android网络开源框架volley(五)——volley的一些细节

    最近又把volley拿出来整理了下。之前没有遇到过的一些小问题又来了，在此记录下：

1、HttpUrlConnection DELETE 方式无法添加body的问题：java.net.ProtocolException: DELETE does not support writing

    这个可以算是一个系统级的bug，为什么这么说，请看这里，这个问题在java8中才得以解决。没办法直接过去，咱就绕过去。查看HttpUrlConnection，我们发现他是一个抽象类，因此可以试试能不能通过它的其他实现来达到我们的目的。最终我们决定使用okhttp这个实现。地址为：https://github.com/square/okhttp。

接着我们还得去看看volley的源码，由于我们的app兼容的最低版本是4.0，因此我们知道最终调用的是HurlStack：

    public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
...
        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();
            } else {
                // Prior to Gingerbread, HttpUrlConnection was unreliable.
                // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }
...
    }

因此我们只需要将HurlStack的相关代码修改即可，如下：

volley.java

public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
...
        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                // old way: stack = new HurlStack();
            		// http://square.github.io/okhttp/
                stack = new HurlStack(null, null, new OkUrlFactory(new OkHttpClient()));
            } else {
                // Prior to Gingerbread, HttpUrlConnection was unreliable.
                // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }
...
    }

HurlStack.java

/**
 * An {@link HttpStack} based on {@link HttpURLConnection}.
 */
public class HurlStack implements HttpStack {

    private final OkUrlFactory mOkUrlFactory; 

    /**
     * @param urlRewriter Rewriter to use for request URLs
     * @param sslSocketFactory SSL factory to use for HTTPS connections
     * @param okUrlFactory solution delete body(https://github.com/square/okhttp)
     */
    public HurlStack(UrlRewriter urlRewriter, SSLSocketFactory sslSocketFactory, OkUrlFactory okUrlFactory) {
        mUrlRewriter = urlRewriter;
        mSslSocketFactory = sslSocketFactory;
        mOkUrlFactory = okUrlFactory;
    }
    /**
     * Create an {@link HttpURLConnection} for the specified {@code url}.
     */
    protected HttpURLConnection createConnection(URL url) throws IOException {
		if(null != mOkUrlFactory){
			return mOkUrlFactory.open(url);
		}
        return (HttpURLConnection) url.openConnection();
    }


    @SuppressWarnings("deprecation")
    /* package */ 
    static void setConnectionParametersForRequest(HttpURLConnection connection,
            Request request) throws IOException, AuthFailureError {
        switch (request.getMethod()) {
            ...
            case Method.DELETE:
                connection.setRequestMethod("DELETE");
                addBodyIfExists(connection, request);
                break;
            ...
            default:
                throw new IllegalStateException("Unknown method type.");
        }
    }

...
}

2015-04-26更新：

再次使用到需要使用到okhttp，回头看下上面的代码，不知道当时怎么想的，使用这么复杂的方法引入Okhttp，估计是脑袋进水了。再来看下这个方法：newRequestQueue(Context context, HttpStack stack)，有两个参数：context和HttpStack，这里是要传入自己的HttpStack就好了。那么我们用OKhttp的实现：

/**
 * An {@link com.android.volley.toolbox.HttpStack HttpStack} implementation which
 * uses OkHttp as its transport.
 */
public class OkHttpStack extends HurlStack {
  private final OkHttpClient client;
 
  public OkHttpStack() {
    this(new OkHttpClient());
  }
 
  public OkHttpStack(OkHttpClient client) {
    if (client == null) {
      throw new NullPointerException("Client must not be null.");
    }
    this.client = client;
  }
 
  @Override protected HttpURLConnection createConnection(URL url) throws IOException {
    return client.open(url);
  }   
}

参考： https://gist.github.com/JakeWharton/5616899

2、关于（修改）volley的缓存

    volley有完整的一套缓存机制。而目前我们想做个简单的需求：部分界面（几乎不会改动的）简单的做一定时间的缓存，研究了下代码发现很容易修改达到自己的目的（有时间在分析下volley的缓存机制，这个一定要做）。简单来说修改一个地方：request.parseNetworkResponse中的

