在看微信支付的SDK时，看到里面有一个单例模式实现采用了内部类的方式，于是查了一下，发现这是一种线程安全的单例实现模式，查询网上资料说，大部分单例都会有多线程环境下的问题，使用内部类可以避免这个问题，因为在多线程环境下，jvm对一个类的初始化会做限制，同一时间只会允许一个线程去初始化一个类，这样就从虚拟机层面避免了大部分单例实现的问题。

public class WXPayDomainSimpleImpl implements IWXPayDomain {
    private WXPayDomainSimpleImpl(){}
    private static class WxpayDomainHolder{
        private static IWXPayDomain holder = new WXPayDomainSimpleImpl();
    }
    public static IWXPayDomain instance(){
        return WxpayDomainHolder.holder;
    }

    public synchronized void report(final String domain, long elapsedTimeMillis, final Exception ex) {
        DomainStatics info = domainData.get(domain);
        if(info == null){
            info = new DomainStatics(domain);
            domainData.put(domain, info);
        }

        if(ex == null){ //success
            if(info.succCount >= 2){    //continue succ, clear error count
                info.connectTimeoutCount = info.dnsErrorCount = info.otherErrorCount = 0;
            }else{
                ++info.succCount;
            }
        }else if(ex instanceof ConnectTimeoutException){
            info.succCount = info.dnsErrorCount = 0;
            ++info.connectTimeoutCount;
        }else if(ex instanceof UnknownHostException){
            info.succCount = 0;
            ++info.dnsErrorCount;
        }else{
            info.succCount = 0;
            ++info.otherErrorCount;
        }
    }

    public synchronized DomainInfo getDomain(final WXPayConfig config) {
        DomainStatics primaryDomain = domainData.get(WXPayConstants.DOMAIN_API);
        if(primaryDomain == null ||
                primaryDomain.isGood()) {
            return new DomainInfo(WXPayConstants.DOMAIN_API, true);
        }

        long now = System.currentTimeMillis();
        if(switchToAlternateDomainTime == 0){   //first switch
            switchToAlternateDomainTime = now;
            return new DomainInfo(WXPayConstants.DOMAIN_API2, false);
        }else if(now - switchToAlternateDomainTime < MIN_SWITCH_PRIMARY_MSEC){
            DomainStatics alternateDomain = domainData.get(WXPayConstants.DOMAIN_API2);
            if(alternateDomain == null ||
                alternateDomain.isGood() ||
                alternateDomain.badCount() < primaryDomain.badCount()){
                return new DomainInfo(WXPayConstants.DOMAIN_API2, false);
            }else{
                return new DomainInfo(WXPayConstants.DOMAIN_API, true);
            }
        }else{  //force switch back
            switchToAlternateDomainTime = 0;
            primaryDomain.resetCount();
            DomainStatics alternateDomain = domainData.get(WXPayConstants.DOMAIN_API2);
            if(alternateDomain != null)
                alternateDomain.resetCount();
            return new DomainInfo(WXPayConstants.DOMAIN_API, true);
        }
    }

    static class DomainStatics {
        final String domain;
        int succCount = 0;
        int connectTimeoutCount = 0;
        int dnsErrorCount =0;
        int otherErrorCount = 0;

        DomainStatics(String domain) {
            this.domain = domain;
        }
        void resetCount(){
            succCount = connectTimeoutCount = dnsErrorCount = otherErrorCount = 0;
        }
        boolean isGood(){ return connectTimeoutCount <= 2 && dnsErrorCount <= 2; }
        int badCount(){
            return connectTimeoutCount + dnsErrorCount * 5 + otherErrorCount / 4;
        }
    }
    private final int MIN_SWITCH_PRIMARY_MSEC = 3 * 60 * 1000;  //3 minutes
    private long switchToAlternateDomainTime = 0;
    private Map<String, DomainStatics> domainData = new HashMap<String, DomainStatics>();
}

另外还有一种单例的写法

public class WXPayConfigImpl extends WXPayConfig{

    private byte[] certData;
    private static WXPayConfigImpl INSTANCE;

    private WXPayConfigImpl() throws Exception{
        String certPath = "D://CERT/common/apiclient_cert.p12";
        File file = new File(certPath);
        InputStream certStream = new FileInputStream(file);
        this.certData = new byte[(int) file.length()];
        certStream.read(this.certData);
        certStream.close();
    }

    public static WXPayConfigImpl getInstance() throws Exception{
        if (INSTANCE == null) {
            synchronized (WXPayConfigImpl.class) {
                if (INSTANCE == null) {
                    INSTANCE = new WXPayConfigImpl();
                }
            }
        }
        return INSTANCE;
    }
}