1. 概述
In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects. The term comes from the mathematical concept of a singleton. - wikipedia
單例模式：是一種對象創(chuàng)建型模式，用來確保程序中一個(gè)類最多只有一個(gè)實(shí)例扔亥，并提供訪問這個(gè)實(shí)例的全局點(diǎn)场躯。
單例模式解決以下類似問題:
- 如何樣保證一個(gè)類只有一個(gè)實(shí)例谈为？
- 如何輕松地訪問類的唯一實(shí)例?
- 一個(gè)類如何控制它的實(shí)例化?
- 如何限制一個(gè)類的實(shí)例數(shù)量?
單例模式的解決方案:
- 隱藏類的構(gòu)造方法，用
private修飾符修飾構(gòu)造方法. - 定義一個(gè)public的靜態(tài)方法(
getInstance()) 踢关，返回這個(gè)類唯一靜態(tài)實(shí)例伞鲫。
2. 適用場景
以下場景可使用單例模式：
- 某些管理類，保證資源的一致訪問性签舞。
- 創(chuàng)建對象時(shí)耗時(shí)過多或耗費(fèi)資源過多秕脓，但又經(jīng)常用到的對象；
- 工具類對象儒搭；
- 頻繁訪問數(shù)據(jù)庫或文件的對象吠架。
Android：
Context.getSystemService()
KeyguardUpdateMonitor.getInstance(mContext)
Calendar.getInstance()
....
其他：
- 日志操作類
- 文件管理器
- 數(shù)據(jù)庫的連接尺
- ...
3.實(shí)現(xiàn)方式
3.1 餓漢式
餓漢式，故名思議搂鲫，很餓傍药，所以在類加載的時(shí)候就直接創(chuàng)建類的實(shí)例。
/**
* Singleton class. Eagerly initialized static instance guarantees thread safety.
*/
public final class Singleton {
/**
* Private constructor so nobody can instantiate the class.
*/
private Singleton() {}
/**
* Static to class instance of the class.
*/
private static final Singleton INSTANCE = new Singleton();
/**
* To be called by user to obtain instance of the class.
*
* @return instance of the singleton.
*/
public static Singleton getInstance() {
return INSTANCE;
}
}
優(yōu)點(diǎn)：
- 多線程安全
缺點(diǎn)：
- 內(nèi)存浪費(fèi)魂仍，類加載之后就被創(chuàng)建了實(shí)例拐辽，但是如果某次的程序運(yùn)行沒有用到，內(nèi)存就被浪費(fèi)了蓄诽。
小結(jié)：
- 適合：單例占用內(nèi)存比較小薛训，初始化時(shí)就會(huì)被用到的情況。
- 不適合：單例占用的內(nèi)存比較大仑氛，或單例只是在某個(gè)特定場景下才會(huì)用到
3.2 懶漢式
? 懶漢式乙埃，故名思議，很懶锯岖，需要用的時(shí)候才創(chuàng)建實(shí)例介袜。
/**
* Singleton class. Eagerly initialized static instance guarantees thread safety.
*/
public final class Singleton {
/**
* Private constructor so nobody can instantiate the class.
*/
private Singleton() {}
/**
* Static to class instance of the class.
*/
private static final Singleton INSTANCE = null;
/**
* To be called by user to obtain instance of the class.
*
* @return instance of the singleton.
*/
public static Singleton getInstance() {
if (INSTANCE == null){
INSTANCE = new Singleton();
}
return INSTANCE;
}
}
優(yōu)點(diǎn)：
- 內(nèi)存節(jié)省，由于此種模式的實(shí)例實(shí)在需要時(shí)創(chuàng)建出吹，如果某次的程序運(yùn)行沒有用到遇伞，就是可以節(jié)省內(nèi)存
缺點(diǎn)：
-
線程不安全，分析如下
線程1 線程2 INSTANCE public static Singleton getInstance() { null public static Singleton getInstance() { null if (INSTANCE == null){ null if (INSTANCE == null){ null INSTANCE = new Singleton(); object1 return INSTANCE; object1 INSTANCE = new Singleton(); object2 return INSTANCE; object2 糟糕的事發(fā)生了捶牢，這里返回2個(gè)不同的實(shí)例鸠珠。
小結(jié)：
- 適合：單線程，內(nèi)存敏感的
- 不適合：多線程
3.3 線程安全的懶漢式
/**
* Thread-safe Singleton class. The instance is lazily initialized and thus needs synchronization
* mechanism.
