网上关于Dagger2的博客很多，但自己还是觉得大多都讲得稀里糊涂，大多只讲了怎么用，但始终是没怎么讲其内部实现流程，懒得搜博客，干脆直接看源码。
简单的一个应用
首先从一个最最简单的例子开始
一个user类，在其构造方法加个Inject注解，功能相当于表示此处提供User的实例化对象
public class User {
private String userName;
private String password;
@Inject
public User() {
// 暂时用无参，便于分析
this.userName = " dfs";
this.password = "fdsa ";
}
@Override
public String toString() {
return "User{" +
"userName='" + userName + '\'' +
", password='" + password + '\'' +
'}';
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}
然后来个随意的注射器MainActivityComponent,@Component()表示此接口为注射器，这里里面有一个往MainActivity注射的方法
@Component()
public interface MainActivityComponent {
void inject(MainActivity mainActivity);
}
然后make下model，生成DaggerMainActivityComponent类，最后应用于MainActivity
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity>>>";
@Inject
User user;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
DaggerMainActivityComponent.builder()
.build()
.inject(this);
Log.i(TAG, "onCreate: >>>>" + user);
}
}
得到结果,显而易见是注射成功了
I/MainActivity>>>: onCreate: >>>>User{userName=' dfs', password='fdsa '}
源码分析
User类的构造方法加了注解Inject后，后面编译时会生成一个对应的枚举
@Generated("dagger.internal.codegen.ComponentProcessor")
public enum User_Factory implements Factory<User> {
INSTANCE;
@Override
public User get() {
return new User();
}
public static Factory<User> create() {
return INSTANCE;
}
}
看了这个，我才知道枚举也能像类一样。。。之前看到书上说枚举可以实现单例，还说是最安全的，但书上却没有给出例子，这里应该就是枚举实现单例的一个应用。对于这个枚举的理解就是工厂，专门生产实例
然后看下Factory这个接口,很简单，就是继承Provider接口
public interface Factory<T> extends Provider<T> {
}
provider里面呢，里面就一个get方法
public interface Provider<T> {
T get();
}
然后MainActivity用了@Inject注解，然后我看到debug对应的目录下生成了MainActivity_MembersInjector类
@Generated("dagger.internal.codegen.ComponentProcessor")
public final class MainActivity_MembersInjector implements MembersInjector<MainActivity> {
private final MembersInjector<AppCompatActivity> supertypeInjector;
private final Provider<User> userProvider;
public MainActivity_MembersInjector(MembersInjector<AppCompatActivity> supertypeInjector, Provider<User> userProvider) {
assert supertypeInjector != null;
this.supertypeInjector = supertypeInjector;
assert userProvider != null;
this.userProvider = userProvider;
}
@Override
public void injectMembers(MainActivity instance) {
if (instance == null) {
throw new NullPointerException("Cannot inject members into a null reference");
}
supertypeInjector.injectMembers(instance);
instance.user = userProvider.get();
}
public static MembersInjector<MainActivity> create(MembersInjector<AppCompatActivity> supertypeInjector, Provider<User> userProvider) {
return new MainActivity_MembersInjector(supertypeInjector, userProvider);
}
}
MainActivity_MembersInjector实现了接口MembersInjector里的injectMembers方法，先调用了supertypeInjector.injectMembers(instance)这里应该有个逐级递归，然后 instance.user = userProvider.get()把user实例赋给MainActivity的user成员变量。对MainActivity_MembersInjector理解就是用来具体实现注入的，并且可以注入多个成员
然后MainActivityComponent也对应生成了一个DaggerMainActivityComponent,其实是对MainActivityComponent的进一步封装
@Generated("dagger.internal.codegen.ComponentProcessor")
public final class DaggerMainActivityComponent implements MainActivityComponent {
private MembersInjector<MainActivity> mainActivityMembersInjector;
private DaggerMainActivityComponent(Builder builder) {
assert builder != null;
initialize(builder);
}
public static Builder builder() {
return new Builder();
}
public static MainActivityComponent create() {
return builder().build();
}
private void initialize(final Builder builder) {
this.mainActivityMembersInjector = MainActivity_MembersInjector.create((MembersInjector) MembersInjectors.noOp(), User_Factory.create());
}
@Override
public void inject(MainActivity mainActivity) {
mainActivityMembersInjector.injectMembers(mainActivity);
}
public static final class Builder {
private Builder() {
}
public MainActivityComponent build() {
return new DaggerMainActivityComponent(this);
}
}
}
DaggerMainActivityComponent给MainActivityComponent加了个Builder，典型运用了Builder模式，便于链式调用，设置参数。这里Builder构造了MembersInjector<MainActivity>这个类型的成员，然后DaggerMainActivityComponent就可以利用mainActivityMembersInjector这个实例去给User变量注入实例。
扩展，User设置为单例注入
上述只是一种场景，注射器每次注入的都是一个全新的对象，并不是同一个对象，接下看看dagger是怎么实现单例注入的
简单应用,在前面代码的基础上
在前面的User类上加了@Singleton
@Singleton
public class User {
...
