在使用ActivityResultContracts.CreateDocument启动系统文件选择器时，onActivityResult的回调函数中没有调用onResult，因此无法获得所选文件的Uri。解决方法是在启动ActivityResultContracts.CreateDocument时，将返回的Uri保存在一个变量中，并在onActivityResult回调函数中手动调用onResult方法。

以下是示例代码：

private ActivityResultLauncher mDocumentLauncher;

private Uri mSelectedDocumentUri;

@Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);

mDocumentLauncher = registerForActivityResult(new ActivityResultContracts.CreateDocument(), uri -> {
    mSelectedDocumentUri = uri;
    // manually call onResult
    onDocumentSelected(uri);
});

// start file picker
mDocumentLauncher.launch("my_document.txt");

}

@Override protected void onActivityResult(int requestCode, int resultCode, Intent data) { super.onActivityResult(requestCode, resultCode, data);

if (requestCode == MY_DOCUMENT_REQUEST_CODE) {
    if (resultCode == RESULT_OK) {
        // do nothing, onResult is already called manually
    } else {
        // handle cancel
    }
}

}

private void onDocumentSelected(Uri uri) { // handle selected uri here Log.d(TAG, "Selected document URI: " + uri); }

需要注意的是，在手动调用onResult时需要保证在主线程中执行。