*
* Note: if created by reflection then a singleton will not be created but multiple options in the
* same classloader
*/
public final class ThreadSafeLazyLoadedSingleton {
private static ThreadSafeLazyLoadedSingleton instance;
private ThreadSafeLazyLoadedSingleton() {
// to prevent instantiating by Reflection call
if (instance != null) {
throw new IllegalStateException("Already initialized.");
}
}
/**
* The instance gets created only when it is called for first time. Lazy-loading
*/
public static synchronized ThreadSafeLazyLoadedSingleton getInstance() {
if (instance == null) {
instance = new ThreadSafeLazyLoadedSingleton();
}
return instance;
}
}
優(yōu)點(diǎn)：
- 多線程安全
缺點(diǎn)：
- 執(zhí)行效率低秋麸，每個(gè)線程在想獲得類的實(shí)例時(shí)候渐排，執(zhí)行g(shù)etInstance()方法都要進(jìn)行同步驯耻。而其實(shí)這個(gè)方法只執(zhí)行一次實(shí)例化代碼就夠了可缚，后面的想獲得該類實(shí)例，直接return就行了知给。方法進(jìn)行同步效率太低要改進(jìn)炼鞠。
小結(jié)：
- 不建議使用此方法,后續(xù)介紹其他方法可兼顧內(nèi)存和多線程安全.
3.4 線程安全的雙重檢查
/**
* Double check locking
* <p/>
* http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
* <p/>
* Broken under Java 1.4.
*
* @author mortezaadi@gmail.com
*/
public final class ThreadSafeDoubleCheckLocking {
private static volatile ThreadSafeDoubleCheckLocking instance;
/**
* private constructor to prevent client from instantiating.
*/
private ThreadSafeDoubleCheckLocking() {
// to prevent instantiating by Reflection call
if (instance != null) {
throw new IllegalStateException("Already initialized.");
}
}
/**
* Public accessor.
*
* @return an instance of the class.
*/
public static ThreadSafeDoubleCheckLocking getInstance() {
// local variable increases performance by 25 percent
// Joshua Bloch "Effective Java, Second Edition", p. 283-284
ThreadSafeDoubleCheckLocking result = instance;
// Check if singleton instance is initialized. If it is initialized then we can return the instance.
if (result == null) {
// It is not initialized but we cannot be sure because some other thread might have initialized it
// in the meanwhile. So to make sure we need to lock on an object to get mutual exclusion.
synchronized (ThreadSafeDoubleCheckLocking.class) {
// Again assign the instance to local variable to check if it was initialized by some other thread
// while current thread was blocked to enter the locked zone. If it was initialized then we can
// return the previously created instance just like the previous null check.
result = instance;
if (result == null) {
// The instance is still not initialized so we can safely (no other thread can enter this zone)
// create an instance and make it our singleton instance.
instance = result = new ThreadSafeDoubleCheckLocking();
}
}
}
return result;
}
}
優(yōu)點(diǎn)：
- 多線程安全
注意點(diǎn)：
- jdk 1.5以下多線程安全不能實(shí)現(xiàn)
小結(jié)：
- 可使用此方法,兼顧內(nèi)存和多線程安全.
