基于arouter-api:1.3.1 arouter-compiler:1.1.4
ARouter 为Android平台中对页面、服务提供路由功能的中间件
下面从源码分析为什么只用通过一个注解就可以得到相应的Activity或Fragment

//服务module中的FragmentService
@Route(path = "/service/fragmentService")
public class FragmentService{}

//服务module中的TestActivity
@Route(path = "/service/test")
public class TestActivity extends BaseActivity{}

//主APP中获取FragmentService实例
Fragment serviceFragment = (Fragment) ARouter.getInstance().build("/service/fragmentService").navigation();

//主APP中跳转服务module中的TestActivity
ARouter.getInstance().build("/service/test").navigation();

image.png

//生成Group对应的代码（loadInto功能主要是在ARouter.init()时把Group放到group集合中）
public class ARouter$$Root$$service_module implements IRouteRoot {
  @Override
  public void loadInto(Map<String, Class<? extends IRouteGroup>> routes) {
    routes.put("service", ARouter$$Group$$service.class);
  }
}

//生成Group对应所有子节点的代码（loadInto功能主要是在APP第一次调用该group下的子节点时把所有的子节点加进集合中）
public class ARouter$$Group$$service implements IRouteGroup {
  @Override
  public void loadInto(Map<String, RouteMeta> atlas) {
    atlas.put("/service/fragmentService", RouteMeta.build(RouteType.FRAGMENT, FragmentService.class, "/service/fragmentservice", "service", null, -1, -2147483648));
    atlas.put("/service/test", RouteMeta.build(RouteType.ACTIVITY, TestActivity.class, "/service/test", "service", null, -1, -2147483648));
  }
}

上面代码为程序编译时生成，在APP运行期间使用ARouter.init(Context);会调用如下代码

//_ARouter中init方法
protected static synchronized boolean init(Application application) {
        mContext = application;
        LogisticsCenter.init(mContext, executor);
        logger.info(Consts.TAG, "ARouter init success!");
        hasInit = true;
        return true;
    }

//LogisticsCenter精简版
public synchronized static void init(Context context, ThreadPoolExecutor tpe) throws HandlerException {
     for (String className : routerMap) {
       if (className.startsWith(ROUTE_ROOT_PAKCAGE + DOT + SDK_NAME + SEPARATOR + SUFFIX_ROOT)) {
       ((IRouteRoot) (Class.forName(className).getConstructor().newInstance())).loadInto(Warehouse.groupsIndex);
       } else if (className.startsWith(ROUTE_ROOT_PAKCAGE + DOT + SDK_NAME + SEPARATOR + SUFFIX_INTERCEPTORS)) {
       ((IInterceptorGroup) (Class.forName(className).getConstructor().newInstance())).loadInto(Warehouse.interceptorsIndex);
       } else if (className.startsWith(ROUTE_ROOT_PAKCAGE + DOT + SDK_NAME + SEPARATOR + SUFFIX_PROVIDERS)) {
       ((IProviderGroup) (Class.forName(className).getConstructor().newInstance())).loadInto(Warehouse.providersIndex);
            }
         }
      }
    
    }

通过反射把生成的文件全限定名取到加入routermap中然后通过反射实例化对应的类调用loadinto方法将group添加进如下集合之中

//Warehouse 精简源码
class Warehouse {
    // group节点集合
    static Map<String, Class<? extends IRouteGroup>> groupsIndex = new HashMap<>();
   // group下子节点集合
    static Map<String, RouteMeta> routes = new HashMap<>();
}

init完毕之后ARouter只把group节点初始化了
在APP调用ARouter.getInstance().build("/service/test").navigation();方法会调用如下源码

//_ARouter 中navigation()方法精简版
final class _ARouter {
  protected Object _navigation(final Context context, final Postcard postcard, final int requestCode, final NavigationCallback callback) {
     switch (postcard.getType()) {
            case ACTIVITY:
                // Build intent
                final Intent intent = new Intent(currentContext, postcard.getDestination());
                intent.putExtras(postcard.getExtras());

                // Set flags.
                int flags = postcard.getFlags();
                if (-1 != flags) {
                    intent.setFlags(flags);
                } else if (!(currentContext instanceof Activity)) {    // Non activity, need less one flag.
                    intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                }

                // Navigation in main looper.
                new Handler(Looper.getMainLooper()).post(new Runnable() {
                    @Override
                    public void run() {
                        if (requestCode > 0) {  // Need start for result
                            ActivityCompat.startActivityForResult((Activity) currentContext, intent, requestCode, postcard.getOptionsBundle());
                        } else {
                            ActivityCompat.startActivity(currentContext, intent, postcard.getOptionsBundle());
                        }

                        if ((0 != postcard.getEnterAnim() || 0 != postcard.getExitAnim()) && currentContext instanceof Activity) {    // Old version.
                            ((Activity) currentContext).overridePendingTransition(postcard.getEnterAnim(), postcard.getExitAnim());
                        }

                        if (null != callback) { // Navigation over.
                            callback.onArrival(postcard);
                        }
                    }
                });

                break;
            case PROVIDER:
                return postcard.getProvider();
            case BOARDCAST:
            case CONTENT_PROVIDER:
            case FRAGMENT:
                Class fragmentMeta = postcard.getDestination();
                try {
                    Object instance = fragmentMeta.getConstructor().newInstance();
                    if (instance instanceof Fragment) {
                        ((Fragment) instance).setArguments(postcard.getExtras());
                    } else if (instance instanceof android.support.v4.app.Fragment) {
                        ((android.support.v4.app.Fragment) instance).setArguments(postcard.getExtras());
                    }

                    return instance;
                } catch (Exception ex) {
                    logger.error(Consts.TAG, "Fetch fragment instance error, " + TextUtils.formatStackTrace(ex.getStackTrace()));
                }
            case METHOD:
            case SERVICE:
            default:
                return null;
        }

        return null;
  }
}

在这里会把传入路径中的group解析出来查询对应的group下子节点是否已经被加到集合中，如果没有加入则通过loadinto加入，然后通过RouteMeta中的属性得到具体页面的类型和class名 进入_ARouter ._navigation()就获得了具体的Activity 其中Fragment需要根据全限定名反射得到实例，此时整个流程结束