java 单例模式构造函数需要私有吗？

最新推荐文章于 2023-10-08 16:10:20 发布

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#java #单例模式 #构造函数是否私有化

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本文讨论了单例模式中构造函数是否应该私有化的问题，并通过分析Glide、EventBus及Runtime等实例，揭示了单例模式的应用场景及其背后的考虑。

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！