HttpHeaderParser（此处突然感慨volley的设计TMD灵活了，想怎么改就怎么改）。HttpHeaderParser修改后的代码如下：

/**
 * 修改后的，用户处理缓存
 */
public class BHHttpHeaderParser {

    /**
     * Extracts a {@link Cache.Entry} from a {@link NetworkResponse}.
     *
     * @param response The network response to parse headers from
     * @return a cache entry for the given response, or null if the response is not cacheable.
     */
    public static Cache.Entry parseCacheHeaders(NetworkResponse response, boolean isCustomCache) {
...
        if(isCustomCache){
        		softExpire = now + Config.HTTP_CACHE_TTL;
        } else {
	        	if (hasCacheControl) {
	            softExpire = now + maxAge * 1000;
	        } else if (serverDate > 0 && serverExpires >= serverDate) {
	            // Default semantic for Expire header in HTTP specification is softExpire.
	            softExpire = now + (serverExpires - serverDate);
	        }
        }
        
        Cache.Entry entry = new Cache.Entry();
        entry.data = response.data;
        entry.etag = serverEtag;
        entry.softTtl = softExpire;
        entry.ttl = entry.softTtl;
        entry.serverDate = serverDate;
        entry.responseHeaders = headers;

        return entry;
    }
...
}

此处大家可以发现，我们主要是根据自定义的变量决定如何修改cache的TTL来达到自己的目的。

3、HttpUrlConnection与PATCH（2015-04-26）

在使用Volley发送PATCH请求的时候，我们可能会遇到这样的问题：Unknown method 'PATCH'; must be one of [OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE]。这个时候你的第一反应是什么呢？是Volley不支持PATCH请求吗？换成OkHttp是不是可以呢？查看了下Volley的源码，在HurlHttp.java中发现如下一段：

/* package */ 
static void setConnectionParametersForRequest(HttpURLConnection connection,
            Request request) throws IOException, AuthFailureError {
        switch (request.getMethod()) {
            case Method.DEPRECATED_GET_OR_POST:
                // This is the deprecated way that needs to be handled for backwards compatibility.
                // If the request's post body is null, then the assumption is that the request is
                // GET.  Otherwise, it is assumed that the request is a POST.
                byte[] postBody = request.getPostBody();
                if (postBody != null) {
                    // Prepare output. There is no need to set Content-Length explicitly,
                    // since this is handled by HttpURLConnection using the size of the prepared
                    // output stream.
                    connection.setDoOutput(true);
                    connection.setRequestMethod("POST");
                    connection.addRequestProperty(HEADER_CONTENT_TYPE,
                            request.getPostBodyContentType());
                    DataOutputStream out = new DataOutputStream(connection.getOutputStream());
                    out.write(postBody);
                    out.close();
                }
                break;
            case Method.GET:
                // Not necessary to set the request method because connection defaults to GET but
                // being explicit here.
                connection.setRequestMethod("GET");
                break;
            case Method.DELETE:
                connection.setRequestMethod("DELETE");
                break;
            case Method.POST:
                connection.setRequestMethod("POST");
                addBodyIfExists(connection, request);
                break;
            case Method.PUT:
                connection.setRequestMethod("PUT");
                addBodyIfExists(connection, request);
                break;
            case Method.HEAD:
                connection.setRequestMethod("HEAD");
                break;
            case Method.OPTIONS:
                connection.setRequestMethod("OPTIONS");
                break;
            case Method.TRACE:
                connection.setRequestMethod("TRACE");
                break;
            case Method.PATCH:
                connection.setRequestMethod("PATCH");
                addBodyIfExists(connection, request);
                break;
            default:
                throw new IllegalStateException("Unknown method type.");
        }
    }

通过这段代码，我们知道，Volley对PATCH还是支持的。在细看下错误这个是有HttpUrlConnection抛出的。因此我们需要在这方面下手。这里有一个参考：

https://github.com/adriancole/retrofit/commit/e704b800878b2e37f5ac98b0139cb4994618ace0

以后有其他关于volley的总结都记录在此。