Java URLConnection：如何找出Web文件的大小？ [英] Java URLConnection : how can I find out the size of a web file?

问题描述

我正在为学校开展项目，我正在实施一个可用于从网上下载文件的工具（带有限制选项）。问题是，我将有一个GUI，我将使用 JProgressBar 小部件，我想展示下载的当前进度。为此，我需要知道文件的大小。如何在下载文件之前获取文件的大小。

I'm working on a project for school, and I'm implementing a tool which can be used to download files from the web ( with a throttling option ). The thing is, I'm gonna have a GUI for it, and I will be using a JProgressBar widget, which I would like to show the current progress of the download. For that I would need to know the size of the file. How do you get the size of the file prior to downloading the file.

推荐答案

任何HTTP响应假设包含Content-Length标头，因此您可以查询此值的URLConnection对象。

Any HTTP response is supposed to contain a Content-Length header, so you could query the URLConnection object for this value.

//once the connection has been opened
List values = urlConnection.getHeaderFields().get("content-Length")
if (values != null && !values.isEmpty()) {

    // getHeaderFields() returns a Map with key=(String) header 
    // name, value = List of String values for that header field. 
    // just use the first value here.
    String sLength = (String) values.get(0);

    if (sLength != null) {
       //parse the length into an integer...
       ...
    }

服务器可能无法始终返回准确的内容长度，因此值可能不准确，但至少你在大多数情况下会得到一些可用值。

It might not always be possible for a server to return an accurate Content-Length, so the value could be inaccurate, but at least you would get some usable value most of the time.

更新：或者，现在我看看URLConnection更完整的javadoc，你可以使用 getContentLength（）方法。

update: Or, now that I look at the URLConnection javadoc more completely, you could just use the getContentLength() method.

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