feign中的Retryer有什么作用

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本文主要研究一下feign的Retryer

Retryer

feign-core-10.2.3-sources.jar!/feign/Retryer.java

public interface Retryer extends Cloneable {  /**   * if retry is permitted, return (possibly after sleeping). Otherwise propagate the exception.   */  void continueOrPropagate(RetryableException e);  Retryer clone();  class Default implements Retryer {    private final int maxAttempts;    private final long period;    private final long maxPeriod;    int attempt;    long sleptForMillis;    public Default() {      this(100, SECONDS.toMillis(1), 5);    }    public Default(long period, long maxPeriod, int maxAttempts) {      this.period = period;      this.maxPeriod = maxPeriod;      this.maxAttempts = maxAttempts;      this.attempt = 1;    }    // visible for testing;    protected long currentTimeMillis() {      return System.currentTimeMillis();    }    public void continueOrPropagate(RetryableException e) {      if (attempt++ >= maxAttempts) {        throw e;      }      long interval;      if (e.retryAfter() != null) {        interval = e.retryAfter().getTime() - currentTimeMillis();        if (interval > maxPeriod) {          interval = maxPeriod;        }        if (interval < 0) {          return;        }      } else {        interval = nextMaxInterval();      }      try {        Thread.sleep(interval);      } catch (InterruptedException ignored) {        Thread.currentThread().interrupt();        throw e;      }      sleptForMillis += interval;    }    /**     * Calculates the time interval to a retry attempt. 
     * The interval increases exponentially with each attempt, at a rate of nextInterval *= 1.5     * (where 1.5 is the backoff factor), to the maximum interval.     *     * @return time in nanoseconds from now until the next attempt.     */    long nextMaxInterval() {      long interval = (long) (period * Math.pow(1.5, attempt - 1));      return interval > maxPeriod ? maxPeriod : interval;    }    @Override    public Retryer clone() {      return new Default(period, maxPeriod, maxAttempts);    }  }  /**   * Implementation that never retries request. It propagates the RetryableException.   */  Retryer NEVER_RETRY = new Retryer() {    @Override    public void continueOrPropagate(RetryableException e) {      throw e;    }    @Override    public Retryer clone() {      return this;    }  };}

• Retryer继承了Cloneable接口，它定义了continueOrPropagate、clone方法；它内置了一个名为Default以及名为NEVER_RETRY的实现

• Default有period、maxPeriod、maxAttempts参数可以设置，默认构造器使用的period为100，maxPeriod为1000，maxAttempts为5；continueOrPropagate方法首先判断attempt是否达到阈值，达到则抛出异常，否则进一步计算interval，然后进行sleep

• NEVER_RETRY的continueOrPropagate直接抛出异常，而clone方法直接返回当前实例

SynchronousMethodHandler

feign-core-10.2.3-sources.jar!/feign/SynchronousMethodHandler.java

final class SynchronousMethodHandler implements MethodHandler {        //......  public Object invoke(Object[] argv) throws Throwable {    RequestTemplate template = buildTemplateFromArgs.create(argv);    Retryer retryer = this.retryer.clone();    while (true) {      try {        return executeAndDecode(template);      } catch (RetryableException e) {        try {          retryer.continueOrPropagate(e);        } catch (RetryableException th) {          Throwable cause = th.getCause();          if (propagationPolicy == UNWRAP && cause != null) {            throw cause;          } else {            throw th;          }        }        if (logLevel != Logger.Level.NONE) {          logger.logRetry(metadata.configKey(), logLevel);        }        continue;      }    }  }        //......}

• SynchronousMethodHandler的invoke的方法首先使用retryer.clone()创建一个retryer，然后在捕获到RetryableException的时候，会执行retryer.continueOrPropagate(e)

