Hands-on: JSON Java API

Standard

JSON (JavaScript Object Notation) is a notation for data communication, as well as XML, for example. Its popularity has grown with the growth of the REST Web Services, and today has long been used in the development of APIs.

In this hands-on, we will learn how to use a JSON Java API, present in Java EE 7. With it, you can parse JSON structures for reading the data, and generate their own structures.

Creating the project

In this hands-on we will use Eclipse. Create a Maven project in New> Other> Maven Project. If you do not have this option, open the Eclipse Marketplace on the IDE itself (Help menu), and look for the plugin “Maven Integration for Eclipse” on your version. At the end of this post, you can find a link to the source code of hands-on.

With the project done, we will add in the pom the following dependencies:

<dependencies>
<dependency>
<groupId>javax.json</groupId>
<artifactId>javax.json-api</artifactId>
<version>1.0</version>
</dependency>
<dependency>
<groupId>org.glassfish</groupId>
<artifactId>javax.json</artifactId>
<version>1.0.4</version>
</dependency>
</dependencies>

With the dependencies created, we will begin to explore the API.

JsonParser

The first class we will talk about is the JsonParser. With this class, we can, from a JSON input, perform a parse of the structure. The code below demonstrates the use of the class:

.....
FileInputStream file = new FileInputStream("dados.json");
JsonParser parser = Json.createParser(file);
while (parser.hasNext()) {
Event evento = parser.next();
switch (evento) {
case KEY_NAME: {
System.out.print(parser.getString() + "=");
break;
}
case VALUE_STRING: {
System.out.println(parser.getString());
break;
}
case VALUE_NUMBER: {
System.out.println(parser.getString());
break;
}
case VALUE_NULL: {
System.out.println("null");
break;
}
case START_ARRAY: {
System.out.println("Inicio do Array de Telefone");
break;
}
case END_ARRAY: {
System.out.println("Final do Array de Telefone");
break;
}
case END_OBJECT: {
System.out.println("Final do Objeto Json");
break;
}
}
}
.....

As we can see in the code above, through the class we followed the whole json structure contained within the file “dados.json”. For example, with a file which has the following structure:

{
“id”:123,
“descricao”:”Produto 1″,
“Classificacao”:{
“nivel”:1,
“subnivel”:2,
“secao”:”eletrodomesticos”
},
“fornecedores”:[
{
“id”:1,
“descricao”:”brastemp”
},
{
“id”:2,
“descricao”:”consul”
},
{
“id”:3,
“descricao”:”eletrolux”
}
]
}

We have the following print on the console:

id:
123
descricao:
Produto 1
Classificacao:
nivel:
1
subnivel:
2
secao:
eletrodomesticos
Final do Objeto Json
fornecedores:
começo de um array
id:
1
descricao:
brastemp
Final do Objeto Json
id:
2
descricao:
consul
Final do Objeto Json
id:
3
descricao:
eletrolux
Final do Objeto Json
final de um array
Final do Objeto Json

JsonGenerator

With the JsonGenerator class, you can generate JSON structures.The usage is made by putting the openings and closings of the tags in a manual way,  through the API methods, generating the structure in a  sequentially way:

.....
JsonGeneratorFactory factory = Json.createGeneratorFactory(properties);
JsonGenerator jsonGen = factory.createGenerator(System.out);
jsonGen.writeStartObject().write("id", 123).write("descricao", "Produto 1").writeStartObject("Classificacao").write("nivel", 1).write("subnivel", 2).write("secao", "eletrodomesticos").writeEnd().writeStartArray("fornecedores").writeStartObject().write("id", 1).write("descricao", "brastemp").writeEnd().writeStartObject().write("id", 2).write("descricao", "consul").writeEnd().writeStartObject().write("id", 3).write("descricao", "eletrolux").writeEnd().writeEnd().writeEnd().close();
.....

The above code will generate an identical Json than shown above.

JsonObjectBuilder

In the example above, although the API facilitates the creation of the JSON, we have some problems. As we have to manually put the openings and closings of the tags, the result is a somewhat laborious code, which requires the developer to careful not generate invalid results. A better alternative is to generate Jsons with the JsonObjectBuilder class, which use a nearest OO API format, and therefore easier to program in the language:

.....
JsonBuilderFactory jBuilderFactory = Json.createBuilderFactory(null);
JsonObjectBuilder jObjectBuilder = jBuilderFactory
.createObjectBuilder();
jObjectBuilder
.add("id", 123)
.add("descricao", "Produto 1")
.add("Classificacao",
jBuilderFactory.createObjectBuilder().add("nivel", 1)
.add("subnivel", 2)
.add("secao", "eletrodomesticos"))
.add("fornecedores",
jBuilderFactory
.createArrayBuilder()
.add(jBuilderFactory.createObjectBuilder()
.add("id", 1)
.add("descricao", "brastemp"))
.add(jBuilderFactory.createObjectBuilder()
.add("id", 2)
.add("descricao", "consul"))
.add(jBuilderFactory.createObjectBuilder()
.add("id", 3)
.add("descricao", "eletrolux")));
JsonObject jObject = jObjectBuilder.build();
JsonWriter jWriterOut = Json.createWriter(System.out);
jWriterOut.writeObject(jObject);
jWriterOut.close();
.....

As in the other example, this code will generate the same JSON shown at the beginning of the post.

Conclusion

In this hands-on, we saw a sample of a JSON manipulation API of the Java language. With it, we can create Jsons more simply, beyond reading them. The reader may be wondering “but it is not easier to use the JAX-RS 2.0 to produce / consume Jsons”? It is true that the JAX-RS 2.0 has brought a simpler interface than the one presented here, where, basically, simply create a POJO to have a ready Json structure. The reader should remember, however, that the JSON is not a unique structure for use with REST services, and therefore, for scenarios where the use of the RS 2.0 is not appropriate, this API can become a good option. Out of curiosity, the JAX-RS 2.0 uses this API “under the hood”.

And so we ended our hands-on. Thanks to all who attended this post, until next time

Source-code (Github)

Consuming Jsons using JAX-RS

API summary

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