最近在项目中将Rxjava升级到Rxjava2之后，对之前的P、M(项目是基于MVP结构)层进行单元测试的时候，出现了如下的问题：

Android RxJava 2 JUnit test - getMainLooper in android.os.Looper not mocked RuntimeException

java.lang.ExceptionInInitializerError
    at io.reactivex.android.schedulers.AndroidSchedulers$1.call(AndroidSchedulers.java:35)
    at io.reactivex.android.schedulers.AndroidSchedulers$1.call(AndroidSchedulers.java:33)
    at io.reactivex.android.plugins.RxAndroidPlugins.callRequireNonNull(RxAndroidPlugins.java:70)
    at io.reactivex.android.plugins.RxAndroidPlugins.initMainThreadScheduler(RxAndroidPlugins.java:40)
    at io.reactivex.android.schedulers.AndroidSchedulers.<clinit>(AndroidSchedulers.java:32)
    …
Caused by: java.lang.RuntimeException: Method getMainLooper in android.os.Looper not mocked. See http://g.co/androidstudio/not-mocked for details.
    at android.os.Looper.getMainLooper(Looper.java)
    at io.reactivex.android.schedulers.AndroidSchedulers$MainHolder.<clinit>(AndroidSchedulers.java:29)
    ...
java.lang.NoClassDefFoundError: Could not initialize class io.reactivex.android.schedulers.AndroidSchedulers
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)

出现这个问题的原因是：AndroidSchedulers.mainThread返回的是LooperScheduler的一个实例，但是LooperScheduler依赖于Android，我们在纯Java的单元测试中是无法使用它的。找到了问题的原因解决起来就容易多了。
我们需要在单元测试运行之前，初始化RxAndroidPlugins让它返回一个我们指定的不依赖于Android的Scheduler。下面给出两种解决方案。

方案一：

在@BeforeClass里面初始化Scheduler:

@BeforeClass
    public static void setUpRxSchedulers(){
        final Scheduler immediate = new Scheduler() {

            @Override
            public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
                return super.scheduleDirect(run, 0, unit);
            }

            @Override
            public Worker createWorker() {
                return new ExecutorScheduler.ExecutorWorker(new Executor() {
                    @Override
                    public void execute(@android.support.annotation.NonNull Runnable command) {
                        command.run();
                    }
                });
            }
        };

        RxJavaPlugins.setInitIoSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
            @Override
            public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                return immediate;
            }
        });
        RxJavaPlugins.setInitComputationSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
            @Override
            public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                return immediate;
            }
        });
        RxJavaPlugins.setInitNewThreadSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
            @Override
            public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                return immediate;
            }
        });
        RxJavaPlugins.setInitSingleSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
            @Override
            public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                return immediate;
            }
        });
        RxAndroidPlugins.setInitMainThreadSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
            @Override
            public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                return immediate;
            }
        });
    }

这样做虽然可以解决问题，但是你需要在每个测试类里面都要添加这样的代码，非常麻烦，有没有简单一点的办法呢？当然是有的，我们可以自定义一个TestRule，在需要的地方直接调用就行了。

方案二：

自定义一个RxSchedulersOverrideRule让它实现TestRule:

public class RxSchedulersOverrideRule implements TestRule {


    private Scheduler immediate = new Scheduler() {

        @Override
        public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
            return super.scheduleDirect(run, 0, unit);
        }

        @Override
        public Worker createWorker() {
            return new ExecutorScheduler.ExecutorWorker(new Executor() {
                @Override
                public void execute(@android.support.annotation.NonNull Runnable command) {
                    command.run();
                }
            });
        }
    };

    @Override
    public Statement apply(final Statement base, Description description) {
        return new Statement() {
            @Override
            public void evaluate() throws Throwable {
                RxJavaPlugins.setInitIoSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
                    @Override
                    public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                        return immediate;
                    }
                });
                RxJavaPlugins.setInitComputationSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
                    @Override
                    public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                        return immediate;
                    }
                });
                RxJavaPlugins.setInitNewThreadSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
                    @Override
                    public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                        return immediate;
                    }
                });
                RxJavaPlugins.setInitSingleSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
                    @Override
                    public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                        return immediate;
                    }
                });
                RxAndroidPlugins.setInitMainThreadSchedulerHandler(new Function<Callable<Scheduler>, Scheduler>() {
                    @Override
                    public Scheduler apply(@NonNull Callable<Scheduler> schedulerCallable) throws Exception {
                        return immediate;
                    }
                });

                try {
                    base.evaluate();
                }finally {
                    RxJavaPlugins.reset();
                    RxAndroidPlugins.reset();
                }
            }
        };
    }
}

在需要的地方直接像下面这样调用即可：

  @ClassRule
    public static RxSchedulersOverrideRule sSchedulersOverrideRule = new RxSchedulersOverrideRule();