Java可以對Url進行Http Request來取得Response，
得到input stream後將檔案儲存下來，
為了避免有些Server會擋程式的Request，
我們必須模仿瀏覽器，在Request中加上User-Agent的Header (或更多的其他Header，模仿的越像越不容易被擋)，
如果圖片的Url是http的話比較簡單，
但如果是https的話，即SSL，那就要取得對方Server網站的憑證，
或是自己實作一個 X509TrustManager，來所有憑證檢查都通過，
如果是JDK 1.8 以下，可能會有此Exception :
Could not generate DH keypair
使用 JDK 1.8 就可解決此問題，
可參考 [Java] 處理無法透過SSL抓取網站資料的問題
以下為程式碼範例，說明都寫在註解中：
import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.security.KeyManagementException;
import java.security.NoSuchAlgorithmException;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;
import javax.net.ssl.HttpsURLConnection;
import javax.net.ssl.SSLContext;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;
public class ImageDownloader {
public static void main(String[] args) {
String fileName_http = downloadImageFromUrl("http://www.image.com/files/8813/5551/7470/cruise-ship.png","D:" + File.separator, "HttpImgTest");
System.out.println("Http的圖片下載: " + fileName_http);
String fileName_https = downloadImageFromUrl("https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Plithocyon_armagnacensis.JPG/220px-Plithocyon_armagnacensis.JPG","D:" + File.separator, "HttpsImgTest");
System.out.println("Https的圖片下載: " + fileName_https);
}
public static String downloadImageFromUrl(String url, String fileDirectoryPath, String fileNameWithoutFormat) {
String filePath = null;
BufferedInputStream in = null;
ByteArrayOutputStream out = null;
HttpURLConnection httpUrlConnection = null;
FileOutputStream file = null;
try {
if (url.startsWith("https://")) {
//HTTPS時
httpUrlConnection = getHttpURLConnectionFromHttps(url);
}
//如果不是HTTPS或是沒成功得到httpUrlConnection，用HTTP的方法
if(httpUrlConnection == null) {
httpUrlConnection = (HttpURLConnection) (new URL(url)).openConnection();
}
// 設置User-Agent，偽裝成一般瀏覽器，不然有些伺服器會擋掉機器程式請求
httpUrlConnection.setRequestProperty("User-Agent",
"Mozilla/5.0 (Linux; Android 4.2.1; Nexus 7 Build/JOP40D) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.166 Safari/535.19");
httpUrlConnection.connect();
String imageType;
if (httpUrlConnection.getResponseCode() == 200) {
//成功取得response，
//取得contentType
String contentType = httpUrlConnection.getHeaderField("Content-Type");
// 只處理image的回應
if ("image".equals(contentType.substring(0, contentType.indexOf("/")))) {
//得到對方Server提供的圖片副檔名，如jpg, png等
imageType = contentType.substring(contentType.indexOf("/") + 1);
if (imageType != null && !"".equals(imageType)) {
//由HttpUrlConnection取得輸入串流
in = new BufferedInputStream(httpUrlConnection.getInputStream());
out = new ByteArrayOutputStream();
//建立串流Buffer
byte[] buffer = new byte[1024];
file = new FileOutputStream(new File(fileDirectoryPath + File.separator + fileNameWithoutFormat + "." + imageType));
int readByte;
while ((readByte = in.read(buffer)) != -1) {
//輸出檔案
out.write(buffer, 0, readByte);
}
byte[] response = out.toByteArray();
file.write(response);
//下載成功後，返回檔案路徑
filePath = fileDirectoryPath + File.separator + fileNameWithoutFormat + "." + imageType;
}
}
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
//關閉各種串流
try {
if (out != null) {
out.close();
}
if (in != null) {
in.close();
}
if (httpUrlConnection != null) {
httpUrlConnection.disconnect();
}
if (file != null) {
file.close();
}
}catch (IOException e) {
e.printStackTrace();
}
}
return filePath;
}
public static HttpURLConnection getHttpURLConnectionFromHttps(String url) {
HttpURLConnection httpUrlConnection = null;
//建立一個信認所有憑證的X509TrustManager，放到TrustManager裡面
TrustManager[] trustAllCerts;
try {
// Activate the new trust manager
trustAllCerts = new TrustManager[] { new X509TrustManager() {
public void checkClientTrusted(X509Certificate[] chain, String authType) throws CertificateException { // TODO Auto-generated method stub
//不作任何事
}
public void checkServerTrusted(X509Certificate[] chain, String authType) throws CertificateException { // TODO Auto-generated method stub
//不作任何事
}
public X509Certificate[] getAcceptedIssuers() {
//不作任何事
return null;
}
} };
//設置SSL設定
SSLContext sslContext = SSLContext.getInstance("SSL");
sslContext.init(null, trustAllCerts, new java.security.SecureRandom());
HttpsURLConnection.setDefaultSSLSocketFactory(sslContext.getSocketFactory());
//跟HTTP一樣，用Url建立連線
httpUrlConnection = (HttpURLConnection) (new URL(url)).openConnection();
} catch (KeyManagementException e) {
e.printStackTrace();
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return httpUrlConnection;
}
}
