實(shí)現(xiàn)單例應(yīng)該注意以下4點(diǎn)：
- 構(gòu)造函數(shù)不對(duì)外開(kāi)放，一般為private;
- 通過(guò)一個(gè)靜態(tài)方法或者枚舉返回單例類(lèi)對(duì)象；
- 確保單例的對(duì)象有且僅有一個(gè)，尤其是在多線(xiàn)程環(huán)境下；
- ?單例類(lèi)對(duì)象在反序列化時(shí)不會(huì)被重新構(gòu)建對(duì)象。
1. 餓漢單列模式
public class Singleton {
private static Singleton instance= new Singleton();
private Singleton() {};
public static Singleton getInstance() {
return instance;
}
}
2. 懶漢單列模式
public class Singleton {
private Singleton() {};
private static Singleton instance= null;
public static synchronized Singleton getInstance() {
if (instance == null) {
instance = new Singleton();
}
return instance;
}
}
3. 懶漢單列模式（雙重校驗(yàn)）
public class Singleton {
private Singleton() {};
private static Singleton instance= null;
public static Singleton getInstance() {
if (instance == null) {
synchronized (Singleton.this){
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
4. 靜態(tài)內(nèi)部類(lèi)單列模式
public class Singleton {
private Singleton() {};
public static Singleton getInstance () {
return SingletonHolder.instance;
}
private static class SingletonHolder {
private static final Singleton instance = new Singleton();
}
}
5. 枚舉單列模式
public enum Singleton {
INSTANCE;
}
6. 容器實(shí)現(xiàn)單列模式
public class Singleton {
private Singleton() {};
private static Map<String, Object> objects = new HashMap<String, Object>();
public static void registe (String key, Object instance) {
if (!objects.containsKey(key)) objects.put(key, instance);
}
public static Object get () {
return objects.get(key);
}
}
注：。。。
