Scanner.nextLine（）何时阻止？

Question

I'm using a Scanner for my socket connection in a Client/Server program.

I want Scanner.next() to block so the Server thread can wait for something to read.

However, it sometimes gives me:

java.util.NoSuchElementException: No line found
    at java.util.Scanner.nextLine(Scanner.java:1585)
    at cscie55.hw4.Server.serviceClient(Server.java:146)
    at cscie55.hw4.Server.main(Server.java:106)

The java documentation says that next() "sometimes" blocks, and it doesn't say whether nextLine() blocks.

Some code online wraps it in an if that tests for null, but I read that it never returns null. What should I do to get blocking IO working properly in my socket?

Specifically,

When does Scanner.nextLine() block rather than throw NoSuchElementException?

Answer 1

The Scanner class will block whenever the InputStream used to construct it would block. It will usually not block when reading from a file because the contents of it are readily available (ie, inputStream.available() > 0 ), they are on your machine after all. However, if you construct a Scanner using an InputStream that may require waiting for data to arrive (eg, reading text from a remote client, waiting for a page to load so you can read its source, etc.) then Scanner#nextLine() will block because it needs to wait for enough information to build the next line to arrive.

Answer 2

It blocks if it's reading from a network and there is no data or no newline present in the input and it hasn't reached EOS. You must be reading from a file.