本文深入解析了Volley框架中的Request类，介绍了其属性、构造方法、重试策略及实现细节。并通过StringRequest实例展示了如何继承Request类以实现具体的网络请求。
源码：
1 /* 2 * Copyright (C) 2011 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.android.volley; 18 19 import android.net.TrafficStats; 20 import android.net.Uri; 21 import android.os.Handler; 22 import android.os.Looper; 23 import android.text.TextUtils; 24 import com.android.volley.VolleyLog.MarkerLog; 25 26 import java.io.UnsupportedEncodingException; 27 import java.net.URLEncoder; 28 import java.util.Collections; 29 import java.util.Map; 30 31 /** 32 * Base class for all network requests. 33 * 34 * @param <T> The type of parsed response this request expects. 35 */ 36 public abstract class Request<T> implements Comparable<Request<T>> { 37 38 /** 39 * Default encoding for POST or PUT parameters. See {@link #getParamsEncoding()}. 40 */ 41 private static final String DEFAULT_PARAMS_ENCODING = "UTF-8"; 42 43 /** 44 * Supported request methods. 45 */ 46 public interface Method { 47 int DEPRECATED_GET_OR_POST = -1; 48 int GET = 0; 49 int POST = 1; 50 int PUT = 2; 51 int DELETE = 3; 52 int HEAD = 4; 53 int OPTIONS = 5; 54 int TRACE = 6; 55 int PATCH = 7; 56 } 57 58 /** An event log tracing the lifetime of this request; for debugging. */ 59 private final MarkerLog mEventLog = MarkerLog.ENABLED ? new MarkerLog() : null; 60 61 /** 62 * Request method of this request. Currently supports GET, POST, PUT, DELETE, HEAD, OPTIONS, 63 * TRACE, and PATCH. 64 */ 65 private final int mMethod; 66 67 /** URL of this request. */ 68 private final String mUrl; 69 70 /** The redirect url to use for 3xx http responses */ 71 private String mRedirectUrl; 72 73 /** The unique identifier of the request */ 74 private String mIdentifier; 75 76 /** Default tag for {@link TrafficStats}. */ 77 private final int mDefaultTrafficStatsTag; 78 79 /** Listener interface for errors. */ 80 private Response.ErrorListener mErrorListener; 81 82 /** Sequence number of this request, used to enforce FIFO ordering. */ 83 private Integer mSequence; 84 85 /** The request queue this request is associated with. */ 86 private RequestQueue mRequestQueue; 87 88 /** Whether or not responses to this request should be cached. */ 89 private boolean mShouldCache = true; 90 91 /** Whether or not this request has been canceled. */ 92 private boolean mCanceled = false; 93 94 /** Whether or not a response has been delivered for this request yet. */ 95 private boolean mResponseDelivered = false; 96 97 /** The retry policy for this request. */ 98 private RetryPolicy mRetryPolicy; 99 100 /** 101 * When a request can be retrieved from cache but must be refreshed from 102 * the network, the cache entry will be stored here so that in the event of 103 * a "Not Modified" response, we can be sure it hasn't been evicted from cache. 104 */ 105 private Cache.Entry mCacheEntry = null; 106 107 /** An opaque token tagging this request; used for bulk cancellation. */ 108 private Object mTag; 109 110 /** 111 * Creates a new request with the given URL and error listener. Note that 112 * the normal response listener is not provided here as delivery of responses 113 * is provided by subclasses, who have a better idea of how to deliver an 114 * already-parsed response. 115 * 116 * @deprecated Use {@link #Request(int, String, com.android.volley.Response.ErrorListener)}. 117 */ 118 @Deprecated 119 public Request(String url, Response.ErrorListener listener) { 120 this(Method.DEPRECATED_GET_OR_POST, url, listener); 121 } 122 123 /** 124 * Creates a new request with the given method (one of the values from {@link Method}), 125 * URL, and error listener. Note that the normal response listener is not provided here as 126 * delivery of responses is provided by subclasses, who have a better idea of how to deliver 127 * an already-parsed response. 