Apache’s HttpClient’s default retry mechanism

  java

maven

        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpclient</artifactId>
            <version>4.5.2</version>
        </dependency>

Exception retry log

2017-01-31 19:31:39.057  INFO 3873 --- [askScheduler-13] o.apache.http.impl.execchain.RetryExec   : I/O exception (org.apache.http.NoHttpResponseException) caught when processing request to {}->http://192.168.99.100:8080: The target server failed to respond
2017-01-31 19:31:39.058  INFO 3873 --- [askScheduler-13] o.apache.http.impl.execchain.RetryExec   : Retrying request to {}->http://192.168.99.100:8080

RetryExec

org/apache/http/impl/execchain/RetryExec.java

/**
 * Request executor in the request execution chain that is responsible
 * for making a decision whether a request failed due to an I/O error
 * should be re-executed.
 * <p>
 * Further responsibilities such as communication with the opposite
 * endpoint is delegated to the next executor in the request execution
 * chain.
 * </p>
 *
 * @since 4.3
 */
@Immutable
public class RetryExec implements ClientExecChain {

    private final Log log = LogFactory.getLog(getClass());

    private final ClientExecChain requestExecutor;
    private final HttpRequestRetryHandler retryHandler;

    public RetryExec(
            final ClientExecChain requestExecutor,
            final HttpRequestRetryHandler retryHandler) {
        Args.notNull(requestExecutor, "HTTP request executor");
        Args.notNull(retryHandler, "HTTP request retry handler");
        this.requestExecutor = requestExecutor;
        this.retryHandler = retryHandler;
    }

    @Override
    public CloseableHttpResponse execute(
            final HttpRoute route,
            final HttpRequestWrapper request,
            final HttpClientContext context,
            final HttpExecutionAware execAware) throws IOException, HttpException {
        Args.notNull(route, "HTTP route");
        Args.notNull(request, "HTTP request");
        Args.notNull(context, "HTTP context");
        final Header[] origheaders = request.getAllHeaders();
        for (int execCount = 1;; execCount++) {
            try {
                return this.requestExecutor.execute(route, request, context, execAware);
            } catch (final IOException ex) {
                if (execAware != null && execAware.isAborted()) {
                    this.log.debug("Request has been aborted");
                    throw ex;
                }
                if (retryHandler.retryRequest(ex, execCount, context)) {
                    if (this.log.isInfoEnabled()) {
                        this.log.info("I/O exception ("+ ex.getClass().getName() +
                                ") caught when processing request to "
                                + route +
                                ": "
                                + ex.getMessage());
                    }
                    if (this.log.isDebugEnabled()) {
                        this.log.debug(ex.getMessage(), ex);
                    }
                    if (!RequestEntityProxy.isRepeatable(request)) {
                        this.log.debug("Cannot retry non-repeatable request");
                        throw new NonRepeatableRequestException("Cannot retry request " +
                                "with a non-repeatable request entity", ex);
                    }
                    request.setHeaders(origheaders);
                    if (this.log.isInfoEnabled()) {
                        this.log.info("Retrying request to " + route);
                    }
                } else {
                    if (ex instanceof NoHttpResponseException) {
                        final NoHttpResponseException updatedex = new NoHttpResponseException(
                                route.getTargetHost().toHostString() + " failed to respond");
                        updatedex.setStackTrace(ex.getStackTrace());
                        throw updatedex;
                    } else {
                        throw ex;
                    }
                }
            }
        }
    }

}

DefaultHttpRequestRetryHandler

org/apache/http/impl/client/DefaultHttpRequestRetryHandler.java

/**
 * The default {@link HttpRequestRetryHandler} used by request executors.
 *
 * @since 4.0
 */
@Immutable
public class DefaultHttpRequestRetryHandler implements HttpRequestRetryHandler {

    public static final DefaultHttpRequestRetryHandler INSTANCE = new DefaultHttpRequestRetryHandler();

    /** the number of times a method will be retried */
    private final int retryCount;

    /** Whether or not methods that have successfully sent their request will be retried */
    private final boolean requestSentRetryEnabled;

    private final Set<Class<? extends IOException>> nonRetriableClasses;

