BasicNetwork详解

BasicNetwork详解

BasicNetwork负责完成网络请求，受NetworkDispatcher类调用。在NetworkDispatcher的run()方法中，是这样调用的。

void run(){
    processRequest();
}

......

void processRequest(Request<?> request) {
    ......
    // Perform the network request.
    NetworkResponse networkResponse = mNetwork.performRequest(request);
    request.addMarker("network-http-complete");
    ......
}

......

Network

它是接口Network的一个实例。老规矩，我们看看这个类的注释：

/** A network performing Volley requests over an {@link HttpStack}. */

它实现了Network接口。我们阅读一下这个类的源码。

/** An interface for performing requests. */
public interface Network {
    /**
     * Performs the specified request.
     *
     * @param request Request to process
     * @return A {@link NetworkResponse} with data and caching metadata; will never be null
     * @throws VolleyError on errors
     */
    NetworkResponse performRequest(Request<?> request) throws VolleyError;
}

注释说明，这个是一个实现请求的接口。只有一个方法-performRequest()。一个入参，泛型Reqest。这个方法的作用是实现指定的请求。返回的是请求的结果为-NetworkResponse。

Request

Request是一个抽象类，所有的请求都是继承于这个，作用就是进行网络请求参数的封装和请求结果的解析。在这里暂时不详细介绍。它有三个重要的方法:

    /**
     * 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
     */
    @Override
    protected void deliverResponse(String response);

    /**
     * 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)}
     */
    @Override
    @SuppressWarnings("DefaultCharset")
    protected Response<String> parseNetworkResponse(NetworkResponse response)

    /**
     * Mark this request as canceled.
     *
     * <p>No callback will be delivered as long as either:
     *
     * <ul>
     *   <li>This method is called on the same thread as the {@link ResponseDelivery} is running on.
     *       By default, this is the main thread.
     *   <li>The request subclass being used overrides cancel() and ensures that it does not invoke
     *       the listener in {@link #deliverResponse} after cancel() has been called in a
     *       thread-safe manner.
     * </ul>
     *
     * <p>There are no guarantees if both of these conditions aren't met.
     */
    @Override
    public void cancel() 

作用如上的注释。

构造函数

    /**
     * @deprecated Should never have been exposed in the API. This field may be removed in a future
     *     release of Volley.
     */
    @Deprecated protected final HttpStack mHttpStack;

    private final BaseHttpStack mBaseHttpStack;

    protected final ByteArrayPool mPool;

    /**
     * @param httpStack HTTP stack to be used
     * @deprecated use {@link #BasicNetwork(BaseHttpStack)} instead to avoid depending on Apache
     *     HTTP. This method may be removed in a future release of Volley.
     */
    @Deprecated
    public BasicNetwork(HttpStack httpStack) {
        // If a pool isn't passed in, then build a small default pool that will give us a lot of
        // benefit and not use too much memory.
        this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE));
    }

    /**
     * @param httpStack HTTP stack to be used
     * @param pool a buffer pool that improves GC performance in copy operations
     * @deprecated use {@link #BasicNetwork(BaseHttpStack, ByteArrayPool)} instead to avoid
     *     depending on Apache HTTP. This method may be removed in a future release of Volley.
     */
    @Deprecated
    public BasicNetwork(HttpStack httpStack, ByteArrayPool pool) {
        mHttpStack = httpStack;
        mBaseHttpStack = new AdaptedHttpStack(httpStack);
        mPool = pool;
    }

    /** @param httpStack HTTP stack to be used */
    public BasicNetwork(BaseHttpStack httpStack) {
        // If a pool isn't passed in, then build a small default pool that will give us a lot of
        // benefit and not use too much memory.
        this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE));
    }

    /**
     * @param httpStack HTTP stack to be used
     * @param pool a buffer pool that improves GC performance in copy operations
     */
    public BasicNetwork(BaseHttpStack httpStack, ByteArrayPool pool) {
        mBaseHttpStack = httpStack;
        // Populate mHttpStack for backwards compatibility, since it is a protected field. However,
        // we won't use it directly here, so clients which don't access it directly won't need to
        // depend on Apache HTTP.
        mHttpStack = httpStack;
        mPool = pool;
    }

额，貌似，遇到了问题，我们要看看HttpStack和ByteArrayPool是个啥东西。

HttpStack

代码不长，我们直接贴出来。

/**
 * An HTTP stack abstraction.
 *
 * @deprecated This interface should be avoided as it depends on the deprecated Apache HTTP library.
 *     Use {@link BaseHttpStack} to avoid this dependency. This class may be removed in a future
 *     release of Volley.
 */
@Deprecated
public interface HttpStack {
    /**
     * Performs an HTTP request with the given parameters.
     *
     * <p>A GET request is sent if request.getPostBody() == null. A POST request is sent otherwise,
     * and the Content-Type header is set to request.getPostBodyContentType().
     *
     * @param request the request to perform
     * @param additionalHeaders additional headers to be sent together with {@link
     *     Request#getHeaders()}
     * @return the HTTP response
     */
    HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
            throws IOException, AuthFailureError;
}

