单例模式

Intent

• Ensure a class has only one instance, and provide a global point of access to it.
• Encapsulated "just-in-time initialization" or "initialization on first use".

Structure

Scheme of Singleton

代码:

// normal single ton, bad for multi-thread
public class SingleTon {
    private static SingleTon instance;

    private SingleTon() {
    }

    public SingleTon getInstance() {
        if (instance == null) instance = new SingleTon();
        return instance;
    }
}

对于多线程的问题:

1. 加synchronized关键字或加锁，但是性能可能下降100倍

// lock
public class SingleTon {
    private static SingleTon instance;
    Lock lock = new ReentrantLock();

    private SingleTon() {
    }

    public SingleTon getInstance() {
        lock.lock();
        try {
            if (instance == null) instance = new SingleTon();
            return instance;
        } finally {
            lock.unlock();
        }

    }
}

2. 使用"急切"创建实例，不用延迟创建(Recommanded)

// Initialization on Demand Holder
public class SingleTon {
    private static SingleTon instance = new SingleTon();

    private SingleTon() {
    }

    public SingleTon getInstance() {
        return instance;
    }
}

3. 使用"双重检查枷锁",在getInstance()中减少使用同步

// double-checked locking
public class SingleTon {
    private volatile static SingleTon instance = new SingleTon();

    private SingleTon() {
    }

    public SingleTon getInstance() {
        if (instance == null) {
            synchronized (SingleTon.class) {
                if (instance == null) instance = new SingleTon();
            }
        }
        return instance;
    }
}

Refenrence

1. Design Patterns
2. Head First 设计模式