Java单例与同步

Question

• 在多线程环境中用Java实现单例的最佳方法是什么？
• 当多个线程同时尝试访问getinstance()方法时会发生什么？
• 我们可以让Singleton的getinstance（）同步化吗？
• 当使用单例类时，真的需要同步吗？

Answer 1

是的，这是必要的。您可以使用以下几种方法通过惰性初始化实现线程安全：

严格的同步：

private static YourObject instance;

public static synchronized YourObject getInstance() {
    if (instance == null) {
        instance = new YourObject();
    }
    return instance;
}

这个解决方案要求每个线程都要同步，而实际上只需要前几个线程同步。

private static final Object lock = new Object();
private static volatile YourObject instance;

public static YourObject getInstance() {
    YourObject r = instance;
    if (r == null) {
        synchronized (lock) {    // While we were waiting for the lock, another 
            r = instance;        // thread may have instantiated the object.
            if (r == null) {  
                r = new YourObject();
                instance = r;
            }
        }
    }
    return r;
}

private static class InstanceHolder {
    private static final YourObject instance = new YourObject();
}

public static YourObject getInstance() {
    return InstanceHolder.instance;
}