Introduction to Dagger 2, Using Dependency Injection in Android: Part 2

This is the part 2 of the article series. In part 1 we understood the need and advantages of dependency injection. We also got an overview of Dagger 2. In this part, we will focus on implementing the DI using Dagger 2 in an android app.
Check Part 1 here.
For the sake of this tutorial, we will break the process in steps and analyze each step one by one. Remember Dagger requires a concentrated approach. So actively follow the below tutorial, asking a lot of questions. For the project structure, see the project repo mentioned below.
let’s get started.
The GitHub repo for the project: https://github.com/MindorksOpenSource/android-dagger2-example
First, we have to define the structure of the Android App. The core classes are as follows:
DataManager
class will provide access to the data of the application.DbHelper
class will be used by DataManager to access the SQLite database.SharedPrefsHelper
will be used byDataManager
to access theSharedPreferences
.- Model classes for retrieving the DB table.
Step 1:
Create a project in android studio with an empty Activity and add the following dependencies in app’s build.gradle
dependencies {
...
compile "com.google.dagger:dagger:2.8"
annotationProcessor "com.google.dagger:dagger-compiler:2.8"
provided 'javax.annotation:jsr250-api:1.0'
compile 'javax.inject:javax.inject:1'
}
Notes: We are using the annotation processor provided by gradle for android. dagger-compiler is the annotation processing repo for generating the dependency graph classes during build time. Other gradle dependencies are provided for the sake of Dagger.
Step 2:
The data part we will be building first. So, create a model class User
.
public class User {
private Long id;
private String name;
private String address;
private String createdAt;
private String updatedAt;
public User() {
}
public User(String name, String address) {
this.name = name;
this.address = address;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getCreatedAt() {
return createdAt;
}
public void setCreatedAt(String createdAt) {
this.createdAt = createdAt;
}
public String getUpdatedAt() {
return updatedAt;
}
public void setUpdatedAt(String updatedAt) {
this.updatedAt = updatedAt;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", address='" + address + '\'' +
", createdAt='" + createdAt + '\'' +
", updatedAt='" + updatedAt + '\'' +
'}';
}
}
Notes: This class will bind the DB table data.
Step 3:
Create few custom annotations: ActivityContext
, ApplicationContext
, DatabaseInfo
, PerActivity
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
public @interface ActivityContext {
}
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
public @interface ApplicationContext {
}
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
public @interface DatabaseInfo {
}
@Scope
@Retention(RetentionPolicy.RUNTIME)
public @interface PerActivity {
}
Notes:
Why are we creating these annotations and what is @Qualifier
and@Scope
?
@Qualifier
annotation is provided by javax inject package and is used to qualify the dependency. For example, a class can ask both, an Application Context and an Activity Context. But both these Objects will be of type Context. So, for Dagger to figure out which variable is to be provided with what, we have to explicitly specify the identifier for it.
Thus @Qualifier
is used to distinguish between objects of the same type but with different instances. In the above code, we have ActivityContext
and ApplicationContext
so that the Context
object being injected can refer to the respectiveContext
type. DatabaseInfo
is used to provide the database name in the class dependency. Since a String
class in being provided as a dependency, it always a good idea to qualify it so that the Dagger can explicitly resolve it.
An alternative to this is @Named
annotation provided by Dagger. @Named
itself is annotated with @Qualifier
. With @Named
we have to provide string identifier for similar class objects and this identifier is used to map the dependency of a class. We will explore the @Named
at the end of this example.
@Scope
is used to specify the scope in which a dependency object persists. If a class getting dependencies, have members injected with classes annotated with a scope, then each instance of that class asking for dependencies will get its own set of member variables.
Step 4:
Create a DbHelper
class that extends the SQLiteOpenHelper
. This class will be responsible for all the DB related operations.
