直接上程式碼：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.READ_PHONE_STATE" />

new Thread(new Runnable() {
@Override
public void run() {
download()
 
;
}
}).start();

//下載具體操作
private boolean download() {
try {
URL url = new URL("http://file.chmsp.com.cn/colligate/file/00100000224821.pdf");
//開啟連線
URLConnection conn = url.openConnection();
//開啟輸入流
InputStream is = conn.getInputStream();
//獲得長度
int contentLength = conn.getContentLength();
Log.e("", "檔案長度 = " + contentLength
 
);
//建立資料夾 MyDownLoad，在儲存卡下
String dirName = Environment.getExternalStorageDirectory() + "/";
//下載後的檔名
String fileName = dirName + "00100000224821.pdf";
File file1 = new File(fileName);
if (file1.exists()) {
Log.e("檔案-----", "檔案已經存在！");
return fileIsExists(fileName);
} else {
//建立位元組流
byte[] bs = new byte[1024];
int 
 
len;
OutputStream os = new FileOutputStream(fileName);
//寫資料
while ((len = is.read(bs)) != -1) {
os.write(bs, 0, len);
}
//完成後關閉流
Log.e("檔案不存在", "下載成功！");
os.close();
is.close();
}
    } catch (Exception e) {
e.printStackTrace();
}
return false;
}
//判斷檔案是否存在
public boolean fileIsExists(String strFile) {
try {
File f = new File(strFile);
if (!f.exists()) {
return false;
}
    } catch (Exception e) {
return false;
}
return true;
}