Retrofit中的享元模式

上次看到Retrofit中的动态代理模式，其中有段代码如下：

  public <T> T create(final Class<T> service) {
    Utils.validateServiceInterface(service);
    if (validateEagerly) {
      eagerlyValidateMethods(service);
    }
    return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] { service },
        new InvocationHandler() {
          private final Platform platform = Platform.get();

          @Override public Object invoke(Object proxy, Method method, Object... args)
              throws Throwable {
            // If the method is a method from Object then defer to normal invocation.
            if (method.getDeclaringClass() == Object.class) {
              return method.invoke(this, args);
            }
            if (platform.isDefaultMethod(method)) {
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            ServiceMethod serviceMethod = loadServiceMethod(method);
            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
            return serviceMethod.callAdapter.adapt(okHttpCall);
          }
        });
  }

其中用到的享元模式就是 loadServiceMethod这个函数：

ServiceMethod loadServiceMethod(Method method) {
    ServiceMethod result;
    synchronized (serviceMethodCache) {
      result = serviceMethodCache.get(method);
      if (result == null) {
        result = new ServiceMethod.Builder(this, method).build();
        serviceMethodCache.put(method, result);
      }
    }
    return result;
  }

可以看到serviceMethodCache就是一个LinkedHashMap(拿数据顺序和存数据顺序一样的HashMap)，这里用LinkedHashMap数据结构个人认为主要是为了存储以后可以快速提取的优势.

private final Map<Method, ServiceMethod> serviceMethodCache = new LinkedHashMap<>();

可以看到主要是从Method转到ServiceMethod，先从serviceMethodCache 缓存里面取，拿到就直接用，拿不到就new 新对象(Builder模式建立新对象)然后存入serviceMethodCache 缓存，这样就避免过多创建对象导致gc，影响性能.