}
然后MainActivityComponent加了个@Singleton注解
@Singleton
@Component
public interface MainActivityComponent {
void inject(MainActivity mainActivity);
}
最后编译一下，应用
public class MainActivity extends AppCompatActivity {
private static final String TAG = "MainActivity>>>";
@Inject
User user1;
@Inject
User user2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
DaggerMainActivityComponent.builder()
.build()
.inject(this);
Log.i(TAG, "onCreate: >>>>user1: " + user1.hashCode()+" user2:"+user2.hashCode());
}
}
结果：
onCreate: >>>>user1: 219956319 user2:219956319
可以看到user1和user2是同一个对象
可是为什么加两个注解就可以实现单例注入呢！！
直接看生成的代码和前一次的有什么不同
其实不同点就在于DaggerMainActivityComponent 里面的initialize方法
@Generated("dagger.internal.codegen.ComponentProcessor")
public final class DaggerMainActivityComponent implements MainActivityComponent {
private Provider<User> userProvider;
private MembersInjector<MainActivity> mainActivityMembersInjector;
private DaggerMainActivityComponent(Builder builder) {
assert builder != null;
initialize(builder);
}
public static Builder builder() {
return new Builder();
}
public static MainActivityComponent create() {
return builder().build();
}
private void initialize(final Builder builder) {
this.userProvider = ScopedProvider.create(User_Factory.create());
this.mainActivityMembersInjector = MainActivity_MembersInjector.create((MembersInjector) MembersInjectors.noOp(), userProvider);
}
@Override
public void inject(MainActivity mainActivity) {
mainActivityMembersInjector.injectMembers(mainActivity);
}
public static final class Builder {
private Builder() {
}
public MainActivityComponent build() {
return new DaggerMainActivityComponent(this);
}
}
}
构造mainActivityMembersInjector需要传入userProvider,原来的userProvider是直接User_Factory.create()生成，然而现在mainActivityMembersInjector构造时传入的userProvider是经ScopedProvider.create封装过的userProvider,所以单例实现关键就是在里面
看一下ScopedProvider的源码
public final class ScopedProvider<T> implements Provider<T> {
private static final Object UNINITIALIZED = new Object();
private final Factory<T> factory;
private volatile Object instance = UNINITIALIZED;
private ScopedProvider(Factory<T> factory) {
assert factory != null;
this.factory = factory;
}
@SuppressWarnings("unchecked") // cast only happens when result comes from the factory
@Override
public T get() {
// double-check idiom from EJ2: Item 71
Object result = instance;
if (result == UNINITIALIZED) {
synchronized (this) {
result = instance;
if (result == UNINITIALIZED) {
instance = result = factory.get();
}
}
}
return (T) result;
}
/** Returns a new scoped provider for the given factory. */
public static <T> Provider<T> create(Factory<T> factory) {
if (factory == null) {
throw new NullPointerException();
}
return new ScopedProvider<T>(factory);
}
}
原来里面用了个单例模式，经典的双重检查DCL双重检查锁定，这样provider提供的实例就是同一个对象了
总结
通过这两个例子，大致可以知道Dagger实现注入的基本流程，首先声明的注解会对应生成对应的工具类，如xxx_Factory工厂类，xxx_MembersInjector成员注入工具类，DaggerXXXComponet注射器类，xxx_Factory提供对象实例，xxx_MembersInjector是将xxx_Factory提供的实例赋给目标变量，DaggerXXXComponet起连接作用，是目标类和注入工具类的中介者。
当然Dagger2还有其它用法，如Module的使用，但我粗虐的实验了下，其中的流程也基本一样，xxx_Factory类型由枚举enum变成了class，其它不同点还有待细看
还有个Scope注解我还没弄懂，有时间继续探索。。。
第一次写源码分析的文章，语言组织可能不尽人意，以后会慢慢提高。。。
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