fix: Map subclass loses @type when serialized with ValueFilter (#3984) by wenshao · Pull Request #4029 · alibaba/fastjson2

wenshao · 2026-03-15T00:18:12Z

What this PR does / why we need it?

当使用 WriteClassName 特性序列化 HashMap 子类时，如果同时设置了 ValueFilter，@type 字段会被静默丢弃，导致反序列化时丢失类型信息。

复现代码：

CustomMap map = new CustomMap(); // extends HashMap
map.put("key1", "value1");
ValueFilter filter = (object, name, value) -> value;
String json = JSON.toJSONString(map, new ValueFilter[]{filter}, JSONWriter.Feature.WriteClassName);
// 实际输出: {"key1":"value1"}  — 缺少 @type
// 期望输出: {"@type":"...CustomMap","key1":"value1"}

Summary of your change

根因： ObjectWriterImplMap.write() 方法在写入 Map 条目前会调用 writeTypeInfo()，但 writeWithFilter() 方法完全缺失了这一逻辑。当存在 Filter 时，write() 会委托给 writeWithFilter()，导致 @type 丢失。

修复： 在 writeWithFilter() 的 startObject() 之后、写入 Map 条目之前，添加与 write() 一致的 writeTypeInfo 判断和调用逻辑：

boolean writeTypeInfo = (fieldType == this.objectType
        && jsonWriter.isWriteMapTypeInfo(object, this.objectClass, features))
        || jsonWriter.isWriteTypeInfo(object, fieldType, features);
if (writeTypeInfo) {
    writeTypeInfo(jsonWriter);
}

测试覆盖：
- 单层 Map 子类 + ValueFilter + WriteClassName
- 无 Filter 时行为不变（对照）
- 嵌套 Map 子类，验证内外层均写入 @type

Please indicate you've done the following:

- Made sure tests are passing and test coverage is added if needed.
- Made sure commit message follow the rule of https://www.conventionalcommits.org/.
- Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

@type

ObjectWriterImplMap.writeWithFilter() was missing the writeTypeInfo call that write() had, causing @type to be silently dropped when any filter was present with WriteClassName enabled. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Copilot

Pull request overview

Fixes missing @type emission when serializing Map subclasses with ValueFilter and WriteClassName enabled, and adds regression tests for the behavior.

Changes:

Add writeTypeInfo handling to ObjectWriterImplMap.writeWithFilter() so @type is written when required.
Add JUnit tests covering WriteClassName with/without ValueFilter, including nested map scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java	Restores type info writing in the filtered map writer path.
core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3984.java	Adds regression tests ensuring `@type` is preserved with filters and nested maps.

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core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java

+        boolean writeTypeInfo = (fieldType == this.objectType && jsonWriter.isWriteMapTypeInfo(object, this.objectClass, features))
+                || jsonWriter.isWriteTypeInfo(object, fieldType, features);
+        if (writeTypeInfo) {
+            writeTypeInfo(jsonWriter);
+        }
+
        features |= jsonWriter.getFeatures();


core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3984.java

+        assertTrue(json.contains("@type"), "JSON should contain @type when WriteClassName is enabled with ValueFilter: " + json);
+        assertTrue(json.contains("CustomMap"), "JSON should contain class name: " + json);


core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3984.java

+        int firstIdx = json.indexOf("@type");
+        int lastIdx = json.lastIndexOf("@type");
+        assertTrue(firstIdx >= 0, "Should contain @type: " + json);
+        assertTrue(firstIdx != lastIdx, "Should contain @type for both outer and inner maps: " + json);


@type

Address review feedback: assert against full qualified type name via CustomMap.class.getName() and count exact @type occurrences instead of substring checks. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Copilot

Pull request overview

Fixes a regression where @type metadata is omitted when serializing HashMap subclasses with WriteClassName if a ValueFilter is also applied (Issue #3984).

Changes:

Add @type emission logic to ObjectWriterImplMap.writeWithFilter() to match write() behavior.
Add regression tests covering filtered/unfiltered and nested Map-subclass serialization with WriteClassName.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java	Ensures type info is written even when filters are present.
core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3984.java	Adds regression coverage for `@type` output with `ValueFilter` and nested map subclasses.

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core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplMap.java

        jsonWriter.startObject();
        Map map = (Map) object;

+        boolean writeTypeInfo = (fieldType == this.objectType && jsonWriter.isWriteMapTypeInfo(object, this.objectClass, features))
+                || jsonWriter.isWriteTypeInfo(object, fieldType, features);
+        if (writeTypeInfo) {
+            writeTypeInfo(jsonWriter);
+        }
+
        features |= jsonWriter.getFeatures();


core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3984.java

+        String json = JSON.toJSONString(map,
+                new ValueFilter[]{valueFilter},
+                JSONWriter.Feature.WriteClassName);
+
+        String expectedType = "\"@type\":\"" + CustomMap.class.getName() + "\"";
+        assertTrue(json.contains(expectedType),
+                "JSON should contain " + expectedType + ", actual: " + json);


core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3984.java

+        String json = JSON.toJSONString(outer,
+                new ValueFilter[]{valueFilter},
+                JSONWriter.Feature.WriteClassName);
+
+        String typeToken = "\"@type\":\"" + CustomMap.class.getName() + "\"";
+        int count = 0;
+        int idx = 0;
+        while ((idx = json.indexOf(typeToken, idx)) != -1) {
+            count++;
+            idx += typeToken.length();
+        }
+        assertEquals(2, count,
+                "Should contain exactly 2 @type entries for outer and inner maps: " + json);


@type

Parse serialized JSON and assert @type via JSONObject.getString() rather than substring matching, making tests resilient to formatting/order changes. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

wenshao requested a review from Copilot March 15, 2026 00:44

Copilot AI reviewed Mar 15, 2026

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Copilot started reviewing on behalf of wenshao March 15, 2026 00:55 View session

wenshao requested a review from Copilot March 15, 2026 01:05

Copilot AI reviewed Mar 15, 2026

View reviewed changes

test: use structural JSON assertions instead of substring matching

aa0004f

Parse serialized JSON and assert @type via JSONObject.getString() rather than substring matching, making tests resilient to formatting/order changes. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Copilot started reviewing on behalf of wenshao March 15, 2026 01:34 View session

wenshao merged commit 14ea6ea into main Mar 15, 2026
62 of 63 checks passed

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fix: Map subclass loses @type when serialized with ValueFilter (#3984)#4029

fix: Map subclass loses @type when serialized with ValueFilter (#3984)#4029
wenshao merged 3 commits intomainfrom
fix/3984-map-writeClassName-with-filter

wenshao commented Mar 15, 2026 •

edited

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		assertTrue(json.contains("@type"), "JSON should contain @type when WriteClassName is enabled with ValueFilter: " + json);
		assertTrue(json.contains("CustomMap"), "JSON should contain class name: " + json);

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wenshao commented Mar 15, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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wenshao commented Mar 15, 2026 •

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