#冲刺创作新星#java内存模型之双重检查锁定与线程安全的延迟初始 原创

public class UnsafeLazyInitialization {
    private static Instance instance;
    public static Instance getInstance() {
        if (instance == null) // 1：A线程执行
            instance = new Instance(); // 2：B线程执行
        return instance;
    }
}
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public class SafeLazyInitialization {
    private static Instance instance;

    public synchronized static Instance getInstance() {
        if (instance == null)
            instance = new Instance();
        return instance;
    }
}
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public class DoubleCheckedLocking { // 1
    private static Instance instance; // 2
    public static Instance getInstance() { // 3
        if (instance == null) { // 4:第一次检查
            synchronized (DoubleCheckedLocking.class) { // 5:加锁
                if (instance == null) // 6:第二次检查
                    instance = new Instance(); // 7:问题的根源出在这里
            } // 8
        } // 9
        return instance; // 10
    } // 11
}
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public class SafeDoubleCheckedLocking {
    private volatile static Instance instance;

    public static Instance getInstance() {
        if (instance == null) {
            synchronized (SafeDoubleCheckedLocking.class) {
                if (instance == null)
                    instance = new Instance(); // instance为volatile，现在没问题了
            }
        }
        return instance;
    }
}
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public class InstanceFactory {
    private static class InstanceHolder {
        public static Instance instance = new Instance();
    }
    public static Instance getInstance() {
        return InstanceHolder.instance ; // 这里将导致InstanceHolder类被初始化
    }
}
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