聊聊HttpClient的重试机制

序

本文主要研究一下HttpClient的重试机制

HttpRequestRetryHandler

org/apache/http/client/HttpRequestRetryHandler.java

public interface HttpRequestRetryHandler {

    /**
     * Determines if a method should be retried after an IOException
     * occurs during execution.
     *
     * @param exception the exception that occurred
     * @param executionCount the number of times this method has been
     * unsuccessfully executed
     * @param context the context for the request execution
     *
     * @return {@code true} if the method should be retried, {@code false}
     * otherwise
     */
    boolean retryRequest(IOException exception, int executionCount, HttpContext context);

}

HttpRequestRetryHandler接口定义了retryRequest方法，它接收IOException、executionCount及context，然后判断是否可以重试

DefaultHttpRequestRetryHandler

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

@Contract(threading = ThreadingBehavior.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;
        }
        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());
    }

}

DefaultHttpRequestRetryHandler实现了HttpRequestRetryHandler接口，其无参构造器默认将InterruptedIOException、UnknownHostException、ConnectException、SSLException设定为不重试的异常，默认retryCount为3，requestSentRetryEnabled为false；其retryRequest方法先判断executionCount是否超出retryCount，接着判断异常类型是否是不重试的异常类型，若request为aborted则返回false，若request非HttpEntityEnclosingRequest则表示幂等请求，返回true，若请求未完全发送则返回true，其余的默认返回false。

RetryExec

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

@Contract(threading = ThreadingBehavior.IMMUTABLE_CONDITIONAL)
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;
                    }
                    throw ex;
                }
            }
        }
    }

}

RetryExec实现了ClientExecChain接口，其execute方法会循环执行requestExecutor.execute，它catch了IOException，对于retryHandler.retryRequest(ex, execCount, context)返回true的，会在通过RequestEntityProxy.isRepeatable(request)判断一下是否是可重复读取的request，不是则抛出NonRepeatableRequestException；对于retryHandler.retryRequest返回false的则针对NoHttpResponseException重新包装一下，将targetHost体现在message里头然后重新抛出

HttpEntityEnclosingRequest

org/apache/http/HttpEntityEnclosingRequest.java

public interface HttpEntityEnclosingRequest extends HttpRequest {

    /**
     * Tells if this request should use the expect-continue handshake.
     * The expect continue handshake gives the server a chance to decide
     * whether to accept the entity enclosing request before the possibly
     * lengthy entity is sent across the wire.
     * @return true if the expect continue handshake should be used, false if
     * not.
     */
    boolean expectContinue();

    /**
     * Associates the entity with this request.
     *
     * @param entity the entity to send.
     */
    void setEntity(HttpEntity entity);

    /**
     * Returns the entity associated with this request.
     *
     * @return entity
     */
    HttpEntity getEntity();

}

HttpEntityEnclosingRequest定义了getEntity、setEntity、expectContinue方法，它的子类有HttpPut、HttpPost、HttpPatch、HttpDelete等

RequestEntityProxy.isRepeatable

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

class RequestEntityProxy implements HttpEntity  {

    private final HttpEntity original;
    private boolean consumed = false;

    public boolean isConsumed() {
        return consumed;
    }

    static boolean isRepeatable(final HttpRequest request) {
        if (request instanceof HttpEntityEnclosingRequest) {
            final HttpEntity entity = ((HttpEntityEnclosingRequest) request).getEntity();
            if (entity != null) {
                if (isEnhanced(entity)) {
                    final RequestEntityProxy proxy = (RequestEntityProxy) entity;
                    if (!proxy.isConsumed()) {
                        return true;
                    }
                }
                return entity.isRepeatable();
            }
        }
        return true;
    }
}

RequestEntityProxy提供了静态方法isRepeatable用于判断该request的entity是否可以重复读取，对于非HttpEntityEnclosingRequest的返回true，是HttpEntityEnclosingRequest类型的话则判断entity.isRepeatable()，若entity是RequestEntityProxy类型的，则通过RequestEntityProxy.isConsumed来判断

entity.isRepeatable()

public interface HttpEntity {

    /**
     * Tells if the entity is capable of producing its data more than once.
     * A repeatable entity's getContent() and writeTo(OutputStream) methods
     * can be called more than once whereas a non-repeatable entity's can not.
     * @return true if the entity is repeatable, false otherwise.
     */
    boolean isRepeatable();

    //......
}    

HttpEntity接口定义了isRepeatable方法，用于表示entity的content及OutputStream是否可以读写多次。其实现类里头，BufferedHttpEntity、ByteArrayEntity、EntityTemplate、FileEntity、SerializableEntity、StringEntity为true，BasicHttpEntity、InputStreamEntity、StreamingHttpEntity为false

小结

HttpRequestRetryHandler接口定义了retryRequest方法，它接收IOException、executionCount及context，然后判断是否可以重试

DefaultHttpRequestRetryHandler实现了HttpRequestRetryHandler接口，其无参构造器默认将InterruptedIOException、UnknownHostException、ConnectException、SSLException设定为不重试的异常，默认retryCount为3，requestSentRetryEnabled为false；其retryRequest方法先判断executionCount是否超出retryCount，接着判断异常类型是否是不重试的异常类型，若request为aborted则返回false，若request非HttpEntityEnclosingRequest则表示幂等请求，返回true，若请求未完全发送则返回true，其余的默认返回false。

RetryExec实现了ClientExecChain接口，其execute方法会循环执行requestExecutor.execute，它catch了IOException，对于retryHandler.retryRequest(ex, execCount, context)返回true的，会在通过RequestEntityProxy.isRepeatable(request)判断一下是否是可重复读取的request，不是则抛出NonRepeatableRequestException

DefaultHttpRequestRetryHandler针对不是幂等请求的HttpEntityEnclosingRequest类型(HttpPut、HttpPost、HttpPatch、HttpDelete)，不会重试；若retryHandler.retryRequest返回可以重试，RetryExec还有一个repeatable的判断，BufferedHttpEntity、ByteArrayEntity、EntityTemplate、FileEntity、SerializableEntity、StringEntity为true，BasicHttpEntity、InputStreamEntity、StreamingHttpEntity为false