当应用想要访问其他应用共享的文件时,请求方应用(客户端)通常会向共享文件的应用(服务器)发送请求。在大多数情况下,请求会在服务器应用中启动一个 Activity
,以显示可共享的文件。
用户选择文件后,服务器应用会将文件的内容 URI 返回给客户端应用。
本课将介绍客户端应用如何从服务器应用请求文件,如何从服务器应用接收文件的内容 URI,以及如何使用内容 URI 打开文件。
发送文件请求
如需从服务器应用请求文件,客户端应用需要使用 Intent
调用 startActivityForResult
,该 intent 包含 ACTION_PICK
等操作和客户端应用能够处理的 MIME 类型。
例如,以下代码段演示了如何将 Intent
发送到服务器应用,以便启动共享文件中所述的 Activity
:
Kotlin
class MainActivity : Activity() { private lateinit var requestFileIntent: Intent private lateinit var inputPFD: ParcelFileDescriptor ... override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) requestFileIntent = Intent(Intent.ACTION_PICK).apply { type = "image/jpg" } ... } ... private fun requestFile() { /** * When the user requests a file, send an Intent to the * server app. * files. */ startActivityForResult(requestFileIntent, 0) ... } ... }
Java
public class MainActivity extends Activity { private Intent requestFileIntent; private ParcelFileDescriptor inputPFD; ... @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); requestFileIntent = new Intent(Intent.ACTION_PICK); requestFileIntent.setType("image/jpg"); ... } ... protected void requestFile() { /** * When the user requests a file, send an Intent to the * server app. * files. */ startActivityForResult(requestFileIntent, 0); ... } ... }
访问请求的文件
服务器应用通过 Intent
将文件的内容 URI 发回给客户端应用。此 Intent
会通过替换 onActivityResult()
传递给客户端应用。客户端应用获得文件的内容 URI 后,即可通过获取其 FileDescriptor
来访问该文件。
只要您正确解析客户端应用收到的内容 URI,在此过程中便可确保文件的安全。解析内容时,必须确保此 URI 不指向目标目录之外的任何内容,以确保不会尝试进行路径遍历。 只有客户端应用应获得对文件的访问权限,而且必须具有服务器应用授予的权限才能访问。这些权限是临时的,因此一旦客户端应用的任务堆栈结束,便无法再从服务器应用外访问该文件。
以下代码段展示了客户端应用如何处理服务器应用发送的 Intent
,以及如何使用内容 URI 获取 FileDescriptor
:
Kotlin
/* * When the Activity of the app that hosts files sets a result and calls * finish(), this method is invoked. The returned Intent contains the * content URI of a selected file. The result code indicates if the * selection worked or not. */ public override fun onActivityResult(requestCode: Int, resultCode: Int, returnIntent: Intent) { // If the selection didn't work if (resultCode != Activity.RESULT_OK) { // Exit without doing anything else return } // Get the file's content URI from the incoming Intent returnIntent.data?.also { returnUri -> /* * Try to open the file for "read" access using the * returned URI. If the file isn't found, write to the * error log and return. */ inputPFD = try { /* * Get the content resolver instance for this context, and use it * to get a ParcelFileDescriptor for the file. */ contentResolver.openFileDescriptor(returnUri, "r") } catch (e: FileNotFoundException) { e.printStackTrace() Log.e("MainActivity", "File not found.") return } // Get a regular file descriptor for the file val fd = inputPFD.fileDescriptor ... } }
Java
/* * When the Activity of the app that hosts files sets a result and calls * finish(), this method is invoked. The returned Intent contains the * content URI of a selected file. The result code indicates if the * selection worked or not. */ @Override public void onActivityResult(int requestCode, int resultCode, Intent returnIntent) { // If the selection didn't work if (resultCode != RESULT_OK) { // Exit without doing anything else return; } else { // Get the file's content URI from the incoming Intent Uri returnUri = returnIntent.getData(); /* * Try to open the file for "read" access using the * returned URI. If the file isn't found, write to the * error log and return. */ try { /* * Get the content resolver instance for this context, and use it * to get a ParcelFileDescriptor for the file. */ inputPFD = getContentResolver().openFileDescriptor(returnUri, "r"); } catch (FileNotFoundException e) { e.printStackTrace(); Log.e("MainActivity", "File not found."); return; } // Get a regular file descriptor for the file FileDescriptor fd = inputPFD.getFileDescriptor(); ... } }
方法 openFileDescriptor()
会返回文件的 ParcelFileDescriptor
。客户端应用从此对象获取 FileDescriptor
对象,然后使用该对象读取文件。
如需了解其他相关信息,请参阅: