如何为外部库类创建单例对象？是否有更优实现方式？

你的当前实现是懒加载+同步方法的单例模式，问题在于每次调用getInstance()都要获取锁，高并发场景下会有性能损耗。下面是几种更优的实现方式：

1. 双重检查锁定（Double-Checked Locking） #

通过两次判空+volatile关键字，既保证懒加载，又避免不必要的同步开销：

import com.amazonaws.auth.AWSCredentialsProvider;
import com.amazonaws.auth.DefaultAWSCredentialsProviderChain;

public class AWSCredentialsProviderSingleton {
    // volatile防止指令重排序，确保实例初始化完成后再被其他线程可见
    private static volatile AWSCredentialsProvider credentials;

    public static AWSCredentialsProvider getInstance() {
        if (credentials == null) {
            synchronized (AWSCredentialsProviderSingleton.class) {
                if (credentials == null) {
                    credentials = new DefaultAWSCredentialsProviderChain();
                }
            }
        }
        return credentials;
    }
}

2. 静态内部类实现 #

利用Java类加载的懒加载特性（内部类只有在被引用时才会加载），天然线程安全，代码更简洁：

import com.amazonaws.auth.AWSCredentialsProvider;
import com.amazonaws.auth.DefaultAWSCredentialsProviderChain;

public class AWSCredentialsProviderSingleton {
    private static class Holder {
        private static final AWSCredentialsProvider INSTANCE = new DefaultAWSCredentialsProviderChain();
    }

    public static AWSCredentialsProvider getInstance() {
        return Holder.INSTANCE;
    }
}

3. 枚举单例 #

这是最简洁且安全的单例实现，天然支持线程安全，还能防止序列化/反射破坏单例：

import com.amazonaws.auth.AWSCredentialsProvider;
import com.amazonaws.auth.DefaultAWSCredentialsProviderChain;

public enum AWSCredentialsProviderSingleton {
    INSTANCE;

    private final AWSCredentialsProvider credentials;

    AWSCredentialsProviderSingleton() {
        credentials = new DefaultAWSCredentialsProviderChain();
    }

    public AWSCredentialsProvider getCredentials() {
        return credentials;
    }
}
// 使用方式：AWSCredentialsProviderSingleton.INSTANCE.getCredentials();

额外说明 #

DefaultAWSCredentialsProviderChain本身是线程安全的，所以我们的核心目标只是确保它只被初始化一次，上面三种方案都能满足需求。另外原类名GetAWSCredentials不够贴合职责，建议改成更清晰的命名（比如上面示例中的AWSCredentialsProviderSingleton）。

内容的提问来源于stack exchange，提问作者lakshmiravali rimmalapudi