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This example shows what to do about input Readers that sometimes return 0 characters.

Short Version: Wrap your Reader in ZeroReader to get the behaviour of JFlex pre 1.6.1.


The specification for the method int read(char[] cbuf, int off, int len) in says:

* Reads characters into a portion of an array.  This method will block
* until some input is available, an I/O error occurs, or the end of the
* stream is reached.

This means, the read method should never return 0. It should either block and return more than 0 characters, should throw an execption, or should return -1 for end-of-stream. Unfortunately, not all implementations of this class follow that protocol. Up to version 1.6.1, JFlex was trying to work around those problems. This worked in most circumstances, but not in all, depending on how exactly the Reader implementation violates the protocol. See also the discussion on

From version 1.6.1, JFlex will not try to work around this issue any more, but instead throw an IOException if 0 characters are returned. This makes the problem explicit and solutions more robust.


The usual solution if you encounter such an IOException is to wrap the offending Reader in your own Reader class that knows under which conditions the original Reader returns 0.

For instance, for FilterReaders, it is usually enough to just request input again when 0 was returned, until new input arrives. For other Readers, you may have to request a specific number of characters to read to get any input. For yet other situations, you may have to request a single character.


The example wrapper ZeroReader in this directory implements the workaround that JFlex used previously: it first requests characters as usual, using the int read(char[], int, int) method. If the result is 0, it requests a single character using the int read() method. Because this method is supposed to always return either a character or -1, most Readers implement it correctly by blocking if no input is available yet (if read() returns 0, it means the character 0 has been read, not that 0 characters have been read). This method is usually too inefficient to use always, but 0-character returns tend to be rare, so the impact of using it only in those cases is small.