blocks|key|3101669|text|就在浏览器中使用它而言，这并不重要。我通常会看到file:///...，但一个、两个或三个'/‘都可以工作。这让我认为(不看java文档)这是正常行为。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3101670|entityMap^0|O|B|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

As far as using it in a browser is concerned, it doesn't matter. I have typically seen <code>file:///...</code> but one, two or three '/' will all work. This makes me think (without looking at the java documentation) that it would be normal behavior.

blocks|key|339420|text|这是一个有趣的问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|339421|首先要做的是:我在JRE6上得到了同样的结果。当我去掉toURL()部分时，我甚至得到了它。|339422|RFC2396实际上不需要两个斜杠。根据第3条：|offset|length|339423|339424|+URI语法取决于方案。通常，绝对URI的编写方式如下：|blockquote|339425|339426|339427|339428|话虽如此，RFC2396已经被RFC3986取代，后者表示|339429|339430|通用URI语法由称为方案、授权、路径、查询和片段的组件的分层序列组成。|339431|339432|URI+=+scheme+":“hier-part+"?”查询hier-part+=+"//“权限路径-abempty/路径-绝对/路径-无根/路径-空|339433|方案和路径组件是必需的，但路径可能为空(无字符)。如果有权限，则路径必须为空或以斜杠("/")字符开头。当权限不存在时，路径不能以两个斜杠字符("//")开头。这些限制为路径产生了五个不同的ABNF规则(3.3节)，其中只有一个与任何给定的URI引用相匹配。|339434|339435|所以，就是这样。由于文件URI没有权限段，因此禁止它们以//开头。|339436|然而，RFC直到2005年才出现，而且Java引用了RFC2396，所以我不知道它为什么要遵循这种约定，因为新RFC之前文件URL总是有两个斜杠。|339437|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ietf.org/rfc/rfc2396.txt|1|http://www.faqs.org/rfcs/rfc3986.html^0|0|0|0|7|0|0|0|0|0|0|0|F|7|1|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1D|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1E|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|1F|8|@]|9|@$F|1G|G|1H|1|1I]]|A|$]]|$1|H|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]|$1|I|3|J|5|K|7|1K|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|1M|8|@]|9|@]|A|$]]|$1|N|3|-4|5|6|7|1N|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1O|8|@]|9|@$F|1P|G|1Q|1|1R]]|A|$]]|$1|Q|3|-4|5|6|7|1S|8|@]|9|@]|A|$]]|$1|R|3|S|5|K|7|1T|8|@]|9|@]|A|$]]|$1|T|3|-4|5|6|7|1U|8|@]|9|@]|A|$]]|$1|U|3|V|5|6|7|1V|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|1W|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1X|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|11|3|12|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|13|3|-4|5|6|7|20|8|@]|9|@]|A|$]]]|14|$15|$5|16|17|18|A|$19|1A]]|1B|$5|16|17|18|A|$19|1C]]]]

That's an interesting question.

First things first: I get the same results on JRE6. I even get that when I lop off the toURL() part.

<a href="http://www.ietf.org/rfc/rfc2396.txt" rel="noreferrer">RFC2396</a> does not actually require two slashes. According to section 3:

<blockquote>
 The URI syntax is dependent upon the
 scheme. In general, absolute URI are
 written as follows:

<pre><code>&lt;scheme&gt;:&lt;scheme-specific-part&gt;
</code></pre>
</blockquote>

Having said that, RFC2396 has been superseded by <a href="http://www.faqs.org/rfcs/rfc3986.html" rel="noreferrer">RFC3986</a>, which states

<blockquote>
 The generic URI syntax consists of a
 hierarchical sequence of omponents
 referred to as the scheme, authority,
 path, query, and fragment.

