java 单例模式构造函数需要私有吗？

青旅半醒 2024-04-17 06:19 29阅读 0赞

先看下标准的单例模式之一的代码写法，单例模式有几种写法，如下：

public class SocketUtil {
        
        private static volatile SocketUtil socketUtil;
    
        /**
         * socketUtil 单例模式 - 双重校验锁
         */
        public static SocketUtil getInstance() {
            if (socketUtil == null) {
                synchronized (SocketUtil.class) {
                    if (socketUtil == null) {
                        socketUtil = new SocketUtil();
                    }
                }
            }
            return socketUtil;
        }
        
        private SocketUtil() {
            
        }
    
    }

构造函数为私有的，那么其他地方就不能再生产多的实例，保证了进程中实例的唯一性；

but， 我看了几个目前安卓端github上最火的轮子Glide、EventBus的源码，发现它们并没有把构造函数私有化，总不能说大佬连最基本的单例都不懂吧，我就纳闷了，上代码为证：

Glide:

/**
     * A singleton to present a simple static interface for building requests with
     * {@link RequestBuilder} and maintaining an {@link Engine}, {@link BitmapPool},
     * {@link com.bumptech.glide.load.engine.cache.DiskCache} and {@link MemoryCache}.
     */
    public class Glide implements ComponentCallbacks2 {
      private static final String DEFAULT_DISK_CACHE_DIR = "image_manager_disk_cache";
      private static final String TAG = "Glide";
      private static volatile Glide glide;
      private static volatile boolean isInitializing;
    
    
      /**
       * Get the singleton.
       *
       * @return the singleton
       */
      @NonNull
      public static Glide get(@NonNull Context context) {
        if (glide == null) {
          synchronized (Glide.class) {
            if (glide == null) {
              checkAndInitializeGlide(context);
            }
          }
        }
    
        return glide;
      }
    
     Glide(
          @NonNull Context context,
          @NonNull Engine engine,
          @NonNull MemoryCache memoryCache,
          @NonNull BitmapPool bitmapPool,
          @NonNull ArrayPool arrayPool,
          @NonNull RequestManagerRetriever requestManagerRetriever,
          @NonNull ConnectivityMonitorFactory connectivityMonitorFactory,
          int logLevel,
          @NonNull RequestOptions defaultRequestOptions,
          @NonNull Map<Class<?>, TransitionOptions<?, ?>> defaultTransitionOptions) {
        this.engine = engine;
        this.bitmapPool = bitmapPool;
        this.arrayPool = arrayPool;
        this.memoryCache = memoryCache;
        this.requestManagerRetriever = requestManagerRetriever;
        ……
      }
    
    }

EventBus:

/**
     * EventBus is a central publish/subscribe event system for Android. Events are posted ({@link #post(Object)}) to the
     * bus, which delivers it to subscribers that have a matching handler method for the event type. To receive events,
     * subscribers must register themselves to the bus using {@link #register(Object)}. Once registered, subscribers
     * receive events until {@link #unregister(Object)} is called. Event handling methods must be annotated by
     * {@link Subscribe}, must be public, return nothing (void), and have exactly one parameter
     * (the event).
     *
     * @author Markus Junginger, greenrobot
     */
    public class EventBus {
    
        /** Log tag, apps may override it. */
        public static String TAG = "EventBus";
    
        static volatile EventBus defaultInstance;
    
        private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
    
    
        /** Convenience singleton for apps using a process-wide EventBus instance. */
        public static EventBus getDefault() {
            if (defaultInstance == null) {
                synchronized (EventBus.class) {
                    if (defaultInstance == null) {
                        defaultInstance = new EventBus();
                    }
                }
            }
            return defaultInstance;
        }
    
    
        /**
         * Creates a new EventBus instance; each instance is a separate scope in which 
           events are delivered. To use a
         * central bus, consider {@link #getDefault()}.
         */
        public EventBus() {
            this(DEFAULT_BUILDER);
        }
    }

再看看jdk中的Runtime这个类的代码，构造方法私有化了，如下：

public class Runtime {
        private static Runtime currentRuntime = new Runtime();
    
    
        /**
         * Returns the runtime object associated with the current Java application.
         * Most of the methods of class <code>Runtime</code> are instance
         * methods and must be invoked with respect to the current runtime object.
         *
         * @return  the <code>Runtime</code> object associated with the current
         *          Java application.
         */
        public static Runtime getRuntime() {
            return currentRuntime;
        }
    
        /** Don't let anyone else instantiate this class */
        private Runtime() {}
    }

哈哈，到底该不该私有化呢？

此问题令小弟夜不能寐，以前一直写单例都没思考过这个问题，最后请教了几个大神，我得出以下结论：

标准的单例模式，构造函数是需要私有化的，但是单例模式是一种思想，代码模式设计的目的是为了服务于业务的，可能某些特俗场景下，需要外界使用构造函数。

好了，请路过的大神多多指点迷津，不胜感激！