设计模式-单例设计模式

定义

应用内只存在类的一个实例

饿汉式

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

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

懒汉式

• 方案一

public class Singleton {
    public static Singleton mInstance ;

    private Singleton() {
    }

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

存在问题：多线程并发时，可能new出多个对象

• 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;
    }
}

存在问题：多线程并发时，每个线程都需要判断一次同步锁，效率较低

• 方案三（推荐）

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;
    }
}

存在问题：存在不加关键字volatile的弊端
volatile的作用:

1. 防止重排序( 1 分配一块内存空间 2 初始化对象（指向内存空间） 3赋值
   多线程时2,3可能会更换顺序，出现意想不到的错误)
2. 线程可见(一个线程改变了公用对象，短时间内，可能对另一个线程不可见)

• 方案四（推荐）

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;
    }
}

静态内部类

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);
    }

}