From 8c7d863828e0bf4b9045075ff3ccd386993dfbb5 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Fri, 10 Jul 2015 22:01:30 -0400
Subject: lazy: Fix Lazy<T> when a provider returns null.

---
 .../java/com/tavianator/sangria/lazy/Lazy.java     | 11 +++++----
 .../java/com/tavianator/sangria/lazy/LazyTest.java | 27 +++++++++++++++++++++-
 2 files changed, 33 insertions(+), 5 deletions(-)

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
index adf531c..ee42a2f 100644
--- a/sangria-lazy/src/main/java/com/tavianator/sangria/lazy/Lazy.java
+++ b/sangria-lazy/src/main/java/com/tavianator/sangria/lazy/Lazy.java
@@ -56,8 +56,10 @@ import javax.inject.Provider;
  * @since 1.2
  */
 public final class Lazy<T> {
+    private static final Object SENTINEL = new Object();
+
     private final Provider<T> provider;
-    private volatile T instance = null;
+    private volatile Object instance = SENTINEL;
 
     @Inject
     Lazy(Provider<T> provider) {
@@ -67,15 +69,16 @@ public final class Lazy<T> {
     /**
      * @return A lazily-produced value of type {@code T}.
      */
+    @SuppressWarnings("unchecked")
     public T get() {
         // Double-checked locking
-        if (instance == null) {
+        if (instance == SENTINEL) {
             synchronized (this) {
-                if (instance == null) {
+                if (instance == SENTINEL) {
                     instance = provider.get();
                 }
             }
         }
-        return instance;
+        return (T) instance;
     }
 }
diff --git a/sangria-lazy/src/test/java/com/tavianator/sangria/lazy/LazyTest.java b/sangria-lazy/src/test/java/com/tavianator/sangria/lazy/LazyTest.java
index ee6200a..a451a96 100644
--- a/sangria-lazy/src/test/java/com/tavianator/sangria/lazy/LazyTest.java
+++ b/sangria-lazy/src/test/java/com/tavianator/sangria/lazy/LazyTest.java
@@ -36,6 +36,7 @@ import com.google.inject.Provider;
 import com.google.inject.spi.DefaultBindingTargetVisitor;
 import com.google.inject.spi.Element;
 import com.google.inject.spi.Elements;
+import com.google.inject.util.Providers;
 import org.junit.Test;
 
 import static com.tavianator.sangria.test.SangriaMatchers.*;
@@ -62,15 +63,26 @@ public class LazyTest {
     @Singleton
     private static class CountingProvider implements Provider<Abstract> {
         int count = 0;
+        private final Provider<Abstract> impl;
 
         @Inject
         CountingProvider() {
+            this.impl = new Provider<Abstract>() {
+                @Override
+                public Abstract get() {
+                    return new Abstract() { };
+                }
+            };
+        }
+
+        CountingProvider(Abstract instance) {
+            this.impl = Providers.of(instance);
         }
 
         @Override
         public Abstract get() {
             ++count;
-            return new Abstract() { };
+            return impl.get();
         }
     }
 
@@ -186,6 +198,19 @@ public class LazyTest {
         assertThat(module, followsBestPractices());
     }
 
+    @Test
+    public void testNull() {
+        Module module = new AbstractModule() {
+            @Override
+            protected void configure() {
+                bind(CountingProvider.class)
+                        .toInstance(new CountingProvider(null));
+            }
+        };
+
+        test(Guice.createInjector(module), HasLazy.class);
+    }
+
     @Test(expected = CreationException.class)
     public void testMissingBinding() {
         Guice.createInjector(new AbstractModule() {
-- 
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