根据Uri获取文档的路径

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根据Url获取文档的绝对路径，解决Android4.4以上版本Uri转换。
Android在4.4之后的版本(包括4.4)中，从相册中选取图片返回Uri进行了改动。所以无法通过该Uri来取得文件路径从而解码图片将其显示出来。
在4.3或以下可以直接用Intent.ACTION_GET_CONTENT打开相册;在4.4或以上,官方建议用ACTION_OPEN_DOCUMENT打开相册
在Android4.4之前得到的Uri为：

  • content://media/external/images/media/8302
  • content://media/external/video/media
  • content://media/external/images/media

而在Android4.4后得到的可能是以下：

  • content://com.android.providers.media.documents/document/image:8302
  • content://com.android.providers.downloads.documents/document/5

以下为Android4.4之后的适配： 

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 /**
 *
 * 专为Android4.4设计的从Uri获取文件绝对路径
 */
@SuppressLint("NewApi")
public static String getPath(final Context context, final Uri uri) {
    final boolean isKitKat = Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT;
    LogUtil.d("uri:" + uri);
    // DocumentProvider
    if (isKitKat && DocumentsContract.isDocumentUri(context, uri)) {
        // ExternalStorageProvider
        if (isExternalStorageDocument(uri)) {
            final String docId = DocumentsContract.getDocumentId(uri);
            final String[] split = docId.split(":");
            final String type = split[0];

            if ("primary".equalsIgnoreCase(type)) {
                return Environment.getExternalStorageDirectory() + "/" + split[1];
            }
            // TODO handle non-primary volumes
        }
        // DownloadsProvider
        else if (isDownloadsDocument(uri)) {

            final String id = DocumentsContract.getDocumentId(uri);
            final Uri contentUri = ContentUris.withAppendedId(Uri.parse("content://downloads/public_downloads"), Long.valueOf(id));

            return getDataColumn(context, contentUri, null, null);
        }
        // MediaProvider
        else if (isMediaDocument(uri)) {
            final String docId = DocumentsContract.getDocumentId(uri);
            final String[] split = docId.split(":");
            final String type = split[0];

            Uri contentUri = null;
            if ("image".equals(type)) {
                contentUri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;
            } else if ("video".equals(type)) {
                contentUri = MediaStore.Video.Media.EXTERNAL_CONTENT_URI;
            } else if ("audio".equals(type)) {
                contentUri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
            }

            final String selection = "_id=?";
            final String[] selectionArgs = new String[]{split[1]};
            LogUtil.d("format uri:" + contentUri);
            return getDataColumn(context, contentUri, selection, selectionArgs);
        }
    }
    // MediaStore (and general)
    else if ("content".equalsIgnoreCase(uri.getScheme())) {
        return getDataColumn(context, uri, null, null);
    }
    // File
    else if ("file".equalsIgnoreCase(uri.getScheme())) {
        return uri.getPath();
    }

    return null;
}

/**
 * Get the value of the data column for this Uri. This is useful for
 * MediaStore Uris, and other file-based ContentProviders.
 *
 * @param context       The context.
 * @param uri           The Uri to query.
 * @param selection     (Optional) Filter used in the query.
 * @param selectionArgs (Optional) Selection arguments used in the query.
 * @return The value of the _data column, which is typically a file path.
 */
public static String getDataColumn(Context context, Uri uri, String selection,
                                   String[] selectionArgs) {

    Cursor cursor = null;
    final String column = "_data";
    final String[] projection = {column};

    try {
        cursor = context.getContentResolver().query(uri, projection, selection, selectionArgs,
                null);
        if (cursor != null && cursor.moveToFirst()) {
            final int column_index = cursor.getColumnIndexOrThrow(column);
            return cursor.getString(column_index);
        }
    } finally {
        if (cursor != null)
            cursor.close();
    }
    return null;

}
/**
  * @param uri The Uri to check.
  * @return Whether the Uri authority is ExternalStorageProvider.
  */
 public static boolean isExternalStorageDocument(Uri uri) {
     return "com.android.externalstorage.documents".equals(uri.getAuthority());
 }

 /**
  * @param uri The Uri to check.
  * @return Whether the Uri authority is DownloadsProvider.
  */
 public static boolean isDownloadsDocument(Uri uri) {
     return "com.android.providers.downloads.documents".equals(uri.getAuthority());
 }

 /**
  * @param uri The Uri to check.
  * @return Whether the Uri authority is MediaProvider.
  */
 public static boolean isMediaDocument(Uri uri) {
     return "com.android.providers.media.documents".equals(uri.getAuthority());
 }

首先我们看一个获取Mp3文档的Uri，其格式类似：content://com.android.providers.media.documents/document/audio%3A39，
然后我根据代码进一步分析，首先看判断条件：isKitKat && DocumentsContract.isDocumentUri(context, uri)，
这里判断了版本号和该Uri是否是文档类Uri，之所以要判断版本号是Uri的生成在Api19以后发送变化，通过官方文档DocumentsContract，我们也可以验证这点，DocumentsContract是在Api19加入的，其定义就是定义文档提供者与平台之间的协议，其主要作用就是关于文档Uri的一系列操作。
下面是其内部实现代码（代码都是在DocumentsContract类中）：

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private static final String PATH_DOCUMENT = "document";
private static final String PATH_TREE = "tree";
public static final String PROVIDER_INTERFACE = "android.content.action.DOCUMENTS_PROVIDER";

/**
   * Test if the given URI represents a {@link Document} backed by a
   * {@link DocumentsProvider}.
   *
   * @see #buildDocumentUri(String, String)
   * @see #buildDocumentUriUsingTree(Uri, String)
   */
  public static boolean isDocumentUri(Context context, @Nullable Uri uri) {
      if (isContentUri(uri) && isDocumentsProvider(context, uri.getAuthority())) {
          final List<String> paths = uri.getPathSegments();
          if (paths.size() == 2) {
              return PATH_DOCUMENT.equals(paths.get(0));
          } else if (paths.size() == 4) {
              return PATH_TREE.equals(paths.get(0)) && PATH_DOCUMENT.equals(paths.get(2));
          }
      }
      return false;
  }
  /** {@hide} */
   public static boolean isContentUri(@Nullable Uri uri) {
      // public static final String SCHEME_CONTENT = "content";     !!add by custom
       return uri != null && ContentResolver.SCHEME_CONTENT.equals(uri.getScheme());
   }

   private static boolean isDocumentsProvider(Context context, String authority) {
    final Intent intent = new Intent(PROVIDER_INTERFACE);
    final List<ResolveInfo> infos = context.getPackageManager()
            .queryIntentContentProviders(intent, 0);
    for (ResolveInfo info : infos) {
        if (authority.equals(info.providerInfo.authority)) {
            return true;
        }
    }
    return false;
}

这里我们可以看出，前提条件是判断是否是contentUri&&documentProvider，然后在进一步判断其pathSegments
是否是document/或者tree/document/开头。其中isDocumentsProvider方法不是特别理解，希望大神指点下。

参考地址：
[1] 解决Android4.4以上版本Uri转换 https://blog.csdn.net/q445697127/article/details/40537945
[2] https://stackoverflow.com/questions/20067508/get-real-path-from-uri-android-kitkat-new-storage-access-framework