Dagger2学习过程中一直以为被 @single注解会有类似static效果（效果相同只是实现方式并不是static，下面会提到双重检查锁）。无意找到如下说明：
@Singleton
means there will be one instance of the object for the lifetime of the component, not for the lifetime of the JVM.
and
Adding a scope annotation to a @Provides
method means that the method will be called at most once within one instance of the component it's installed into. Without that, the @Provides
method would be called again every time a Provider
's get()
method is called, even within one component instance. Every class that @Injects
a RestRepository
would get a new instance
被@single dagger2会在生成的DaggerxxComponent文件的初始化方法中以 ScopedProvider 包裹提供依赖：
DaggerxxComponent.java
private void initialize(final Builder builder) {
this.providesAnimProvider = ScopedProvider.create(AnimModule_ProvidesAnimFactory.create(builder.animModule));
}
ScopeProvider.java 单例模式 - 双重检查锁模式
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;
}
Adding a scope annotation to a @Providesmethod means that the method will be called at most once within one instance of the component it's installed into.
达到以上效果是通过ScopeProvider的get方法控制的。
