonNoAnnotations() {}

    public AnotherPersonNoAnnotations(String lastName, String firstName) {
        this.lastName = lastName;
        this.firstName = firstName;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    ...

When we try to read the object, we do this:
AnotherPerson p = objectMapper.readValue(new File("JohnDoe.txt"), AnotherPerson.class);
And it fails spectacularly!
Exception in thread "main" com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "givenName" (class com.oopuniversity.json.model.AnotherPersonNoAnnotations), not marked as ignorable (2 known properties: "lastName", "firstName"])
 at [Source: {"givenName":"John","surName":"Doe"}; line: 1, column: 15] (through reference chain: com.oopuniversity.json.model.AnotherPersonNoAnnotations["givenName"])
at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:62)
...
The actual error will be followed by a whole bunch more stuff, but the important information is right at the top.  And notice:  This is an extremely useful error message.  It tells you exactly what went wrong and why.  It's an "UnrecognizedPropertyException" which means exactly what it says:  Jackson did not recognize a property it found.  Then it flat out tells you that 'givenName' is the problem.  It also tells you which properties it can recognize.  Then to top it off it shows you the exact JSON it was trying to read.  I wish all libraries were this good at telling us what went wrong.
So now we know that our object supports "lastName" and "firstName" but the data contains "givenName" and "surName".  Fixing this is quite simple.
Not to worry, even though we have different names for our properties we'll be fine.  This is far from an uncommon situation and Jackson has it handled.  All we need to do is add a couple of  'annotations' to our code.  Annotations are a clever addition that was made to Java years ago.  Classes, properties and methods can be annotated in order to add new behaviors.  In this case, we'll use them to inform Jackson that we are expecting different field names.  We change our 'AnotherPerson' class so that the declarations look like this:
public class AnotherPerson {
    @JsonProperty("givenName")
    private String firstName;
    @JsonProperty("surName")
    private String lastName;
I think that's pretty self-explanatory.  We're telling Jackson that when it's processing our file, it should think of our 'firstName' field as though it was called 'givenName', and of course the same thing applies to 'lastName' and 'surName'.  These annotations affect both reading and writing and now our class, while it uses our preferred variable names internally, looks to the outside world exactly like the Person class listed above.
Now our attempt to read the file works beautifully:
AnotherPerson{firstName='John', lastName='Doe'}
This is not all there is to say about  JSON processing with Jackson, but it is certainly a good start.  I'm sure I'll be talking about it more in posts to come.  I mentioned but did not talk about using this stuff to communicate between running systems (via RESTful services) and that's a key area that many developers will want to understand.
A code repository with working examples can be found here.
https://github.com/OOPUniversity/JSON_Basics