在看微信支付的SDK時猜欺，看到里面有一個單例模式實現(xiàn)采用了內(nèi)部類的方式，于是查了一下拷窜，發(fā)現(xiàn)這是一種線程安全的單例實現(xiàn)模式开皿，查詢網(wǎng)上資料說，大部分單例都會有多線程環(huán)境下的問題篮昧，使用內(nèi)部類可以避免這個問題赋荆，因為在多線程環(huán)境下，jvm對一個類的初始化會做限制懊昨，同一時間只會允許一個線程去初始化一個類窄潭，這樣就從虛擬機(jī)層面避免了大部分單例實現(xiàn)的問題。

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>();
}

另外還有一種單例的寫法

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;
    }
}