Android源碼學習之單例模式應用及優點介紹

2020-04-11 12:36:43

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單例模式定義：
Ensure a class has only one instance, and provide a global point of access to it.
動態確保某一個類只有一個實例，而且自行實例化并向整個系統提供這個實例。

如上圖所示（截取自《Head First Design Patterns》一書）。

通過使用private的構造函數確保了在一個應用中產生一個實例，并且是自行實例化（在Singleton中自己使用new Singleton（））。
具體單例模式有什么優點呢：
由于單例模式在內存中只有一個實例，減少了內存開銷。
單例模式可以避免對資源的多重占用，例如一個寫文件時，由于只有一個實例存在內存中，避免對同一個資源文件的同時寫操作。
單例模式可以再系統設置全局的訪問點，優化和共享資源訪問。
其中使用到單例模式時，考慮較多的就是多線程的情況下如何防止被多線程同時創建等問題，其中《Head First Design Patterns》使用到“double-checked locking”來降低使用synchronization。

復制代碼代碼如下:

 
public class Singleton { 
/* The volatile keyword ensures that multiple threads 
* handle the uniqueInstance variable correctly when it 
* is being initialized to the Singleton instance. 
* */ 
private volatile static Singleton uniqueInstance; 
private Singleton() {} 
public static Singleton getInstance() { 
if(uniqueInstance == null) { 
synchronized (Singleton.class) { 
if(uniqueInstance == null) { 
uniqueInstance = new Singleton(); 
} 
} 
} 
return uniqueInstance; 
} 
} 

在Android源碼中，使用到單例模式的例子很多，如：
一、如InputMethodManager類

復制代碼代碼如下:

 
public final class InputMethodManager { 
static final boolean DEBUG = false; 
static final String TAG = "InputMethodManager"; 
static final Object mInstanceSync = new Object(); 
static InputMethodManager mInstance; 
final IInputMethodManager mService; 
final Looper mMainLooper; 

創建唯一的實例static InputMethodManager mInstance;

復制代碼代碼如下:

 
/** 
* Retrieve the global InputMethodManager instance, creating it if it 
* doesn't already exist. 
* @hide 
*/ 
static public InputMethodManager getInstance(Context context) { 
return getInstance(context.getMainLooper()); 
} 
/** 
* Internally, the input method manager can't be context-dependent, so 
* we have this here for the places that need it. 
* @hide 
*/ 
static public InputMethodManager getInstance(Looper mainLooper) { 
synchronized (mInstanceSync) { 
if (mInstance != null) { 
return mInstance; 
} 
IBinder b = ServiceManager.getService(Context.INPUT_METHOD_SERVICE); 
IInputMethodManager service = IInputMethodManager.Stub.asInterface(b); 
mInstance = new InputMethodManager(service, mainLooper); 
} 
return mInstance; 
} 

防止多線程同時創建實例：

復制代碼代碼如下:

synchronized (mInstanceSync) { 
if (mInstance != null) { 
return mInstance; 
} 

當沒有創建實例對象時，調用mInstance = new InputMethodManager(service, mainLooper);
其中類構造函數如下所示：

復制代碼代碼如下:

 
InputMethodManager(IInputMethodManager service, Looper looper) { 
mService = service; 
mMainLooper = looper; 
mH = new H(looper); 
mIInputContext = new ControlledInputConnectionWrapper(looper, 
mDummyInputConnection); 
if (mInstance == null) { 
mInstance = this; 
} 
} 

二、BluetoothOppManager類

復制代碼代碼如下:

 
public class BluetoothOppManager { 
private static final String TAG = "BluetoothOppManager"; 
private static final boolean V = Constants.VERBOSE; 
// 創建private static類實例 
private static BluetoothOppManager INSTANCE; 
/** Used when obtaining a reference to the singleton instance. */ 
private static Object INSTANCE_LOCK = new Object(); 
。。。 
/** 
* Get singleton instance. 
*/ 
public static BluetoothOppManager getInstance(Context context) { 
synchronized (INSTANCE_LOCK) { 
if (INSTANCE == null) { 
INSTANCE = new BluetoothOppManager(); 
} 
INSTANCE.init(context); 
return INSTANCE; 
} 
} 

三、AccessibilityManager類

復制代碼代碼如下:

 
public final class AccessibilityManager { 
private static final boolean DEBUG = false; 
private static final String LOG_TAG = "AccessibilityManager"; 
/** @hide */ 
public static final int STATE_FLAG_ACCESSIBILITY_ENABLED = 0x00000001; 
/** @hide */ 
public static final int STATE_FLAG_TOUCH_EXPLORATION_ENABLED = 0x00000002; 
static final Object sInstanceSync = new Object(); 
private static AccessibilityManager sInstance; 
... 
/** 
* Get an AccessibilityManager instance (create one if necessary). 
* 
* @hide 
*/ 
public static AccessibilityManager getInstance(Context context) { 
synchronized (sInstanceSync) { 
if (sInstance == null) { 
IBinder iBinder = ServiceManager.getService(Context.ACCESSIBILITY_SERVICE); 
IAccessibilityManager service = IAccessibilityManager.Stub.asInterface(iBinder); 
sInstance = new AccessibilityManager(context, service); 
} 
} 
return sInstance; 
} 
/** 
* Create an instance. 
* 
* @param context A {@link Context}. 
* @param service An interface to the backing service. 
* 
* @hide 
*/ 
public AccessibilityManager(Context context, IAccessibilityManager service) { 
mHandler = new MyHandler(context.getMainLooper()); 
mService = service; 
try { 
final int stateFlags = mService.addClient(mClient); 
setState(stateFlags); 
} catch (RemoteException re) { 
Log.e(LOG_TAG, "AccessibilityManagerService is dead", re); 
} 
} 