Package dagger

Interface Lazy<T>

public interface Lazy<T>

A handle to a lazily-computed value. Each Lazy computes its value on the first call to get() and remembers that same value for all subsequent calls to get().

All implementations are expected to be thread-safe and compute their value at most once.

Example

The differences between direct injection, provider injection and lazy injection are best demonstrated with an example. Start with a module that computes a different integer for each use:

   @Module
   final class CounterModule {
     int next = 100;

     @Provides Integer provideInteger() {
       System.out.println("computing...");
       return next++;
     }
   }
 

Direct Injection

This class injects that integer and prints it 3 times:

   final class DirectCounter {
     @Inject Integer value;

     void print() {
       System.out.println("printing...");
       System.out.println(value);
       System.out.println(value);
       System.out.println(value);
     }
   }
 

Injecting a DirectCounter and invoking print() reveals that the value is computed before it is required:

   computing...
   printing...
   100
   100
   100
 

Provider Injection

This class injects a provider for the integer. It calls Provider.get() 3 times and prints each result:

   final class ProviderCounter {
     @Inject Provider<Integer> provider;

     void print() {
       System.out.println("printing...");
       System.out.println(provider.get());
       System.out.println(provider.get());
       System.out.println(provider.get());
     }
   }
 

Injecting a ProviderCounter and invoking print() shows that a new value is computed each time Provider.get() is used:

   printing...
   computing...
   100
   computing...
   101
   computing...
   102
 

Lazy Injection

This class injects a Lazy for the integer. Like the provider above, it calls Lazy.get() 3 times and prints each result:

   final class LazyCounter {
     @Inject Lazy<Integer> lazy;

     void print() {
       System.out.println("printing...");
       System.out.println(lazy.get());
       System.out.println(lazy.get());
       System.out.println(lazy.get());
     }
   }
 

Injecting a LazyCounter and invoking print() shows that a new value is computed immediately before it is needed. The same value is returned for all subsequent uses:

   printing...
   computing...
   100
   100
   100
 

Lazy != Singleton

Note that each injected Lazy is independent, and remembers its value in isolation of other Lazy instances. In this example, two LazyCounter objects are created and print() is called on each:

   final class LazyCounters {
     @Inject LazyCounter counter1;
     @Inject LazyCounter counter2;

     void print() {
       counter1.print();
       counter2.print();
     }
   }
 

The output demonstrates that each Lazy works independently:

   printing...
   computing...
   100
   100
   100
   printing...
   computing...
   101
   101
   101
 

Use @Singleton to share one instance among all clients, and Lazy for lazy computation in a single client.

Method Summary

Modifier and Type	Method	Description
T	get()	Return the underlying value, computing the value if necessary.

Method Details

get

T get()

Return the underlying value, computing the value if necessary. All calls to the same Lazy instance will return the same result.