RFR: 8341566: Adding factory for non-synchronized CharSequence Reader [v2]

[Chen Liang]

On Mon, 7 Oct 2024 17:39:33 GMT, Markus KARG <duke at openjdk.org> wrote:

>>> keep the reference to the char sequence "forever"
>> 
>> Note that this only happens if the `Reader` instance is kept forever; but a `Reader` is supposed to be used and closed and discarded instead of being held indefinitely after closing. If users are bad enough, they can never call `close` and the non-final `cs` is still leaked.
>> 
>> The right way of GC freeing is to let the GC free the reader and the char sequence together, instead of allowing to free when a client keeps this reader instance erroneously after closing.
>
> I really do see you point, nevertheless I think we should be kind to "bad" users, too. In the end, there is *no* specification which explicitly tells "bad" users that readers are support to be short-living (despite it being rather obvious), while `close` explicitly mentions freeing *all* resources, so it is rather unexpected to keep something past `close`, even for short term. NB: `StringReader` applies the very same behavior, too (using `s` and `str` variables), that's where I copied the code from originally. It is not a big problem for us to have two variables, neither that the name of the variable is `source` (in fact, I do like `source` even more than `cs`, but this is just my personal preference). To sum up, I really dislike the idea to keep the reference for any longer than essentially needed. If it's just about the name of the variable, I could simply switch them if you prefer that.

On a linux x64 build:

./javap -p java.io.Reader$2
Compiled from "Reader.java"
class java.io.Reader$2 extends java.io.Reader {
  private final int length;
  private java.lang.CharSequence cs;
  private int next;
  private int mark;
  final java.lang.CharSequence val$source;
  java.io.Reader$2(java.lang.CharSequence);
  private void ensureOpen() throws java.io.IOException;
  public int read() throws java.io.IOException;
  public int read(char[], int, int) throws java.io.IOException;
  public long skip(long) throws java.io.IOException;
  public boolean ready() throws java.io.IOException;
  public boolean markSupported();
  public void mark(int) throws java.io.IOException;
  public void reset() throws java.io.IOException;
  public void close();
}


The javap in the built binaries of this patch shows that this anonymous class already keeps the `CharSequence val$source` in a final field; therefore, your setting the `cs` field to `null` will not help GC or anything of that sort.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21371#discussion_r1790782348