Dagger Core Semantics

Dagger Core Semantics

This series of documents describes the semantics of Dagger. It’s not intended for learning how to use Dagger, but rather for what Dagger is.

Dagger is roughly divided into a “core”, plus several extensions. This document describes the core, and subsequent documents describe the extensions.

Topics are presented incrementally; initially, we present a simplified view of Dagger, and as we introduce new topics, we’ll note where previous statements must be amended.

[TOC]

Keys

A qualifier is a Java annotation that’s annotated @Qualifier. For example,

@Qualifier
@interface Blue {}

@Qualifier
@interface Name {
  String value();
}

define @Blue and @Name as qualifiers.

We also use the term qualifier for the concrete use of the qualifier in code. For example:

@Name("Aaron")
@Name("Brett")

are two different instantiations of the qualifier @Name. It is usually clear from context whether “qualifier” means the annotation definition or instantiation.

A key is a pair of a Java type and an optional qualifier instantiation. For example,

int
@Blue Foo
@Name("Giannis") Map<Integer, String>

are all keys.

We often refer to a key as @Q T; by this, we mean that Q is a qualifier and T is a type.

Two keys are the same if they have the same type (after unboxing) and the same qualifier. For example,

@Blue int
@Blue Integer

are the same, but

@Green int

is different from both.

It is always an error to apply multiple qualifiers to the same type.

Bindings

A binding is a function that tells Dagger how to compute an instance of a particular key. This key, which is the function’s binding key, is essentially the return type of the function (see Signatures below). It can be either user-defined or synthetic:

A user-defined binding is a Java method that is any of the following:

These are the basic user-defined bindings; other Dagger features like multibindings and producers introduce new kinds of user-defined bindings.

A synthetic binding is a binding that Dagger generates, subject to constraints that will be discussed later.

Signatures

Recall that the signature of a Java method describes the inputs and outputs of that method; these inputs and outputs are, of course, Java types. In our case, we will also include the annotations as part of the signature, both for the method parameters and the method itself. For example,

// Java signature = (@Blue Set<Bar>, @Nullable Provider<Integer>) -> @Nullable Foo
@Nullable Foo foo(@Blue Set<Bar> setOfBar, @Nullable Provider<Integer> i);

This signature induces a key signature, that is, a signature where the inputs and outputs are Dagger keys, as follows.

First, we compute the declared key signature by simply dropping all annotations that are not qualifiers. For example:

// declared key signature = (@Blue Set<Bar>, Provider<Integer>) -> Foo
@Nullable Foo foo(@Blue Set<Bar> setOfBar, @Nullable Provider<Integer> i);

Then, we form the key signature by unwrapping arguments that are Provider<T>, Lazy<T>, or Provider<Lazy<T>>. For example:

// key signature = (@Blue Set<Bar>, int) -> Foo
@Nullable Foo foo(@Blue Set<Bar> setOfBar, @Nullable Provider<Integer> i);

The output key is called the binding key and the input keys are called dependency keys; we’ll sometimes refer to the declared binding key or declared dependency keys, which are the output and inputs keys of the declared key signature.

Note: For @Inject constructors, we treat the return type of the Java signature as the type of the constructor’s class. Unlike the other binding methods, @Inject constructors’ binding keys cannot have qualifiers.

Here are some more examples of key signatures:

// key signature = (int) -> Foo
@Provides Foo foo(int x) { ... }

// key signature = (FooImpl) -> @Green Foo
@Binds @Green abstract Foo greenFoo(FooImpl impl);

// key signature = () -> int
@Provides int count() { return 5; }

// key signature = (Bar) -> Foo
class Foo {
  @Inject
  Foo(Bar bar) { ... }
}

// key signature = (Bar, @Blue Baz, @Green int) -> Foo
@Provides Foo foo(
    Bar bar, @Blue Baz baz, @Green Provider<Integer> greenIntProvider) { ... }

Binding Logic

The binding logic of a binding is the function that the binding represents. It is a (possibly fictitious) Java function with the same signature as the binding, except ignoring qualifiers.

Of the bindings discussed so far:

Modules

A module is a class annotated @Module, which contains zero or more @Provides or @Binds methods; these methods are the bindings of the module.