@Singleton
public class DbHelper extends SQLiteOpenHelper {
//USER TABLE
public static final String USER_TABLE_NAME = "users";
public static final String USER_COLUMN_USER_ID = "id";
public static final String USER_COLUMN_USER_NAME = "usr_name";
public static final String USER_COLUMN_USER_ADDRESS = "usr_add";
public static final String USER_COLUMN_USER_CREATED_AT = "created_at";
public static final String USER_COLUMN_USER_UPDATED_AT = "updated_at";
@Inject
public DbHelper(@ApplicationContext Context context,
@DatabaseInfo String dbName,
@DatabaseInfo Integer version) {
super(context, dbName, null, version);
}
@Override
public void onCreate(SQLiteDatabase db) {
tableCreateStatements(db);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS " + USER_TABLE_NAME);
onCreate(db);
}
private void tableCreateStatements(SQLiteDatabase db) {
try {
db.execSQL(
"CREATE TABLE IF NOT EXISTS "
+ USER_TABLE_NAME + "("
+ USER_COLUMN_USER_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
+ USER_COLUMN_USER_NAME + " VARCHAR(20), "
+ USER_COLUMN_USER_ADDRESS + " VARCHAR(50), "
+ USER_COLUMN_USER_CREATED_AT + " VARCHAR(10) DEFAULT " + getCurrentTimeStamp() + ", "
+ USER_COLUMN_USER_UPDATED_AT + " VARCHAR(10) DEFAULT " + getCurrentTimeStamp() + ")"
);
} catch (SQLException e) {
e.printStackTrace();
}
}
protected User getUser(Long userId) throws Resources.NotFoundException, NullPointerException {
Cursor cursor = null;
try {
SQLiteDatabase db = this.getReadableDatabase();
cursor = db.rawQuery(
"SELECT * FROM "
+ USER_TABLE_NAME
+ " WHERE "
+ USER_COLUMN_USER_ID
+ " = ? ",
new String[]{userId + ""});
if (cursor.getCount() > 0) {
cursor.moveToFirst();
User user = new User();
user.setId(cursor.getLong(cursor.getColumnIndex(USER_COLUMN_USER_ID)));
user.setName(cursor.getString(cursor.getColumnIndex(USER_COLUMN_USER_NAME)));
user.setAddress(cursor.getString(cursor.getColumnIndex(USER_COLUMN_USER_ADDRESS)));
user.setCreatedAt(cursor.getString(cursor.getColumnIndex(USER_COLUMN_USER_CREATED_AT)));
user.setUpdatedAt(cursor.getString(cursor.getColumnIndex(USER_COLUMN_USER_UPDATED_AT)));
return user;
} else {
throw new Resources.NotFoundException("User with id " + userId + " does not exists");
}
} catch (NullPointerException e) {
e.printStackTrace();
throw e;
} finally {
if (cursor != null)
cursor.close();
}
}
protected Long insertUser(User user) throws Exception {
try {
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put(USER_COLUMN_USER_NAME, user.getName());
contentValues.put(USER_COLUMN_USER_ADDRESS, user.getAddress());
return db.insert(USER_TABLE_NAME, null, contentValues);
} catch (Exception e) {
e.printStackTrace();
throw e;
}
}
private String getCurrentTimeStamp() {
return String.valueOf(System.currentTimeMillis() / 1000);
}
}
Notes: This class introduces few annotation.
@Singleton
ensure a single instance of a class globally. So, there will be only oneDbHelper
class instance for the app and whenever a class asks forDbHelper
as a dependency, it will be provided with the same instance that is maintained in the Dagger’s dependency graph.@Inject
on the constructor instructs the Dagger to accumulate all the parameter dependencies when the class is being constructed.@ApplicationContext
Qualifier
facilitatesDbHelper
to get the context object of the application from dagger’s dependency graph@DatabaseInfo
qualifier helps the dagger to distinguish betweenString
andInteger
Dependencies from existing same types in the dependency graph.
We will again come back to this discussion when we deal with the module.
Rest of the contents of this class is standard SQLiteOpenHelper
. This class creates a user
table and inserts/reads it.
Step 5:
Create a SharedPrefsHelper
to deal with the shared preferences.
@Singleton
public class SharedPrefsHelper {
public static String PREF_KEY_ACCESS_TOKEN = "access-token";
private SharedPreferences mSharedPreferences;
@Inject
public SharedPrefsHelper(SharedPreferences sharedPreferences) {
mSharedPreferences = sharedPreferences;
}
public void put(String key, String value) {
mSharedPreferences.edit().putString(key, value).apply();
}
public void put(String key, int value) {
mSharedPreferences.edit().putInt(key, value).apply();
}
public void put(String key, float value) {
mSharedPreferences.edit().putFloat(key, value).apply();
}
public void put(String key, boolean value) {
mSharedPreferences.edit().putBoolean(key, value).apply();
}
public String get(String key, String defaultValue) {
return mSharedPreferences.getString(key, defaultValue);
}
public Integer get(String key, int defaultValue) {
return mSharedPreferences.getInt(key, defaultValue);
}
public Float get(String key, float defaultValue) {
return mSharedPreferences.getFloat(key, defaultValue);
}
public Boolean get(String key, boolean defaultValue) {
return mSharedPreferences.getBoolean(key, defaultValue);
}
public void deleteSavedData(String key) {
mSharedPreferences.edit().remove(key).apply();
}
}
Notes: This class is annotated with @Singleton
to make this a singleton class in the dependency graph of Dagger.
This class also gets SharedPreferences
dependency through Dagger which is expressed by the @Inject
on the constructor.
How is this dependency provided? It is explained later in this example.
