Java URLConnection :我怎样才能找出网络文件的大小?

问题描述

我正在为学校做一个项目，我正在实现一个可用于从网络下载文件的工具(带有限制选项).问题是，我将有一个 GUI，我将使用一个 JProgressBar 小部件，我想显示下载的当前进度.为此，我需要知道文件的大小.在下载文件之前如何获取文件的大小.

应该任何 HTTP 响应都包含 Content-Length 标头，因此您可以查询 URLConnection 对象以获取此值.p>

//一旦打开连接列表值 = urlConnection.getHeaderFields().get("content-Length")if (values != null && !values.isEmpty()) {//getHeaderFields() 返回带有 key=(String) 标头的 Map//name, value = 该标头字段的字符串值列表.//这里只使用第一个值.String sLength = (String) values.get(0);如果(sLength！= null){//将长度解析为整数......}

服务器可能无法始终返回准确的 Content-Length，因此该值可能不准确，但至少在大多数情况下您会得到一些可用值.p>

更新: 或者，现在我更完整地查看了 URLConnection javadoc，您可以只使用 getContentLength() 方法.

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.

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...
       ...
    }

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.

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

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