java 并发实践 16.7

Question

此代码用于演示双重检查锁定反模式:

@NotThreadSafe
public class DoubleCheckedLocking {
    private static Resource resource;

    public static Resource getInstance() {
        if (resource == null) {
            synchronized (DoubleCheckedLocking.class) {
                if (resource == null)
                    resource = new Resource();
            }
        }
        return resource;
    }
}

我可以通过修改它来避免这个问题吗:

@NotThreadSafe
public class DoubleCheckedLocking {
    private static Resource resource;

    public static Resource getInstance() {
        if (resource == null) {
            synchronized (DoubleCheckedLocking.class) {
                if (resource == null){
                    Resource r=new Resource();
                    resource = r;
                }
            }
        }
        return resource;
    }
}

据我所知，

Resource r=new Resource();
resource = r;

该编译器应该为此提供先发生关系。

Answer 1

据我所知，唯一已知的双重检查锁定模式(对于 JDK5 及更高版本)有效的实现使用了“volatile”关键字。 See Fixing Double-Checked Locking using Volatile