其实刚开始看volley代码的时候看到这三个类也不知道有啥区别，有点懵。先来看一看Http的响应报文：
HTTP的响应报文由状态行、消息报头、空行、响应正文组成。响应报头后面会讲到，响应正文是服务器返回的资源的内容，先来看看状态行。
状态行
1、状态行格式如下：
HTTP-Version Status-Code Reason-Phrase CRLF
例：HTTP/1.1 200 OK
其中，HTTP-Version表示服务器HTTP协议的版本；Status-Code表示服务器发回的响应状态代码；Reason-Phrase表示状态代码的文本描述。
状态代码有三位数字组成，第一个数字定义了响应的类别，且有五种可能取值：
1.100~199：指示信息，表示请求已接收，继续处理
2.200~299：请求成功，表示请求已被成功接收、理解、接受
3.300~399：重定向，要完成请求必须进行更进一步的操作
4.400~499：客户端错误，请求有语法错误或请求无法实现
5.500~599：服务器端错误，服务器未能实现合法的请求
常见的状态码如下：
1.200 OK：客户端请求成功
2.400 Bad Request：客户端请求有语法错误，不能被服务器所理解
3.401 Unauthorized：请求未经授权，这个状态代码必须和WWW-Authenticate报头域一起使用
4.403 Forbidden：服务器收到请求，但是拒绝提供服务
5.500 Internal Server Error：服务器发生不可预期的错误
6.503 Server Unavailable：服务器当前不能处理客户端的请求，一段时间后可能恢复正常
在volley的HurlStack通过HttpURLConnection访问网络，首先返回的是HttpResponse对象，代码如下：
if (!hasResponseBody(request.getMethod(), responseCode)) {
return new HttpResponse(responseCode, convertHeaders(connection.getHeaderFields()));
}
return new HttpResponse(responseCode, convertHeaders(connection.getHeaderFields()),
connection.getContentLength(), inputStreamFromConnection(connection));
根据请求方式和响应值来判断是否包含一个body也就是实体内容。convertHeaders()Map<String, List<String>>类型的响应头转化为 List<Header> HttpResponse的构造方法如下：
/**
* Construct a new HttpResponse.
*
* @param statusCode the HTTP status code of the response
* @param headers the response headers
* @param contentLength the length of the response content. Ignored if there is no content.
* @param content an {@link InputStream} of the response content. May be null to indicate that
* the response has no content.
*/
public HttpResponse(
int statusCode, List<Header> headers, int contentLength, InputStream content) {
mStatusCode = statusCode;
mHeaders = headers;
mContentLength = contentLength;
mContent = content;
}
成员变量是什么注释说的很清楚~。将HttpResponse返回到BasicNetwork之后，根据返回状态码进行情况判断创建不同的NetworkResponse对象返回到NetworkDispatcher。
return new NetworkResponse(statusCode, responseContents, false,
SystemClock.elapsedRealtime() - requestStart, responseHeaders);
上面这种返回是响应值为200时候的。NetworkResponse有好几个重载的构造方法如下：
/**
* Creates a new network response.
* @param statusCode the HTTP status code
* @param data Response body
* @param headers Headers returned with this response, or null for none
* @param notModified True if the server returned a 304 and the data was already in cache
* @param networkTimeMs Round-trip network time to receive network response
* @deprecated see {@link #NetworkResponse(int, byte[], boolean, long, List)}. This constructor
* cannot handle server responses containing multiple headers with the same name.
* This constructor may be removed in a future release of Volley.
*/
@Deprecated
public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
boolean notModified, long networkTimeMs) {
this(statusCode, data, headers, toAllHeaderList(headers), notModified, networkTimeMs);
}
/**
* Creates a new network response.
* @param statusCode the HTTP status code
* @param data Response body
* @param notModified True if the server returned a 304 and the data was already in cache
* @param networkTimeMs Round-trip network time to receive network response
* @param allHeaders All headers returned with this response, or null for none
*/
public NetworkResponse(int statusCode, byte[] data, boolean notModified, long networkTimeMs,
List<Header> allHeaders) {
this(statusCode, data, toHeaderMap(allHeaders), allHeaders, notModified, networkTimeMs);
}
/**
* Creates a new network response.
* @param statusCode the HTTP status code
* @param data Response body
* @param headers Headers returned with this response, or null for none
* @param notModified True if the server returned a 304 and the data was already in cache
* @deprecated see {@link #NetworkResponse(int, byte[], boolean, long, List)}. This constructor
* cannot handle server responses containing multiple headers with the same name.
* This constructor may be removed in a future release of Volley.
*/
@Deprecated
public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
boolean notModified) {
this(statusCode, data, headers, notModified, 0);
}
/**
* Creates a new network response for an OK response with no headers.
* @param data Response body
*/
public NetworkResponse(byte[] data) {
this(HttpURLConnection.HTTP_OK, data, false, 0, Collections.<Header>emptyList());
}
/**
* Creates a new network response for an OK response.
* @param data Response body
* @param headers Headers returned with this response, or null for none
* @deprecated see {@link #NetworkResponse(int, byte[], boolean, long, List)}. This constructor
* cannot handle server responses containing multiple headers with the same name.
* This constructor may be removed in a future release of Volley.
*/
@Deprecated
public NetworkResponse(byte[] data, Map<String, String> headers) {
this(HttpURLConnection.HTTP_OK, data, headers, false, 0);
}
private NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
List<Header> allHeaders, boolean notModified, long networkTimeMs) {
this.statusCode = statusCode;
this.data = data;
this.headers = headers;
if (allHeaders == null) {
this.allHeaders = null;
} else {
this.allHeaders = Collections.unmodifiableList(allHeaders); //作用返回一个只读的集合
}
this.notModified = notModified;
this.networkTimeMs = networkTimeMs;
}
到NetworkResponse这里 ，已经有了响应状态，响应内容，响应头等。返回到NetworkDispatcher之后，又调用Request的parseNetworkResponse方法将NetworkResponse转化为Response<?>。parseNetworkResponse是一个抽象方法，StringRequest里实现如下：
@Override
protected Response<String> parseNetworkResponse(NetworkResponse response) {
String parsed;
try {
//将byte[]数组根据指定编码转化为String
parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
} catch (UnsupportedEncodingException e) {
parsed = new String(response.data);
}
//HttpHeaderParser.parseCacheHeaders(response) 根据返回值header是否缓存进行缓存。
return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
}
通过parseNetworkResponse转化为Response对象。代码如下：
/** Parsed response, or null in the case of error. */
public final T result;
/** Cache metadata for this response, or null in the case of error. */
public final Cache.Entry cacheEntry;
/** Detailed error information if <code>errorCode != OK</code>. */
public final VolleyError error;
/** Returns a successful response containing the parsed result. */
public static <T> Response<T> success(T result, Cache.Entry cacheEntry) {
return new Response<T>(result, cacheEntry);
}
private Response(T result, Cache.Entry cacheEntry) {
this.result = result;
this.cacheEntry = cacheEntry;
this.error = null;
}
这样就完成了转换。
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