On Wed, Apr 25, 2018 at 4:16 PM, <[email protected]> wrote:
> Hello,
>
> Today I ran into a bizarre issue where my application indented its JSON
> improperly or not at all, sometimes always wrong, and sometimes only
> partially wrong, and sometimes changing what kind of wrong from REST call to
> REST call.
>
> After researching the problem, I noticed that PrettyPrinter and Indenter are
> not thread-safe, but do not mention this in their API documentation, as far
> as I could read during my investigation (using Jackson 2.9.5).
Have another look at `PrettyPrinter` Javadoc, this is expalined there, in second
paragraph (wrt `Instantiatable`).
> I am not 100% sure I have fixed this properly in my code, because it's
> extremely complicated. It appears that ObjectMapper is thread-safe, meaning
> you can re-use a static ObjectMapper in multiple parts of the code, however,
> it would seem that calling mapper.setDefaultPrettyPrinter(...) would
> potentially mean that it's not-thread-safe anymore based on the issue I was
> seeing
>
> I had to make a factory method of sorts to generate a pretty-printer:
>
> public static DefaultPrettyPrinter getPrinter() {
> DefaultPrettyPrinter.Indenter indenter =
> new DefaultIndenter(" ", DefaultIndenter.SYS_LF);
> DefaultPrettyPrinter printer = new DefaultPrettyPrinter();
> printer.indentObjectsWith(indenter);
> printer.indentArraysWith(indenter);
> return printer;
> }
>
> Then I needed this in my JsonFactory:
>
> public static class CustomJsonFactory extends JsonFactory {
> private static final long serialVersionUID = -3521368899846283407L;
>
> @Override
> protected JsonGenerator _createGenerator(Writer out, IOContext ctxt)
> throws IOException {
> DefaultPrettyPrinter printer = getPrinter();
> return super._createGenerator(out,
> ctxt).setPrettyPrinter(printer);
> }
>
> @Override
> protected JsonGenerator _createUTF8Generator(OutputStream out,
> IOContext ctxt) throws IOException {
> DefaultPrettyPrinter printer = getPrinter();
> return super._createUTF8Generator(out,
> ctxt).setPrettyPrinter(printer);
> }
> }
>
> However, I am not sure if this fix will help on the static copies of
> ObjectMapper I set up in the code, for doing stuff like internally
> converting YAML to Objects, Objects to JSON strings for log messages, etc.
>
> If anybody has some more experience how this should work, I'd love
> recommendations, and I would be happy to help write some kind of
> documentation patch if I can understand how this should be done properly to
> avoid breakage.
So: to make sure `PrettyPrinter` works in thread-safe manner, it needs
to either:
1. Be stateless (like `MinimalPrettyPrinter`), immutable. OR
2. Implement `com.fasterxml.jackson.core.util.Instantiatable`,
implement `createInstance()` method
that creates stateful instance that is not shared across threads
`DefaultPrettyPrinter` does the latter. If you want to sub-class it,
however, you MUST override `createInstance()`
method; otherwise it will probably not work the way you want (since by
default it can not create instance of sub-class).
So use of `DefaultPrettyPrinter` should be fine without any extra work.
Assuming that all code paths in Jackson do use `createInstance`. So
one thing that could be useful is to see how
your code is creating generators. With that, it should be possible to
create actual test case with custom `PrettyPrinter`
that verifies that a copy must have been made (keep a flag or
something, set on "blueprint" instance, cleared for
actual instances created).
-+ Tatu +-
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