一、不使用Dagger2
1、编写一个类：
public class Test3 {
public Test3() {
}
public void sayHelloWorld(){
Log.d("Test3","Hello World");
}
}
2、在MainActivity中使用Test3这个类
public class MainActivity extends AppCompatActivity {
Test3 test3;
@Override
protected void onCreate(Bundle savedInstanceState) {
setContentView(R.layout.activity_main);
test3 = new Test3();
}
}
3、成功运行代码，可用看到控制台输出了日志信息：
2022-11-12 14:47:12.411 2973-2973 Test3 com.example.dagger2application D Hello World
二、使用Dagger2
1、添加依赖
dependencies {
...代码省略...
//dagger2依赖
api 'com.google.dagger:dagger:2.44'
annotationProcessor 'com.google.dagger:dagger-compiler:2.44'
}
2、编写一个类，添加注解@Inject，使得Dagger2能找到它。
public class Test3 {
//添加Dagger2注解
@Inject
public Test3() {
}
public void sayHelloWorld(){
Log.d("Test3","Hello World");
}
}
3、新增一个接口，添加@Singleton和@Component注解和inject方法。
//用这个标注标识是一个单例
@Singleton
//用这个标注标识是一个组件
@Component()
public interface MainActivityComponent {
//这个组件要注入的对象。这个inject标注的意思是
//后面的参数对象里面有被@Inject标注的属性，需要这个连接器来进行依赖注入。
void inject(MainActivity activity);
}
4、在MainActivity中使用Test3这个类。
public class MainActivity extends AppCompatActivity {
@Inject
Test3 test3;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//使用组件进行构造、注入
DaggerMainActivityComponent.builder().build().inject(this);
test3.sayHelloWorld();
}
}
5、成功运行代码，可用看到控制台输出了日志信息：
2022-11-12 14:47:12.411 2973-2973 Test3 com.example.dagger2application D Hello World
三、查看使用Dagger2新增了那些临时类
打开build/generated/ap_generaed_sources/debug/out/com/example/dagger2application目录：
image.png
可以发现该目录新增了三个类，分别是DaggerMainActivityComponent、MainActivity_MembersInjector和Test3_Factory，这三个类是Dagger2框架帮我们自动生成的。分别来看下这三个类的源码详情。
1、DaggerMainActivityComponent类的全部代码：
package com.example.dagger2application;
import dagger.internal.DaggerGenerated;
@DaggerGenerated
@SuppressWarnings({
"unchecked",
"rawtypes"
})
public final class DaggerMainActivityComponent {
private DaggerMainActivityComponent() {
}
public static Builder builder() {
return new Builder();
}
public static MainActivityComponent create() {
return new Builder().build();
}
public static final class Builder {
private Builder() {
}
public MainActivityComponent build() {
return new MainActivityComponentImpl();
}
}
private static final class MainActivityComponentImpl implements MainActivityComponent {
private final MainActivityComponentImpl mainActivityComponentImpl = this;
private MainActivityComponentImpl() {
}
@Override
public void inject(MainActivity activity) {
injectMainActivity(activity);
}
private MainActivity injectMainActivity(MainActivity instance) {
MainActivity_MembersInjector.injectTest3(instance, new Test3());
return instance;
}
}
}
2、MainActivity_MembersInjector 类的全部代码：
package com.example.dagger2application;
import dagger.MembersInjector;
import dagger.internal.DaggerGenerated;
import dagger.internal.InjectedFieldSignature;
import dagger.internal.QualifierMetadata;
import javax.inject.Provider;
@QualifierMetadata
@DaggerGenerated
@SuppressWarnings({
"unchecked",
"rawtypes"
})
public final class MainActivity_MembersInjector implements MembersInjector<MainActivity> {
private final Provider<Test3> test3Provider;
public MainActivity_MembersInjector(Provider<Test3> test3Provider) {
this.test3Provider = test3Provider;
}
public static MembersInjector<MainActivity> create(Provider<Test3> test3Provider) {
return new MainActivity_MembersInjector(test3Provider);
}
@Override
public void injectMembers(MainActivity instance) {
injectTest3(instance, test3Provider.get());
}
@InjectedFieldSignature("com.example.dagger2application.MainActivity.test3")
public static void injectTest3(MainActivity instance, Test3 test3) {
instance.test3 = test3;
}
}
3、Test3_Factory类的全部代码：
// Generated by Dagger (https://dagger.dev).
package com.example.dagger2application;
import dagger.internal.DaggerGenerated;
import dagger.internal.Factory;
import dagger.internal.QualifierMetadata;
import dagger.internal.ScopeMetadata;
@ScopeMetadata
@QualifierMetadata
@DaggerGenerated
@SuppressWarnings({
"unchecked",
"rawtypes"
})
public final class Test3_Factory implements Factory<Test3> {
@Override
public Test3 get() {
return newInstance();
}
public static Test3_Factory create() {
return InstanceHolder.INSTANCE;
}
public static Test3 newInstance() {
return new Test3();
}
private static final class InstanceHolder {
private static final Test3_Factory INSTANCE = new Test3_Factory();
}
}
四、总结
1、结合Dagger2框架新增的三个类我们可以发现，在MainActivity的onCreate方法中，Dagger2框架实现依赖注入的流程如下所示：
image.png
2、MainActivity的onCreate方法关键代码：
public class MainActivity extends AppCompatActivity {
...代码省略...
@Override
protected void onCreate(Bundle savedInstanceState) {
...代码省略...
DaggerMainActivityComponent.builder().build().inject(this);
}
}
3、DaggerMainActivityComponent的builder方法关键代码：
public final class DaggerMainActivityComponent {
public static Builder builder() {
return new Builder();
}
}
4、DaggerMainActivityComponent_Builder的builder方法关键代码：
public final class DaggerMainActivityComponent {
public static final class Builder {
...代码省略...
public MainActivityComponent build() {
return new MainActivityComponentImpl();
}
}
}
5、MainActivityComponentImpl的inject方法会进一步调用injectMainActivity方法：
private static final class MainActivityComponentImpl implements MainActivityComponent {
...代码省略...
@Override
public void inject(MainActivity activity) {
injectMainActivity(activity);
}
private MainActivity injectMainActivity(MainActivity instance) {
MainActivity_MembersInjector.injectTest3(instance, new Test3());
return instance;
}
}
6、MainActivity_MembersInjector的injectTest3方法如下所示
public final class MainActivity_MembersInjector implements MembersInjector<MainActivity> {
...代码省略...
@InjectedFieldSignature("com.example.dagger2application.MainActivity.test3")
public static void injectTest3(MainActivity instance, Test3 test3) {
instance.test3 = test3;
}
}
经过上面代码的调用流程分析，基本可以明白Daggger2是如何实现依赖注入的原理了。