128 */ 129 public Request(int method, String url, Response.ErrorListener listener) { 130 mMethod = method; 131 mUrl = url; 132 mIdentifier = createIdentifier(method, url); 133 mErrorListener = listener; 134 setRetryPolicy(new DefaultRetryPolicy()); 135 136 mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url); 137 } 138 139 /** 140 * Return the method for this request. Can be one of the values in {@link Method}. 141 */ 142 public int getMethod() { 143 return mMethod; 144 } 145 146 /** 147 * Set a tag on this request. Can be used to cancel all requests with this 148 * tag by {@link RequestQueue#cancelAll(Object)}. 149 * 150 * @return This Request object to allow for chaining. 151 */ 152 public Request<?> setTag(Object tag) { 153 mTag = tag; 154 return this; 155 } 156 157 /** 158 * Returns this request's tag. 159 * @see Request#setTag(Object) 160 */ 161 public Object getTag() { 162 return mTag; 163 } 164 165 /** 166 * @return this request's {@link com.android.volley.Response.ErrorListener}. 167 */ 168 public Response.ErrorListener getErrorListener() { 169 return mErrorListener; 170 } 171 172 /** 173 * @return A tag for use with {@link TrafficStats#setThreadStatsTag(int)} 174 */ 175 public int getTrafficStatsTag() { 176 return mDefaultTrafficStatsTag; 177 } 178 179 /** 180 * @return The hashcode of the URL's host component, or 0 if there is none. 181 */ 182 private static int findDefaultTrafficStatsTag(String url) { 183 if (!TextUtils.isEmpty(url)) { 184 Uri uri = Uri.parse(url); 185 if (uri != null) { 186 String host = uri.getHost(); 187 if (host != null) { 188 return host.hashCode(); 189 } 190 } 191 } 192 return 0; 193 } 194 195 /** 196 * Sets the retry policy for this request. 197 * 198 * @return This Request object to allow for chaining. 199 */ 200 public Request<?> setRetryPolicy(RetryPolicy retryPolicy) { 201 mRetryPolicy = retryPolicy; 202 return this; 203 } 204 205 /** 206 * Adds an event to this request's event log; for debugging. 207 */ 208 public void addMarker(String tag) { 209 if (MarkerLog.ENABLED) { 210 mEventLog.add(tag, Thread.currentThread().getId()); 211 } 212 } 213 214 /** 215 * Notifies the request queue that this request has finished (successfully or with error). 216 * 217 * <p>Also dumps all events from this request's event log; for debugging.</p> 218 */ 219 void finish(final String tag) { 220 if (mRequestQueue != null) { 221 mRequestQueue.finish(this); 222 onFinish(); 223 } 224 if (MarkerLog.ENABLED) { 225 final long threadId = Thread.currentThread().getId(); 226 if (Looper.myLooper() != Looper.getMainLooper()) { 227 // If we finish marking off of the main thread, we need to 228 // actually do it on the main thread to ensure correct ordering. 229 Handler mainThread = new Handler(Looper.getMainLooper()); 230 mainThread.post(new Runnable() { 231 @Override 232 public void run() { 233 mEventLog.add(tag, threadId); 234 mEventLog.finish(this.toString()); 235 } 236 }); 237 return; 238 } 239 240 mEventLog.add(tag, threadId); 241 mEventLog.finish(this.toString()); 242 } 243 } 244 245 /** 246 * clear listeners when finished 247 */ 248 protected void onFinish() { 249 mErrorListener = null; 250 } 251 252 /** 253 * Associates this request with the given queue. The request queue will be notified when this 254 * request has finished. 255 * 256 * @return This Request object to allow for chaining. 257 */ 258 public Request<?> setRequestQueue(RequestQueue requestQueue) { 259 mRequestQueue = requestQueue; 260 return this; 261 } 262 263 /** 264 * Sets the sequence number of this request. Used by {@link RequestQueue}. 265 * 266 * @return This Request object to allow for chaining. 267 */ 268 public final Request<?> setSequence(int sequence) { 269 mSequence = sequence; 270 return this; 271 } 272 273 /** 274 * Returns the sequence number of this request. 275 */ 276 public final int getSequence() { 277 if (mSequence == null) { 278 throw new IllegalStateException("getSequence called before setSequence"); 279 } 280 return mSequence; 281 } 282 283 /** 284 * Returns the URL of this request. 