RetryableException

feign-core-10.2.3-sources.jar!/feign/RetryableException.java

public class RetryableException extends FeignException {  private static final long serialVersionUID = 1L;  private final Long retryAfter;  private final HttpMethod httpMethod;  /**   * @param retryAfter usually corresponds to the {@link feign.Util#RETRY_AFTER} header.   */  public RetryableException(int status, String message, HttpMethod httpMethod, Throwable cause,      Date retryAfter) {    super(status, message, cause);    this.httpMethod = httpMethod;    this.retryAfter = retryAfter != null ? retryAfter.getTime() : null;  }  /**   * @param retryAfter usually corresponds to the {@link feign.Util#RETRY_AFTER} header.   */  public RetryableException(int status, String message, HttpMethod httpMethod, Date retryAfter) {    super(status, message);    this.httpMethod = httpMethod;    this.retryAfter = retryAfter != null ? retryAfter.getTime() : null;  }  /**   * Sometimes corresponds to the {@link feign.Util#RETRY_AFTER} header present in {@code 503}   * status. Other times parsed from an application-specific response. Null if unknown.   */  public Date retryAfter() {    return retryAfter != null ? new Date(retryAfter) : null;  }  public HttpMethod method() {    return this.httpMethod;  }}

• RetryableException继承了FeignException，它的构造器会接收retryAfter，该参数可以为null

FeignException

feign-core-10.2.3-sources.jar!/feign/FeignException.java

public class FeignException extends RuntimeException {        //......  static FeignException errorReading(Request request, Response response, IOException cause) {    return new FeignException(        response.status(),        format("%s reading %s %s", cause.getMessage(), request.httpMethod(), request.url()),        cause,        request.requestBody().asBytes());  }        //......}

• FeignException定义了errorReading静态方法，它创建的是FeignException

ErrorDecoder

feign-core-10.2.3-sources.jar!/feign/codec/ErrorDecoder.java

public interface ErrorDecoder {        //......  public static class Default implements ErrorDecoder {    private final RetryAfterDecoder retryAfterDecoder = new RetryAfterDecoder();    @Override    public Exception decode(String methodKey, Response response) {      FeignException exception = errorStatus(methodKey, response);      Date retryAfter = retryAfterDecoder.apply(firstOrNull(response.headers(), RETRY_AFTER));      if (retryAfter != null) {        return new RetryableException(            response.status(),            exception.getMessage(),            response.request().httpMethod(),            exception,            retryAfter);      }      return exception;    }    private  T firstOrNull(Map> map, String key) {      if (map.containsKey(key) && !map.get(key).isEmpty()) {        return map.get(key).iterator().next();      }      return null;    }  }  static class RetryAfterDecoder {    static final DateFormat RFC822_FORMAT =        new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss 'GMT'", US);    private final DateFormat rfc822Format;    RetryAfterDecoder() {      this(RFC822_FORMAT);    }    RetryAfterDecoder(DateFormat rfc822Format) {      this.rfc822Format = checkNotNull(rfc822Format, "rfc822Format");    }    protected long currentTimeMillis() {      return System.currentTimeMillis();    }    /**     * returns a date that corresponds to the first time a request can be retried.     *     * @param retryAfter String in     *        Retry-After format     */    public Date apply(String retryAfter) {      if (retryAfter == null) {        return null;      }      if (retryAfter.matches("^[0-9]+$")) {        long deltaMillis = SECONDS.toMillis(Long.parseLong(retryAfter));        return new Date(currentTimeMillis() + deltaMillis);      }      synchronized (rfc822Format) {        try {          return rfc822Format.parse(retryAfter);        } catch (ParseException ignored) {          return null;        }      }    }  }        //......}

• ErrorDecoder提供了Default的默认实现，其decode方法会使用RetryAfterDecoder来计算retryAfter值，在该值不为null时会返回RetryableException；RetryAfterDecoder的apply方法会根据retryAfter来计算retryAfter日期，该retryAfter参数是从response的名为Retry-After的header读取而来

小结

• Retryer继承了Cloneable接口，它定义了continueOrPropagate、clone方法；它内置了一个名为Default以及名为NEVER_RETRY的实现

• Default有period、maxPeriod、maxAttempts参数可以设置，默认构造器使用的period为100，maxPeriod为1000，maxAttempts为5；continueOrPropagate方法首先判断attempt是否达到阈值，达到则抛出异常，否则进一步计算interval，然后进行sleep

• NEVER_RETRY的continueOrPropagate直接抛出异常，而clone方法直接返回当前实例

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