Ankur
c438afc33b
37 runnable examples covering the eight feature posts on ankurm.com, verified against Jackson 3.2.1 on Temurin 21.0.5. Every output committed under docs/ was produced by run-all.sh. Also documents 11 places where the published snippets do not compile or do not behave as printed against a real Jackson 3 build - most notably that writeValueAsString(List<Base>) silently drops the polymorphic type discriminator, so the post's serialised output cannot be read back.
81 lines
2.9 KiB
Markdown
81 lines
2.9 KiB
Markdown
# Part 6 — Streaming API and the tree model
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Post: <https://ankurm.com/jackson-streaming-api-jsonnode/>
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Renames that break the post's code verbatim:
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| Jackson 2 | Jackson 3 |
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|---|---|
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| `com.fasterxml.jackson.core.JsonFactory` | `tools.jackson.core.json.JsonFactory` (note the extra `.json`) |
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| `parser.getCurrentName()` | `parser.currentName()` |
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| `parser.getText()` | `parser.getString()` |
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| `JsonToken.FIELD_NAME` | `JsonToken.PROPERTY_NAME` |
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| `gen.writeNumberField` / `writeStringField` | `gen.writeNumberProperty` / `writeStringProperty` |
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| `node.asText()` | `node.asString()` |
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| `factory.createParser(file)` | `factory.createParser(ObjectReadContext.empty(), file)` |
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The comparison post's performance section gives the import as
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`tools.jackson.core.JsonFactory`, which does not resolve — the class is in the
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`.json` subpackage.
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## G01 — filtering a large array
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[`G01StreamingParserFilter.java`](../src/main/java/com/ankurm/jackson3/part6streaming/G01StreamingParserFilter.java)
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```
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ERROR: Database connection failed
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Total errors found: 1
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```
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## G02 — writing a million records
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[`G02StreamingGenerator.java`](../src/main/java/com/ankurm/jackson3/part6streaming/G02StreamingGenerator.java)
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The post's loop, kept at its full 1,000,000 iterations, with the heap delta measured so
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"constant memory usage" is a number.
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```
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records written : 1000000
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file size : 30 MB
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elapsed : 129 ms
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heap delta : 0 MB <- the document is never held in memory
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```
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## G03 — the tree model
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[`G03TreeModelNavigation.java`](../src/main/java/com/ankurm/jackson3/part6streaming/G03TreeModelNavigation.java)
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```
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Customer: Alice
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KB-01 x2
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MS-42 x1
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Has discount: false
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path(missing) : (class MissingNode)
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get(missing) : null
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deep path : '<default>'
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treeToValue : CustomerRecord[name=Alice, tier=gold]
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```
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The `path(missing)` / `get(missing)` pair is the whole argument for `path()`: it returns
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a `MissingNode` that chains safely, where `get()` returns `null` and the next call NPEs.
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## G04 — the decision table, measured
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[`G04ThreeApproachesMeasured.java`](../src/main/java/com/ankurm/jackson3/part6streaming/G04ThreeApproachesMeasured.java)
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The post closes with a table asserting relative memory and verbosity. This runs the same
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filter three ways over a generated 20 MB file.
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```
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input file : 20 MB, 200000 entries
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data binding (readValue) errors=4000 524 ms heap delta 47 MB
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tree model (readTree) errors=4000 341 ms heap delta 102 MB
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streaming (JsonParser) errors=4000 78 ms heap delta 1 MB
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```
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Single-shot measurements on a 2-core container, not JMH — treat the ordering as the
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result, not the absolute figures. Two things do stand out and are stable across runs:
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the tree model costs roughly twice the heap of data binding for the same document, and
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streaming is the only approach whose heap does not scale with input size.
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