Step 6:
Create DataManager
class
@Singleton
public class DataManager {
private Context mContext;
private DbHelper mDbHelper;
private SharedPrefsHelper mSharedPrefsHelper;
@Inject
public DataManager(@ApplicationContext Context context,
DbHelper dbHelper,
SharedPrefsHelper sharedPrefsHelper) {
mContext = context;
mDbHelper = dbHelper;
mSharedPrefsHelper = sharedPrefsHelper;
}
public void saveAccessToken(String accessToken) {
mSharedPrefsHelper.put(SharedPrefsHelper.PREF_KEY_ACCESS_TOKEN, accessToken);
}
public String getAccessToken(){
return mSharedPrefsHelper.get(SharedPrefsHelper.PREF_KEY_ACCESS_TOKEN, null);
}
public Long createUser(User user) throws Exception {
return mDbHelper.insertUser(user);
}
public User getUser(Long userId) throws Resources.NotFoundException, NullPointerException {
return mDbHelper.getUser(userId);
}
}
Notes: This class expresses the dependencies of Application Context
, DbHelper
and SharedPrefsHelper
in the contructor. It provides all the apis to access the data of the application.
Step 7:
Create DemoApplication
class that extends android.app.Application
public class DemoApplication extends Application {
protected ApplicationComponent applicationComponent;
@Inject
DataManager dataManager;
public static DemoApplication get(Context context) {
return (DemoApplication) context.getApplicationContext();
}
@Override
public void onCreate() {
super.onCreate();
applicationComponent = DaggerApplicationComponent
.builder()
.applicationModule(new ApplicationModule(this))
.build();
applicationComponent.inject(this);
}
public ApplicationComponent getComponent(){
return applicationComponent;
}
}
Add this class in AndroidManifest.xml
<application
...
android:name=".DemoApplication"
...
</application>
Notes: This class gets DataManager
through DI. The interesting part in this class is the ApplicationComponent
. Let’s go on with the steps before we cover this point.
Step 8:
Create class MainActivity
public class MainActivity extends AppCompatActivity {
@Inject
DataManager mDataManager;
private ActivityComponent activityComponent;
private TextView mTvUserInfo;
private TextView mTvAccessToken;
public ActivityComponent getActivityComponent() {
if (activityComponent == null) {
activityComponent = DaggerActivityComponent.builder()
.activityModule(new ActivityModule(this))
.applicationComponent(DemoApplication.get(this).getComponent())
.build();
}
return activityComponent;
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getActivityComponent().inject(this);
mTvUserInfo = (TextView) findViewById(R.id.tv_user_info);
mTvAccessToken = (TextView) findViewById(R.id.tv_access_token);
}
@Override
protected void onPostCreate(@Nullable Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
createUser();
getUser();
mDataManager.saveAccessToken("ASDR12443JFDJF43543J543H3K543");
String token = mDataManager.getAccessToken();
if(token != null){
mTvAccessToken.setText(token);
}
}
private void createUser(){
try {
mDataManager.createUser(new User("Ali", "1367, Gurgaon, Haryana, India"));
}catch (Exception e){e.printStackTrace();}
}
private void getUser(){
try {
User user = mDataManager.getUser(1L);
mTvUserInfo.setText(user.toString());
}catch (Exception e){e.printStackTrace();}
}
}
Also create activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
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"
android:gravity="center"
android:orientation="vertical">
<TextView
android:id="@+id/tv_user_info"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textColor="@android:color/black"
android:padding="10dp"/>
<TextView
android:id="@+id/tv_access_token"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textColor="@android:color/black"
android:padding="10dp"/>
</LinearLayout>
Now let’s hold on for some time and review the Dagger.
- To provide the dependency for a class we have to create a Module class. This class defines the methods that provide the dependency. A Module class is identified by
@Module
and the dependency provider method in identified by@Provides
. - We then have to create an interface that serves as a connection between a class that expresses the dependency through
@Inject
and a class that provides the dependency i.e. annotated with@Module
. - In order to figure out the dependencies that a Module has to provide, we have to scan all the classes in the graph that needs to be provided with dependencies and then figure out the least number of objects that has to be provided.
Let’s move back to the example steps to understand the above statements.
Step 9:
We have to provide the dependencies expressed in the DemoApplication
class. This class needs DataManager
and to provide this class we have to provide the dependencies expressed by DataManager
, which as mentioned in the constructor are Context
, DbHelper
, and SharedPrefsHelper
. We then move further in the graph.