285 */ 286 public String getUrl() { 287 return (mRedirectUrl != null) ? mRedirectUrl : mUrl; 288 } 289 290 /** 291 * Returns the URL of the request before any redirects have occurred. 292 */ 293 public String getOriginUrl() { 294 return mUrl; 295 } 296 297 /** 298 * Returns the identifier of the request. 299 */ 300 public String getIdentifier() { 301 return mIdentifier; 302 } 303 304 /** 305 * Sets the redirect url to handle 3xx http responses. 306 */ 307 public void setRedirectUrl(String redirectUrl) { 308 mRedirectUrl = redirectUrl; 309 } 310 311 /** 312 * Returns the cache key for this request. By default, this is the URL. 313 */ 314 public String getCacheKey() { 315 return mMethod + ":" + mUrl; 316 } 317 318 /** 319 * Annotates this request with an entry retrieved for it from cache. 320 * Used for cache coherency support. 321 * 322 * @return This Request object to allow for chaining. 323 */ 324 public Request<?> setCacheEntry(Cache.Entry entry) { 325 mCacheEntry = entry; 326 return this; 327 } 328 329 /** 330 * Returns the annotated cache entry, or null if there isn't one. 331 */ 332 public Cache.Entry getCacheEntry() { 333 return mCacheEntry; 334 } 335 336 /** 337 * Mark this request as canceled. No callback will be delivered. 338 */ 339 public void cancel() { 340 mCanceled = true; 341 } 342 343 /** 344 * Returns true if this request has been canceled. 345 */ 346 public boolean isCanceled() { 347 return mCanceled; 348 } 349 350 /** 351 * Returns a list of extra HTTP headers to go along with this request. Can 352 * throw {@link AuthFailureError} as authentication may be required to 353 * provide these values. 354 * @throws AuthFailureError In the event of auth failure 355 */ 356 public Map<String, String> getHeaders() throws AuthFailureError { 357 return Collections.emptyMap(); 358 } 359 360 /** 361 * Returns a Map of POST parameters to be used for this request, or null if 362 * a simple GET should be used. Can throw {@link AuthFailureError} as 363 * authentication may be required to provide these values. 364 * 365 * <p>Note that only one of getPostParams() and getPostBody() can return a non-null 366 * value.</p> 367 * @throws AuthFailureError In the event of auth failure 368 * 369 * @deprecated Use {@link #getParams()} instead. 370 */ 371 @Deprecated 372 protected Map<String, String> getPostParams() throws AuthFailureError { 373 return getParams(); 374 } 375 376 /** 377 * Returns which encoding should be used when converting POST parameters returned by 378 * {@link #getPostParams()} into a raw POST body. 379 * 380 * <p>This controls both encodings: 381 * <ol> 382 * <li>The string encoding used when converting parameter names and values into bytes prior 383 * to URL encoding them.</li> 384 * <li>The string encoding used when converting the URL encoded parameters into a raw 385 * byte array.</li> 386 * </ol> 387 * 388 * @deprecated Use {@link #getParamsEncoding()} instead. 389 */ 390 @Deprecated 391 protected String getPostParamsEncoding() { 392 return getParamsEncoding(); 393 } 394 395 /** 396 * @deprecated Use {@link #getBodyContentType()} instead. 397 */ 398 @Deprecated 399 public String getPostBodyContentType() { 400 return getBodyContentType(); 401 } 402 403 /** 404 * Returns the raw POST body to be sent. 405 * 406 * @throws AuthFailureError In the event of auth failure 407 * 408 * @deprecated Use {@link #getBody()} instead. 409 */ 410 @Deprecated 411 public byte[] getPostBody() throws AuthFailureError { 412 // Note: For compatibility with legacy clients of volley, this implementation must remain 413 // here instead of simply calling the getBody() function because this function must 414 // call getPostParams() and getPostParamsEncoding() since legacy clients would have 415 // overridden these two member functions for POST requests. 