它是一个接口，类说明是：一个抽象的Http栈。又说了：应该避免使用此接口，因为它取决于已弃用的Apache HTTP库。 使用{@link BaseHttpStack}来避免这种依赖。 此类可能会在未来的Volley版本中删除。这是，因为，谷歌android5.0之后更换了将HttpClient更换成了HttpURLConnection。关于这部分，我们先暂时分析到这里，稍后我们在后面详细讲讲网络请求的实现。

ByteArrayPool

类说明：

/**
 * ByteArrayPool is a source and repository of <code>byte[]</code> objects. Its purpose is to supply
 * those buffers to consumers who need to use them for a short period of time and then dispose of
 * them. Simply creating and disposing such buffers in the conventional manner can considerable heap
 * churn and garbage collection delays on Android, which lacks good management of short-lived heap
 * objects. It may be advantageous to trade off some memory in the form of a permanently allocated
 * pool of buffers in order to gain heap performance improvements; that is what this class does.
 *
 * <p>A good candidate user for this class is something like an I/O system that uses large temporary
 * <code>byte[]</code> buffers to copy data around. In these use cases, often the consumer wants the
 * buffer to be a certain minimum size to ensure good performance (e.g. when copying data chunks off
 * of a stream), but doesn't mind if the buffer is larger than the minimum. Taking this into account
 * and also to maximize the odds of being able to reuse a recycled buffer, this class is free to
 * return buffers larger than the requested size. The caller needs to be able to gracefully deal
 * with getting buffers any size over the minimum.
 *
 * <p>If there is not a suitably-sized buffer in its recycling pool when a buffer is requested, this
 * class will allocate a new buffer and return it.
 *
 * <p>This class has no special ownership of buffers it creates; the caller is free to take a buffer
 * it receives from this pool, use it permanently, and never return it to the pool; additionally, it
 * is not harmful to return to this pool a buffer that was allocated elsewhere, provided there are
 * no other lingering references to it.
 *
 * <p>This class ensures that the total size of the buffers in its recycling pool never exceeds a
 * certain byte limit. When a buffer is returned that would cause the pool to exceed the limit,
 * least-recently-used buffers are disposed.
 */

真正的代码不多，主要注释很详细。 大概的意思是它是一个Byte[]池，作用是能够更快速的对byte[]进行操作，节省I/O，减小内存分配和垃圾回收的系统消耗。提供了两个方法：

• public synchronized byte[] getBuf(int len)
  如果一个缓冲区在请求的大小中可用，则返回该缓冲区中的缓冲区;如果池中的缓冲区不可用，则返回一个新的缓冲区。
• public synchronized void returnBuf(byte[] buf)
  返回池的缓冲区，如果池超过其分配的大小，则丢弃旧的缓冲区。

回到构造函数。有两个被废弃的，我们不分析了。另外的两个的区别是，一个不带ByteArrayPool。我们直接分析复杂的那个。

    /** @param httpStack HTTP stack to be used */
    public BasicNetwork(BaseHttpStack httpStack) {
        // If a pool isn't passed in, then build a small default pool that will give us a lot of
        // benefit and not use too much memory.
        this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE));
    }

    /**
     * @param httpStack HTTP stack to be used
     * @param pool a buffer pool that improves GC performance in copy operations
     */
    public BasicNetwork(BaseHttpStack httpStack, ByteArrayPool pool) {
        mBaseHttpStack = httpStack;
        // Populate mHttpStack for backwards compatibility, since it is a protected field. However,
        // we won't use it directly here, so clients which don't access it directly won't need to
        // depend on Apache HTTP.
        //填充mHttpStack以实现向后兼容性，因为它是受保护的字段。 但是，我们不会在这里直接使用它，因此不直接访问它的客户端不需要依赖Apache HTTP。
        mHttpStack = httpStack;
        mPool = pool;
    }

可以知道，现在我们的网络请求都是使用mBaseHttpStack。
然后我们来看看重写的一个方法-performRequest，从名字可以看出来，这个方法用来实现网络请求。代码有点多，额，我也没办法。读吧。

