设计模式是前人在开发过程中总结的一些经验，我们在开发过程中依据实际的情况，套用合适的设计模式，能够使程序结构更加简单。利于程序的扩展和维护。但也不是没有使用设计模式的程序就不好。如简单的程序就不用了，有种画蛇添足的感觉。
单例模式能够说是全部模式中最简单的一种，它自始至终仅仅能创建一个实例，能够有两种形式，分别为懒汉式和饿汉式
一、饿汉式。非常easy，一開始就创建了实例，实际上究竟会不会被调用也无论
package com.dzt.singleton; /**
* 饿汉式。线程安全
*
* @author Administrator
*
*/
public class SingletonHungry { private static SingletonHungry instance = new SingletonHungry(); private SingletonHungry() { } public static SingletonHungry getInstance() {
return instance;
}
}
二、懒汉式，因为是线程不安全的，在多线程中处理会有问题。所以须要加同步
package com.dzt.singleton; /**
* 懒汉式，这是线程不安全的，假设有多个线程在运行，有可能会创建多个实例
*
* @author Administrator
*
*/
public class SingletonIdler { private static SingletonIdler instance = null; private SingletonIdler() { } public static SingletonIdler getInstance() {
if (instance == null) {
instance = new SingletonIdler();
}
return instance;
}
}
加了同步之后的代码，每次进来都要推断下同步锁。比較费时。还能够进行改进
package com.dzt.singleton; /**
* 懒汉式
*
* @author Administrator
*
*/
public class SingletonIdler { private static SingletonIdler instance = null; private SingletonIdler() { } public synchronized static SingletonIdler getInstance() {
if (instance == null) {
instance = new SingletonIdler();
}
return instance;
}
}
加同步代码块，仅仅会推断一次同步，假设已经创建了实例就不会推断，降低了时间
package com.dzt.singleton; /**
* 懒汉式
*
* @author Administrator
*
*/
public class SingletonIdler { private static SingletonIdler instance = null; private SingletonIdler() { } public static SingletonIdler getInstance() {
if (instance == null) {
synchronized (SingletonIdler.class) {
if (instance == null)
instance = new SingletonIdler();
}
}
return instance;
}
}
单例模式在Androidd原生应用中也有使用，如Phone中
NotificationMgr.java类
private static NotificationMgr sInstance;
private NotificationMgr(PhoneApp app) {
mApp = app;
mContext = app;
mNotificationManager = (NotificationManager) app
.getSystemService(Context.NOTIFICATION_SERVICE);
mStatusBarManager = (StatusBarManager) app
.getSystemService(Context.STATUS_BAR_SERVICE);
mPowerManager = (PowerManager) app
.getSystemService(Context.POWER_SERVICE);
mPhone = app.phone; // TODO: better style to use mCM.getDefaultPhone()
// everywhere instead
mCM = app.mCM;
statusBarHelper = new StatusBarHelper();
}
static NotificationMgr init(PhoneApp app) {
synchronized (NotificationMgr.class) {
if (sInstance == null) {
sInstance = new NotificationMgr(app);
// Update the notifications that need to be touched at startup.
sInstance.updateNotificationsAtStartup();
} else {
Log.wtf(LOG_TAG, "init() called multiple times! sInstance = "
+ sInstance);
}
return sInstance;
}
}