Add Schedulers.reset() for better testing

Resolves ReactiveX#3985

Author: ZacSweers

src/main/java/rx/schedulers/Schedulers.java

@@ -16,7 +16,10 @@
 package rx.schedulers;
 
 import rx.Scheduler;
-import rx.internal.schedulers.*;
+import rx.annotations.Experimental;
+import rx.internal.schedulers.ExecutorScheduler;
+import rx.internal.schedulers.GenericScheduledExecutorService;
+import rx.internal.schedulers.SchedulerLifecycle;
 import rx.internal.util.RxRingBuffer;
 import rx.plugins.RxJavaPlugins;
 import rx.plugins.RxJavaSchedulersHook;
@@ -32,7 +35,14 @@ public final class Schedulers {
     private final Scheduler ioScheduler;
     private final Scheduler newThreadScheduler;
 
-    private static final Schedulers INSTANCE = new Schedulers();
+    private static Schedulers INSTANCE;
+
+    private static synchronized Schedulers getInstance() {
+        if (INSTANCE == null) {
+            INSTANCE = new Schedulers();
+        }
+        return INSTANCE;
+    }
 
     private Schedulers() {
         RxJavaSchedulersHook hook = RxJavaPlugins.getInstance().getSchedulersHook();
@@ -86,7 +96,7 @@ public static Scheduler trampoline() {
      * @return a {@link Scheduler} that creates new threads
      */
     public static Scheduler newThread() {
-        return INSTANCE.newThreadScheduler;
+        return getInstance().newThreadScheduler;
     }
 
     /**
@@ -101,7 +111,7 @@ public static Scheduler newThread() {
      * @return a {@link Scheduler} meant for computation-bound work
      */
     public static Scheduler computation() {
-        return INSTANCE.computationScheduler;
+        return getInstance().computationScheduler;
     }
 
     /**
@@ -118,7 +128,7 @@ public static Scheduler computation() {
      * @return a {@link Scheduler} meant for IO-bound work
      */
     public static Scheduler io() {
-        return INSTANCE.ioScheduler;
+        return getInstance().ioScheduler;
     }
 
     /**
@@ -141,13 +151,27 @@ public static TestScheduler test() {
     public static Scheduler from(Executor executor) {
         return new ExecutorScheduler(executor);
     }
+
+    /**
+     * Resets the current {@link Schedulers} instance.
+     * <p>
+     * This API is experimental. Resetting the schedulers is dangerous
+     * during application runtime and also bad code could invoke it in
+     * the middle of an application life-cycle and really break applications
+     * if not used cautiously.
+     */
+    @Experimental
+    public static void reset() {
+        shutdown();
+        INSTANCE = null;
+    }
 
     /**
      * Starts those standard Schedulers which support the SchedulerLifecycle interface.
      * <p>The operation is idempotent and threadsafe.
      */
     /* public test only */ static void start() {
-        Schedulers s = INSTANCE;
+        Schedulers s = getInstance();
         synchronized (s) {
             if (s.computationScheduler instanceof SchedulerLifecycle) {
                 ((SchedulerLifecycle) s.computationScheduler).start();
@@ -170,7 +194,7 @@ public static Scheduler from(Executor executor) {
      * <p>The operation is idempotent and threadsafe.
      */
     public static void shutdown() {
-        Schedulers s = INSTANCE;
+        Schedulers s = getInstance();
         synchronized (s) {
             if (s.computationScheduler instanceof SchedulerLifecycle) {
                 ((SchedulerLifecycle) s.computationScheduler).shutdown();
@@ -189,4 +213,4 @@ public static void shutdown() {
             RxRingBuffer.SPMC_POOL.shutdown();
         }
     }
-}
+}

src/test/java/rx/schedulers/ResetSchedulersTest.java

@@ -0,0 +1,51 @@
+package rx.schedulers;
+
+
+import org.junit.Test;
+import rx.Scheduler;
+import rx.internal.schedulers.*;
+import rx.plugins.RxJavaPlugins;
+import rx.plugins.RxJavaSchedulersHook;
+
+import static org.junit.Assert.assertTrue;
+
+public class ResetSchedulersTest {
+
+    @Test
+    public void reset() {
+        RxJavaSchedulersHook originalHook = RxJavaPlugins.getInstance().getSchedulersHook();
+        RxJavaPlugins.getInstance().reset();
+
+        final TestScheduler testScheduler = new TestScheduler();
+        RxJavaPlugins.getInstance().registerSchedulersHook(new RxJavaSchedulersHook() {
+            @Override
+            public Scheduler getComputationScheduler() {
+                return testScheduler;
+            }
+
+            @Override
+            public Scheduler getIOScheduler() {
+                return testScheduler;
+            }
+
+            @Override
+            public Scheduler getNewThreadScheduler() {
+                return testScheduler;
+            }
+        });
+        Schedulers.reset();
+
+        assertTrue(Schedulers.io().equals(testScheduler));
+        assertTrue(Schedulers.computation().equals(testScheduler));
+        assertTrue(Schedulers.newThread().equals(testScheduler));
+
+        RxJavaPlugins.getInstance().reset();
+        RxJavaPlugins.getInstance().registerSchedulersHook(originalHook);
+        Schedulers.reset();
+
+        assertTrue(Schedulers.io() instanceof CachedThreadScheduler);
+        assertTrue(Schedulers.computation() instanceof EventLoopsScheduler);
+        assertTrue(Schedulers.newThread() instanceof rx.internal.schedulers.NewThreadScheduler);
+    }
+
+}