For the purposes of this initial exposition, modules may not have instance methods or fields. In the Parameterization section, they will be allowed to have instance methods or fields, but this is generally discouraged.

Modules may include other modules via the includes field on their annotation. The transitive closure of a module is the transitive set of modules that are included by that module, and the transitive bindings of a module is the union of all bindings in modules in the transitive closure.

Components

A component is an interface or abstract class annotated @Component.

Dagger will generate implementations of components.

The modules of a component are the transitive closure of the list of modules specified in the modules field on its annotation.

The entry points of the component are its abstract, zero-arg, non-void methods. It is an error for a component to have any abstract methods that have arguments or are void. Each entry point has an entry point key that is defined as the key associated with the method’s return type (analogous to a binding key).

The bindings of a component are the union of the set of bindings in all of the component’s modules.

Here is a complete example with all of the features described so far:

@Qualifier @interface Blue {}

@Module(includes = N.class)
abstract class M {
  @Provides static int v() { ... }
  @Provides static Foo foo(X x, int v) { ... }
  @Binds abstract Bar bar(BarImpl impl);
}

@Module
final class N {
  @Provides @Blue static Y y(X x) { ... }
}

class X {
  @Inject
  X() { ... }
}

class BarImpl implements Bar {
  @Inject
  BarImpl(@Blue Y y, int v) { ... }
}

@Component(modules = M.class)
interface C {
  Foo foo();
  Bar bar();
}

In the above example:

Note that X and BarImpl are included as bindings for C even though they do not appear as functions in C’s included modules. This is because any @Inject-annotated constructor is implicitly included in any component that needs it, i.e. has a dependency key of that constructor’s class.

Resolution: Construction of the Graph

Dagger constructs a bipartite graph for each component.

The nodes of the graph are:

The edges of the graph are directed edges consisting of:

The graph is well-formed if:

If a key node has zero inbound edges, it is a missing binding; if it has more than one inbound edge, it is a duplicate binding.

The above example component generates the following graph. The key nodes are light red, the binding nodes are light blue, and the entry point nodes are light green.

digraph d_front_back {
  rankdir="LR";
  graph [size="9;9", style=invis, ranksep=2]
  node [ style=filled, nodesep=0.2 ]
  edge [ weight=1.2 ]

  subgraph cluster_bindings {
    rank=same;
    "M\#v()" [ color="lightblue" ];
    "M\#foo()" [ color="lightblue" ];
    "M\#bar()" [ color="lightblue" ];
    "N\#y()" [ color="lightblue" ];
    "BarImpl()" [ color="lightblue" ];
    "X()" [ color="lightblue" ];
    "C#foo" [ color="lightgreen" ];
    "C#bar" [ color="lightgreen" ];
  }
  subgraph cluster_keys {
    rank=same;
    X [ color="mistyrose" ];
    "@Blue Y" [ color="mistyrose" ];
    int [ color="mistyrose" ];
    Foo [ color="mistyrose" ];
    Bar [ color="mistyrose" ];
    BarImpl [ color="mistyrose" ];
  }
  X -> "M\#foo()"
  int -> "M\#foo()"
  BarImpl -> "M\#bar()"
  X -> "N\#y()"
  "@Blue Y" -> "BarImpl()"
  int -> "BarImpl()"
  Foo -> "C#foo"
  Bar -> "C#bar"
  "X()" -> X
  "N\#y()" -> "@Blue Y"
  "M\#v()" -> int
  "M\#foo()" -> Foo
  "M\#bar()" -> Bar
  "BarImpl()" -> BarImpl
}

Code Generation and Runtime

Given a well-formed graph of a component, Dagger generates an implementation for that component interface, which has the following runtime behavior.

Implementing the Component

Dagger generates an implementation of the component in the same package as the component, whose name is prefixed with Dagger. If the component was a nested class, named C1.C2...Cn, then the generated implementation replaces dots with underscores: DaggerC1_C2_..._Cn.

The generated component class has a private zero-arg constructor, and Dagger will generate a special public method #create() that constructs the component.

For each entry point, Dagger will resolve it by following the graph along reverse edges, which will produce a subset of the graph.

