From af52e472b4f0b00b0d00c04b3fa54a79f94b9a49 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Sun, 14 Sep 2014 23:56:51 -0400
Subject: lazy: Implement Lazy<T>, very much like Dagger's.

---
 .../java/com/tavianator/sangria/lazy/Lazy.java     | 81 ++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 sangria-lazy/src/main/java/com/tavianator/sangria/lazy/Lazy.java

(limited to 'sangria-lazy/src/main/java/com/tavianator/sangria/lazy/Lazy.java')

diff --git a/sangria-lazy/src/main/java/com/tavianator/sangria/lazy/Lazy.java b/sangria-lazy/src/main/java/com/tavianator/sangria/lazy/Lazy.java
new file mode 100644
index 0000000..adf531c
--- /dev/null
+++ b/sangria-lazy/src/main/java/com/tavianator/sangria/lazy/Lazy.java
@@ -0,0 +1,81 @@
+/****************************************************************************
+ * Sangria                                                                  *
+ * Copyright (C) 2014 Tavian Barnes <tavianator@tavianator.com>             *
+ *                                                                          *
+ * Licensed under the Apache License, Version 2.0 (the "License");          *
+ * you may not use this file except in compliance with the License.         *
+ * You may obtain a copy of the License at                                  *
+ *                                                                          *
+ * http://www.apache.org/licenses/LICENSE-2.0                               *
+ *                                                                          *
+ * Unless required by applicable law or agreed to in writing, software      *
+ * distributed under the License is distributed on an "AS IS" BASIS,        *
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. *
+ * See the License for the specific language governing permissions and      *
+ * limitations under the License.                                           *
+ ****************************************************************************/
+
+package com.tavianator.sangria.lazy;
+
+import javax.inject.Inject;
+import javax.inject.Provider;
+
+/**
+ * A lazily-loaded dependency. Like a {@link Provider}, calling {@link #get()} will produce an instance of {@code T}.
+ * Unlike a {@link Provider}, the same instance will be returned for every future call to {@link #get()}. Different
+ * {@code Lazy} instances are independent and will return different instances from {@link #get()}.
+ *
+ * <p>
+ * {@link Lazy} works automatically for unqualified bindings, as long as just-in-time bindings are enabled. For
+ * qualified bindings, or if explicit bindings are requred, use {@link LazyBinder}:
+ * </p>
+ *
+ * <pre>
+ * // Either separately...
+ * bind(Dependency.class)
+ *         .annotatedWith(Names.named("name"))
+ *         .to(RealDependency.class);
+ *
+ * LazyBinder.create(binder())
+ *         .bind(Dependency.class)
+ *         .annotatedWith(Names.named("name"));
+ *
+ * // ... or in one go
+ * LazyBinder.create(binder())
+ *         .bind(Dependency.class)
+ *         .annotatedWith(Names.named("name"))
+ *         .to(RealDependency.class);
+ *
+ * ...
+ *
+ * {@literal @}Inject {@literal @}Named("name") Lazy&lt;Dependency&gt; lazy;
+ * </pre>
+ *
+ * @author Tavian Barnes (tavianator@tavianator.com)
+ * @version 1.2
+ * @since 1.2
+ */
+public final class Lazy<T> {
+    private final Provider<T> provider;
+    private volatile T instance = null;
+
+    @Inject
+    Lazy(Provider<T> provider) {
+        this.provider = provider;
+    }
+
+    /**
+     * @return A lazily-produced value of type {@code T}.
+     */
+    public T get() {
+        // Double-checked locking
+        if (instance == null) {
+            synchronized (this) {
+                if (instance == null) {
+                    instance = provider.get();
+                }
+            }
+        }
+        return instance;
+    }
+}
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