    @Override
    public NetworkResponse performRequest(Request<?> request) throws VolleyError {
        long requestStart = SystemClock.elapsedRealtime();
        while (true) {
            HttpResponse httpResponse = null;
            byte[] responseContents = null;
            List<Header> responseHeaders = Collections.emptyList();
            try {
                // Gather headers.
                Map<String, String> additionalRequestHeaders =
                        getCacheHeaders(request.getCacheEntry());
                httpResponse = mBaseHttpStack.executeRequest(request, additionalRequestHeaders);
                int statusCode = httpResponse.getStatusCode();

                responseHeaders = httpResponse.getHeaders();
                // Handle cache validation.
                if (statusCode == HttpURLConnection.HTTP_NOT_MODIFIED) {
                    Entry entry = request.getCacheEntry();
                    if (entry == null) {
                        return new NetworkResponse(
                                HttpURLConnection.HTTP_NOT_MODIFIED,
                                /* data= */ null,
                                /* notModified= */ true,
                                SystemClock.elapsedRealtime() - requestStart,
                                responseHeaders);
                    }
                    // Combine cached and response headers so the response will be complete.
                    List<Header> combinedHeaders = combineHeaders(responseHeaders, entry);
                    return new NetworkResponse(
                            HttpURLConnection.HTTP_NOT_MODIFIED,
                            entry.data,
                            /* notModified= */ true,
                            SystemClock.elapsedRealtime() - requestStart,
                            combinedHeaders);
                }

                // Some responses such as 204s do not have content.  We must check.
                InputStream inputStream = httpResponse.getContent();
                if (inputStream != null) {
                    responseContents =
                            inputStreamToBytes(inputStream, httpResponse.getContentLength());
                } else {
                    // Add 0 byte response as a way of honestly representing a
                    // no-content request.
                    responseContents = new byte[0];
                }

                // if the request is slow, log it.
                long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
                logSlowRequests(requestLifetime, request, responseContents, statusCode);

                if (statusCode < 200 || statusCode > 299) {
                    throw new IOException();
                }
                return new NetworkResponse(
                        statusCode,
                        responseContents,
                        /* notModified= */ false,
                        SystemClock.elapsedRealtime() - requestStart,
                        responseHeaders);
            } catch (SocketTimeoutException e) {
                attemptRetryOnException("socket", request, new TimeoutError());
            } catch (MalformedURLException e) {
                throw new RuntimeException("Bad URL " + request.getUrl(), e);
            } catch (IOException e) {
                int statusCode;
                if (httpResponse != null) {
                    statusCode = httpResponse.getStatusCode();
                } else {
                    throw new NoConnectionError(e);
                }
                VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
                NetworkResponse networkResponse;
                if (responseContents != null) {
                    networkResponse =
                            new NetworkResponse(
                                    statusCode,
                                    responseContents,
                                    /* notModified= */ false,
                                    SystemClock.elapsedRealtime() - requestStart,
                                    responseHeaders);
                    if (statusCode == HttpURLConnection.HTTP_UNAUTHORIZED
                            || statusCode == HttpURLConnection.HTTP_FORBIDDEN) {
                        attemptRetryOnException(
                                "auth", request, new AuthFailureError(networkResponse));
                    } else if (statusCode >= 400 && statusCode <= 499) {
                        // Don't retry other client errors.
                        throw new ClientError(networkResponse);
                    } else if (statusCode >= 500 && statusCode <= 599) {
                        if (request.shouldRetryServerErrors()) {
                            attemptRetryOnException(
                                    "server", request, new ServerError(networkResponse));
                        } else {
                            throw new ServerError(networkResponse);
                        }
                    } else {
                        // 3xx? No reason to retry.
                        throw new ServerError(networkResponse);
                    }
                } else {
                    attemptRetryOnException("network", request, new NetworkError());
                }
            }
        }
    }

    /** Logs requests that took over SLOW_REQUEST_THRESHOLD_MS to complete. */
    private void logSlowRequests(
            long requestLifetime, Request<?> request, byte[] responseContents, int statusCode) {
        if (DEBUG || requestLifetime > SLOW_REQUEST_THRESHOLD_MS) {
            VolleyLog.d(
                    "HTTP response for request=<%s> [lifetime=%d], [size=%s], "
                            + "[rc=%d], [retryCount=%s]",
                    request,
                    requestLifetime,
                    responseContents != null ? responseContents.length : "null",
                    statusCode,
                    request.getRetryPolicy().getCurrentRetryCount());
        }
    }

    /**
     * Attempts to prepare the request for a retry. If there are no more attempts remaining in the
     * request's retry policy, a timeout exception is thrown.
     *
     * @param request The request to use.
     */
    private static void attemptRetryOnException(
            String logPrefix, Request<?> request, VolleyError exception) throws VolleyError {
        RetryPolicy retryPolicy = request.getRetryPolicy();
        int oldTimeout = request.getTimeoutMs();

        try {
            retryPolicy.retry(exception);
        } catch (VolleyError e) {
            request.addMarker(
                    String.format("%s-timeout-giveup [timeout=%s]", logPrefix, oldTimeout));
            throw e;
        }
        request.addMarker(String.format("%s-retry [timeout=%s]", logPrefix, oldTimeout));
    }

    private Map<String, String> getCacheHeaders(Cache.Entry entry) {
        // If there's no cache entry, we're done.
        if (entry == null) {
            return Collections.emptyMap();
        }

