java - URL、扫描器和分隔符 : How does this Java Line of Code Works?

Question

谁能帮我理解这行代码是如何工作的:

String s = new Scanner(new URL("http://example.com").openStream(), "UTF-8").useDelimiter("\\A").next();

该代码用于直接从网页读取。扫描器对象究竟是如何转换为字符串的，以及我们为什么使用定界符。

谢谢。

Answer 1

这是发生的事情，滥用缩进

     new Scanner(                           // A new scanner is created
             new URL("http://example.com")  // the scanner takes a Stream 
                                            // which is obtained from a URL
          .openStream(),                    // - openStream returns the stream
       "UTF-8")                             // Now the scanner can parse the        
                                            // stream character by character
                                            // with UTF-8 encoding

     .useDelimiter("\\A")                   // Now the scanner set as 
                                            // delimiter the [Regexp for \A][1]
                                            // \A stands for :start of a string!

   .next();                                 // Here it returns the first(next) 
                                            // token that is before another
                                            // start of string. 
                                            // Which, I'm not sure 
                                            // what it will be

From the Java documentation

A simple text scanner which can parse primitive types and strings using regular expressions. A Scanner breaks its input into tokens using a delimiter pattern, which by default matches whitespace. The resulting tokens may then be converted into values of different types using the various next methods.

所以您只是将 \A 替换为分隔符(而不是空格)。但是 \A has a specific meaning when evaluating as regular expression!

如果您的流仅包含以下文本

\Ahello world!\A Goodbye!\A

您的代码将返回整行 \Ahello world!\A Goodbye!\A

如果您想去除反斜杠后跟大写字母 A 的序列，那么您应该使用 \\\\A。

感谢@Faux Pas 指出这一点!