Assume first that this is an acyclic graph with no binding methods that take Provider or Lazy. Beginning with the roots of this subgraph, Dagger will execute the binding logic for each binding, passing the result of each computation into the subsequent binding. If one binding is used by several others, its binding logic will be re-evaluated each time it is needed. When Dagger reaches the terminal point of this process (the entry point), Dagger returns the value it has computed.

In the earlier example, Dagger will generate code that roughly looks like

public final class DaggerC implements C {
  private DaggerC() {}

  public static C create() {
    return new DaggerC();
  }

  @Override
  public Foo foo() {
    return new Foo(new X(), M.v());
  }

  @Override
  public Bar bar() {
    return new BarImpl(N.y(new X()), M.v());
  }
}

Note that M.v() is called twice, once for each place it’s used.

Provider and Lazy

If a binding method has an argument of Provider, then instead of actually executing the binding logic, Dagger will generate a class that defers the computation of that logic and all of its dependencies. For example, suppose in the earlier example that the constructor for BarImpl was:

class BarImpl {
  @Inject
  BarImpl(@Blue Provider<Y> y, int v) { ... }
}

Dagger might generate code looking like:

public final class DaggerC implements C {
  ... // as before

  @Override
  public Bar bar() {
    return new BarImpl(() -> N.y(new X()), M.v());
  }
}

If a binding method has an argument of Lazy, then similarly, Dagger will generate a class that defers the computation of that logic and all of its dependencies, and caches the result. For example, suppose in the example that the constructor for BarImpl was:

class BarImpl {
  @Inject
  BarImpl(@Blue Lazy<Y> y) { ... }
}

Dagger might generate code looking like:

public final class DaggerC implements C {
  ... // as before

  @Override
  public Bar bar() {
    return new BarImpl(DoubleCheck.provider(() -> N.y(new X())), M.v());
  }
}

where DoubleCheck is a Dagger utility class that caches the result of a provider call.

Note: Dagger will construct a new Lazy instance every time it is requested; so although Lazy itself caches its contents, if multiple bindings request a Lazy of the same type, they’ll get different instances.

Cycles

If a well-formed graph has a cycle, then necessarily the cycle contains an argument to a binding that is a Provider or Lazy. In this case, the implementation of this cycle can be broken with an extra layer of indirection: Dagger starts by constructing a dummy version of that Provider or Lazy, continuing around the cycle by making providers (never actually executing any binding logic), until it reaches the end of the cycle and it fills in the original Provider.

For example:

@Module
final class M {
  @Provides static A a(Provider<C> c) { return new A(); }
  @Provides static B b(A a) { return new B(); }
  @Provides static C c(B b) { return new C(); }
}

@Component(modules = M.class)
interface X {
  A a();
}

Dagger might generate code that looks like:

public final class DaggerX implements X {
  @Override
  public A a() {
    DelegateProvider<A> a = new DelegateProvider<>();
    Provider<B> b = () -> M.b(a.get());
    Provider<C> c = () -> M.c(b.get());
    a.setDelegate(() -> M.a(c));
    return a.get();
  }
}

Exceptions

Exceptions are not caught by Dagger. Since Dagger generates code that is essentially composition of functions, if any binding’s logic throws an exception, then it will propagate out to the caller of the entry point.

The one exception to this behavior is with Provider or Lazy; since both of these types defer computation, if a binding’s logic throws an exception in some ancestor dependency of a Provider or Lazy injection, then the get() method will throw that exception, and the user could catch it there. However, it’s often considered a code smell to catch exceptions this way, since binding methods should rarely throw exceptions.

Parameterization: Component Builders

So far, a component is self-contained: it can be constructed at runtime with no arguments.

Dagger allows components to be parameterized: new bindings can be added to the component whose logic is passed at runtime, to a “component builder”, before constructing the component.

The builder of a component is an interface or abstract class annotated @Component.Builder, that’s nested inside a component.

Component builders must have a build method, which is an abstract zero-arg method returning the component.

All other abstract methods on the builder must be setter methods, that is, single-arg methods returning the builder class itself. There are three kinds of setter methods, which will be discussed separately.

Bound instances

A bound instance is the argument of a builder setter method annotated with @BindsInstance. This binds the value of the instance passed at runtime to the declared key of the argument; that is, it generates a binding.

