定義

應(yīng)用內(nèi)只存在類的一個實例

餓漢式

public class Singleton {
    public  static Singleton mInstance = new Singleton();

    private Singleton() {
    }
    
    public  static Singleton getInstance(){
        return mInstance;
    }
}

懶漢式

• 方案一

public class Singleton {
    public static Singleton mInstance ;

    private Singleton() {
    }

    public static Singleton getInstance(){
        if (mInstance == null) {
            mInstance = new Singleton();
        } 
        return mInstance;
    }
}

存在問題：多線程并發(fā)時脂信，可能new出多個對象

• 方案二

public class Singleton {
    public static Singleton mInstance ;

    private Singleton() {
    }

    public synchronized static Singleton getInstance(){
        if (mInstance == null) {
            mInstance = new Singleton();
        }
        return mInstance;
    }
}

存在問題：多線程并發(fā)時癣蟋，每個線程都需要判斷一次同步鎖，效率較低

• 方案三（推薦）

public class Singleton {
    public static Singleton mInstance ;

    private Singleton() {
    }

    public  static Singleton getInstance(){
        if (mInstance == null) {
            synchronized (Singleton.class){
                if (mInstance == null) {
                    mInstance = new Singleton();
                }
            }
        }
        return mInstance;
    }
}

存在問題：存在不加關(guān)鍵字volatile的弊端
volatile的作用:

1. 防止重排序( 1 分配一塊內(nèi)存空間 2 初始化對象（指向內(nèi)存空間） 3賦值
   多線程時2,3可能會更換順序狰闪，出現(xiàn)意想不到的錯誤)
2. 線程可見(一個線程改變了公用對象疯搅，短時間內(nèi)，可能對另一個線程不可見)

• 方案四（推薦）

public class Singleton {
    public volatile static Singleton mInstance ;

    private Singleton() {
    }

    public  static  Singleton getInstance(){
        if (mInstance == null) {
            synchronized (Singleton.class){
                if (mInstance == null) {
                    mInstance = new Singleton();
                }
            }
        }
        return mInstance;
    }
}

靜態(tài)內(nèi)部類

public class Singleton {
    public static Singleton mInstance;

    private Singleton() {
    }

    public static Singleton getInstance() {
        return SingletonHolder.mInstance;
    }

    static class SingletonHolder {
        static Singleton mInstance = new Singleton();
    }
}

容器管理

public class Singleton {
    public static Map<String,Object> mSingleMap = new HashMap<>();

    static {
        mSingleMap.put("activity_manager",new Singleton());
    }

    private Singleton() {
    }

    public static Object getInstance(String name) {
        return  mSingleMap.get(name);
    }

}