对于设计模式,我个人认为如果整篇写原理的话,应该很乏味有难懂,所以我比较喜欢白话似的解释和代码的结合。废话不多说。
在Android中,很多地方用到了单例。 比如Android-Universal-Image-Loader中的单例
private volatile static ImageLoader instance; /** Returns singleton class instance */ public static ImageLoader getInstance() { if (instance == null) { synchronized (ImageLoader.class) { if (instance == null) { instance = new ImageLoader(); } } } return instance; }比如EventBus中的单例
private static volatile EventBus defaultInstance; public static EventBus getDefault() { if (defaultInstance == null) { synchronized (EventBus.class) { if (defaultInstance == null) { defaultInstance = new EventBus(); } } } return defaultInstance; }上面的单例都是比较规规矩矩的,当然实际上有很多单例都是变了一个样子,单本质还是单例。 如InputMethodManager 中的单例
static InputMethodManager sInstance; public static InputMethodManager getInstance() { synchronized (InputMethodManager.class) { if (sInstance == null) { IBinder b = ServiceManager.getService(Context.INPUT_METHOD_SERVICE); IInputMethodManager service = IInputMethodManager.Stub.asInterface(b); sInstance = new InputMethodManager(service, Looper.getMainLooper()); } return sInstance; } }AccessibilityManager 中的单例,看代码这么长,其实就是进行了一些判断,还是一个单例
private static AccessibilityManager sInstance; public static AccessibilityManager getInstance(Context context) { synchronized (sInstanceSync) { if (sInstance == null) { final int userId; if (Binder.getCallingUid() == Process.SYSTEM_UID || context.checkCallingOrSelfPermission( Manifest.permission.INTERACT_ACROSS_USERS) == PackageManager.PERMISSION_GRANTED || context.checkCallingOrSelfPermission( Manifest.permission.INTERACT_ACROSS_USERS_FULL) == PackageManager.PERMISSION_GRANTED) { userId = UserHandle.USER_CURRENT; } else { userId = UserHandle.myUserId(); } IBinder iBinder = ServiceManager.getService(Context.ACCESSIBILITY_SERVICE); IAccessibilityManager service = IAccessibilityManager.Stub.asInterface(iBinder); sInstance = new AccessibilityManager(context, service, userId); } } return sInstance; }最后,我们应用一下单例模式。典型的一个应用就是管理我们的Activity,下面这个可以作为一个工具类,代码也很简单,也不做什么解释了。
public class ActivityManager { private static volatile ActivityManager instance; private Stack<Activity> mActivityStack = new Stack<Activity>(); private ActivityManager(){ } public static ActivityManager getInstance(){ if (instance == null) { synchronized (ActivityManager.class) { if (instance == null) { instance = new ActivityManager(); } } return instance; } public void addActicity(Activity act){ mActivityStack.push(act); } public void removeActivity(Activity act){ mActivityStack.remove(act); } public void killMyProcess(){ int nCount = mActivityStack.size(); for (int i = nCount - 1; i >= 0; i--) { Activity activity = mActivityStack.get(i); activity.finish(); } mActivityStack.clear(); android.os.Process.killProcess(android.os.Process.myPid()); } }单例模式的优缺点分析 优点:客户端使用单例模式的实例的时候,只需要调用一个单一的方法即可生成一个唯一的实例,有利于节约资源。
缺点:首先单例模式很难实现序列化,这就导致采用单例模式的类很难被持久化,当然也很难通过网络传输;其次由于单例采用静态方法,无法在继承结构中使用。