请求某个分享的文件

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。

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