我试图在 Android 应用程序中做一个非常简单的依赖注入。我使用 dagger 2 作为 DI 工具。
问题是没有发生注入:
这是我的代码:
//看到 Motor.java 的所有敬畏。
public class Motor {
private int rpm;
public Motor(){
this.rpm = 10; //default will be 10
}
public int getRpm(){
return rpm;
}
public void accelerate(int value){
rpm = rpm + value;
}
public void brake(){
rpm = 0;
}
}
这是使用电机类的 Vehicle.java:
import javax.inject.Inject;
public class Vehicle {
private Motor motor;
@Inject
public Vehicle(Motor motor){
this.motor = motor;
}
public void increaseSpeed(int value){
motor.accelerate(value);
}
public void stop(){
motor.brake();
}
public int getSpeed(){
return motor.getRpm();
}
}
然后我创建了一个 VehicleModule.java 类来定义我的提供者:
import javax.inject.Singleton;
import dagger.Module;
import dagger.Provides;
@Module
public class VehicleModule {
@Provides
@Singleton
Motor provideMotor(){
return new Motor();
}
@Provides @Singleton
Vehicle provideVehicle(){
return new Vehicle(new Motor());
}
}
然后我有一个带注释的接口组件,定义如下:
import javax.inject.Singleton;
import Modules.VehicleModule;
import dagger.Component;
@Singleton
@Component(modules = {VehicleModule.class})
public interface VehicleComponent {
Vehicle provideVehicle();
}
这是我应该注入的Android mainactivity 类,但它不是,任何人都可以帮忙:
import javax.inject.Inject;
public class MainActivity extends ActionBarActivity {
@Inject
Vehicle vehicle;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toast.makeText(this, String.valueOf(vehicle.getSpeed()), Toast.LENGTH_SHORT).show();
}
}
我在车辆上得到一个空指针异常:
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int com.example.uen229.myapplication.Vehicle.getSpeed()' on a null object reference
顺便说一下,我的 gradle 依赖项如下所示:
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
compile 'com.android.support:appcompat-v7:22.2.0'
compile 'com.google.dagger:dagger:2.0'
}