Android设计模式--单例模式

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何为单例模式

所谓单例就是该类只有一个实例，并向外只提供这个实例，不提供构造方法

单例模式在android源码中的应用的例子

1.InputMethodManager

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;    /**     * 检索InputMethodManager的实例 instance, 如果不存在就创建     * @hide     */    static public InputMethodManager getInstance(Context context) {        return getInstance(context.getMainLooper());    }    /**     * 在内部，输入管理器不能依赖上下文，所以需要当前这个方法，然后调用上面的方法传入mainLooper     * @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;    }

为了防止多线程同时创建多个实例（此处使用synchronized ）

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

2.BluetoothOppManager

public class BluetoothOppManager {    private static final String TAG = "BluetoothOppManager";    private static final boolean V = Constants.VERBOSE;    // 创建private static类实例    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;        }    }

3.AccessibilityManager

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);        }    }

怎么写一个单例

1.懒汉式

public static class SingleTon{        /**         * volatile（不稳定的） 关键字能保证多个线程 在处理 instance的时候正确的初始化         */        private volatile static SingleTon instance;        private SingleTon(){}        public static SingleTon getInstance(){            if(instance == null){                synchronized (SingleTon.class){                    if(instance == null){                        instance = new SingleTon();                    }                }            }            return  instance;        }    }

2.饿汉式

 public static class SingleTon{        /**         * volatile（不稳定的） 关键字能保证多个线程 在处理 instance的时候正确的初始化         */        private volatile static SingleTon instance;        private SingleTon(){}        public synchronized static SingleTon getInstance(){            instance = new SingleTon();            return  instance;        }    }

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