正确使用Android SQLiteStatement的simpleQueryForBlobFileDescriptor()

发布于 2024-12-04 15:13:56 字数 550 浏览 2 评论 0原文

Android 的 SQLiteStatement 中的 simpleQueryForBlobFileDescriptor() 函数对我来说是个谜。而且似乎没有人在任何地方使用它。看起来这对于单个 blob 访问来说是一种快速方法，并且在性能方面（如果文档可信的话），当用于从数据库中流式传输 blob 数据时，SQLiteStatement 和 Parcel 的组合应该会产生非常快的结果。我不知道，因为我无法让这个东西工作，以便我可以测试...对此的任何帮助或见解将不胜感激：

SQLiteStatement get = mDb.compileStatement(
    "SELECT blobColumn" + 
    " FROM tableName" +
    " WHERE _id = 1" +
    " LIMIT 1"
);

ParcelFileDescriptor result = get.simpleQueryForBlobFileDescriptor();
// now what?

我可能应该注意到我正在使用并且只真正关心 Honeycomb (3.2)并提前感谢您的任何见解。

原文

The function simpleQueryForBlobFileDescriptor() in Android's SQLiteStatement is a mystery to me. And no one seems to use it anywhere. It seems as though it would be a fast method for individual blob access and in terms of performance (if the documentation is to be believed) the combination of SQLiteStatement and Parcel should yield a very fast result when used to stream blob data out of a db. I don't know because I can't get the thing to work so that I could test... Any help with this or insight would be appreciated:

SQLiteStatement get = mDb.compileStatement(
    "SELECT blobColumn" + 
    " FROM tableName" +
    " WHERE _id = 1" +
    " LIMIT 1"
);

ParcelFileDescriptor result = get.simpleQueryForBlobFileDescriptor();
// now what?

I should probably note that I am using and only really care about Honeycomb (3.2) and thank you in advance for any insight.

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薄荷港 2024-12-11 15:13:56

SQLiteStatement get = mDb.compileStatement(
    "SELECT blobColumn" + 
    " FROM tableName" +
    " WHERE _id = 1" +
    " LIMIT 1"
);

ParcelFileDescriptor result = get.simpleQueryForBlobFileDescriptor();
FileInputStream fis = new FileInputStream(result.getFileDescriptor()); // read like any other

我可以验证这对于我的用例来说非常快。

我发现这特别有用的地方（如果您需要像我一样创建自己的自定义离线 webview 文件缓存）是在内容提供者的 openFile 覆盖中，您可以直接返回描述符（或者理想情况下覆盖 openTypedAssetFile()）

<强>例子：
files 是一个数据库助手， getFileDescriptor(String) 与上面的代码类似

public AssetFileDescriptor openTypedAssetFile(Uri uri, String mimeTypeFilter, android.os.Bundle opts) throws FileNotFoundException {
    ParcelFileDescriptor pfd = files.getFileDescriptor(uri.toString());
    if (pfd != null) {
        return new AssetFileDescriptor(pfd, 0, AssetFileDescriptor.UNKNOWN_LENGTH);
    }
    return null;
}

，或者是解决类似问题的另一个很好的用途：
（您想要将自定义文件从数据库提供给网络视图）

private WebViewClient webViewClient = new WebViewClient() {
    @Override
    public WebResourceResponse shouldInterceptRequest(WebView view, String url) {

        ParcelFileDescriptor pfd = files.getFileDescriptor(url);
        if (pfd != null) {
            return new WebResourceResponse(null, null, new FileInputStream(pfd.getFileDescriptor()));
        }

        return null; 
    }

SQLiteStatement get = mDb.compileStatement(
    "SELECT blobColumn" + 
    " FROM tableName" +
    " WHERE _id = 1" +
    " LIMIT 1"
);

ParcelFileDescriptor result = get.simpleQueryForBlobFileDescriptor();
FileInputStream fis = new FileInputStream(result.getFileDescriptor()); // read like any other

and I can verify that this is very fast for my use cases.

Where I found this particularly useful (if you need to create your own custom offline webview file cache like I did) was in the openFile override for a content provider where you can just return the descriptor straight up (or ideally override openTypedAssetFile())

examples:
files is a DB helper and getFileDescriptor(String) is similar to the code above

public AssetFileDescriptor openTypedAssetFile(Uri uri, String mimeTypeFilter, android.os.Bundle opts) throws FileNotFoundException {
    ParcelFileDescriptor pfd = files.getFileDescriptor(uri.toString());
    if (pfd != null) {
        return new AssetFileDescriptor(pfd, 0, AssetFileDescriptor.UNKNOWN_LENGTH);
    }
    return null;
}

or another great use for a similar problem:
(where you want to serve custom file from a db to a webview)

private WebViewClient webViewClient = new WebViewClient() {
    @Override
    public WebResourceResponse shouldInterceptRequest(WebView view, String url) {

        ParcelFileDescriptor pfd = files.getFileDescriptor(url);
        if (pfd != null) {
            return new WebResourceResponse(null, null, new FileInputStream(pfd.getFileDescriptor()));
        }

        return null; 
    }

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