416 Map<String, String> postParams = getPostParams(); 417 if (postParams != null && postParams.size() > 0) { 418 return encodeParameters(postParams, getPostParamsEncoding()); 419 } 420 return null; 421 } 422 423 /** 424 * Returns a Map of parameters to be used for a POST or PUT request. Can throw 425 * {@link AuthFailureError} as authentication may be required to provide these values. 426 * 427 * <p>Note that you can directly override {@link #getBody()} for custom data.</p> 428 * 429 * @throws AuthFailureError in the event of auth failure 430 */ 431 protected Map<String, String> getParams() throws AuthFailureError { 432 return null; 433 } 434 435 /** 436 * Returns which encoding should be used when converting POST or PUT parameters returned by 437 * {@link #getParams()} into a raw POST or PUT body. 438 * 439 * <p>This controls both encodings: 440 * <ol> 441 * <li>The string encoding used when converting parameter names and values into bytes prior 442 * to URL encoding them.</li> 443 * <li>The string encoding used when converting the URL encoded parameters into a raw 444 * byte array.</li> 445 * </ol> 446 */ 447 protected String getParamsEncoding() { 448 return DEFAULT_PARAMS_ENCODING; 449 } 450 451 /** 452 * Returns the content type of the POST or PUT body. 453 */ 454 public String getBodyContentType() { 455 return "application/x-www-form-urlencoded; charset=" + getParamsEncoding(); 456 } 457 458 /** 459 * Returns the raw POST or PUT body to be sent. 460 * 461 * <p>By default, the body consists of the request parameters in 462 * application/x-www-form-urlencoded format. When overriding this method, consider overriding 463 * {@link #getBodyContentType()} as well to match the new body format. 464 * 465 * @throws AuthFailureError in the event of auth failure 466 */ 467 public byte[] getBody() throws AuthFailureError { 468 Map<String, String> params = getParams(); 469 if (params != null && params.size() > 0) { 470 return encodeParameters(params, getParamsEncoding()); 471 } 472 return null; 473 } 474 475 /** 476 * Converts <code>params</code> into an application/x-www-form-urlencoded encoded string. 477 */ 478 private byte[] encodeParameters(Map<String, String> params, String paramsEncoding) { 479 StringBuilder encodedParams = new StringBuilder(); 480 try { 481 for (Map.Entry<String, String> entry : params.entrySet()) { 482 encodedParams.append(URLEncoder.encode(entry.getKey(), paramsEncoding)); 483 encodedParams.append('='); 484 encodedParams.append(URLEncoder.encode(entry.getValue(), paramsEncoding)); 485 encodedParams.append('&'); 486 } 487 return encodedParams.toString().getBytes(paramsEncoding); 488 } catch (UnsupportedEncodingException uee) { 489 throw new RuntimeException("Encoding not supported: " + paramsEncoding, uee); 490 } 491 } 492 493 /** 494 * Set whether or not responses to this request should be cached. 495 * 496 * @return This Request object to allow for chaining. 497 */ 498 public final Request<?> setShouldCache(boolean shouldCache) { 499 mShouldCache = shouldCache; 500 return this; 501 } 502 503 /** 504 * Returns true if responses to this request should be cached. 505 */ 506 public final boolean shouldCache() { 507 return mShouldCache; 508 } 509 510 /** 511 * Priority values. Requests will be processed from higher priorities to 512 * lower priorities, in FIFO order. 513 */ 514 public enum Priority { 515 LOW, 516 NORMAL, 517 HIGH, 518 IMMEDIATE 519 } 520 521 /** 522 * Returns the {@link Priority} of this request; {@link Priority#NORMAL} by default. 523 */ 524 public Priority getPriority() { 525 return Priority.NORMAL; 526 } 527 528 /** 529 * Returns the socket timeout in milliseconds per retry attempt. (This value can be changed 530 * per retry attempt if a backoff is specified via backoffTimeout()). If there are no retry 531 * attempts remaining, this will cause delivery of a {@link TimeoutError} error. 532 */ 533 public final int getTimeoutMs() { 534 return mRetryPolicy.getCurrentTimeout(); 535 } 536 537 /** 538 * Returns the retry policy that should be used for this request. 