Context
has to beApplicationContext
DbHelper
needsContext
,dbName
, andversion
. This does not have any further branching.SharedPrefsHelper
needsSharedPreferences
We now accumulate the superset of all these dependencies, which turn out to be: Context
, dbName
, version
, and SharedPreferences
Now to provide these dependencies we create ApplicationModule
@Module
public class ApplicationModule {
private final Application mApplication;
public ApplicationModule(Application app) {
mApplication = app;
}
@Provides
@ApplicationContext
Context provideContext() {
return mApplication;
}
@Provides
Application provideApplication() {
return mApplication;
}
@Provides
@DatabaseInfo
String provideDatabaseName() {
return "demo-dagger.db";
}
@Provides
@DatabaseInfo
Integer provideDatabaseVersion() {
return 2;
}
@Provides
SharedPreferences provideSharedPrefs() {
return mApplication.getSharedPreferences("demo-prefs", Context.MODE_PRIVATE);
}
}
Note: We have annotated this class with @Module
and all the methods with @Provides
.
Let’s explore this class:
- In the constructor, we have passed the
Application
instance. This instance is used to provide other dependencies. - This class provides all the dependencies that we listed in the above step.
Step 10:
We create a Component which links the DemoApplication
dependency and the ApplicationModule
@Singleton
@Component(modules = ApplicationModule.class)
public interface ApplicationComponent {
void inject(DemoApplication demoApplication);
@ApplicationContext
Context getContext();
Application getApplication();
DataManager getDataManager();
SharedPrefsHelper getPreferenceHelper();
DbHelper getDbHelper();
}
Notes: ApplicationComponent
is an interface that is implemented by Dagger. Using @Component
we specify the class to be a Component.
Here we have written a method inject
where we pass the DemoApplication
instance. Why do we do it?
When the dependencies are provided through field injection i.e. @inject
on the member variables, we have to tell the Dagger to scan this class through the implementation of this interface.
This class also provides methods that are used to access the dependencies that exist in the dependency graph.
Step 11:
Similarly, we have to create the module for MainActivity
and it’s component. Which follow the same procedures as in the above step.
@Module
public class ActivityModule {
private Activity mActivity;
public ActivityModule(Activity activity) {
mActivity = activity;
}
@Provides
@ActivityContext
Context provideContext() {
return mActivity;
}
@Provides
Activity provideActivity() {
return mActivity;
}
}
@PerActivity
@Component(dependencies = ApplicationComponent.class, modules = ActivityModule.class)
public interface ActivityComponent {
void inject(MainActivity mainActivity);
}
Note: ActivityComponent
specify ApplicationComponent
and ActivityModule
. ApplicationComponent
is added to use the graph that has already been generated in the previous step and do exists because the DemoApplication
class persists till the application is running.
@PerActivity
is a scope and is used to tell the Dagger that the Context
and Activity
provided by the ActivityModule
will be instantiated each time an Activity
is created. So, these objects persist till that activity lives and each activity has its own set of these.
We may ask that the DataManager
will then be created with each Activity
. But that is not true because we have annotated the DataManager
class with @Singleton
. Which brings the class in the global scope and thus is accessed whenever a dependency is expressed.
Now let’s revisit the DemoApplication
class and MainActivity
class. These classes don’t have a constructor and Android System is responsible for instantiating these. To get the dependency we use the OnCreate
method as it is called once when they are instantiated.
applicationComponent = DaggerApplicationComponent
.builder()
.applicationModule(new ApplicationModule(this))
.build();
applicationComponent.inject(this);
DaggerApplicationComponent
is the generated class by the Dagger, implementing the ApplicationComponent
. We provide the ApplicationModule
class that is used to construct the dependencies.
We have also called the inject
method of applicationComponent
and passed the instance of the DemoApplication
class. This is done to use it for providing the DataManager
.ApplicationComponent
instance is retained so as to access all the classes that are available in the dependency graph and is express for access.
public ActivityComponent getActivityComponent() {
if (activityComponent == null) {
activityComponent = DaggerActivityComponent.builder()
.activityModule(new ActivityModule(this))
.applicationComponent(DemoApplication.get(this).getComponent())
.build();
}
return activityComponent;
}
Similar process is applied in the MainActivity
as well. The only difference is that, we also provide ApplicationComponent
that is required for dependencies resolution mentioned in ActivityModule
.
This completes the Example project.
Hope you have got a working knowledge of Dagger.
The GitHub repo for this example project: https://github.com/MindorksOpenSource/android-dagger2-example
As we mentioned about @Named("string")
annotation, we just have to replace @ApplicationContext
and @ActivityContext
with something like @Named("application_context")
and @Named("activity_context")
every where. But personally I don’t like this implementation. It requires to maintain a String
tag.
Note: If due to some reason we have to inject dependencies through field injection for classes other that android constructed classes then define a component interface for that class and call it’s inject method in class’s constructor. I would suggest to figure out a way not to do this but try to use the constructor with @Inject
for the injection.
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