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152 lines
4.6 KiB
152 lines
4.6 KiB
/*
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* Copyright (C) 2012 The Dagger Authors.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package dagger;
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/**
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* A handle to a lazily-computed value. Each {@code Lazy} computes its value on
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* the first call to {@link #get()} and remembers that same value for all
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* subsequent calls to {@code get()}.
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*
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* <p>All implementations are expected to be thread-safe and compute their value at most once.
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*
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* <h2>Example</h2>
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* The differences between <strong>direct injection</strong>, <strong>provider
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* injection</strong> and <strong>lazy injection</strong> are best demonstrated
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* with an example. Start with a module that computes a different integer for
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* each use:<pre><code>
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* {@literal @Module}
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* final class CounterModule {
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* int next = 100;
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*
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* {@literal @Provides} Integer provideInteger() {
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* System.out.println("computing...");
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* return next++;
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* }
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* }
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* </code></pre>
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*
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* <h3>Direct Injection</h3>
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* This class injects that integer and prints it 3 times:<pre><code>
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* final class DirectCounter {
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* {@literal @Inject} Integer value;
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*
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* void print() {
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* System.out.println("printing...");
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* System.out.println(value);
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* System.out.println(value);
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* System.out.println(value);
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* }
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* }
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* </code></pre>
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* Injecting a {@code DirectCounter} and invoking {@code print()} reveals that
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* the value is computed <i>before</i> it is required:<pre><code>
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* computing...
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* printing...
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* 100
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* 100
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* 100
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* </code></pre>
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*
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* <h3>Provider Injection</h3>
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* This class injects a {@linkplain javax.inject.Provider provider} for the
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* integer. It calls {@code Provider.get()} 3 times and prints each result:
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* <pre><code>
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* final class ProviderCounter {
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* {@literal @Inject Provider<Integer> provider;}
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*
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* void print() {
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* System.out.println("printing...");
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* System.out.println(provider.get());
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* System.out.println(provider.get());
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* System.out.println(provider.get());
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* }
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* }
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* </code></pre>
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* Injecting a {@code ProviderCounter} and invoking {@code print()} shows that
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* a new value is computed each time {@code Provider.get()} is used:<pre><code>
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* printing...
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* computing...
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* 100
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* computing...
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* 101
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* computing...
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* 102
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* </code></pre>
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*
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* <h3>Lazy Injection</h3>
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* This class injects a {@code Lazy} for the integer. Like the provider above,
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* it calls {@code Lazy.get()} 3 times and prints each result:<pre><code>
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* final class LazyCounter {
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* {@literal @Inject Lazy<Integer> lazy;}
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*
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* void print() {
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* System.out.println("printing...");
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* System.out.println(lazy.get());
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* System.out.println(lazy.get());
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* System.out.println(lazy.get());
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* }
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* }
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* </code></pre>
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* Injecting a {@code LazyCounter} and invoking {@code print()} shows that a new
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* value is computed immediately before it is needed. The same value is returned
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* for all subsequent uses:<pre><code>
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* printing...
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* computing...
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* 100
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* 100
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* 100
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* </code></pre>
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*
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* <h3>Lazy != Singleton</h3>
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* Note that each injected {@code Lazy} is independent, and remembers its value
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* in isolation of other {@code Lazy} instances. In this example, two {@code
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* LazyCounter} objects are created and {@code print()} is called on each:
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* <pre><code>
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* final class LazyCounters {
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* {@literal @Inject} LazyCounter counter1;
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* {@literal @Inject} LazyCounter counter2;
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*
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* void print() {
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* counter1.print();
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* counter2.print();
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* }
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* }
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* </code></pre>
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* The output demonstrates that each {@code Lazy} works independently:
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* <pre><code>
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* printing...
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* computing...
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* 100
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* 100
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* 100
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* printing...
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* computing...
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* 101
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* 101
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* 101
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* </code></pre>
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* Use {@link javax.inject.Singleton @Singleton} to share one instance among all
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* clients, and {@code Lazy} for lazy computation in a single client.
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*/
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public interface Lazy<T> {
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/**
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* Return the underlying value, computing the value if necessary. All calls to
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* the same {@code Lazy} instance will return the same result.
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*/
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T get();
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}
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