539 */ 540 public RetryPolicy getRetryPolicy() { 541 return mRetryPolicy; 542 } 543 544 /** 545 * Mark this request as having a response delivered on it. This can be used 546 * later in the request's lifetime for suppressing identical responses. 547 */ 548 public void markDelivered() { 549 mResponseDelivered = true; 550 } 551 552 /** 553 * Returns true if this request has had a response delivered for it. 554 */ 555 public boolean hasHadResponseDelivered() { 556 return mResponseDelivered; 557 } 558 559 /** 560 * Subclasses must implement this to parse the raw network response 561 * and return an appropriate response type. This method will be 562 * called from a worker thread. The response will not be delivered 563 * if you return null. 564 * @param response Response from the network 565 * @return The parsed response, or null in the case of an error 566 */ 567 abstract protected Response<T> parseNetworkResponse(NetworkResponse response); 568 569 /** 570 * Subclasses can override this method to parse 'networkError' and return a more specific error. 571 * 572 * <p>The default implementation just returns the passed 'networkError'.</p> 573 * 574 * @param volleyError the error retrieved from the network 575 * @return an NetworkError augmented with additional information 576 */ 577 protected VolleyError parseNetworkError(VolleyError volleyError) { 578 return volleyError; 579 } 580 581 /** 582 * Subclasses must implement this to perform delivery of the parsed 583 * response to their listeners. The given response is guaranteed to 584 * be non-null; responses that fail to parse are not delivered. 585 * @param response The parsed response returned by 586 * {@link #parseNetworkResponse(NetworkResponse)} 587 */ 588 abstract protected void deliverResponse(T response); 589 590 /** 591 * Delivers error message to the ErrorListener that the Request was 592 * initialized with. 593 * 594 * @param error Error details 595 */ 596 public void deliverError(VolleyError error) { 597 if (mErrorListener != null) { 598 mErrorListener.onErrorResponse(error); 599 } 600 } 601 602 /** 603 * Our comparator sorts from high to low priority, and secondarily by 604 * sequence number to provide FIFO ordering. 605 */ 606 @Override 607 public int compareTo(Request<T> other) { 608 Priority left = this.getPriority(); 609 Priority right = other.getPriority(); 610 611 // High-priority requests are "lesser" so they are sorted to the front. 612 // Equal priorities are sorted by sequence number to provide FIFO ordering. 613 return left == right ? 614 this.mSequence - other.mSequence : 615 right.ordinal() - left.ordinal(); 616 } 617 618 @Override 619 public String toString() { 620 String trafficStatsTag = "0x" + Integer.toHexString(getTrafficStatsTag()); 621 return (mCanceled ? "[X] " : "[ ] ") + getUrl() + " " + trafficStatsTag + " " 622 + getPriority() + " " + mSequence; 623 } 624 625 private static long sCounter; 626 /** 627 * sha1(Request:method:url:timestamp:counter) 628 * @param method http method 629 * @param url http request url 630 * @return sha1 hash string 631 */ 632 private static String createIdentifier(final int method, final String url) { 633 return InternalUtils.sha1Hash("Request:" + method + ":" + url + 634 ":" + System.currentTimeMillis() + ":" + (sCounter++)); 635 } 636 }
Request<T>中的泛型T，是指解析response以后的结果。在上一篇文章中我们知道，ResponseDelivery会把response分派给对应的request(中文翻译就是，把响应分派给对应的请求)。在我们定义的请求中，需要重写parseNetworkResponse(NetworkResponse response)这个方法，解析请求，解析出来的结果，就是T类型的。
首先是一些属性
