在进行http请求时，难免会遇到请求失败的情况，失败后需要重新请求，尝试再次获取数据。
Apache的HttpClient提供了异常重试机制，在该机制中，我们可以很灵活的定义在哪些异常情况下进行重试。
重试前提
被请求的方法必须是幂等的：就是多次请求服务端结果应该是准确且一致的。
适合的方法：比如根据ID，修改人员姓名，无论请求多次结果都是一样，这就是幂等。
不适合的方法：比如减少账号50元，多次请求将多次扣减。
实现方式
想要实现异常重试，需要实现它提供的一个接口 HttpRequestRetryHandler ，并实现 retryRequest 方法。然后set到HttpClient中。该httpClient就具备了重试机制。
HttpClient自身提供了 StandardHttpRequestRetryHandler 和 DefaultHttpRequestRetryHandler 两个实现类。 DefaultHttpRequestRetryHandler 继承自 DefaultHttpRequestRetryHandler ， StandardHttpRequestRetryHandler 默认几个方法为幂等，如PUT、GET、HEAD等除POST外的方法，如果自定义可以参考它的实现方式。
代码如下：
import java.io.IOException;
import java.io.InterruptedIOException;
import java.net.ConnectException;
import java.net.UnknownHostException; import javax.net.ssl.SSLException; import org.apache.commons.lang3.ObjectUtils;
import org.apache.commons.lang3.StringUtils;
import org.apache.http.Consts;
import org.apache.http.HttpEntityEnclosingRequest;
import org.apache.http.HttpRequest;
import org.apache.http.HttpStatus;
import org.apache.http.ParseException;
import org.apache.http.client.HttpRequestRetryHandler;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.client.protocol.HttpClientContext;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.protocol.HttpContext;
import org.apache.http.util.EntityUtils; /**
* @author *
* @date 2017年5月18日 上午9:17:30
*
* @Description
*/
public class HttpPostUtils {
/**
*
* @param uri
* the request address
* @param json
* the request data that must be a JSON string
* @param retryCount
* the number of times this method has been unsuccessfully
* executed
* @param connectTimeout
* the timeout in milliseconds until a connection is established
* @param connectionRequestTimeout
* the timeout in milliseconds used when requesting a connection
* from the connection manager
* @param socketTimeout
* the socket timeout in milliseconds, which is the timeout for
* waiting for data or, put differently, a maximum period
* inactivity between two consecutive data packets
* @return null when method parameter is null, "", " "
* @throws IOException
* if HTTP connection can not opened or closed successfully
* @throws ParseException
* if response data can not be parsed successfully
*/
public String retryPostJson(String uri, String json, int retryCount, int connectTimeout,
int connectionRequestTimeout, int socketTimeout) throws IOException, ParseException {
if (StringUtils.isAnyBlank(uri, json)) {
return null;
} HttpRequestRetryHandler httpRequestRetryHandler = new HttpRequestRetryHandler() { @Override
public boolean retryRequest(IOException exception, int executionCount, HttpContext context) {
if (executionCount > retryCount) {
// Do not retry if over max retry count
return false;
}
if (exception instanceof InterruptedIOException) {
// An input or output transfer has been terminated
return false;
}
if (exception instanceof UnknownHostException) {
// Unknown host 修改代码让不识别主机时重试，实际业务当不识别的时候不应该重试，再次为了演示重试过程，执行会显示retryCount次下面的输出
System.out.println("不识别主机重试"); return true;
}
if (exception instanceof ConnectException) {
// Connection refused
return false;
}
if (exception instanceof SSLException) {
// SSL handshake exception
return false;
}
HttpClientContext clientContext = HttpClientContext.adapt(context);
HttpRequest request = clientContext.getRequest();
boolean idempotent = !(request instanceof HttpEntityEnclosingRequest);
if (idempotent) {
// Retry if the request is considered idempotent
return true;
}
return false;
}
}; CloseableHttpClient client = HttpClients.custom().setRetryHandler(httpRequestRetryHandler).build();
HttpPost post = new HttpPost(uri);
// Create request data
StringEntity entity = new StringEntity(json, ContentType.APPLICATION_JSON);
// Set request body
post.setEntity(entity); RequestConfig config = RequestConfig.custom().setConnectTimeout(connectTimeout)
.setConnectionRequestTimeout(connectionRequestTimeout).setSocketTimeout(socketTimeout).build();
post.setConfig(config);
// Response content
String responseContent = null;
CloseableHttpResponse response = null;
try {
response = client.execute(post, HttpClientContext.create());
if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
responseContent = EntityUtils.toString(response.getEntity(), Consts.UTF_8.name());
}
} finally {
if (ObjectUtils.anyNotNull(response)) {
response.close();
}
if (ObjectUtils.anyNotNull(client)) {
client.close();
}
}
return responseContent;
}
在实现的 retryRequest 方法中，遇到不识别主机异常，返回 true ，请求将重试。最多重试请求retryCount次。