A bound instance binding is a user-defined binding for each of the @BindsInstance methods on the component builder; with

When bound instance bindings are present, we need a special synthetic binding to represent the component, whose

Here is a complete example with bound instances:

@Qualifier @interface Blue {}

@Module(includes = N.class)
abstract class M {
  @Provides static Foo foo(X x, int v) { ... }
  @Binds abstract Bar bar(BarImpl impl);
}

@Module
final class N {
  @Provides @Blue static Y y(X x) { ... }
}

class BarImpl {
  @Inject
  BarImpl(@Blue Y y, int v) { ... }
}

@Component(modules = M.class)
interface C {
  Foo foo();
  Bar bar();

  @Component.Builder
  interface B {
    @BindsInstance B x(X x);
    @BindsInstance B v(int v);
    C build();
  }
}

In the above example:

Resolution and Runtime

Dagger generates a builder class that is a standard Java builder: it collects all of the bound instances passed to it; and then constructs a component object, which passes references to all of those instances to the constructor of the component.

Dagger generates a component class that maintains references to all of these instances passed by the builder. These references describe the bound instance bindings on the component; i.e., the binding for a bound instance, which has signature component -> some bound instance key, is just a field lookup on the component for that instance.

Moreover, note that all generated code below is inside the component itself; therefore, every method has access to the component instance itself (via the this parameter).

Finally, Dagger generates a public static method #builder() that constructs the component builder.

For the earlier example, Dagger would generate code that roughly looks like:

public final class DaggerC implements C {
  private final X x;
  private final int v;

  private DaggerC(X x, int v) {
    this.x = x;
    this.v = v;
  }

  @Override
  public Foo foo() {
    return new Foo(x, v);
  }

  @Override
  public Bar bar() {
    return new BarImpl(N.y(x), v);
  }

  public static B builder() {
    return new Builder();
  }

  static final class Builder implements B {
    private X x;
    private int v;

    @Override
    public B x(X x) {
      this.x = checkNotNull(x);
      return this;
    }

    @Override
    public B v(int v) {
      this.v = v;
      return this;
    }

    @Override
    public C build() {
      return new DaggerC(checkNotNull(x), v);
    }
  }
}

Component dependencies

The dependencies of a component are the classes listed in the component’s dependencies field of its annotation. These classes may be arbitrary classes, but what Dagger cares about is the zero-arg non-void instance methods, whose return types will be bound.

When a component dependency is listed in the component annotation, it must have a corresponding setter method in the component builder.

Component dependencies add two new kinds of bindings:

Component dependencies are simply syntactic sugar for a certain use case for bound instances. Suppose that we have the following interface:

interface D {
  @Q1 T1 t1();
  ...
  @Qn Tn tn();
  // possibly other, non-void or non-zero-arg methods
}

Then specifying D as a component dependency as follows:

@Component(dependencies = D.class, modules = ...)
interface C {
  ...

  @Component.Builder
  interface B {
    B d(D d);
    ...
    C build();
  }
}

is equivalent to writing a module:

@Module
final class DModule {
  @Provides @Q1 static T1 t1(D d) { return d.t1(); }
  ...
  @Provides @Qn static Tn tn(D d) { return d.tn(); }
}

and binding D as an instance:

@Component(modules = {DModule.class, ...})
interface C {
  ...

  @Component.Builder
  interface B {
    @BindsInstance B d(D d);
    ...
    C build();
  }
}

This is really truly equivalent: not only does it produce the same graph of bindings, but it also provides clients with the same runtime API for the component and its builder.

Module instances

When modules were presented earlier, they were not allowed instance methods or fields. We now describe a way that Dagger does allow modules to have instance methods or fields.

Warning: it’s discouraged for modules to have state; this can lead to unpredictable behavior. Moreover, module instances in general are rarely useful. We have considered removing them.

If a builder setter method specifies a module, then Dagger generates a new binding:

A module binding is a synthetic binding for a module; with

Then, given a binding in that module that is an instance method, Dagger treats its signature’s first argument as the module itself (i.e., the this parameter of the module).

Warning: module bindings are not allowed to be injected by user code; it is only a figment of Dagger’s imagination.

Module instances are also redundant with bound instances. Any instance fields of a module can be bound directly as instances, so that instance methods that refer to them can inject those types directly, and hence can be static methods.