1 /** 2 * Base class for all network requests. 3 * 请求基类 4 * @param <T> The type of parsed response this request expects. 5 * T为响应类型 6 */ 7 public abstract class Request<T> implements Comparable<Request<T>> { 8 9 /** 10 * Default encoding for POST or PUT parameters. See {@link #getParamsEncoding()}. 11 * 默认编码 12 */ 13 private static final String DEFAULT_PARAMS_ENCODING = "UTF-8"; 14 15 /** 16 * Supported request methods. 17 * 支持的请求方式 18 */ 19 public interface Method { 20 int DEPRECATED_GET_OR_POST = -1; 21 int GET = 0; 22 int POST = 1; 23 int PUT = 2; 24 int DELETE = 3; 25 int HEAD = 4; 26 int OPTIONS = 5; 27 int TRACE = 6; 28 int PATCH = 7; 29 } 30 31 /** 32 * An event log tracing the lifetime of this request; for debugging. 33 * 用于跟踪请求的生存时间，用于调试 34 * */ 35 private final MarkerLog mEventLog = MarkerLog.ENABLED ? new MarkerLog() : null; 36 37 /** 38 * Request method of this request. Currently supports GET, POST, PUT, DELETE, HEAD, OPTIONS, 39 * TRACE, and PATCH. 40 * 当前请求方式 41 */ 42 private final int mMethod; 43 44 /** 45 * URL of this request. 46 * 请求地址 47 */ 48 private final String mUrl; 49 50 /** 51 * The redirect url to use for 3xx http responses 52 * 重定向地址 53 */ 54 private String mRedirectUrl; 55 56 /** 57 * The unique identifier of the request 58 * 该请求的唯一凭证 59 */ 60 private String mIdentifier; 61 62 /** 63 * Default tag for {@link TrafficStats}. 64 * 流量统计标签 65 */ 66 private final int mDefaultTrafficStatsTag; 67 68 /** 69 * Listener interface for errors. 70 * 错误监听器 71 */ 72 private final Response.ErrorListener mErrorListener; 73 74 /** 75 * Sequence number of this request, used to enforce FIFO ordering. 76 * 请求序号，用于fifo算法 77 */ 78 private Integer mSequence; 79 80 /** 81 * The request queue this request is associated with. 82 * 请求所在的请求队列 83 */ 84 private RequestQueue mRequestQueue; 85 86 /** 87 * Whether or not responses to this request should be cached. 88 * 是否使用缓存响应请求 89 */ 90 private boolean mShouldCache = true; 91 92 /** 93 * Whether or not this request has been canceled. 94 * 该请求是否被取消 95 */ 96 private boolean mCanceled = false; 97 98 /** 99 * Whether or not a response has been delivered for this request yet. 100 * 该请求是否已经被响应 101 */ 102 private boolean mResponseDelivered = false; 103 104 /** 105 * A cheap variant of request tracing used to dump slow requests. 106 * 一个简单的变量，跟踪请求，用来抛弃过慢的请求 107 * 请求产生时间 108 */ 109 private long mRequestBirthTime = 0; 110 111 /** Threshold at which we should log the request (even when debug logging is not enabled). */ 112 private static final long SLOW_REQUEST_THRESHOLD_MS = 3000; 113 114 /** 115 * The retry policy for this request. 116 * 请求重试策略 117 */ 118 private RetryPolicy mRetryPolicy; 119 120 /** 121 * When a request can be retrieved from cache but must be refreshed from 122 * the network, the cache entry will be stored here so that in the event of 123 * a "Not Modified" response, we can be sure it hasn't been evicted from cache. 124 * 缓存记录。当请求可以从缓存中获得响应，但必须从网络上更新时。我们保留这个缓存记录，所以一旦从网络上获得的响应带有Not Modified 125 * (没有更新)时，来保证这个缓存没有被回收. 126 */ 127 private Cache.Entry mCacheEntry = null; 128 129 /** 130 * An opaque token tagging this request; used for bulk cancellation. 131 * 用于自定义标记，可以理解为用于请求的分类 132 */ 133 private Object mTag;
接下来看构造方法
/** * Creates a new request with the given method (one of the values from {@link Method}), * URL, and error listener. Note that the normal response listener is not provided here as * delivery of responses is provided by subclasses, who have a better idea of how to deliver * an already-parsed response. * 根据请求方式，创建新的请求(需要地址，错误监听器等参数) */ public Request(int method, String url, Response.ErrorListener listener) { mMethod = method; mUrl = url; mIdentifier = createIdentifier(method, url);//为请求创建唯一凭证 mErrorListener = listener;//设定监听器 setRetryPolicy(new DefaultRetryPolicy());//设置默认重试策略 mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);//设置流量标志 }
首先是请求方式，请求地址的设定，这是作为一个请求必须有的。然后是监听器的设定，注意这里只是这是了ErrorListner，说明errorListener是必须的，但是正确响应，我们有可能不处理。这样设定是合理的，因为出错了，我们必须处理，至于请求成功，我们可以不处理。那么我们想处理成功的请求怎么办呢，这需要在子类中重写构造方法(例如StringRequest)。
然后是创建了一个唯一凭证
/** * sha1(Request:method:url:timestamp:counter) * @param method http method * @param url http request url * @return sha1 hash string * 利用请求方式和地址，进行sha1加密，创建该请求的唯一凭证 */ private static String createIdentifier(final int method, final String url) { return InternalUtils.sha1Hash("Request:" + method + ":" + url + ":" + System.currentTimeMillis() + ":" + (sCounter++)); }
