Observable.just("123")
        .map(new Function<String, Integer>() {
    @Override
    public Integer apply(String s) throws Exception {
        String i = null;
        i.length();
        return 1;
    }
}).subscribe();

这种情况下, 由于在subscribe时没有传入异常处理时的Consumer, 所以在这里会崩掉.
看一下源码:

public final Disposable subscribe() {
    // 会传入一个默认的 Functions.ERROR_CONSUMER
    return subscribe(Functions.emptyConsumer()
            , Functions.ERROR_CONSUMER
            , Functions.EMPTY_ACTION, Functions.emptyConsumer());
}

public static final Consumer<Throwable> ERROR_CONSUMER = new Consumer<Throwable>() {
    @Override
    public void accept(Throwable error) {
        // 捕获到异常后会掉还用 RxJavaPlugins.onError.onError(); 
        RxJavaPlugins.onError(error);
    }
};

public static void onError(Throwable error) {
    // 注意errorHandler
    Consumer<Throwable> f = errorHandler;

    if (error == null) {
        error = new NullPointerException("onError called with null. Null values are generally not allowed in 2.x operators and sources.");
    }

    // errorHandler不为空, 调用它的 accept() 方法, 并 return
    if (f != null) {
        try {
            f.accept(error);
            return;
        } catch (Throwable e) {
            e.printStackTrace(); // NOPMD
            uncaught(e);
        }
    }
    
    // 如果执行到这里就会抛出异常
    error.printStackTrace(); // NOPMD
    uncaught(error);
}

// RxjavaPlugins
static volatile Consumer<Throwable> errorHandler;

public static void setErrorHandler(Consumer<Throwable> handler) {
    if (lockdown) {
        throw new IllegalStateException("Plugins can't be changed anymore");
    }
    errorHandler = handler;
}

可以利用 上面的方法设置errorHandler, 这样当你, subscribr()不传入errorConsumer时, 就会捕获到异常不会崩掉