背景

与公司外服务采用http交互，使用httpClient 4.5.2版本的默认重试策略DefaultHttpRequestRetryHandler，发现对方接口连接超时（3秒）后， 并不会重试 而是直接抛出ConnectTimeoutException，不符合预期。

结论先行

httpClient默认重试策略DefaultHttpRequestRetryHandler针对连接超时和获取数据超时并不会重试，需要自定义重试策略。

问题分析

详细场景

采用HttpClient 4.5.2发起http请求，代码如下，使用方式正常

//http设置，retryHandler采用组件提供默认的重试策略DefaultHttpRequestRetryHandler，重试次数为3
return HttpClients.custom()
                .setConnectionManager(****)
                .setDefaultRequestConfig(****)
                .setRetryHandler(new DefaultHttpRequestRetryHandler())
                .build();

//请求发起主要代码
HttpPost httpPost = new HttpPost(url);
httpPost.setEntity(new UrlEncodedFormEntity(nameValuePairs, UTF_8));
CloseableHttpResponse response = httpClient.execute(request);
HttpEntity entity = response.getEntity();
……其他关闭资源代码

请求后过了超时时间3s得到ConnectTimeoutException异常，且log中没有重试日志，堆栈如下：

Caused by: org.apache.http.conn.ConnectTimeoutException: Connect to www.XXX.com:443 [www.XXX.com/XX.1.XX.12] failed: connect timed out
	at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:150)
	at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.connect(PoolingHttpClientConnectionManager.java:353)
	at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:380)
	at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:236)
	at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:184)
	at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:88)
	at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110)
	at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:184)
	at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82)
	at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:107)
	at com.sankuai.meituan.resv.platform.utils.HttpUtil.getResult(HttpUtil.java:124)
	... 36 more
Caused by: java.net.SocketTimeoutException: connect timed out
	at java.net.PlainSocketImpl.socketConnect(Native Method)
	at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
	at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
	at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
	at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
	at java.net.Socket.connect(Socket.java:589)
	at org.apache.http.conn.ssl.SSLConnectionSocketFactory.connectSocket(SSLConnectionSocketFactory.java:337)
	at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:141)
	... 46 more

原因分析

出现这个问题，只好看默认重试策略的代码了，DefaultHttpRequestRetryHandler实现了HttpRequestRetryHandler，其中有这么一个方法public boolean retryRequest(final IOException exception,final int executionCount,final HttpContext context)，判断是否需要重试。

那么我们试图请求facebook（实验机器访问不了）来复现连接超时的情况，并进行断点调试，看这个方法的返回值。抛出的异常符合预期，即为ConnectTimeoutException，断点情况如下：

该方法的确返回了false，即不会重试。

代码分析

实例化DefaultHttpRequestRetryHandler时会初始化nonRetriableClasses，结果如上图圈红出所示，然后发现这个继承关系public class ConnectTimeoutException extends InterruptedIOException {
，所以进入if分支返回false。

举一反三读取数据超时

连接超时不会重试，那读取数据超时是否会重试呢，根据继承关系public class SocketTimeoutException extends java.io.InterruptedIOException 判断是不会重试的。我们通过测试来复现下。

复现方式：http请求本地某个服务，设置SocketTimeout为3s，在服务端打上断点，让client调用超时。 结果的确没有重试，异常堆栈如下，

Caused by: java.net.SocketTimeoutException: Read timed out
	at java.net.SocketInputStream.socketRead0(Native Method)
	at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
	at java.net.SocketInputStream.read(SocketInputStream.java:171)
	at java.net.SocketInputStream.read(SocketInputStream.java:141)
	at org.apache.http.impl.io.SessionInputBufferImpl.streamRead(SessionInputBufferImpl.java:137)
	at org.apache.http.impl.io.SessionInputBufferImpl.fillBuffer(SessionInputBufferImpl.java:153)
	at org.apache.http.impl.io.SessionInputBufferImpl.readLine(SessionInputBufferImpl.java:282)
	at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:140)
	at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:57)
	at org.apache.http.impl.io.AbstractMessageParser.parse(AbstractMessageParser.java:259)
	at org.apache.http.impl.DefaultBHttpClientConnection.receiveResponseHeader(DefaultBHttpClientConnection.java:163)
	at org.apache.http.impl.conn.CPoolProxy.receiveResponseHeader(CPoolProxy.java:167)
	at org.apache.http.protocol.HttpRequestExecutor.doReceiveResponse(HttpRequestExecutor.java:273)
	at org.apache.http.protocol.HttpRequestExecutor.execute(HttpRequestExecutor.java:125)
	at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:271)
	at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:184)
	at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:88)
	at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110)
	at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:184)
	at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82)
	at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:107)

解决方案

自定义重试策略，某些异常时让它返回true重试就好。

private class MyHttpRequestRetryHandler implements HttpRequestRetryHandler{

        @Override
        public boolean retryRequest(IOException exception, int executionCount, HttpContext context) {
            if (executionCount > retryTimes){
                return false;
            }
            if (exception instanceof InterruptedIOException
                    || exception instanceof NoHttpResponseException) {
                // Timeout or 服务端断开连接
                return true;
            }
            // Unknown host
            if (exception instanceof UnknownHostException) {
                return false;
            }
            // SSL handshake exception
            if (exception instanceof SSLException) {
                return false;
            }

            final HttpClientContext clientContext = HttpClientContext.adapt(context);
            final HttpRequest request = clientContext.getRequest();
            boolean idempotent = !(request instanceof HttpEntityEnclosingRequest);
            if (idempotent) {
                // Retry if the request is considered idempotent
                return true;
            }
            return false;
        }
    }

几个超时时间设置参数

    /** 从连接池获取连接超时时间 **/
    private int connectionRequestTimeout = 500;

    /** 连接一个url的连接等待时间,建立socket连接时间 **/
    private int connectTimeout = 3000;

    /** 传输数据等待返回的超时时间 **/
    private int socketTimeout = 3000;

反思总结

coding时认为默认重试策略遇到连接超时是会重试的，但实际没有进行这种异常case的测试，也没有看默认策略的代码实现，所以线上出现超时并没有重试的情况。

总结 ：为异常流程开发的处理方式，测试的时候虽然会麻烦些，但应该覆盖到，保证线上真正出现异常时恢复流程能正常work。