URLConnection的使用

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An URLConnection for HTTP (RFC 2616) used to send and receive data over the web. Data may be of any type and length. This class may be used to send and receive streaming data whose length is not known in advance.

Uses of this class follow a pattern:

1. Obtain a new HttpURLConnection by calling URL.openConnection() and casting the result to HttpURLConnection.
2. Prepare the request. The primary property of a request is its URI. Request headers may also include metadata such as credentials, preferred content types, and session cookies.
3. Optionally upload a request body. Instances must be configured with setDoOutput(true) if they include a request body. Transmit data by writing to the stream returned by getOutputStream().
4. Read the response. Response headers typically include metadata such as the response body‘s content type and length, modified dates and session cookies. The response body may be read from the stream returned by getInputStream(). If the response has no body, that method returns an empty stream.
5. Disconnect. Once the response body has been read, the HttpURLConnection should be closed by calling disconnect(). Disconnecting releases the resources held by a connection so they may be closed or reused.

For example, to retrieve the webpage at http://www.android.com/:

   URL url = new URL("http://www.android.com/");    HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();    try {      InputStream in = new BufferedInputStream(urlConnection.getInputStream());      readStream(in);     finally {      urlConnection.disconnect();    }  }

Secure Communication with HTTPS

Response Handling

If the HTTP response indicates that an error occurred, getInputStream() will throw an IOException. Use getErrorStream() to read the error response. The headers can be read in the normal way using getHeaderFields(),

Posting Content

For best performance, you should call either setFixedLengthStreamingMode(int) when the body length is known in advance, orsetChunkedStreamingMode(int) when it is not. Otherwise HttpURLConnection will be forced to buffer the complete request body in memory before it is transmitted, wasting (and possibly exhausting) heap and increasing latency.

For example, to perform an upload:

   HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();    try {      urlConnection.setDoOutput(true);      urlConnection.setChunkedStreamingMode(0);      OutputStream out = new BufferedOutputStream(urlConnection.getOutputStream());      writeStream(out);      InputStream in = new BufferedInputStream(urlConnection.getInputStream());      readStream(in);     finally {      urlConnection.disconnect();    }  }

Performance

When transferring large amounts of data to or from a server, use streams to limit how much data is in memory at once. Unless you need the entire body to be in memory at once, process it as a stream (rather than storing the complete body as a single byte array or string).

To reduce latency, this class may reuse the same underlying Socket for multiple request/response pairs. As a result, HTTP connections may be held open longer than necessary. Calls to disconnect() may return the socket to a pool of connected sockets. This behavior can be disabled by setting the http.keepAlive system property to false before issuing any HTTP requests. The http.maxConnections property may be used to control how many idle connections to each server will be held.

By default, this implementation of HttpURLConnection requests that servers use gzip compression and it automatically decompresses the data for callers of getInputStream(). The Content-Encoding and Content-Length response headers are cleared in this case. Gzip compression can be disabled by setting the acceptable encodings in the request header:

   urlConnection.setRequestProperty("Accept-Encoding", "identity");  

Setting the Accept-Encoding request header explicitly disables automatic decompression and leaves the response headers intact; callers must handle decompression as needed, according to the Content-Encoding header of the response.

getContentLength() returns the number of bytes transmitted and cannot be used to predict how many bytes can be read fromgetInputStream() for compressed streams. Instead, read that stream until it is exhausted, i.e. when read() returns -1.

Handling Network Sign-On

   HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();    try {      InputStream in = new BufferedInputStream(urlConnection.getInputStream());      if (!url.getHost().equals(urlConnection.getURL().getHost())) {        // we were redirected! Kick the user out to the browser to sign on?            ...    } finally {      urlConnection.disconnect();    }  }

HTTP Authentication

   Authenticator.setDefault(new Authenticator() {      protected PasswordAuthentication getPasswordAuthentication() {        return new PasswordAuthentication(username, password.toCharArray());          });  }

Sessions with Cookies

   CookieManager cookieManager = new CookieManager();    CookieHandler.setDefault(cookieManager);  

The default CookieManager keeps all accepted cookies in memory. It will forget these cookies when the VM exits. Implement CookieStore to define a custom cookie store.

In addition to the cookies set by HTTP responses, you may set cookies programmatically. To be included in HTTP request headers, cookies must have the domain and path properties set.

By default, new instances of HttpCookie work only with servers that support RFC 2965 cookies. Many web servers support only the older specification, RFC 2109. For compatibility with the most web servers, set the cookie version to 0.

For example, to receive www.twitter.com in French:

   HttpCookie cookie = new HttpCookie("lang", "fr");    cookie.setDomain("twitter.com");    cookie.setPath("/");    cookie.setVersion(0);    cookieManager.getCookieStore().add(new URI("http://twitter.com/"), cookie);  

HTTP Methods

HttpURLConnection uses the GET method by default. It will use POST if setDoOutput(true) has been called. Other HTTP methods (OPTIONS, HEAD, PUT, DELETE and TRACE) can be used with setRequestMethod(String).

Proxies

IPv6 Support

This class includes transparent support for IPv6. For hosts with both IPv4 and IPv6 addresses, it will attempt to connect to each of a host‘s addresses until a connection is established.

URLConnection的使用

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原文地址：http://my.oschina.net/doz/blog/525732