由上面的方法可以看出，这个凭证和当前时间有关，因此是独一无二的
接着是设置重试策略，这个类等下再介绍，接下来是流量标志的设置，所谓流量标志，是用于调试日志记录的，不是重点
/** * @return The hashcode of the URL's host component, or 0 if there is none. * 返回url的host(主机地址)部分的hashcode，如果host不存在，返回0 */ private static int findDefaultTrafficStatsTag(String url) { if (!TextUtils.isEmpty(url)) { Uri uri = Uri.parse(url); if (uri != null) { String host = uri.getHost(); if (host != null) { return host.hashCode(); } } } return 0; }
我们再回过头来看这个重试策略。
volley为重试策略专门定义了一个类，这样我们就可以根据需要实现自己的重试策略了，至于源码内部，为我们提供了一个默认的重试策略DefaultRetryPolicy()
要介绍重试策略，我们先看重试策略的基类RetryPolicy
/** * Retry policy for a request. * 请求重试策略类 */ public interface RetryPolicy { /** * Returns the current timeout (used for logging). * 获得当前时间，用于日志 */ public int getCurrentTimeout(); /** * Returns the current retry count (used for logging). * 返回当前重试次数，用于日志 */ public int getCurrentRetryCount(); /** * Prepares for the next retry by applying a backoff to the timeout. * @param error The error code of the last attempt. * @throws VolleyError In the event that the retry could not be performed (for example if we * ran out of attempts), the passed in error is thrown. * 重试实现 */ public void retry(VolleyError error) throws VolleyError; }
重要的是retry()这个方法，我们来看DefaultRetryPolicy里面这个方法的具体实现
/** * Prepares for the next retry by applying a backoff to the timeout. * @param error The error code of the last attempt. */ @Override public void retry(VolleyError error) throws VolleyError { mCurrentRetryCount++;//当前重试次数 mCurrentTimeoutMs += (mCurrentTimeoutMs * mBackoffMultiplier);//当前超出时间 if (!hasAttemptRemaining()) {//是否已经到达最大重试次数 throw error; } } /** * Returns true if this policy has attempts remaining, false otherwise. * 是否还重试 */ protected boolean hasAttemptRemaining() { return mCurrentRetryCount <= mMaxNumRetries;//最大重试次数 }
可以看到，在默认的重试策略中，只是简单地统计了重试的次数，然后，在超出最大次数以后，抛出异常。
就这么简单，那么究竟volley是怎么实现重试的呢？
实际上，当从队列中取出一个request去进行网络请求的时候，我们是写在一个死循环里面的(在以后的代码可以看到，这样不贴出来以免类过多造成困扰)。
一旦请求失败，就会调用上面的retry()方法，但是没有跳出循环。直到请求成功获得response，才return。如果一直请求失败，根据上面的重试策略，最后会抛出VolleyError异常,这个异常不处理，而是通过throws向外抛，从而结束死循环。
从程序设计的角度来说，通过抛出异常结束死循环，显得不是那么的优雅(通常我们用设置标记的方法结束循环)，但是在volley中使用了这个方式，原因是对于这个异常，要交给程序员自己处理，虽然这样使异常传递的过程变得复杂，但是增加了程序的灵活性。
最终的异常，我们会在Request<T>的parseNetworkError()和deliverError()方法里面处理，parseNetworkError()用于解析Volleyerror，deliverError()方法回调了上面一开始就提到的ErrorListener
/** * Subclasses can override this method to parse 'networkError' and return a more specific error. * * <p>The default implementation just returns the passed 'networkError'.</p> * * @param volleyError the error retrieved from the network * @return an NetworkError augmented with additional information * 解析网络错误 */ public VolleyError parseNetworkError(VolleyError volleyError) { return volleyError; } /** * Delivers error message to the ErrorListener that the Request was * initialized with. * * @param error Error details * 分发网络错误 */ public void deliverError(VolleyError error) { if (mErrorListener != null) { mErrorListener.onErrorResponse(error); } }
其实除了上面两个处理错误的方法，还有两个方法用于处理成功响应,是必须要继承的
/** * Subclasses must implement this to parse the raw network response * and return an appropriate response type. This method will be * called from a worker thread. The response will not be delivered * if you return null. * @param response Response from the network * @return The parsed response, or null in the case of an error * 解析响应 */ public abstract Response<T> parseNetworkResponse(NetworkResponse response); <pre name="code" class="java">/** * Subclasses must implement this to perform delivery of the parsed * response to their listeners. The given response is guaranteed to * be non-null; responses that fail to parse are not delivered. * @param response The parsed response returned by * {@link #parseNetworkResponse(NetworkResponse)} * 分发响应 */ public abstract void deliverResponse(T response);
