fix(core): avoid json-format output invoking constructors. #3111 (#3116)

fastjson2 may create a default instance via no-arg constructor when building
writers, which can trigger side effects (e.g. singleton guards) during
`options json-format true`.

Use a custom ObjectWriterProvider that disables default value instantiation
and share the JSON config via ObjectView.toJsonString(), reused by
McpObjectVOFilter.

Fixes #3111

Author: hengyunabc

core/src/main/java/com/taobao/arthas/core/mcp/util/McpObjectVOFilter.java

@@ -1,8 +1,5 @@
 package com.taobao.arthas.core.mcp.util;
 
-import com.alibaba.fastjson2.JSON;
-import com.alibaba.fastjson2.JSONFactory;
-import com.alibaba.fastjson2.JSONWriter;
 import com.alibaba.fastjson2.filter.ValueFilter;
 import com.taobao.arthas.core.GlobalOptions;
 import com.taobao.arthas.core.command.model.ObjectVO;
@@ -93,13 +90,7 @@ private String drawObjectView(ObjectVO objectVO) {
      */
     private String drawJsonView(Object object) {
         try {
-            JSONWriter.Context context = JSONFactory.createWriteContext();
-            context.setMaxLevel(4097);
-            context.config(JSONWriter.Feature.IgnoreErrorGetter, true);
-            context.config(JSONWriter.Feature.ReferenceDetection, true);
-            context.config(JSONWriter.Feature.IgnoreNonFieldGetter, true);
-            context.config(JSONWriter.Feature.WriteNonStringKeyAsString, true);
-            return JSON.toJSONString(object, context);
+            return ObjectView.toJsonString(object);
         } catch (Exception e) {
             logger.debug("ObjectView-style serialization failed, using toString: {}", e.getMessage());
             return objectToString(object);

core/src/main/java/com/taobao/arthas/core/view/ObjectView.java

@@ -4,6 +4,9 @@
 import com.alibaba.arthas.deps.org.slf4j.LoggerFactory;
 import com.alibaba.fastjson2.JSON;
 import com.alibaba.fastjson2.JSONWriter;
+import com.alibaba.fastjson2.writer.FieldWriter;
+import com.alibaba.fastjson2.writer.ObjectWriterCreator;
+import com.alibaba.fastjson2.writer.ObjectWriterProvider;
 import com.taobao.arthas.common.ArthasConstants;
 import com.taobao.arthas.core.GlobalOptions;
 import com.taobao.arthas.core.command.model.ObjectVO;
@@ -25,6 +28,25 @@ public class ObjectView implements View {
     public static final int MAX_DEEP = 4;
     private static final Logger logger = LoggerFactory.getLogger(ObjectView.class);
     private final static int MAX_OBJECT_LENGTH = ArthasConstants.MAX_HTTP_CONTENT_LENGTH;
+    private static final ObjectWriterProvider JSON_OBJECT_WRITER_PROVIDER = new ObjectWriterProvider(
+            new ObjectWriterCreator() {
+                @Override
+                protected void setDefaultValue(List<FieldWriter> fieldWriters, Class objectClass) {
+                    // fastjson2 默认会通过无参构造函数创建一个对象来提取字段默认值（用于 NotWriteDefaultValue 等能力），
+                    // 这可能触发目标对象的构造逻辑（比如单例守卫、资源初始化等），在 Arthas 里属于不可接受的副作用。
+                    // 这里直接禁用该行为，只基于现有对象进行序列化。
+                }
+            });
+
+    public static String toJsonString(Object object) {
+        JSONWriter.Context context = new JSONWriter.Context(JSON_OBJECT_WRITER_PROVIDER);
+        context.setMaxLevel(4097);
+        context.config(JSONWriter.Feature.IgnoreErrorGetter,
+                JSONWriter.Feature.ReferenceDetection,
+                JSONWriter.Feature.IgnoreNonFieldGetter,
+                JSONWriter.Feature.WriteNonStringKeyAsString);
+        return JSON.toJSONString(object, context);
+    }
 
     private final Object object;
     private final int deep;
@@ -54,7 +76,7 @@ public String draw() {
         StringBuilder buf = new StringBuilder();
         try {
             if (GlobalOptions.isUsingJson) {
-                return JSON.toJSONString(object, JSONWriter.Feature.IgnoreErrorGetter);
+                return toJsonString(object);
             }
             renderObject(object, 0, deep, buf);
             return buf.toString();

core/src/test/java/com/taobao/arthas/core/view/ObjectViewTest.java

@@ -1,5 +1,6 @@
 package com.taobao.arthas.core.view;
 
+import com.taobao.arthas.core.GlobalOptions;
 import org.junit.Assert;
 import org.junit.Test;
 
@@ -206,6 +207,22 @@ public void testEnumList() {
         Assert.assertEquals(expected, objectView.draw());
     }
 
+    @Test
+    public void testJsonFormatDoNotCreateNewInstance() {
+        boolean old = GlobalOptions.isUsingJson;
+        try {
+            JsonFormatSingleton singleton = JsonFormatSingleton.getInstance();
+            GlobalOptions.isUsingJson = true;
+
+            ObjectView objectView = new ObjectView(new Object[] { singleton }, 3);
+            String output = objectView.draw();
+            Assert.assertFalse(output.startsWith("ERROR DATA!!!"));
+            Assert.assertEquals(1, JsonFormatSingleton.constructorCalls);
+        } finally {
+            GlobalOptions.isUsingJson = old;
+        }
+    }
+
     @Test
     public void testDate() {
         Date d = new Date(1531204354961L - TimeZone.getDefault().getRawOffset()
@@ -324,4 +341,24 @@ public void setJ(String j) {
     public enum EnumDemo {
         DEMO;
     }
+
+    public static class JsonFormatSingleton {
+        private static final JsonFormatSingleton INSTANCE;
+        private static volatile int constructorCalls = 0;
+
+        static {
+            INSTANCE = new JsonFormatSingleton();
+        }
+
+        private JsonFormatSingleton() {
+            constructorCalls++;
+            if (constructorCalls > 1) {
+                throw new IllegalStateException("JsonFormatSingleton is created!");
+            }
+        }
+
+        public static JsonFormatSingleton getInstance() {
+            return INSTANCE;
+        }
+    }
 }