Java URLConnection :我怎样才能找出网络文件的大小? [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.

解决方案

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.

解决方案

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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