我正在尝试使用ImageIO从网上读取图像：
URL url = new URL(location);
bi = ImageIO.read(url);
当 location 是以实际图片结尾的网址（例如http://www.lol.net/1.jpg）时，上述代码有效。但是，当网址是重定向时（例如http://www.lol.net/redirection，导致http://www.lol.net/1.jpg），上面的代码会在 bi 中返回 null 。
两个问题。一，为什么会发生这种情况？是因为ImageIO库试图根据URL字符串找到合适的ImageReader吗？第二，这个限制最干净的解决方案是什么？请注意，我需要 BufferedImage 输出而不是 Image 输出。
编辑：对于想要测试它的人，我正在尝试阅读的网址为http://graph.facebook.com/804672289/picture，该网址已转换为 http://profile.ak.fbcdn.net/hprofile-ak-snc4/hs351.snc4/41632_804672289_6662_q.jpg
编辑2 ：我在上一次编辑中不正确。网址为https://graph.facebook.com/804672289/picture。如果我用http替换https，上面的代码工作正常。所以我的新问题是如何让它与HTTPS一起工作，所以我不需要做替换。
答案 0 :(得分：4)
对我而言，http://www.lol.net/1.jpg似乎不直接指向图像。
正如@Bozho指出的那样，ImageIO使用默认的URL.openConnection（因为地址以“http”开头）返回HttpURLConnection，默认情况下为setFollowRedirects(true)。
关于你的编辑，这段代码似乎对我有用：
URL url = new URL("http://graph.facebook.com/804672289/picture");
BufferedImage bi = ImageIO.read(url);
System.out.println(bi);
// Prints: BufferedImage@43b09468: type = 5 ColorModel: #pixelBits = 24 numComponents = 3 color space = java.awt.color.ICC_ColorSpace@7ddf5a8f transparency = 1 has alpha = false isAlphaPre = false ByteInterleavedRaster: width = 50 height = 50 #numDataElements 3 dataOff[0] = 2
我怀疑你的错误是在其他地方。
答案 1 :(得分：4)
我遇到了与https图片链接相同的问题。问题是，当我读取https链接时，在代码中，它将返回200.但实际上它是301.为了解决这个问题，我使用了＆＃34; curl＆＃34; user-agent，这样我就可以获得301并迭代直到找到最终的链接。请参阅以下代码：
希望这有助于@Eldad-Mor。
private InputStream getInputStream(String url) throws IOException {
InputStream stream = null;
try {
stream = handleRedirects(url);
byte[] bytes = IOUtils.toByteArray(stream);
return new ByteArrayInputStream(bytes);
} finally {
if (stream != null)
stream.close();
}
}
/**
* Handle redirects in the URL Manually. Method calls itself if redirects are found until there re no more redirects.
*
* @param url URL
* @return input stream
* @throws IOException
*/
private InputStream handleRedirects(String url) throws IOException {
HttpURLConnection.setFollowRedirects(false);
URL obj = new URL(url);
HttpURLConnection conn = (HttpURLConnection) obj.openConnection();
conn.setRequestProperty("User-Agent", "curl/7.30.0");
conn.connect();
boolean redirect = false;
LOG.info(conn.getURL().toString());
int status = conn.getResponseCode();
if (status != HttpURLConnection.HTTP_OK) {
if (status == HttpURLConnection.HTTP_MOVED_TEMP
|| status == HttpURLConnection.HTTP_MOVED_PERM
|| status == HttpURLConnection.HTTP_SEE_OTHER)
redirect = true;
}
if (redirect) {
String newUrl =conn.getHeaderField("Location");
conn.getInputStream().close();
conn.disconnect();
return handleRedirects(newUrl);
}
return conn.getInputStream();
}
答案 2 :(得分：2)
关于您的编辑2：
ImageIo.read（url），不关心url类型。即，http或https。
您只需将网址传递给此方法即可，但如果您要传递https网址，则需要执行某些步骤来验证SSL证书。请找到以下示例。
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.security.cert.X509Certificate;
import javax.imageio.ImageIO;
import javax.net.ssl.HttpsURLConnection;
import javax.net.ssl.SSLContext;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;
/**
* @author Syed CBE
*
*/
public class Main {
public static void main(String[] args) {
int height = 0,width = 0;
String imagePath="https://www.sampledomain.com/sampleimage.jpg";
System.out.println("URL=="+imagePath);
InputStream connection;
try {
URL url = new URL(imagePath);
if(imagePath.indexOf("https://")!=-1){
final SSLContext sc = SSLContext.getInstance("SSL");
sc.init(null, getTrustingManager(), new java.security.SecureRandom());
HttpsURLConnection.setDefaultSSLSocketFactory(sc.getSocketFactory());
connection = url.openStream();
}
else{
connection = url.openStream();
}
BufferedImage bufferedimage = ImageIO.read(connection);
width = bufferedimage.getWidth();
height = bufferedimage.getHeight();
System.out.println("width="+width);
System.out.println("height="+height);
} catch (MalformedURLException e) {
System.out.println("URL is not correct : " + imagePath);
} catch (IOException e) {
System.out.println("IOException Occurred : "+e);
}
catch (Exception e) {
System.out.println("Exception Occurred : "+e);
}
}
private static TrustManager[] getTrustingManager() {
TrustManager[] trustAllCerts = new TrustManager[] { new X509TrustManager() {
@Override
public java.security.cert.X509Certificate[] getAcceptedIssuers() {
return null;
}
@Override
public void checkClientTrusted(X509Certificate[] certs, String authType) {
}
@Override
public void checkServerTrusted(X509Certificate[] certs, String authType) {
}
} };
return trustAllCerts;
}
}
// This Code will work only for http, you can make it work for https but you need additional code to add SSL certificate using keystore
public static void getImage(String testUrl) {
String testUrl="https://sampleurl.com/image.jpg";
HttpGet httpGet = new HttpGet(testUrl);
System.out.println("HttpGet is ---->"+httpGet.getURI());
HttpResponse httpResponse = null;
HttpClient httpClient=new DefaultHttpClient();
try {
httpResponse = httpClient.execute(httpGet);
InputStream stream=httpResponse.getEntity().getContent();
BufferedImage sourceImg = ImageIO.read(stream);
System.out.println("------ source -----"+sourceImg.getHeight());
sourceImg.getWidth();
System.out.println(sourceImg);
}catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
System.out.println("------ MalformedURLException -----"+e);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
System.out.println("------ IOEXCEPTION -----"+e);
}
}
答案 3 :(得分：0)
目前可以使用Apache HttpClient，对我来说它运行正常。
public static void main(String args[]) throws Exception{
String url = "http://graph.facebook.com/804672289/picture?width=800";
HttpClient client = HttpClients.createDefault();
HttpResponse response = client.execute(new HttpGet(url));
BufferedImage image = ImageIO.read(input);
if(image == null){
throw new RuntimeException("Ooops");
} else{
System.out.println(image.getHeight());
}
}