请求共享文件

当一个应用想要访问另一个应用共享的文件时,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,该请求会在服务器应用中启动一个 Activity,用于显示它可以共享的文件。用户选择一个文件后,服务器应用会将该文件的内容 URI 返回给客户端应用。

本课程介绍客户端应用如何向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,以及如何使用该内容 URI 打开文件。

发送文件请求

要从服务器应用请求文件,客户端应用会调用 startActivityForResult,其中包含一个 Intent,该 Intent 包含诸如 ACTION_PICK 之类的操作以及客户端应用可以处理的 MIME 类型。

例如,以下代码片段演示了如何向服务器应用发送 Intent,以启动 共享文件 中描述的 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

服务器应用会在一个 Intent 中将文件的内容 URI 发回给客户端应用。此 Intent 会在其重写 onActivityResult() 方法中传递给客户端应用。客户端应用获取文件的内容 URI 后,便可通过获取其 FileDescriptor 来访问该文件。

仅当您正确解析客户端应用接收到的内容 URI 时,此过程中的文件安全性才能得以保障。解析内容时,您必须确保此 URI 不指向预期目录之外的任何内容,以确保不会发生路径遍历。只有客户端应用才能访问该文件,并且仅限于服务器应用授予的权限。权限是临时的,因此一旦客户端应用的任务堆栈完成,该文件将无法再在服务器应用外部访问。

以下代码片段演示了客户端应用如何处理服务器应用发送的 Intent,以及客户端应用如何使用内容 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。客户端应用可从此对象获取一个 FileDescriptor 对象,然后可以使用该对象读取文件。

如需了解其他相关信息,请参阅