<pre><code> URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]

 hier-part = "//" authority path-abempty
 / path-absolute
 / path-rootless
 / path-empty
</code></pre>
 
 The scheme and path components are
 required, though the path may be empty
 (no characters). When authority is
 present, the path must either be empty
 or begin with a slash ("/") character.
 When authority is not present, the
 path cannot begin with two slash
 characters ("//"). These restrictions
 result in five different ABNF rules
 for a path (Section 3.3), only one of
 which will match any given URI
 reference.
</blockquote>

So, there you go. Since file URIs have no authority segment, they're forbidden from starting with //.

However, that RFC didn't come around until 2005, and Java references RFC2396, so I don't know why it's following this convention, as file URLs before the new RFC have always had two slashes.

blocks|key|339454|text|要回答为什么你可以两者兼得：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|339455|file:/path/file
file:///path/file
file://localhost/path/file|code-block|syntax|javascript|339456|RFC3986+(3.2.2.+Host)声明：|offset|length|339457|339458|“如果URI方案为主机定义了默认值，则当未定义主机子组件或注册名称为空(长度为零)时，该默认值适用。例如，定义”文件“URI方案时，没有颁发机构、空主机和"localhost”都表示最终用户的计算机，而"http“方案将缺少颁发机构或空主机视为无效。”|blockquote|339459|339460|339461|因此，“文件”方案将file:///path/file转换为具有最终用户机器的上下文，即使授权机构是空主机也是如此。|style|CODE|339462|entityMap|0|LINK|mutability|MUTABLE|url|https://www.rfc-editor.org/rfc/rfc3986#section-3.2.2^0|0|0|0|L|0|0|0|0|0|0|A|H|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|14|8|@]|9|@$I|15|J|16|1|17]]|A|$]]|$1|K|3|-4|5|6|7|18|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|19|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|P|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1C|8|@$I|1D|J|1E|S|T]]|9|@]|A|$]]|$1|U|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|11]]]]

To answer why you can have both:
<pre class="lang-none prettyprint-override"><code>file:/path/file
file:///path/file
file://localhost/path/file
</code></pre>
<a href="https://www.rfc-editor.org/rfc/rfc3986#section-3.2.2" rel="nofollow noreferrer">RFC3986 (3.2.2. Host)</a> states:
<blockquote>
&quot;If the URI scheme defines a default for host, then that default applies when the host subcomponent is undefined or when the registered name is empty (zero length). For example, the &quot;file&quot; URI scheme is defined so that no authority, an empty host, and &quot;localhost&quot; all mean the end-user's machine, whereas the &quot;http&quot; scheme considers a missing authority or empty host invalid.&quot;
</blockquote>
So the &quot;file&quot; scheme translates <code>file:///path/file</code> to have a context of the end-user's machine even though the authority is an empty host.

The system I'm running on is Windows XP, with JRE 1.6.

I do this : 

<pre><code>public static void main(String[] args) {
 try {
 System.out.println(new File("C:\\test a.xml").toURI().toURL());
 } catch (Exception e) {
 e.printStackTrace();
 } 
}
</code></pre>

and I get this : <code>file:/C:/test%20a.xml</code>

How come the given URL doesn't have two slashes before the <code>C:</code> ? I expected <code>file://C:...</code>. Is it normal behaviour?

<hr>

EDIT :

From Java source code : java.net.URLStreamHandler.toExternalForm(URL)

<pre><code> result.append(":");
 if (u.getAuthority() != null &amp;&amp; u.getAuthority().length() &gt; 0) {
 result.append("//");
 result.append(u.getAuthority());
 }
</code></pre>

It seems that the Authority part of a file URL is null or empty, and thus the double slash is skipped. So what is the authority part of a URL and is it really absent from the file protocol?

Java : File.toURI().toURL() on Windows file

Windows

Java

我运行的系统是Windows XP，带有JRE 1.6。我这样做：public static void main(String[] args) {    try {        System.out.println(new File("C:\\test a.xml").toURI().toURL());    } c...

问Java : Windows文件上的File.toURI().toURL()
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