3.5 靜態(tài)內(nèi)部類
public class Singleton {
private Singleton() {
}
/**
* 類級的內(nèi)部類，也就是靜態(tài)的成員式內(nèi)部類赃阀，該內(nèi)部類的實(shí)例與外部類的實(shí)例
* 沒有綁定關(guān)系，而且只有被調(diào)用到時(shí)才會(huì)裝載观游，從而實(shí)現(xiàn)了延遲加載驮俗。
*/
public static Singleton getInstance() {
return SingletonLazyHolder.instance;
}
private static class SingletonLazyHolder {
/**
* 靜態(tài)初始化器王凑，由JVM來保證線程安全
*/
private final static Singleton instance = new Singleton();
}
}
優(yōu)點(diǎn)：
- 多線程安全
注意點(diǎn)：
- jdk 1.5以下多線程安全不能實(shí)現(xiàn)
小結(jié)：
- 可使用此方法,兼顧內(nèi)存和多線程安全.
3.6 枚舉
public enum EnumSingleton {
INSTANCE;
@Override
public String toString() {
return getDeclaringClass().getCanonicalName() + "@" + hashCode();
}
}
小結(jié)：
- 可使用此方法,兼顧內(nèi)存和多線程安全.同時(shí)這個(gè)也是Effective Java推薦使用的方法索烹。注意枚舉也是jdk 1.5開始加入的。
4.總結(jié)
單例占用內(nèi)存比較小渊额，初始化時(shí)就會(huì)被用到的情況 - 推薦使用方法 3.1
多線程安全和內(nèi)存占用大旬迹，特定場景下采用奔垦，推薦使用方法 3.4.3.5,3.6. 使用時(shí)注意jdk的版本仑嗅。個(gè)人推薦使用 3.4.3.5仓技。
5.Android代碼實(shí)例
5.1 Dialer,使用方法3.2
packages/apps/Dialer/java/com/android/incallui/InCallPresenter.java
private static InCallPresenter sInCallPresenter;
/** Inaccessible constructor. Must use getRunningInstance() to get this singleton. */
@VisibleForTesting
InCallPresenter() {}
public static synchronized InCallPresenter getInstance() {
if (sInCallPresenter == null) {
sInCallPresenter = new InCallPresenter();
}
return sInCallPresenter;
}
//其他無關(guān)代碼省略
5.2 Email,使用方法3.5
packages/apps/Dialer/java/com/android/incallui/InCallPresenter.java
public class NotificationControllerCreatorHolder {
private static NotificationControllerCreator sCreator =
new NotificationControllerCreator() {
@Override
public NotificationController getInstance(Context context){
return null;
}
};
public static void setNotificationControllerCreator(
NotificationControllerCreator creator) {
sCreator = creator;
}
public static NotificationControllerCreator getNotificationControllerCreator() {
return sCreator;
}
public static NotificationController getInstance(Context context) {
return getNotificationControllerCreator().getInstance(context);
}
}
有興趣的可以自己在找找案例看看阔逼。
6.有參數(shù)的單例
android上有很多需要Context參數(shù)的單例場景嗜浮。先不要急危融，看看Android源碼的實(shí)例：
packages/apps/Email/src/com/android/email/EmailNotificationController.java
private static EmailNotificationController sInstance;
/** Singleton access */
public static synchronized EmailNotificationController getInstance(Context context) {
if (sInstance == null) {
sInstance = new EmailNotificationController(context, Clock.INSTANCE);
}
return sInstance;
}
其實(shí)也很簡單嗎吉殃，但是這里面有個(gè)小問題楷怒，如果傳遞參數(shù)是敏感的鸠删，是需要替換的，那就需要在處理一下：
public final class Singleton {
private Context context;
private static volatile Singleton instance;
private Singleton(Context context) {
this.context = context;
if (instance != null) {
throw new IllegalStateException("Already initialized.");
}
}
public static Singleton getInstance(Context context) {
Singleton result = instance;
if (result == null) {
synchronized (Singleton.class) {
result = instance;
if (result == null) {
instance = result = new Singleton(context);
}
}
//這里要注意重新賦值
instance.context = context;
}
return result;
}
}
鳴謝
- Initialization-on-demand holder idiom
- Singleton pattern
- Head First 設(shè)計(jì)模式
- java-design-patterns
