单例模式java

静态方法

// 这种方法，多线程下会有并发问题
public class SingletonDemo {
    private static SingletonDemo instance = null;
    private SingletonDemo() {}
    public static synchronized SingletonDemo getInstance() {
        if (instance == null) {
            instance = new SingletonDemo();
        }
        return instance;
    }
}

加上内置锁

// 这种方法的问题在于每次都需要同步，非常耗性能
public class SingletonDemo {
    private static SingletonDemo instance = null;
    private SingletonDemo() {}
    public static synchronized SingletonDemo getInstance() {
        synchronized (SingletonDemo.class) {
            if (instance == null) {
                instance = new SingletonDemo();
            }
        }
        return instance;
    }
}

再加一层null判断

// 这种就是最典型的double-check单例
// double-check依然存在一个问题，sync内置锁只保证了原子性和可见性，new操作会出现指令重排问题，当线程1出现先引用分配了内存时，线程2检查发现对象不为空，但此时线程1还没创建完对象，线程2拿到的是一个空对象返回处理了，会导致空异常。
public class SingletonDemo {
    private static SingletonDemo instance = null;
    private SingletonDemo() {}
    public static synchronized SingletonDemo getInstance() {
        if (instance == null) {
            synchronized (SingletonDemo.class) {
                if (instance == null) {
                    instance = new SingletonDemo();
                }
            }
        }
        return instance;
    }
}

加volatile关键字

// volatile 可以禁止指令重排
// 缺点，太复杂了
public class SingletonDemo {
    private volatile static SingletonDemo instance = null;
    private SingletonDemo() {}
    public static synchronized SingletonDemo getInstance() {
        if (instance == null) {
            synchronized (SingletonDemo.class) {
                if (instance == null) {
                    instance = new SingletonDemo();
                }
            }
        }
        return instance;
    }
}

类加载时就初始化

// 类加载时初始化就没有上面同步的问题
// 但这个私有构造函数可以通过反射序列化可以被调用，破坏了单例性
public class SingletonDemo {
    private static SingletonDemo instance = new SingletonDemo();
    private SingletonDemo() {}
    public static synchronized SingletonDemo getInstance() {
        return instance;
    }
}

控制反射序列化情况

// 添加readResolve方法防止反射问题
public class SingletonDemo {
    private static SingletonDemo instance = new SingletonDemo();
    private SingletonDemo() {}
    public static synchronized SingletonDemo getInstance() {
        return instance;
    }
    private Object readResolve() throws ObjectStreamException {    
        return instance;  
    }
}

静态内部类

// 现在使用最为普遍的一种方式，清晰简单
public class SingletonDemo {
    private SingletonDemo() {}
    private static class Holder {
        private static SingletonDemo instance = new SingletonDemo();
    }
    public static SingletonDemo getInstance() {
        return Holder.instance;
    }
}

枚举类型

// 单元素的枚举类型已经成为实现Singleton的最佳方法
// enum中的实例被保证只会被实例化一次
enum SingletonEnum {
    RESOURCE;
    private Resource instance;
    SingletonEnum() {
        instance = new Resource();
    }
    public Resource getInstance() {
        return instance;
    }
}
class Resource {
}
public class Test {
    public static void main(String[] args) {
        SingletonEnum.RESOURCE.getInstance();
    }
}