This is an android example of how to parse JSON file in GSON.
Gson is a Java library that can be used to convert Java Objects into their JSON representation.
Gson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of. There are a few open-source projects that can convert Java objects to JSON.
However, most of them require that you place Java annotations in your classes; something that you can not do if you do not have access to the source-code. Most also do not fully support the use of Java Generics. This library considers both of these as very important design goals.
In this lesson you will learn how to parse a local JSON file using GSON. GSON is the most popular JSON parsing library. You can use it to parse all sorts of JSON data in android and java in general. In this particular we wanna show you how to specifically parse local JSON data. By local we mean data stored under the raw folder inside the res directory.
Step 1: Install Gson
The first step is to install GSON. As popular as it is, GSON is a third party library. It was originally created by Google. It’s been around for almost a decade so you can trust even in a production app.
Simply declare it in your app/build.gradle and sync.
Use the following implementation statement to install it. You can use a later version.
//Install our GSON
//You can use a later version
implementation 'com.google.code.gson:gson:2.5'Step 2: Add JSON file
The second step is to add some JSON data to the raw directory. In your res directory create a raw folder and inside it add the shown JSON file. What you are seeing is a JSON object. It has three properties: a module name, a teacher and a semester. The Teacher itself is a JSON object. The module name is a string, while the semester is an integer.
/raw/course.json
{
"module_name": "Mobile Application Development",
"teacher": {
"first_name": "Nguyên",
"last_name": "Baylatry"
},
"semester": 8
}
Step 3: Design Layout
Next design your main layout with a bunch of TextViews inside a RelativeLayout. The target layout is the activity_main xml layout. These textviews will show the individual values of our JSON object. Our root element is the RelativeLayout. Then 2, 3 and 4 represent our TextViews. That’s all we need to design our layout.
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="fr.esilv.gsonexample.MainActivity">
<TextView
android:id="@+id/moduleNameTextView"
style="@style/TextAppearance.AppCompat.Large"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
<TextView
android:id="@+id/teacherTextView"
style="@style/TextAppearance.AppCompat.Medium"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/moduleNameTextView"/>
<TextView
android:id="@+id/semesterNameTextView"
style="@style/TextAppearance.AppCompat.Medium"
android:layout_width="wrap_content"
android:layout_below="@+id/teacherTextView"
android:layout_height="wrap_content"/>
</RelativeLayout>
Step 4: Create Models
Create two model class as shown below:
(a). Teacher.java
A Teacher class to represent a single teacher. Each teacher will have a first name and a last name. Take note of the SerializedName annotations decorated on those properties and keys passed to them.
package fr.esilv.gsonexample.models;
import com.google.gson.annotations.SerializedName;
public class Teacher {
@SerializedName("first_name")
private String firstName;
@SerializedName("last_name")
private String lastName;
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
}
(b). Course.java
We also have a course model class. It has the module name, teacher and semester as the properties. Only the module name in this case will be serialized. Our whole JSON object will be mapped to this particular class. You can see it also includes the Teacher object which too has properties serialized.
package fr.esilv.gsonexample.models;
import com.google.gson.annotations.SerializedName;
public class Course {
@SerializedName("module_name")
private String moduleName;
private Teacher teacher;
private int semester;
public String getModuleName() {
return moduleName;
}
public void setModuleName(String moduleName) {
this.moduleName = moduleName;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
public int getSemester() {
return semester;
}
public void setSemester(int semester) {
this.semester = semester;
}
}
Step 5: Load JSON
Our last step will be to load our JSON. We do this inside of our MainActivity. Proceed to the class, add imports. Then extend the AppCompatActivity. As instance fields, declare our TextViews.
Then inside the onCreate method, reference them. To load our JSON data, invoke the initialize data function. In that function we load our JSON data, by first invoking openRawResources method. Inside the method we pass the ID of the JSON file.
This function will return an InputStream. We then instantiate a ByteArrayOutputStream. We will be writing the data we are reading from InputStream into this ByteArrayOutputStream. We do this, inside a try-catch block, so that we catch any IO Exception. We then convert the ByteArrayOutputStream object into a string, using the toString method.
Then by invoking the from JSON method, and passing the ByteArrayOutputStream object, we will get a Course object. That’s how load a JSON file from the raw folder using Google JSON library.
MainActivity.java
package fr.esilv.gsonexample;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.TextView;
import com.google.gson.Gson;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import fr.esilv.gsonexample.models.Course;
import fr.esilv.gsonexample.models.Teacher;
public class MainActivity extends AppCompatActivity {
private TextView moduleNameTextView;
private TextView teacherTextView;
private TextView semesterNameTextView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
moduleNameTextView = (TextView) findViewById(R.id.moduleNameTextView);
teacherTextView = (TextView) findViewById(R.id.teacherTextView);
semesterNameTextView = (TextView) findViewById(R.id.semesterNameTextView);
initializeData();
}
private void initializeData() {
//get the raw Json as A Stream
InputStream courseAsInputStream = getResources().openRawResource(R.raw.course);
//get a String from the Stream
ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
int i;
try {
i = courseAsInputStream.read();
while (i != -1) {
byteArrayOutputStream.write(i);
i = courseAsInputStream.read();
}
courseAsInputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
String courseAsString = byteArrayOutputStream.toString();
//parse the String as an Object using Gson
Gson gson = new Gson();
Course course = gson.fromJson(courseAsString, Course.class);
//populate the View
if (course != null) {
moduleNameTextView.setText(course.getModuleName());
Teacher teacher = course.getTeacher();
teacherTextView.setText(getString(R.string.teacher, teacher.getFirstName(), teacher.getLastName()));
semesterNameTextView.setText(getString(R.string.semester, course.getSemester()));
}
}
}
Reference
Download full code here.
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