parseNetworkResponse(NetworkResponse response)用于将网络response解析为本地response，解析出来的response,会交给deliverResponse(T response)方法。
为什么要解析，其实上面已经说过，要将结果解析为T类型。至于这两个方法，其实是在ResponseDelivery响应分发器里面调用的。
看完初始化方法，我们来看结束请求的方法finish()，有时候我们想要主动终止请求，例如停止下载文件，又或者请求已经成功了，我们从队列中去除这个请求
1 /** 2 * Notifies the request queue that this request has finished (successfully or with error). 3 * 提醒请求队列，当前请求已经完成(失败或成功) 4 * <p>Also dumps all events from this request's event log; for debugging.</p> 5 * 6 */ 7 public void finish(final String tag) { 8 if (mRequestQueue != null) { 9 mRequestQueue.finish(this);//该请求完成 10 } 11 if (MarkerLog.ENABLED) {//如果开启调试 12 final long threadId = Thread.currentThread().getId();//线程id 13 if (Looper.myLooper() != Looper.getMainLooper()) {//请求不是在主线程 14 // If we finish marking off of the main thread, we need to 15 // actually do it on the main thread to ensure correct ordering. 16 //如果我们不是在主线程记录log，我们需要在主线程做这项工作来保证正确的顺序 17 Handler mainThread = new Handler(Looper.getMainLooper()); 18 mainThread.post(new Runnable() { 19 @Override 20 public void run() { 21 mEventLog.add(tag, threadId); 22 mEventLog.finish(this.toString()); 23 } 24 }); 25 return; 26 } 27 //如果在主线程，直接记录 28 mEventLog.add(tag, threadId); 29 mEventLog.finish(this.toString()); 30 } else {//不开启调试 31 long requestTime = SystemClock.elapsedRealtime() - mRequestBirthTime; 32 if (requestTime >= SLOW_REQUEST_THRESHOLD_MS) { 33 VolleyLog.d("%d ms: %s", requestTime, this.toString()); 34 } 35 } 36 }
上面主要是做了一些日志记录的工作，最重要的是调用了mRequestQueue的finish()方法，来从队列中去除这个请求。
看完上面的介绍以后，大家是否注意到，Request<T>继承了Comparable<Request<T>>接口，为什么要继承这个接口了，我们当然要来看compareTo()方法了
/** * Our comparator sorts from high to low priority, and secondarily by * sequence number to provide FIFO ordering. */ @Override public int compareTo(Request<T> other) { Priority left = this.getPriority(); Priority right = other.getPriority(); // High-priority requests are "lesser" so they are sorted to the front. // Equal priorities are sorted by sequence number to provide FIFO ordering. return left == right ? this.mSequence - other.mSequence : right.ordinal() - left.ordinal(); }
这个方法比较了两个请求的优先级，如果优先级相等，就按照顺序
实现这个接口的目的，正如上一篇文章提到的，有的请求比较重要，希望早点执行，也就是说让它排在请求队列的前头
通过比较方法，我们就可以设定请求在请求队列中排队顺序的根据，从而让优先级高的排在前面。
OK，Request<T>就基本介绍完了，当然有些属性，例如缓存mCacheEntry，mRedirectUrl重定向地址等我们还没有用到，我们先记住它们，在以后会使用到的。
其实Request<T>类并不复杂，主要就是一些属性的设置，这些属性有的比较难考虑到，例如优先级，重定向地址，自定义标记，重试策略等。
最后，我们通过StringRequest来看一下Request<T>类的具体实现
/** * A canned request for retrieving the response body at a given URL as a String. */ public class StringRequest extends Request<String> { private final Listener<String> mListener; /** * Creates a new request with the given method. * * @param method the request {@link Method} to use * @param url URL to fetch the string at * @param listener Listener to receive the String response * @param errorListener Error listener, or null to ignore errors */ public StringRequest(int method, String url, Listener<String> listener, ErrorListener errorListener) { super(method, url, errorListener); mListener = listener; }
上面的构造方法中，添加了一个新的接口Listener<String> listener,用于监听成功的response
然后是parseNetworkResponse(NetworkResponse response)，这个方法
@Override public Response<String> parseNetworkResponse(NetworkResponse response) { String parsed; try { parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers)); } catch (UnsupportedEncodingException e) { parsed = new String(response.data); } return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response)); }
可以看到，将NetworkResponse解析为String类型的了，然后再构造成对应的本地response
@Override public void deliverResponse(String response) { mListener.onResponse(response); }
至于deliverResponse(String response)，则调用了构造方法里面要求的，新的监听器。
到此为止，对于Request<T>的介绍就结束了，由于Request<T>和其他类的耦合并不是特别重，相信是比较容易理解。
在下一篇文章中，我们会来看RequestQueue队列，看看这个队列的作用到底是什么，我们为什么要创建一个队列来保存request而不是直接每个request开启一个线程去加载网络数据。
扫一扫
1360
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