    /**
     * Create the request retry handler using the specified IOException classes
     *
     * @param retryCount how many times to retry; 0 means no retries
     * @param requestSentRetryEnabled true if it's OK to retry requests that have been sent
     * @param clazzes the IOException types that should not be retried
     * @since 4.3
     */
    protected DefaultHttpRequestRetryHandler(
            final int retryCount,
            final boolean requestSentRetryEnabled,
            final Collection<Class<? extends IOException>> clazzes) {
        super();
        this.retryCount = retryCount;
        this.requestSentRetryEnabled = requestSentRetryEnabled;
        this.nonRetriableClasses = new HashSet<Class<? extends IOException>>();
        for (final Class<? extends IOException> clazz: clazzes) {
            this.nonRetriableClasses.add(clazz);
        }
    }

    /**
     * Create the request retry handler using the following list of
     * non-retriable IOException classes: <br>
     * <ul>
     * <li>InterruptedIOException</li>
     * <li>UnknownHostException</li>
     * <li>ConnectException</li>
     * <li>SSLException</li>
     * </ul>
     * @param retryCount how many times to retry; 0 means no retries
     * @param requestSentRetryEnabled true if it's OK to retry non-idempotent requests that have been sent
     */
    @SuppressWarnings("unchecked")
    public DefaultHttpRequestRetryHandler(final int retryCount, final boolean requestSentRetryEnabled) {
        this(retryCount, requestSentRetryEnabled, Arrays.asList(
                InterruptedIOException.class,
                UnknownHostException.class,
                ConnectException.class,
                SSLException.class));
    }

    /**
     * Create the request retry handler with a retry count of 3, requestSentRetryEnabled false
     * and using the following list of non-retriable IOException classes: <br>
     * <ul>
     * <li>InterruptedIOException</li>
     * <li>UnknownHostException</li>
     * <li>ConnectException</li>
     * <li>SSLException</li>
     * </ul>
     */
    public DefaultHttpRequestRetryHandler() {
        this(3, false);
    }
    /**
     * Used {@code retryCount} and {@code requestSentRetryEnabled} to determine
     * if the given method should be retried.
     */
    @Override
    public boolean retryRequest(
            final IOException exception,
            final int executionCount,
            final HttpContext context) {
        Args.notNull(exception, "Exception parameter");
        Args.notNull(context, "HTTP context");
        if (executionCount > this.retryCount) {
            // Do not retry if over max retry count
            return false;
        }
        if (this.nonRetriableClasses.contains(exception.getClass())) {
            return false;
        } else {
            for (final Class<? extends IOException> rejectException : this.nonRetriableClasses) {
                if (rejectException.isInstance(exception)) {
                    return false;
                }
            }
        }
        final HttpClientContext clientContext = HttpClientContext.adapt(context);
        final HttpRequest request = clientContext.getRequest();

        if(requestIsAborted(request)){
            return false;
        }

        if (handleAsIdempotent(request)) {
            // Retry if the request is considered idempotent
            return true;
        }

        if (!clientContext.isRequestSent() || this.requestSentRetryEnabled) {
            // Retry if the request has not been sent fully or
            // if it's OK to retry methods that have been sent
            return true;
        }
        // otherwise do not retry
        return false;
    }

    /**
     * @return {@code true} if this handler will retry methods that have
     * successfully sent their request, {@code false} otherwise
     */
    public boolean isRequestSentRetryEnabled() {
        return requestSentRetryEnabled;
    }

    /**
     * @return the maximum number of times a method will be retried
     */
    public int getRetryCount() {
        return retryCount;
    }

    /**
     * @since 4.2
     */
    protected boolean handleAsIdempotent(final HttpRequest request) {
        return !(request instanceof HttpEntityEnclosingRequest);
    }

    /**
     * @since 4.2
     *
     * @deprecated (4.3)
     */
    @Deprecated
    protected boolean requestIsAborted(final HttpRequest request) {
        HttpRequest req = request;
        if (request instanceof RequestWrapper) { // does not forward request to original
            req = ((RequestWrapper) request).getOriginal();
        }
        return (req instanceof HttpUriRequest && ((HttpUriRequest)req).isAborted());
    }

}

The default retry is 3 times, and if all three times fail, NoHttpResponseException or other exceptions are thrown.

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