RFR: 8356870: HotSpotDiagnosticMXBean.dumpThreads and jcmd Thread.dump_to_file updates

[Alan Bateman]

On Sun, 25 May 2025 05:16:36 GMT, Chen Liang <liach at openjdk.org> wrote:

>> Updates the thread dump generated by HotSpotDiagnosticMXBean.dumpThreads and jcmd Thread.dump_to_file to include thread state and lock information. Also update the HotSpotDiagnosticMXBean.dumpThreads API description to link to a description of the JSON format dump as that format is intended to be parseable/read by tools.
>> 
>> This PR is dependent on [pull/25425](https://github.com/openjdk/jdk/pull/25425). As noted in that PR, the changes accumulated in the loom repo, and have been split up to make it easier to review. The change to src/hotspot and TheadShapshot are in pull/25425.
>> 
>> The changes include some re-implementation of ThreadDumper. This is because it used PrintStream and didn't fail if there was an I/O error, e.g. file system full. Furthermore, the indentation to pretty print the json was fragile and hard to maintain so this is changed to use a supporting writer class to do this.
>> 
>> Test coverage is significantly expanded, including updating the test library that is used by several tests to parse the thread dump.
>> 
>> Testing: tier1-6
>
> src/java.base/share/classes/jdk/internal/vm/ThreadDumper.java line 247:
> 
>> 245:         var jsonWriter = new JsonWriter(textWriter);
>> 246: 
>> 247:         jsonWriter.startObject();  // top-level object
> 
> Is this anticipated to be used in early bootstrap so we use `startObject` `endObject` instead of using functional code or try-with-resources that create scopes?

There were several prototypes that use AutoCloseable and other shapes. The intention is to keep as a simple as possible, this is not the focus of the change, and we should be able to eventually replace all this with a better writer.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25429#discussion_r2106095592