URL、URLConnection和URLPermission
URL對象代表統一資源定位器,它是對指向互聯網“資源”的指針。資源可以是簡單的文件或目錄,也可以是對更爲複雜對象的引用,例如對數據庫或搜索引擎的查詢。在通常情況下,URL可以由協議名、主機、端口和資源組成。
URL類提供了多個構造器用於創建URL對象,一旦獲得了URL對象之後,就可以調用如下方法來訪問URL對應的資源。
方法 | |
---|---|
String getFile() | 獲取該URL的資源名 |
String getHost() | 獲取該URL的主機名 |
String getPath() | 獲取該URL的路徑部分 |
int getPort() | 獲取該URL的端口號 |
String getProtocol() | 獲取該URL的協議名稱 |
String getQuery() | 獲取該URL的查詢字符串部分 |
URLConnection openConnection() | 返回一個URLConnection對象,它代表了與URL所引用的遠程對象的連接 |
InputStream openStream() | 打開於此URL的連接,並返回一個用於讀取該URL資源的InputStream |
測試URL類方法的代碼:
public class URLTest {
public static void main(String[] args) throws Exception{
URL url = new URL("http://img2.ph.126.net/Mpr7kRqQ51wTt7KHYmJI6A==/6619508599956323849.jpg");
System.out.println("URL資源名:" + url.getFile());
System.out.println("URl主機名:" + url.getHost());
System.out.println("URL路徑:" + url.getPath());
System.out.println("URL端口號:" + url.getPort());
System.out.println("URL協議:" + url.getProtocol());
System.out.println("URL查詢字符串部分:" + url.getQuery());
}
}
測試結果:
URL資源名:/Mpr7kRqQ51wTt7KHYmJI6A==/6619508599956323849.jpg
URl主機名:img2.ph.126.net
URL路徑:/Mpr7kRqQ51wTt7KHYmJI6A==/6619508599956323849.jpg
URL端口號:-1
URL協議:http
URL查詢字符串部分:null
使用openStream()方法可以讀取該URL資源的InputStream,通過該方法可以非常方便地讀取遠程資源——甚至實現多線程下載。
實現步驟:
1.創建URL對象。
2.獲取指定URL對象所指向資源的大小(通過getContentLength()方法獲得)。
3.在本地磁盤上創建一個與網絡資源具有相同大小的空文件。
4.計算每個線程應該下載網絡資源的哪個部分(從哪個字節開始,到哪個字節結束)。
5.依次創建、啓動多個線程來下載網絡資源的指定部分。
下載工具類代碼的實現:
public class DownUtil {
//定義下載資源路徑
private String path;
//指定所下載的文件保存位置
private String targetFile;
//定義需要使用多少個線程下載資源
private int threadNum;
//定義下載的線程對象
private DownThread[] threads;
//定義下載的文件的總大小
private int fileSize;
public DownUtil(String path, String targetFile, int threadNum){
this.path = path;
this.threadNum = threadNum;
//初始化threads數組
threads = new DownThread[threadNum];
this.targetFile = targetFile;
}
public void download() throws Exception{
URL url = new URL(path);
//URLConnection表示應用程序和URL之間的通信連接
//HttpURLConnection表示與URL之間的http連接
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
//連接超時設置
conn.setConnectTimeout(5 * 1000);
//請求獲取Request-URI所標識的資源
conn.setRequestMethod("GET");
//客戶端的配置,需求
//setRequestProperty()可以不設置
conn.setRequestProperty("Accept", "image/gif, image/jpeg, image/pjpeg, "
+ "application/x-shockwave-flash, application/xaml+xml,"
+ "application/vnd.ms-xpsdocument, application/x-ms-xbap,"
+ "application/x-ms-application, application/vnd.ms-excel,"
+ "application/vnd.ms-powerpoint, application/msword, */*");
//支持語言
conn.setRequestProperty("Accept-Language", "zh-CN");
//返回的字符
conn.setRequestProperty("Charset", "UTF-8");
//連接方式
conn.setRequestProperty("Connection", "Keep-Alive");
//得到文件大小
fileSize = conn.getContentLength();
conn.disconnect();
//每個線程所需下載的大小
int currentPartSize = fileSize / threadNum + 1;
//保存文件
RandomAccessFile file = new RandomAccessFile(targetFile, "rw");
//設置本地文件的大小
file.setLength(fileSize);
file.close();
for(int i = 0; i < threadNum; i++){
//計算每個線程下載的開始位置
int startPos = i * currentPartSize;
//每個線程使用一個RandomAccessFile進行下載
RandomAccessFile currentPart = new RandomAccessFile(targetFile, "rw");
//定位該線程的下載位置
currentPart.seek(startPos);
//創建下載線程
threads[i] = new DownThread(startPos, currentPartSize, currentPart);
//啓動下載線程
threads[i].start();
}
}
//獲取下載的完成百分比
public double getCompleteRate(){
//統計多個線程已經下載的總大小
int sumSize = 0;
for(int i = 0; i < threadNum; i++){
sumSize += threads[i].length;
}
//返回已經完成的百分比
return sumSize * 1.0 / fileSize;
}
private class DownThread extends Thread{
//當前線程的下載位置
private int startPos;
//定義當前線程負責下載的文件大小
private int currentPartSize;
//當前線程需要下載的文件塊
private RandomAccessFile currentPart;
//定義該線程已下載的字節數
private int length;
public DownThread(int startPos, int currentPartSize, RandomAccessFile currentPart) {
this.startPos = startPos;
this.currentPartSize = currentPartSize;
this.currentPart = currentPart;
}
public void run(){
try{
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
//連接超時設置
conn.setConnectTimeout(5 * 1000);
//請求獲取Request-URI所標識的資源
conn.setRequestMethod("GET");
//客戶端的配置,需求
conn.setRequestProperty("Accept", "image/gif, image/jpeg, image/pjpeg, "
+ "application/x-shockwave-flash, application/xaml+xml,"
+ "application/vnd.ms-xpsdocument, application/x-ms-xbap,"
+ "application/x-ms-application, application/vnd.ms-excel,"
+ "application/vnd.ms-powerpoint, application/msword, */*");
//支持語言
conn.setRequestProperty("Accept-Language", "zh-CN");
//返回的字符
conn.setRequestProperty("Charset", "UTF-8");
InputStream inStream = conn.getInputStream();
//跳過startPos個字節,表明該線程只下載自己負責的那部分文件
inStream.skip(this.startPos);
byte[] buffer = new byte[1024];
int hasRead = 0;
//讀取網絡數據,並寫入本地文件
while (length < currentPartSize &&
(hasRead = inStream.read(buffer)) != -1){
currentPart.write(buffer, 0, hasRead);
//累計該線程下載的總大小
length += hasRead;
}
currentPart.close();
inStream.close();
}catch (Exception e){
e.printStackTrace();
}
}
}
}
主程序代碼:
public class MultiThreadDown {
public static void main(String[] args) throws Exception{
//初始化DownUtil對象
final DownUtil downUtil = new DownUtil("http://s1.dwstatic.com/group1/"
+ "M00/65/09/5e4f2b1472094ccaf7b7f2fda38b01d9.gif",
"1.png", 4);
downUtil.download();
new Thread(() -> {
while(downUtil.getCompleteRate() < 1){
//每隔0.1秒查詢一次任務得完成速度
//GUI程序中可根據該進度來繪製進度條
System.out.println("已完成:" + downUtil.getCompleteRate());
try{
Thread.sleep(1000);
}catch (Exception e){}
}
}).start();;
}
}
運行程序可以下載到一張圖片。