如何为改造 2 制作单例？

Question

如果存在多个retrofit调用，我该如何做一个retrofit的单例，这样类内就不会出现重复代码，从而摆脱不必要的代码。

Answer 1

这是一个例子，但是！尽管这可能很闪亮且易于使用，但单例是邪恶的。如果可能，尽量避免使用它们。一种解决方法是使用依赖注入。

无论如何。

public class Api {
    private static Api instance = null;
    public static final String BASE_URL = "your_base_url";

    // Keep your services here, build them in buildRetrofit method later
    private UserService userService;

    public static Api getInstance() {
        if (instance == null) {
            instance = new Api();
        }

        return instance;
    }

    // Build retrofit once when creating a single instance
    private Api() {
        // Implement a method to build your retrofit
        buildRetrofit(BASE_URL);
    }

    private void buildRetrofit() {
        Retrofit retrofit = ...

        // Build your services once
        this.userService = retrofit.create(UserService.class);
        ...
    }

    public UserService getUserService() {
        return this.userService;
    }
    ...
}

现在您将所有内容都集中在一个地方。使用它。

UserService userService = Api.getInstance().getUserService();

Answer 2

要实现单例类，最简单的方法是将类的构造函数设为私有。

1. 渴望初始化：

在Eager初始化中，Singleton Class的实例是在类加载时创建的，这是创建单例类最简单的方法。

public class SingletonClass {

private static volatile SingletonClass sSoleInstance = new SingletonClass();

    //private constructor.
    private SingletonClass(){}

    public static SingletonClass getInstance() {
        return sSoleInstance;
    }
}

1. 延迟初始化：

此方法将检查是否已创建该类的任何实例？如果是，那么我们的方法 (getInstance()) 将返回那个旧实例，如果不是，那么它会在 JVM 中创建单例类的新实例并返回该实例。这种方法称为延迟初始化。

public class SingletonClass {

    private static SingletonClass sSoleInstance;

    private SingletonClass(){}  //private constructor.

    public static SingletonClass getInstance(){
        if (sSoleInstance == null){ //if there is no instance available... create new one
            sSoleInstance = new SingletonClass();
        }

        return sSoleInstance;
   }
}

还有其他一些东西，例如 Java 反射 API、线程安全和序列化安全 Singleton。

请查看此参考以获取更多详细信息和对单例对象创建的深入了解。

https://medium.com/@kevalpatel2106/digesting-singleton-design-pattern-in-java-5d434f4f322#.6gzisae2u

Answer 3

public class Singleton {
private volatile static Singleton singleton;
private Singleton(){}
public static Singleton getSingleton(){
    if (singleton == null) {
        synchronized (Singleton.class) {
            if (singleton == null) {
                singleton = new Singleton();
            }
        }
    }
    return singleton;
}

Answer 4

public class Singleton  {

    private static Singleton INSTANCE = null;

    // other instance variables can be here

    private Singleton() {};

    public static Singleton getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new Singleton();
        }
        return(INSTANCE);
    }

    // other instance methods can follow 
}



import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class RetrofitClient {

    private static Retrofit retrofit = null;

    public static Retrofit getClient(String baseUrl) {
        if (retrofit==null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(baseUrl)
                    .addConverterFactory(GsonConverterFactory.create())
                    .build();
        }
        return retrofit;
    }
}




@Module
public class NetworkModule {

    @Provides
    @Singleton
    public Gson gson() {
        GsonBuilder gsonBuilder = new GsonBuilder();
        return gsonBuilder.create();
    }

    @Provides
    @Singleton
    public HttpLoggingInterceptor loggingInterceptor() {
        HttpLoggingInterceptor interceptor = new HttpLoggingInterceptor(
                message -> Timber.i(message));
        interceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
        return interceptor;
    }

    @Provides
    @Singleton
    public Cache cache(File cacheFile) {
        return new Cache(cacheFile, 10 * 1000 * 1000); //10MB Cache
    }

    @Provides
    @Singleton
    public File cacheFile(@ApplicationContext Context context) {
        return new File(context.getCacheDir(), "okhttp_cache");
    }

    @Provides
    @Singleton
    public OkHttpClient okHttpClient(HttpLoggingInterceptor loggingInterceptor, Cache cache) {
        return new OkHttpClient.Builder()
                .addInterceptor(loggingInterceptor)
                .cache(cache)
                .build();
    }

    @Provides
    @Singleton
    public Retrofit retrofit(OkHttpClient okHttpClient, Gson gson) {
        return new Retrofit.Builder()
                .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                .addConverterFactory(GsonConverterFactory.create(gson))
                .client(okHttpClient)
                .baseUrl("you/base/url")
                .build();
    }
}

Answer 5

我在 kotlin 中尝试过：

class RetrofitClient{

    companion object
    {
        var retrofit:Retrofit?=null;
        fun getRetrofitObject():Retrofit?
        {
            if(retrofit==null)
            {
                synchronized(RetrofitClient ::class.java)
                {
                    retrofit=Retrofit.Builder()
                        .addConverterFactory(GsonConverterFactory.create())
                        .baseUrl("YOUR_BASE_URL")
                        .build()
                }
            }
            return retrofit

        }

    }
}

然后：

var service:ServicesInterface?= RetrofitClient.getRetrofitObject()?.create(ServicesInterface::class.java)