        Map<String, String> headers = new HashMap<>();

        if (entry.etag != null) {
            headers.put("If-None-Match", entry.etag);
        }

        if (entry.lastModified > 0) {
            headers.put(
                    "If-Modified-Since", HttpHeaderParser.formatEpochAsRfc1123(entry.lastModified));
        }

        return headers;
    }

    protected void logError(String what, String url, long start) {
        long now = SystemClock.elapsedRealtime();
        VolleyLog.v("HTTP ERROR(%s) %d ms to fetch %s", what, (now - start), url);
    }

    /** Reads the contents of an InputStream into a byte[]. */
    private byte[] inputStreamToBytes(InputStream in, int contentLength)
            throws IOException, ServerError {
        PoolingByteArrayOutputStream bytes = new PoolingByteArrayOutputStream(mPool, contentLength);
        byte[] buffer = null;
        try {
            if (in == null) {
                throw new ServerError();
            }
            buffer = mPool.getBuf(1024);
            int count;
            while ((count = in.read(buffer)) != -1) {
                bytes.write(buffer, 0, count);
            }
            return bytes.toByteArray();
        } finally {
            try {
                // Close the InputStream and release the resources by "consuming the content".
                if (in != null) {
                    in.close();
                }
            } catch (IOException e) {
                // This can happen if there was an exception above that left the stream in
                // an invalid state.
                VolleyLog.v("Error occurred when closing InputStream");
            }
            mPool.returnBuf(buffer);
            bytes.close();
        }
    }

    /**
     * Converts Headers[] to Map&lt;String, String&gt;.
     *
     * @deprecated Should never have been exposed in the API. This method may be removed in a future
     *     release of Volley.
     */
    @Deprecated
    protected static Map<String, String> convertHeaders(Header[] headers) {
        Map<String, String> result = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
        for (int i = 0; i < headers.length; i++) {
            result.put(headers[i].getName(), headers[i].getValue());
        }
        return result;
    }

    /**
     * Combine cache headers with network response headers for an HTTP 304 response.
     *
     * <p>An HTTP 304 response does not have all header fields. We have to use the header fields
     * from the cache entry plus the new ones from the response. See also:
     * http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
     *
     * @param responseHeaders Headers from the network response.
     * @param entry The cached response.
     * @return The combined list of headers.
     */
    private static List<Header> combineHeaders(List<Header> responseHeaders, Entry entry) {
        // First, create a case-insensitive set of header names from the network
        // response.
        Set<String> headerNamesFromNetworkResponse = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
        if (!responseHeaders.isEmpty()) {
            for (Header header : responseHeaders) {
                headerNamesFromNetworkResponse.add(header.getName());
            }
        }

        // Second, add headers from the cache entry to the network response as long as
        // they didn't appear in the network response, which should take precedence.
        List<Header> combinedHeaders = new ArrayList<>(responseHeaders);
        if (entry.allResponseHeaders != null) {
            if (!entry.allResponseHeaders.isEmpty()) {
                for (Header header : entry.allResponseHeaders) {
                    if (!headerNamesFromNetworkResponse.contains(header.getName())) {
                        combinedHeaders.add(header);
                    }
                }
            }
        } else {
            // Legacy caches only have entry.responseHeaders.
            if (!entry.responseHeaders.isEmpty()) {
                for (Map.Entry<String, String> header : entry.responseHeaders.entrySet()) {
                    if (!headerNamesFromNetworkResponse.contains(header.getKey())) {
                        combinedHeaders.add(new Header(header.getKey(), header.getValue()));
                    }
                }
            }
        }
        return combinedHeaders;
    }

首先，调用mBaseHttpStack的excuteRequest()方法，得到请求的结果。然后根据响应码，判断是不是304(HttpURLConnection.HTTP_NOT_MODIFIED)，如果是，表明使用上一次请求的结果即可。取request中的cacheEntry的值，创建NetworkResponse，并返回。如果不是，读取接下来的数据流。用 inputStreamToBytes() 方法将接下来的数据流转换成byte[]（注释：就是在这里，我们使用到了byteArrayPool）。
继续判断状态码，如果是小于200或大于299，则表明出现异常了，直接抛出一个IOException异常。在异常catch代码中，我们创建一个异常的NetworkResponse。如果不是，继续往下走。我们根据状态码和内容，创建一个新的NetworkResponse，并返回。好了，BaseNetwork源码就分析完毕。